The Exhaustive Guide To Back Pressure

[gvfc2]

here is the site this is all taken from http://genesisforums.org/hyundai-genesis-coupe-performance/7146-back-pressure-needed.html

Backpressure: The myth and why it's wrong.

I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?
I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door.

 

[gvfc2]

Another good read on back pressure:

Destroying a myth.

Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

 

[gvfc2]

B]More detailed explanation on back pressure:[/B]

Exhaust backpressure can cause a variety of problems. A plugged catalytic converter can strangle engine breathing and cause a big drop in engine performance and fuel economy. And if the converter plugs up completely, it can make the engine stall. The same thing can happen if a muffler, resonator or double walled exhaust pipe collapses internally. Anything that restricts exhaust flow will create excessive backpressure in the exhaust system.

EXHAUST BACKPRESSURE SYMPTOMS

The classic symptoms of too much backpressure include things like a lack of high speed power, poor fuel economy and even overheating. Anything that backs up exhaust pressure into the engine will also back up heat. About a third of the heat produced by combustion goes out the tailpipe as waste heat, so if the heat can't escape it can overload the cooling system and make the engine run hotter than normal, especially at highway speeds.

If there is a complete blockage in the exhaust, the engine may start and idle fine for a minute or two, then die as backpressure builds up and strangles the engine. In some instances, backpressure may buildup to such a degree that it blow out a pipe connector or the converter shell. That makes diagnosis a lot easier, but in most cases you may not be sure if there is an exhaust restriction or not. So in these instances, you need to measure backpressure.

HOW TO MEASURE EXHAUST BACKPRESSURE

To measure exhaust backpressure, you need a pressure gauge with a scale that reads zero to 15 psi, or zero to 100 kPa or higher (note: 1 psi equals 6.89 kPa, and 1 kPa equals 0.145 psi). If you don't have a low pressure gauge, you can buy a basic exhaust backpressure test kit for around $60. If you want to be really accurate you can use a digital manometer or pressure gauge that displays pressure readings in a variety of different units of measurement (psi, kPa, inches Hg, inches H2O, bar, etc.). A tool that reads from 0 to 15 psi will typically cost around $170.

Measuring exhaust backpressure is not as easy as it sounds because there is no quick and easy way to tap into the exhaust system. If the engine has an air pump, you can tap into the exhaust system at the air pump check valve. Disconnect the check valve and install a pressure gauge. For accurate test results, however, the check valve must connect to the exhaust system ahead of the converter. Note: if the air pump plumbing hooks up at the converter, this technique won't give you reliable results.

You can also check backpressure by removing an oxygen sensor from the exhaust manifold, and connecting a hose fitting to your pressure gauge. If the O2 sensor is fairly easy to reach and the vehicle is fairly new, it should come out without too much effort. But on an older vehicle, O2 sensors can be difficult to remove. And there is always the risk of damaging the sensor.

A third option is to drill a small hole into the exhaust pipe just ahead of the converter and attach a fitting for your pressure gauge or manometer. This may be easier than trying to remove a 10 year old O2 sensor, but it also means you'll have to plug the hole afterwards with a self-tapping screw or a small spot weld.

On some diesel engines (Ford diesel trucks, for example), there is a backpressure sensor in the exhaust system that measures exhaust backpressure directly. You can see the actual value by using a scan tool and looking at the exhaust backpressure sensor PID.

BACKPRESSURE READINGS

Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal.

A partially restricted converter, muffler or pipe may flow enough exhaust at idle not to cause a problem, but chokes breathing at higher engine speeds. So to test this possibility, you need to rev and hold the engine at 2000 rpm. A "good" reading on most engines at 2000 rpm should be 3 psi (20 to 21 kPa) or less. Again, there may be some vehicles that will read a little higher that don't have a problem, but the reading should not be significantly higher.

Pay close attention to what the backpressure reading does while you are holding it at 2000 rpm. If it remains steady, chances are there is no restriction. But if the reading gradually increases, it means backpressure is building up and there may be a blockage.

If you want to rev the engine higher, say to 4000 rpm and hold it, the backpressure numbers will shoot up. Most stock exhaust systems will show backpressure readings from 4 to 8 psi (27 to 55 kPa), or even higher. As before, if the backpressure reading is unusually high or it continues to climb at a steady rpm, it usually means there is an abnormal restriction causing an unhealthy increase in backpressure.

