Getting HP from an exhaust? - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Performance, Brainstorming & Tuning Talk about modifications, or anything else associated with performance enhancements. Have a new idea for performance/reliability? Post it here. No idea is stupid! (please use Detailing and Appearance for cosmetic ideas)

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Old 10-17-2004, 07:14 PM   #11
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Quote:
Originally Posted by Sol
My downpipe/cat/ubend combo probably yielded me 10 to 15 HP.
More importantly it wont be a restriction when you start pumping more air through the motor allowing more HP to be gained from mods.
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Old 10-18-2004, 12:33 AM   #12
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Well I just picked up my front ported manifold. Hopefully gonna bolt it on tommorow. Only cost me $40, its my first step. Next will be headers. After a cam that is
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Old 10-18-2004, 04:28 AM   #13
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40 dollars..wow..nice deal there! defintally do let us know on how it all goes!
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Old 10-18-2004, 12:55 PM   #14
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Quote:
Originally Posted by BonneMeMN
Backpressure is the amount of restriction in an exhaust. Backpressure doesn't make more torque. Backpressure can only make the gasses exiting the combustion chamber stay there more then not.

Scavenging, or the Scavenging effect, is how a good exhaust system or header design gives the engine more power. Performance exhaust is all about getting the exhaust gasses out from the combustion chamber as quick as possible. The exhaust pipe has VERY little effect on this. All engines benefit from this, highly tuned ones get A LOT.

Header design is the most important part of power in an exaust system. The key to the most power is getting it to exit fast. That usually means keeping it as fast as possible while giving ample room for al the gasses (diamter). The biggest constraint is the collector design, and length of the collector. As long as possible, while having very narow angles between primaries, and flow benched joints.

The exhaust diameter is the same, as long as you can get the air out as quick as possible, it doesn't matter what size it is, but if you make the diameter 3x the size of your collector, what'* the point?
I think i read this almost word for word somewhere...
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Old 10-18-2004, 02:34 PM   #15
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I just typed it myself. I'm on [H]ard Forums a lot, and there was a huge debate about exhaust that spawned off of the "ricer cold air intake" thread.
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Old 10-18-2004, 02:38 PM   #16
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If anyone wants to know what a good exhaust is, click the link in Rogue'* signature for his Mod List. He'* the second fastest Bonneville here, and hasn't made major changes. Please note that he'* also running a catalytic converter
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Old 10-18-2004, 09:08 PM   #17
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Quote:
Originally Posted by BonneMeMN
Too add confusion to this place, here'* my opinions.


Go to the local strip. Are they out there bolting pipes up or taking them off?
There is an important difference between tuning your exhaust for drag racing and driving a car on the street.

If your only concern is the quickest WOT run you can do...then go for open headers. If you have any interest in part throttle response you're going to want exhaust pipes.

Quote:
Originally Posted by BonneMeMN
Backpressure is the amount of restriction in an exhaust. Backpressure doesn't make more torque. Backpressure can only make the gasses exiting the combustion chamber stay there more then not.

Scavenging, or the Scavenging effect, is how a good exhaust system or header design gives the engine more power. Performance exhaust is all about getting the exhaust gasses out from the combustion chamber as quick as possible. The exhaust pipe has VERY little effect on this. All engines benefit from this, highly tuned ones get A LOT.
The idea is to maximise the velocity of the exhaust. As the gases cool they contract. Having a slight restriction in the form of a smaller diameter helps maintain the velocity although it does add slight backpressure. Of course we're not seeking backpressure...what we're looking for is velocity...backpressure is a byproduct.

If you have backpressure as the result of a restiction (GP U-bend for example) it'* not going to help you at all...it will hurt you.

By maintaining the highest possible velocity, and therefore having slight backpressure (1 to 1.5 psi), then we would have an exhaust pulse with a median velocity at its centre, a higher velocity on the leading edge and a slower velocity at the trailing edge. The lower velocity at the trailing edge will help draw the next exhaust pulse down the pipe...or scavenge.

Quote:
Originally Posted by BonneMeMN
Header design is the most important part of power in an exaust system. The key to the most power is getting it to exit fast. That usually means keeping it as fast as possible while giving ample room for al the gasses (diamter). The biggest constraint is the collector design, and length of the collector. As long as possible, while having very narow angles between primaries, and flow benched joints.
The primary pipe diameter and length play a big part in determining your power band. As well, the diameter and length of the collector will play an equaly big part in determining the rpm range in which the exhaust is best at scavenging and best at being unrestrictive.

The longer your collector, in general and assuming a reasonably sized piep, the more your power band will be skewed towards low rpm power and conversly the shorter the collector the higher in the rpm range will be the peak exhaust efficiency.

Quote:
Originally Posted by BonneMeMN
The exhaust diameter is the same, as long as you can get the air out as quick as possible, it doesn't matter what size it is, but if you make the diameter 3x the size of your collector, what'* the point?
As above, the exhaust diameter plays a big part in the velocity of the gases, all the more so as they contract while cooling.

While at the track with open headers, you'll loose a bit in the lower rpms versus a closed exhaust (assuming it'* reasonably well sized) but you'll more than make that up in the higher rpm range.

For those that have driven their cars to the exhaust shop with open headers will surely know how unresponsive the car is on the street in that configuration.

My $0.02...

Cheers,
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Old 10-18-2004, 11:28 PM   #18
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Good points Foghorn, you are correct, the tricky parts of exhaust theory are heat and expansion issues.

Rogue has headers, they're pretty, and very complex. and a high flow cat. Both are for high flow, meaning his mufflers are the highest restriction, however he wants a quiet sleeper.
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Old 10-19-2004, 02:43 AM   #19
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All of the exhaust theories are true, but we do have to remember that we need more power in order to benefit from running open headers. Cutouts are an effective way of running open headers on the track.
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