Question about exhaust pipes

[Scout]

A friend of mine has kids who are into go karts, and they've been racing on private tracks, so I got my kart out of storage and started messing with it. So! I've been reading threads here all morning, and thinking about my setup, which led me to looking at carbs, and the info for one carb said not to use it with a weenie pipe. So, I googled weenie pipe, and what I found was a short little exhaust just like the one I am using.
Ok, now my question is, why would someone say not to use a weenie pipe? Too short? Not enough scavenging?

My kart has a 212 hemi predator, billet flywheel and rod, black mamba jr cam, and domed piston which I am going to put the stock flat top back in. And a carb from the next size up engine, which I think I should replace with either the stocker or maybe a flatslide.

Should I keep the weenie pipe or use something longer?

 

[Rat]

Aside from shorter pipes being more prone to throwing a flame on decel, I got nothing.

I knock together my own exhausts and prefer a longer larger diameter setup.

 

[Denny]

It all has to do with scavenging. There is such a thing as too much flow on the exhaust side.

 

[Rat]

General rule of thumb:
The longer the exhaust is the larger the diameter needs to be to maintain a specific tuned pressure constant.

The shorter the exhaust is the smaller it needs to be to maintain a tuned constant.

It all has to do with EGT and wave length, a long pipe that is too small becomes a problem because the pressure waves tend to pile up or collide unto themselves in a way that is like jamming a plug in the end of the pipe.

There is a balance to be found, doesn't matter if it's 2t or 4t, there is always a certain length and diameter any engine will wake up angry on... but the spec is different for every engine and comes down to flow characteristics, upgrades, and EGT.

Exhaust Gas Temp changes the pressure wave speed down the pipe large or small which is why the oldest straight pipe tuned length method is to throw on the longest you can fit realistically, stick a hand in front of the end while the engine is running after running it up to peak operating temp, and then cut the pipe back an inch at a time until the end gasses are nolonger cold and just barely discernable as slightly warm (more lukewarm)

 

[Rat]

A friend of mine has kids who are into go karts, and they've been racing on private tracks, so I got my kart out of storage and started messing with it. So! I've been reading threads here all morning, and thinking about my setup, which led me to looking at carbs, and the info for one carb said not to use it with a weenie pipe. So, I googled weenie pipe, and what I found was a short little exhaust just like the one I am using.
Ok, now my question is, why would someone say not to use a weenie pipe? Too short? Not enough scavenging?

My kart has a 212 hemi predator, billet flywheel and rod, black mamba jr cam, and domed piston which I am going to put the stock flat top back in. And a carb from the next size up engine, which I think I should replace with either the stocker or maybe a flatslide.

Should I keep the weenie pipe or use something longer?

It did occur to me that the mini bike straight pipe standard is 1" OD at 14-18 inches lenth measure from the gasket face of the flange to end. I cannot fathom it being any different for a kart being that it's the same engine on a bigger chassis.

 

[Whitetrashrocker]

Old hot rodder tricks.

 

[Rat]

Old hot rodder tricks.

A lot of my methods are old school, far older than myself. Gasket making, exhaust tuning, carb tricks... you name it. Old school works and it's usually easy as hell fast hacks

 

[panchothedog]

For collectors bolted onto open headers ( like on a 1/4 mile drag car) thick was to put some cheap paint on them, make a few passes. Right where the paint quit burning off was the proper length to cut them. Seems like it would work for a kart pipe as well.

 

[anderkart]

Scout:

You could research & post questions at this other forum with guys that actually race karts with engines like yours at tracks every week.

Predator

4cycle.com

 

[bob58o]

As far as exhaust length tuning, the math shows all our tubes are too short. Pressure waves travel back and forth in the exhaust and help pull exhaust gas out of the chamber and pull fresh intake charge into the chamber.

Wave tuning involves harmonics. The shorter the pipe, the more times the wave travels back and forth. By the time we get to around 18” I think we are on the 6th harmonic or something.

In general, long pipes are better for low rpm.
Slower revolution time, more distance traveled by pressure wave.

Shorter pipes better for high rpm. Faster revolution time, less distance traveled by pressure wave.

I don’t remember the formula, but it figures in the speed of sound (pressure wave), the length of the exhaust, rpm, time between valve events …

 

[Rat]

As far as exhaust length tuning, the math shows all our tubes are too short. Pressure waves travel back and forth in the exhaust and help pull exhaust gas out of the chamber and pull fresh intake charge into the chamber.

Wave tuning involves harmonics. The shorter the pipe, the more times the wave travels back and forth. By the time we get to around 18” I think we are on the 6th harmonic or something.

In general, long pipes are better for low rpm.
Slower revolution time, more distance traveled by pressure wave.

Shorter pipes better for high rpm. Faster revolution time, less distance traveled by pressure wave.

I don’t remember the formula, but it figures in the speed of sound (pressure wave), the length of the exhaust, rpm, time between valve events …

This is a good simplified write up...unfortunately it is slightly over simplified.

EGT effects the wave, as does pipe diameter relevant to length.