INSPECTING THE EXHAUST SYSTEM

Higher than normal backpressure readings mean something is restricting the flow of exhaust out the tailpipe. Though the converter is usually the trouble spot, restrictions can also occur inside mufflers and resonators if a baffle collapses or the fiberglass sound-absorbing roving clogs up an internal passageway. Double-wall exhaust pipes can also collapse internally causing a blockage.

If the system has a blockage, inspect the exhaust system end to end for any obvious signs of damage like a crushed pipe, severe corrosion, etc. You can thunk the converter to see if it rattles inside (indicating the catalyst substrate is broken).

The next step would be to disconnect the exhaust pipe just behind the converter to see if that makes a difference in the backpressure readings. The readings will go down a bit when the exhaust system aft of the converter is disconnected, but if you don't see any drop it means the converter is probably plugged. The other possibility is that the head pipe between the exhaust manifold and converter has collapsed internally.

If you see a big drop in the backpressure readings when the exhaust system aft of the converter is disconnected, it means the converter is flowing okay and the blockage is somewhere in the rest of the system (bad muffler, resonator or tailpipe).

VACUUM TEST ANOTHER WAY TO CHECK FOR BACKPRESSURE PROBLEMS

Another way to check for a backpressure problem is to check intake vacuum at the engine. It's much easier to hook up a vacuum gauge to a vacuum hose or port than it is to remove an O2 sensor. Vacuum gauges typically display readings in inches of Hg (Mercury). One inch Hg equals a pressure reading of 0.49 psi or 3.38 kPa.

Normal atmospheric pressure is around 14.7 lbs. per square inch at seal level. This will vary a bit with temperature and humidity, and it goes down at higher elevations. Vacuum is created inside the engine's intake manifold by the intake stoke of the pistons trying to overcome the restriction created by the throttle butterfly. On most engines will develop 16 to 22 inches Hg of vacuum at idle (except for most diesels which have no intake vacuum because they have no throttle plate). The vacuum reading at idle will depend on engine wear, throttle opening, camshaft overlap, exhaust backpressure, air temperature and density.

To check vacuum at the intake manifold, start with the engine off, and disable the EGR valve by removing or disconnecting its hose or one of its solenoids. Connect a vacuum gauge to a ported vacuum source on the intake manifold or throttle body. Start the engine and note the vacuum reading at idle with the transmission in neutral.

If the idle vacuum reading is lower than normal, or it continues to drop while the engine idles, an exhaust restriction is causing exhaust pressure to build up and backup into the engine.

If you increase engine speed, the vacuum reading will drop slightly, then stabilize and rise back up to within 2 to 3 inches of the vacuum reading you noted at idle. Any sudden drop of over 10 inches Hg of vacuum may indicate a blockage problem. Erratic swings of the vacuum indicator may indicate periodic blockages caused by loose components temporarily blocking the exhaust system.

Vacuum readings can also be affected by other factors such as weak or broken valve springs, overadvanced or retarded valve timing and/or ignition timing. So if the needle on the gauge is bouncing around, it could indicate a mechanical problem in the engine.

REDUCING BACKPRESSURE

Reducing exhaust backpressure can improve fuel economy and performance. Reducing restrictions in the exhaust system allows the exhaust to flow more easily so the engine can breathe more efficiently. The most common modification is to replace the stock muffler with a low restriction aftermarket performance muffler, or to replace the entire stock exhaust system from the catalytic converter back with a free-flowing aftermarket performance exhaust system

 

[gvfc2]

math involved

Exhaust Backpressure Study

Replacing the stock production exhaust system with a low-restriction, free-flow one is usually one of the first modifications made to any vehicle in the name of performance. We all know they're louder, but how much performance do they really add? We've all seen supposed dyno tests, usually run by the exhaust manufacturer's themselves on their own dyno, indicating vast power gains, and psychologically, we always equate a healthy exhaust rumble with increased power in the seat of the pants, but how much power are we really gaining? To find out, we're running a simple backpressure study, and our results will be posted here as they come. Admittedly this study is not totally scientific as there are many uncontrolled variables, but it should be sufficient to provide a rough estimate.

It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.

