I've always been taught that exhaust coming out of a straight pipe/cutout wont have needed backpressure and will therefore have less torque/hp than it would if it were going through a muffler.

What do you guys think?

 

I don't know if that is always true, but i always look at it like if there is more air coming out than is going into the motor (or if it is coming out easier/faster) then you will be down on power. I think balance between intake and exhaust is important, but i'm no expert.

 

Why do ya think Pro Stock runs open headers?
Ya think if mufflers made more HP they would be running them?
Backpressure is not your friend.

 

here are back to back runs on my car.


http://www.mofbody.com/blkchevyz/web...6hpnumbers.htm

my setup was flp long tubes, off road pipes with two 2 1/2 in cut outs in the t pipe, going to 4 inch mufflex

as you can see i did loose a decent amount in the low mid range but gained a bunch in the top end.

 

oh and back when i had shorty headers and 3inch catback with the dump right before the axle i gained 20 hp and didnt loose any low end.
i think i started to gain right around 3400

 

From Race Engine builders, this is what I have been told......

Backpressure in very small quantities is good for stock engines. Usually stock manifolds and a high flow muffler satisfy that, even without a Cat. With a Cat, you can get away without a muffler. Any other builds such as head work, PCM tune and heavy Intake modifications, bottom end or anything else more major, then yes, the lower the better. -Tim

 

PaganEgyptian,
Click the link above and look at the difference in HP with cutouts open.

 

What "air" is coming out of the engine? There are gaseous products of combustion - water, carbon monoxide, carbon dioxide, unburned hydrcarbons, and a huge volume of nitrogen. The gaseous products of combustion, because of their extreme temperatures, and the fact that they include the high molar count as the result of the reaction of all the liquid and gaseous components that went into the combustion chamber, have a huge volume compared to the volume of air going in. And by definition, they have the same mass (maybe a tiny, tiny bit less ). How can there be "more air coming out than is going into the motor"? Not sure I understand your point. Maybe you could expand on this concept.

 

Now look at blkchevyz before and after dyno numbers. He starts making gains right above 4k rpm. After you leave 1st gear, what rpm range are you in after every shift if your WOT? 4k rpm and above. So basically, he may be losing a bit out of the hole, but after that, he's better off.

Ken R.

 

You are correct Injuneer, it was a bad way to word it. I was trying to argue something more towards a "balance." 1 7/8 headers into open air with the stock intake and heads setup just isn't going to work. Likewise an engine with afr227's and a single plane intake manifold with stock GM "log" exhaust manifolds wouldnt be in your best interest either. Hope that gets my point across better, though i think its a pretty obvious concept.

 

But the problem has nothing to do with back pressure. It has to do with the reduced velocity in the oversize primary tubes, and the loss of scavenging. I still don't understand your point, as obvious as you may feel it is.

 

blkchevyz Do you have any pics of your exhaust setup?

 

There is always some confusion about this. Backpressure, per se, is always bad. But it is also the inevitable byproduct of a properly sized and designed exhaust system. Rather than ask "why do race cars run open headers", if you ask yourself "why do race cars run headers at all, and why don't they run the biggest headers possible" you will have your answer. If you could design an exhaust system that would do everything else a good set of headers does AND have no back pressure, that would be ideal. But it can't be done, hence the myth that some backpressure is needed.

Rich

 

no sorry, that was a pretty old setup, i have true duals now.

 

I wish I would have been told sooner...damned LS1s and their wacky rule-bending ways

Mike