This is where smaller diameter can be an asset when extra length doesn't fit. Likewise a larger diameter becomes necessary for longer pipes to prevent it from being restricted by the waves piling up.

Truth be told the BEST material for learning this stuff (aside from the complex math and formulas) is actually for 2cycle engines and directly in regard to designing expansion chambers for a certain powerband entry, peak, length, and fall if rate.

While 2cycle tuning is 98% completely irrelevant in the OHV/OHC world, the exhaust tuning principles are specific to exhaust, thus the engine fuel type and function is not largely part of the equation at all.

One would simply skip/skim read the parts specific to 2cycle expansion chamber shape/size and focus more on the pertinent info regarding flow characteristics in relation to diameter, length, and EGT.

On a side note, I prefer the sound of a longer larger diameter, the extra torque and lower idle speed is quite nice as well. My 208 does not sound anything like what one would expect when they think " It's a 208cc snow thrower engine"

 

[bob58o]

I’ve haven’t done much looking into the derivation of this formula, but for single stage exhaust diameter on single cylinder engine…

RPM (Peak Torque) =
(CrossSectionalAreaOfPipe * 88,200) / Displacement

Example… 212cc (12.94 cubic inches)
4000 RPM Peak TQ

4000 = (CSA * 88,200) / 12.94
51,760 = CSA * 88,200
CSA = 51,760 / 88,200
CSA = 0.59 sq inches

Area = pi * r^2
0.59 = 3.1416 * r^2
r^2 = 0.59/3.1416
r^2 = 0.188
r = 0.4336”
Diameter = 0.867”

ID of pipe for 212 cc with peak TQ at 4000 RPM = 0.867”

Round that up to around 7/8” ID.

 

[Rat]

Round that up to around 7/8” ID.

Stands to reason then with the 1" OD header and pretty close to 7/8" ID exhausts on all the major and minor Kart Part retailers as well as all the cheap getting sites.

 

[bob58o]

L = Length in mm
ET = exhaust valve opening (BBDC)+ 180

L = (21,590 x ET) / RPM

a CL1 cam exhaust valve opens about 80 degrees before BDC and maybe makes peak power at 5500 RPM

ET = 80 + 180 = 260

L = (21,590 x 260) / 5500

L = 1,021mm
L = 40.1 inches

So perhaps for a 212cc trying to tune for 4000-5500 rpm, a 0.875” ID pipe x 40” long.

 

[bob58o]

This guy has some cool pipes.

 

[Rat]

So perhaps for a 212cc trying to tune for 4000-5500 rpm, a 0.875” ID pipe x 40” long.

208cc stepped pipe, 7/8>1¼ >2 w/1⅛" core baffle

Intake
Open 30 Close 76
Lift 234
CL 112
Duration @ 050 > 220

Exhaust
Open 67 close 42
Lift 240
CL 105
Duration @ 050 > 223

 

[bob58o]

With that cam, ET would change to 67+180 = 247. This formula only uses exhaust opening.

L = (21,590mm x 247) / 5500
L= 38”

Retarding the exhaust opening from 80 BBDC to 67 BBDC would shorten the exhaust pipe from 40.1” to 38” according to this formula if trying to give a resonance boost at 5500 RPM.

A mod2 cam exhaust opens at 83 BBDC and you might want to tune for 7000 RPM

L= (21,590 x (83+180)) / 7000
L = 811mm
L = 31.9”

 

[Rat]

With that cam, ET would change to 67+180 = 247. This formula only uses exhaust opening.

L = (21,590mm x 247) / 5500
L= 38”

Retarding the exhaust opening from 80 BBDC to 67 BBDC would shorten the exhaust pipe from 40.1” to 38” according to this formula if trying to give a resonance boost at 5500 RPM.

A mod2 cam exhaust opens at 83 BBDC and you might want to tune for 7000 RPM

L= (21,590 x (83+180)) / 7000
L = 811mm
L = 31.9”

She pulls 4800-5200 before the 30 series drags it down, I've only spun it to 6k-6500 a couple times. My total pipe length is somewhere in the range of 34-38 ish... I went with what fit the way I wanted without issues and said stuff anything else.

 

[bob58o]

When I had that Engine Sim Software I was able to play around with exhaust lengths to try to fix dips in TQ/HP curves. That software was almost as much fun as MS Paint.

 

[Rat]

When I had that Engine Sim Software I was able to play around with exhaust lengths to try to fix dips in TQ/HP curves. That software was almost as much fun as MS Paint.

Sarcasm?
M$ Paint was only kinda fun back in 98 when it was pretty much the only choice available... which reminds me I need to hunt down a thread; I've got a really good CAD type software suggestion for someone.

[Edit]

Oh it's You... It's a fairly large program and tends to push a basic system pretty hard (idk what you're running for a PC/Laptop) but it's called "Blender"
It's free, and pretty much a full package CAD program that can even do animations (handy for playing with swingarm and driveline geometry options) it will do just about anything you tell it (round pipe bend angles especially)

It's been a while since I've used it but I doubt much has changed.