1 inch Hg backpressure = 1 HP lost

For reference, we have the following conversions factors:

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar

Our test vehicle is a '97 Buick Regal GS with 3800SC engine transversely mounted. It's exhaust system consists of a cast iron exhaust manifold on the left side of the engine which connects into a tuned tubular header on the right side, both banks connected to a single downpipe into a catalyst. The output of the catalyst runs into a resonator and then into a single muffler; all pipes are 2.25 inch. The exhaust system is very similar in the Pontiac GTP, the differences being that the GTP splits into 2 mufflers after the resonator. Our sources indicate that the GTP system results in approximately 3 in Hg less backpressure than the Regal, hence there is 3 less horsepower loss.

To measure system backpressure, a sample tube was mounted before the catalyst into the downtube

A flexible hose is run from the sample tube and attached to a pressure
gauge inside the car for monitoring.

We first ran the test with the complete full factory exhaust, and next dropped the entire system from the catalyst back.

The final test was run with the muffler removed. Only the catalyst, resonator, and the majority of 2.25 tubing up to the muffler remained.

Next we took a look at the restrictive U bend that houses the post O2 sensor. It's function is to protect the O2 sensor from damage by positioning it straight up.

This restrictive U bend is completely removed, and replaced with a straight 3" piece. (Note this 3" piece is not the actual replacement pipe - it's just a scrap piece for the photo.)

This U bent tube is replaced by this straight 3" pipe. The O2 sensor is mounted to the side - less protected, but it'll be OK unless you go off-roading as we recently did!

Results & Conclusions
We ran three tests, observing three runs with each configuration and averaging the three. Peak backpressure occurred near the engine RPM redline of 5700-6000 rpm, at a max boost of approximately 7-8 psi. We took all our readings at WOT immediately before the 1-2 shift. Although we performed our tests on a Buick Regal GS, we predict a GTP will have similar results, taking into account the 3 in Hg difference. We realize our tests are not totally scientific, and they were not meant to be. Our goal is to obtain a ballpark estimate which, as the saying goes, is "good enough for government work." [Before you government employees start flaming us - one of our associates worked for the US Army Corps of Engineers for several years, so we know how it is. 8^) ]

1. Full factory exhaust system of catalyst, resonator & muffler: 28-30 in Hg = 28-30HP lost [system is whisper quiet]
2. Only catalyst in place, no resonator or muffler after the cat: 13-14 in Hg = 13-14HP lost, thus approximately 14-17 HP gained over stock full exhaust [system is unbearably loud and shakes the entire car, conversation is impossible]
3. No muffler, just resonator, catalyst & tubing 20 in Hg = 20HP lost, thus approximately 10HP gained over stock full exhaust [system is bearable, but has some bad resonances and drones at particular RPMs.]
4. "U" pipe replaced with straight 3" pipe Testing to be determined.

The catalyst was never removed as we were only interested in achieving an optimal cat-back system. We can see from our results that the muffler is costing approximately 10HP loss while the resonator accounts for a 6HP drop, with everything from catalyst to the engine costing 14-15HP.

Therefore, it's evident that at best, a free-flow system will gain perhaps 10HP - and that's for a noisy system, while one which controls irritating resonances and drones better would probably gain less than that. Therefore, a 5-7HP gain from a cat-back exhaust system is probably in the ballpark for achievable gains.

Will removing the catalyst help? Definitely, but that's illegal for street use and probably more importantly to some folks out there, it sets an OBDII Malf code. Replacing the factory catalyst with a high-flow unit will not result in a significant increase either, as those "high-flow" units outflow a production unit by a couple of inches of Hg at best. In fact, our sources indicate that the catalyst on these cars are actually one of the least restrictive available. With a FWD platform, we're stuck with uneven header lengths due to the transverse mounted engine, limiting one's ability to truly optimize the header design. Therefore, it is probably more fruitful and definitely more cost effective to examine the situation after the catalyst.

The difficulty in designing an effective exhaust system is in minimizing backpressure while achieving a desirable exhaust tone with minimal resonances and drones. It has been suggested that replacing both the resonator and muffler with a single large staight-thru muffler (with dual outlets for the GTP) may be the best solution.

Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the *muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder build up pressure in the manifold that affects the next cylinder that uses the manifold.

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.

Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

 

[gvfc2]

I decided to post all this since there has been alot of people asking about back pressure.

