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Old 11-12-2018, 03:38 PM
propain37 propain37 is offline
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Default Custom Exhaust

If I am to build a custom exhaust header, what dimensions should the pipe be and how should it be bent. Is there an online resource I can look at?
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Old 11-12-2018, 05:31 PM
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Depends on the engine, and the kind of tube you're using.
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Old 11-12-2018, 08:04 PM
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Quote:
Originally Posted by propain37 View Post
If I am to build a custom exhaust header, what dimensions should the pipe be and how should it be bent. Is there an online resource I can look at?
What engine
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Old 11-14-2018, 12:56 AM
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gx160/ gx200
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Old 11-14-2018, 07:55 AM
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Overall this is kinda a open ended question...
What's the end goal? and what's the vehicle for in which to be used?...
Since this is four stroke... It's basically how ever you want it...

Typically most have a 3/4" to start from the port then a 1" for the rest. 1" will do fine... Just the exhaust flange gets tight space with the nuts on the studs...
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Old 11-14-2018, 10:35 AM
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A formula I found for 8 cylinder engines exhaust pipes (I also use it for these single cylinder engines) is below.

cross sectional area of pipe (sq inches) = Displacement (cubic inches) x RPM / 88,200

196cc = 11.9607 cubic inches

CSA = 11.9607 x RPM / 88,200

If you turn the engine 5,200 RPM (no governor), then....
CSA = 11.9607 x 5,200 / 88,200
CSA = 0.7052 sq inches

CSA = Pi*r^2
CSA = 3.14159 * r^2
r^2 = CSA / 3.14159

r^2 = 0.678 / 3.14159
r^2 = 0.22446 inches
r = 0.47377 inches

Internal Diameter = 0.9475"

So a pipe with ID of 0.95" should be big enough for a 196cc engine spinning 5.2k RPM.
Something like 1-1/8" OD pipe with 0.090" wall thickness, which has an internal diameter of 0.945"


If only turning 3600 RPM.... A pipe with internal diameter of 0.79" would be better. Something like a 1" pipe with wall thickness of 0.90", which has an internal diameter of 0.82".

A happy middle ground might be a 1" pipe with 0.065" wall thickness, which has an inner diameter of 0.87". That would be a pipe "tuned" for around 4,400 RPM.

A two or three staged header would probably be best.
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Old 11-14-2018, 11:51 AM
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since it's an approximation.. okay..
but you need to be aware that 88200 is A LIE!

That exhaust formula is basically just something to hold on to in the dark alley of making exhausts.
It has essentially no use at all without modifying each and every bit (hidden [as it'd be in that lie of 88200] or visible) according to the engine in question.

length of the exhaust is as important as diameter
since length means time, means temperature difference, means density, means flowrate...
can of worms.
sharper bends cause worse turbulences than a wide curve etc, etc...

if all that matters would be diameter,
then all would be lost anyways,
since you unlikely ever change the exhaust PORT diameter and so all would be dictated there

As long as your pipe is reasonably short, and bigger in diameter than the port.. you have a good enough pipe..

if you want.. read jack's post here:
https://www.physicsforums.com/thread...design.811716/

it's likely a much better explanation than what I would be able to come up with anyways.

'sid
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Old 11-14-2018, 08:58 PM
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For the purposes of this thread, build yourself a header out of tubing that is about the same inside diameter as the exhaust port (or just sightly larger) and make any bends with a wider radius rather than smaller. Length is practically irrelevant since you won't have room to make it the theoretically ideal length anyway, let alone too long. That's partly why some kart racers use the looped pipes in order to get more length.
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Old 11-15-2018, 07:28 AM
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Quote:
Originally Posted by itsid View Post
since it's an approximation.. okay..
but you need to be aware that 88200 is A LIE!
As a professor told me in a first semester course, Physics is taught (and learned) through a series of approximations and lies.

And if alone in a dark alley, go with the thicker wall tubing (more mass to better protect yourself with).

And I didn't get into length, because wave tuning a pipe for a kart using the 1st, 2nd, or 3rd harmonic makes the pipe too long to be practical.
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