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Tractor Pulling Discussion Forum

Exaust pipes for power?

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Mperfection

05-01-2005 18:59:46




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I have been readin up on race engine building and in every book to date they talk about the 5th cycle law in a engine. It states that with a properly tuned exaust, exaust sonic velocity is capable of filling 150% of the cylenders volume or more. Now i relize a race engine, 6,000rpm's, special intakes, cams ect. but they say the key is a plenty long pipe welded behind the header and that affects torque dramatically. No short zoomies! Just wondering if anyone has found any hp with pipes and stock manifolds. I see Denny sells a "torque tube" but everyone tells me Denny's just torque'n their wallets!

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Hank (MO)

05-02-2005 04:59:37




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 Re: Exaust pipes for power? in reply to Mperfection, 05-01-2005 18:59:46  
Past experience with a high rpm (11000 redline) motorcycle engine. Running open manifold gives you no torque at low rpm (below 6000). However, once you reached 6000 rpm you had to let off the gas or it would "wheelie over" at full throttle 1'st through 3'rd gear.
Current experience with low rpm flathead 6 cylinder tractor. Running open manifold gives you no torque. Engine doesn't run at high enough rpm to see the HP gains that it might had running this way. Long exhaust tube (5'... mine goes down, does a bend then comes up) with no muffler runs fine. Too big a diameter and you've got no torque again. Too little diameter and you start losing HP at operating rpm.
Exhaust gases flowing through the tube has a certain amount of inertia. It's this inertia that helps the exhausting cylinder a little. The inertia is proportional to the velocity. Higher velocity = more inertia. HOWEVER, friction is also proportional to velocity. More velocity = more restriction to air flow. So what you end up with is a trade off between torque and HP.
How to chose the right diameter? Lots and lots of dyno time and tubing changes, or go with stock diameter or your gut feeling.
How to chose the right length? Crayon trick seems to work for many.
my 0.02$

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CASEPOWER

05-01-2005 19:16:59




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 Re: Exaust pipes for power? in reply to Mperfection, 05-01-2005 18:59:46  
We did an article in Old Power Pride magazine on that subject. We put a tractor on the dyno and did some testing also. You are right, as no pipe after the manifold lost something like 3 HP. It actually had better HP with the stock muffler. I can"t remember the gain, but it was a couple of HP, with a 20" + inch pipe. If you get to short, you loose power. From what we learned on the dyno, the method of using a crayon, or paint, to mark the pipe, than cut it just above the un-affected area, will get you as closed to max HP that you will acquire with a stock manifold. I can also say that running the pipes from each cylinder separate and not connecting them together with a collector, will drop HP. I wouldn"t recommend it unless it was looks you where after, not performance. We are getting ready to start another series of dyno test with a stock engine and making many cheap changes, dyno testing in between all, than posting the results in OPP.

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Mperfection

05-02-2005 20:15:22




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 Re: Exaust pipes for power? in reply to CASEPOWER, 05-01-2005 19:16:59  
Crayon trick? I heard from the older generations to cut it where it turns the pipe blue but what is this crayon trick? Thanks for the info i have about a 24" 2"pipe now.



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Hank (MO)

05-03-2005 04:17:09




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 Re: Exaust pipes for power? in reply to Mperfection, 05-02-2005 20:15:22  
This trick I heard from CASEPOWER & others.
Leave your exhaust long... too long. Draw a line down the exhaust pipe all the way to the manifold with a crayon. Start up the engine and let it come to temperature. The exhaust heat will melt the crayon... but only up to a certain length of the pipe. Where it has stopped melting the crayon, is where you want to cut your pipe.
I can tell you a number or reasons why I think this won't work, will make no sense, or is theoretically irrelevant, but people with more experience than myself say it works. So there you go.

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