I was told today it was needed in automobiles. maybe some but not alot.

so i did a google search and came up with this.

please dont delete or anything except maybe move it to a more appropriate place

 

[Doc Sprocket]

I can't thank you enough for posting this! I was getting tired of feeling like the only voice in the crowd, trying like heck to get folks to understand... Back pressure is bad! How many times have I said this?

I am stickying this immediately!

 

[machinist@large]

This needs a mandatory sticky!!!!!!

gvfc2, do you have more footnotes for all this? I don't mean to tear it down (anything but!!), it's just that you dug up some good, relevant information, and if we have the ibid & op. cit. for it all, the rest of us could dig into our hearts content.

Moderators? I hereby motion this thread for a "Sticky". Anyone want to second it?

Pat

EDIT; it seems TS has beaten me to it. That's what I get for looking at other recommended forums around here.....

 

[Doc Sprocket]

LOL... WAAAY ahead of you, buddy!

 

[gvfc2]

i dont have anymore info but i will post as i find it.

 

[machinist@large]

LOL... WAAAY ahead of you, buddy!

You know, I could really start pointing fingers right about now; you want us to look in on another forum that might be interesting, then dis us for not being on top of things here because we did as YOU ASKED US TO DO!!!!

(Now, where did I put that recipe......)

 

[landuse]

Nice one gvfc2!! Thanks for that.

 

[gvfc2]

no problem

 

[J_Walker]

Back pressure is a weird myth, that doesn't exactly exist.. When people refer to back pressure they are most likely talking about the "low" pressure, or the vacuum you can create using the exhaust gases as they exit as more are being expelled from the exhaust valve ports..

While I4's don't normally use to much of this Low pressure.. Being a huge twin fan, I myself noticed the more "low pressure" you can get [suction] the more low end torque you can get..

And what about thumpers? they have expansion chambers for a reason.. I must of missed the part in this post about these things..?

 

[gvfc2]

If you really found out what goes on with expansion chambers, you know it's a fairly complicated thing. Sonic-speed shock waves are timed to arrive just in time to stuff the fuel/air that escaped through the two-stroke's exhaust port back into the cylinder just before closing.

Four stroke exhausts can work in similar ways, but because the four-stroke is more like a positive pump, and not so dependent on the dynamics of gas inertia just to even run, the effect an exhaust system will have on them is going to be more subdued. And there have been "expansion chambers" built for four-strokes for years. They're called megaphones.

In very simplified form, first understand that the exhaust opens, then the intake opens, then the exhaust closes, then the intake closes. The period when they are both open is the valve overlap, and happens as the piston goes over top center at the end of the exhaust stroke. At this time, the piston has just finished shoving the exhaust out past the exhaust valve and is starting down, but because gas has inertia, some left over exhaust is still leaving, and we don't want to close the valve too soon. At the same time, back a bit before we got to top dead center, we opened the intake. The last time the intake was closed, we slammed the door on a column of moving air and fuel, and it piled up against the valve under the pressure of its own weight. Once a condition is reached where the remaining exhaust pressure is lower than that built up in the intake, we can open the intake without exhaust going back up into the intake. In fact, if we time things right, we can have the exhaust help the intake fill the cylinder.

When the exhaust opens, the gas flows out in a pulse, a wave of high pressure in front of the pulse, and a pocket of low pressure trailing behind. If we open the intake as this low pressure trailer is still in the cylinder, it makes it simpler to get the intake charge in.

But what can the exhaust system do? As a high pressure wave exits the pipe, or moves into a significantly larger section of it, it creates a wave of low pressure that starts back toward the exhaust valve. If that wave gets to the valve just before it closes it will help clear the last bit of exhaust from the cylinder, and that will help pull in the fresh charge. The timing and magnitude of this effect can be tailored by changing the length and configuration of the exhaust system.

Using reversing cones is intended to create pressure waves to do some other voodoo in the system. Exhaust builders are messing with some really interesting ideas now that I haven't seen any theory on, liked stepped pipes and the use of internal rings or small expansion chambers in the header, so it gets deeper. But a pipe isn't going to give a four-stroke the massive, off-ON-off kind of hit a two-stroke has, and really, you wouldn't want it to. That would change one of the big things that makes them better in the first place.

 

[gvfc2]

I am sure some of this was covered in my previous posts but heres some more info. also covers some other aspects of the exhuast system


We've seen too much misinformation regarding exhaust theory. What kind of misinformation? For starters, there are a lot of people in the "Bigger is Better" camp. We're talking about exhaust pipe diameters. Even the big magazine editors are boldly smattering statements like, "For a turbo car, you can't get an exhaust pipe that's too big." Also, terms like "back pressure" and the statement, "An engine needs back pressure to run properly!" really rub us the wrong way.

Let's start from the beginning. What is an exhaust system? Silly question? Not hardly. Exhaust systems carry out several functions. Among them are: (1) Getting hot, noxious exhaust gasses from your engine to a place away from the engine compartment; (2) Significantly attenuating noise output from the engine; and (3) In the case of modern cars, reduce exhaust emissions.

Hardware

In order to give you a really good idea of what makes up an exhaust system, let's start with what exhaust gas travels through to get out of your car, as well as some terms and definitions:

After your air/fuel mixture (or nitrous/fuel mixture) burns, you will obviously have some leftovers consisting of a few unburned hydrocarbons (fuel), carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, phosphorus, and the occasional molecule of a heavy metal, such as lead or molybdenum. These are all in gaseous form, and will be under a lot of pressure as the piston rushes them out of the cylinder and into the exhaust manifold or header. They will also be hotter 'n Hades. (After all, this was the explosion of an air/fuel mixture, right?) An exhaust manifold is usually made of cast iron, and its' primary purpose is to funnel several exhaust ports into one, so you don't need four exhaust pipes sticking out the back of your Civic.

Exhaust manifolds are usually pretty restrictive to the flow of exhaust gas, and thus waste a lot of power because your pistons have to push on the exhaust gasses pretty hard to get them out. So why does virtually every new automobile sold have exhaust manifolds? Because they are cheap to produce, and easy to install. Real cheap. Real easy. Like me.

"Ok," you ask, "so now what?" Ah, good thing you asked. The performance alternative to the exhaust manifold is a header. What's the difference? Where a manifold usually has several holes converging into a common chamber to route all your gasses, a header has precisely formed tubes that curve gently to join your exhaust ports to your exhaust pipe. How does this help? First of all, as with any fluid, exhaust gasses must be treated gently for maximum horsepower production. You don't want to just slam-bang exhaust gas from your engine into the exhaust system. No way, Jo-se'! Just as the body of your '94 Eclipse is beautiful, swoopy, and aerodynamic, so must be the inside of your exhaust system.

Secondly, a header can be "tuned" to slightly alter your engines' characteristics. We'll go in-depth into header tuning a little later.

Nextly, exhaust gasses exit from your manifold or header, travel through a bit of pipe, then end up in the catalytic converter, or "cat". The cat's main job is to help clean up some of the harmful chemicals from your exhaust gas so they don't end up in your lungs. In most cars, they also do a great job of quieting things down and giving any exhaust system a deeper, mellow tone. You'll see a lot of Self-Proclaimed Master Technicians (SPMT's) telling people that removing a cat will get you tons of power. There's room for debate on this, but in our experience, removing a catalytic converter from a new car won't gain you much in the horsepower department. It can also get you a $1500 fine if the EPA finds out! If you drive an OBD-II equipped car, you'll also get that **** annoying CHECK ENGINE light burnin' up your dashboard. (And for all you racers concerned with OBD-II's fabled "limp mode", you can put your fears to rest.)

From the catalytic converter, the exhaust gasses go through a bit more pipe and then into a muffler, or system consisting of several mufflers and/or resonators.

Are you a muff?

Exhaust gases leave the engine under extremely high pressure. If we allowed exhaust gasses escape to the atmosphere directly from the exhaust port, you can well imagine how loud and cop-attracting the noise would be. For the same reason gunshots are loud, engine exhaust is loud. Sure, it might be cool to drive around on the street with that testosterone producing, chest-thumping, 150 decibel roar coming from your car… for about 5.3 seconds. (Not 5.2 or 5.4 seconds… 5.3.) Even the gentleman's gentleman has gotta use a muffler, or system of mufflers, on their exhaust.

Again, you may hear a few SPMT's tell you that "Borla mufflers make horsepower!" Or "An engine needs some backpressure to run properly!" Nonsense. A muffler can no more "make" horsepower than Wile E. Coyote can catch roadrunners. Any technician with any dyno experience will tell you that the best mufflers are no mufflers at all!

Types of Muff

Mufflers can take care of the silencing chores by three major methods: Absorption, Restriction, and Reflection. Mufflers can use one method, or all three, to attenuate sound that is not so pleasing to the ears of the Highway Patrol.

The absorption method is probably the least effective at quelling engine roar, but the benefit is that "absorbers" are also best at letting exhaust gas through. Good examples of absorbers are the mufflers found in GReddy BL-series exhausts, DynoMax UltraFlow, and the good old-fashioned Cherry Bomb glasspack.

Absorption mufflers are also the simplest. All of the above named mufflers utilize a simple construction consisting of a perforated tube that goes through a can filled with a packing material, such as fiberglass or steel wool. This is similar to simply punching holes in your exhaust pipe, then wrapping it up with insulation. Neat, huh?

Another trick absorption mufflers use to kill off noise is, well, tricky. For example, the Hooker Aero Chamber muffler is a straight-through design, with a catch. Instead of a simple, perforated tube, there is a chamber inside the muffler that is much larger than the rest of the exhaust pipe. This design abates sound more efficiently than your standard straight-through because when the exhaust gasses enter this large chamber they slow down dramatically. This gives them more time to dwell in the sound insulation, and thus absorb more noise. The large chamber gently tapers back into the smaller size of your exhaust pipe, and the exhaust gasses are sent on their merry way to the tailpipe.

Restriction

Doesn't that word just make your skin crawl? It's right up there in the same league with words like "maim" and "rape".

Obviously, a restrictive muffler doesn't require much engineering expertise, and is almost always the least expensive to manufacture. Thus, we find restrictive mufflers on almost all OEM exhaust systems. We won't waste much time on the restrictive muffler except to say that if you got 'em, you might not want to flaunt 'em.

Reflection

Probably the most sophisticated type of muffler is the reflector. They often utilize absorption principles in conjunction with reflection to make the ultimate high-performance silencer. Remember any of your junior high school math? Specifically, that like numbers cancel each other when on a criss-cross? That's the same principal used by the reflective muffler. Sound is a wave. And when two like waves collide, they will "cancel" each other and leave nothing to call a corpse but a spot of low-grade heat.

There are numerous engineering tricks used in the reflective muffler. Hedman Hedders makes a muffler that looks a lot like a glasspack. In fact, it is a glasspack with a catch. The outer casing is sized just-so, so that high-pitched engine sound (what we deem "noise") is reflected back into the core of the muffler… where those sound waves meet their maker as they slam right into a torrent of more sound waves of like wavelength coming straight from the engine. And, this muffler is packed with a lot of fiberglass to help absorb any straggling noise that might be lagging behind.

 

[gvfc2]

The Exhaust Pulse

To gain a more complete understanding of how mufflers and headers do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving. A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! Whoa, bet you didn't know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good 4-into-1 header, such as the ones sold by Gude, are optimal for that high winding horsepower you've always dreamed of. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 8,000 RPM, your Integra GS-R is making 280 pulses per second. There's a lot more to be gained by minimizing pumping losses as this busy time than optimizing torque production during the slow season.

General Rules of Thumb with Headers

You will undoubtedly see a variety of headers at your local speed shop. While you won't be able to determine the optimal power range of the headers by eyeballing them, you'll find that in general, the best high-revving horsepower can be had with headers utilizing larger diameter, shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street driveability. With four cylinder engines, these are also usually of the Tri-Y design, such as the DC Sports and Lightspeed headers.

Do Mufflers "Make" Horsepower?

The answer, simply, is no. The most efficient mufflers can only employ the same scavenging effect as a header, to help slightly overcome the loss of efficiency introduced into the system as back pressure. But I have yet to see an engine that made more power with a muffler than an open header exhaust. "So," you ask, "what the hell is the best flowing muffler I can buy?"

According to the flowbench, two of the best flowing units you can buy are the Walker Dyno Max and the Cyclone Sonic. They even slightly out flow the straight through designs from HKS and GReddy BL series. Amongst the worst, are the Thrush Turbo and Flow Master mufflers. We'll flow some of the newer mufflers as they become available at our local Chief auto.

Resonators

On your typical cat-back exhaust system, you'll see a couple of bulges in the piping that are apparently mini-mufflers out to help the big muffler that hangs out back. These are called Helmholtz Resonators and are very similar to glasspacks. The main difference is that firstly, there is no sound-absorbing fiberglass or steel wool in a Resonator. And secondly, their main method of silencing is the reflective principle, not absorption. An easy way to tell the difference between a glasspack and a true Helmholtz Resonator is to "ping" one with your finger. A glasspack will make a dull thud, and a true Resonator will make a clear "ping!" sound.

Turbos

Another object that might be sitting in your exhaust flow is a turbine from a turbocharger. If that is the case, we envy you.

Not only that, but turbos introduce a bit of backpressure to your exhaust system, thus making it a bit quieter. All of the typical scavenging rules still apply, but with a twist. Mufflers work really well now! Remember, one of the silencing methods is restriction, and a turbine is just that, a restriction.

This is actually where the term "turbo muffler" is coined. Since a turbine does a pretty good job of silencing, OEM turbo mufflers can do a lot less restricting to quiet things down. Of course, aftermarket manufacturers took advantage of this performance image and branded a lot of their products with the "turbo" name in order to drum up more business from the high performance crowd. We're sad to say that the term "turbo" has been bastardized in this respect, and would like that to serve as a warning. A "turbo" muffler is not necessarily a high-performance muffler.

Pipe Sizing

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.

Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 � inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!

Other Rules

A lot of the time, you'll hear someone talking about how much hotter the exhaust system on a turbo car gets than a naturally aspirated car. Well, if you are catching my drift so far, you'll know that this is a bunch of BS. The temperature of exhaust gas is controlled by air/fuel mixture, spark, and cam timing. Not the turbo hanging off the exhaust manifold.

When designing an exhaust system, turbocharged engines follow the same rules as naturally aspirated engines. About the only difference is that the turbo engine will require quite a bit less silencing.

Another thing to keep in mind is that, even though it would be really super cool to get a 4 inch, mandrel bent exhaust system installed under your car, keep in mind that all of that beautiful art work won't do you a bit of good if the piping is so big that it gets punctured as you drag it over a speed bump! A good example of this is the 3 inch, cat back system sold by Thermal Research and Development for the Talon/Laser/Eclipse cars. The piping is too big to follow the stock routing exactly, and instead of going up over the rear suspension control arms, it hangs down below the mechanicals, right there in reach of large rocks! So when designing your Ultimate Exhaust System, do be careful!

 

[machinist@large]

Maybe this will help??



I really like the amount of in depth info you're putting out here. Even better, you're trying to back it up with real #'s, where ever you can, and best rules/ best practices everywhere else.

Here's one for the large truck/ old iron crowd; back in the early '90's, I had a coworker post a '75 Chevy 1 ton pickup for sale for a couple of hundred bucks. He had bought it for the engine (454 big block)(7.4L dis. for the metric folks). came with a 350 small block (5.7L), internal status unknown (core engine), the original Turbo Hydramatic 400 series auto box, and the rest of the factory equipment. I bit.

The SB was basically scrap; fortunately the farm had another truck that the State Police had red flagged for to many safety violations with a rebuilt 4 bolt main engine hanging in limbo. At the same time, my employer got flushed by the banking industries refusal to regulate itself (sound familiar? That was in 1991......)

I had time; pieced the two trucks together over the six & a half months it took to find a job. When I got back to work, I wanted to finish getting the beast roadworthy. And walked right into the trap you just explained in depth. The generic, "HIGH PERFORMANCE" kit. Two really tiny "Turbo" mufflers, and a box of loose exhaust pipe with a couple of generic flex mounts to try to figure out where it might fit under the truck.

Well, I managed to kind of get it to fit; the truck ran, but had so little power, I thought the tranny was shot. To back up a bit, Dad had bought a new truck to replace the one that got flagged; 1988 Chevy 1 ton dually, with a 15,500 lb GVWR. Under the much newer GCVW rating system, it's rated for 28,000 lbs.

Where this comparo is relevant, the '88 had the exact same basic drive train. A little better state of tune on the 350 V8, same TH400 automatic trans., same basic GM Corp. 14 bolt full floater rear end (a LOT heavier housing, but all the internals are identical).

One truck ('75) weighing ~3800 lbs, one truck ('88) weighing just a little over 7000 lbs. The '88 is the race truck of the pair, and ~2 weeks of chasing combines with wagons had the '75 on it's (supposed) last legs.

First clue; smell really HOT, burning metal coming from under the truck. Crawl under and take a look; the two formerly oval shaped "Turbo" mufflers are each burned blue/grey/blackish, and swollen up to about the size of one of these newer fridge size mini kegs. HMMMMMM.

Limp the rig into town to the old school muffler/ general mechanic shop we used to have (the ones where they actually knew what they were doing).

Throw the truck up on the hoist, tell him what I've got, and he tells ME where I got the "kit", and offers to help me sue, if I wanted to waste that kind of time. Right about now, all I see is large amounts of money being pissed away in the wind, so I bite the bullet, and ask.

"How much?" I had over $350.00 into this supposed "High Performance" kit; this was supposed to be a cheap way to get another truck (backup) for the farm.

Manifolds back, same package as your new truck, $200.00. Why, I ask. If the "HI-PO" stuff isn't working, why should the stock parts help? "Money back guarantee; if this doesn't fix it, we'll swap your junk back on". Some choice...

One hour later, truck had the RV/BUS/MOTOR HOME spec exhaust; two separate mufflers, each about 8" in dia. and over 3 feet long; 1 piece crease free pipes from the manifolds to the muffs, the same leaving the muffs going to the rear. Factory 2" dia. start to finish. Suddenly, an engine rated at only 150 HP felt like a whole lot bigger mill. Nice and quiet, to!!!

Connection to what you've been presenting here? Even with it's flaws, a stock system that functions is a whole lot better than a "TURBO" "HI-PERF" mystery kit/package that doesn't. Be curious what your work would recomend for the '88 truck; long story short, it looks like it's mine now... Another basket case to get running......

 

[gvfc2]

so pretty much dont trust the "HI-PER" stuff right? well some "HI-PER" stuff with actually make it the tiniest bit better but not much. why waste money on the "HI-PER" stuff when the stock is just as good.

is what im getting from what you just said machinist@large

 

[machinist@large]

so pretty much dont trust the "HI-PER" stuff right? well some "HI-PER" stuff with actually make it the tiniest bit better but not much. why waste money on the "HI-PER" stuff when the stock is just as good.

is what im getting from what you just said machinist@large

No, what I'm saying is that there is so much overblown hype out there, that hearing about someone who's trying to get to the bottom of it is awesome!!!

In the case of my old truck, I got burned by the hype; someone with a little experience said, "lets put something on it that we KNOW works". He didn't claim it was the best in the world. All he told me was that what he wanted to do was put a known package back on the truck. A system that was at least known to function.

You have to remember, that all took place back in the early 1990's; very few outfit's had the dyno technologies we do today to actually test this type of stuff. The reason I stated that I would be curious to see what all the research you are doing today could do for the (slightly) newer truck is because 1] when Dad basically abandoned it in place approx. 10 years ago, it still had the original factory system on it. 2] in my initial walk around it the other day, that system is returning to the original ore state, and will need to be replaced. If your research has tips/tricks/benefits that really work, I'm interested in hearing about it.

What I don't want is something that promises the Sun,Moon, and the Stars that is nothing but snake oil.

And here's the thing; if, in the course of your research, you don't find a "perfect match" for my truck, just be upfront about it. Just because you don't have all the answers doesn't mean you can't give some well thought out pointers.

All I was saying in my other post is that I learned that smoke and mirrors cost me 1 1/2 times more than something that might not have even come close to being perfect, but at least actually functioned as advertised, and did so for almost 16 years. If you want to claim that I'm a stick in the mud, living under a rock if I show some scepticism, and would like to have concrete answers? Well, there's nothing i can do about that, except keep asking questions.

Here's a start of some questions for you; 1988 350 Chevy small block (5.7L), extreme duty emissions (dual air pumps, factory exempt from needing catalytic converters), 4 barrel Quadrajet carb, non electric (no computer controls), small primaries, secondary barrels large enough that you could probably slide a Red Bull can thru if the throttles and venturi's weren't there. Dual exhaust, no crossover. Looking for max torque possible between 1200 and 3800 RPM. Think all day draft horse, not some high strung quarter horse.

Any ideas?

 

[gvfc2]

no ideas yet but i will keep searching and eventually have an answer for you