Cutouts and Exhaust/Backpressure Theory

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.

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?

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/webdocs/306hpnumbers.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

here are back to back runs on my car.


http://www.mofbody.com/blkchevyz/webdocs/306hpnumbers.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 a decent amount in the low mid range but gained a bunch in the top end.


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

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.

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.

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.

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.

....1 7/8 headers into open air with the stock intake and heads setup just isn't going to work.....

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

blkchevyz Do you have any pics of your exhaust setup?
no sorry, that was a pretty old setup, i have true duals now.

1 7/8 headers into open air with the stock intake and heads setup just isn't going to work.

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

Mike

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.

Agree - it's a common misconception. There's power in primary sizing & length & collector length tuning because it affects cylinder scavenging & filling, but it's hard for the average guy to do, so we use whatever's commercially available with the limitations and advantages that come with it, and all the while, trying to keep the vehicle street legal (or not). If you take a near stock car with all matched components and suddenly dump the exhaust, it might affect torque and driveability in the same way as when the guys in highschool would put a dual-quad tunnel ram on their 289 mustang, then expect it to be able to cruise at the drive-thru and use it for work 5 times a week. The exhaust should be tuned and matched to the cam & heads as part of the total system and set up for how the car is going to be used like the rest of the parts. The rules change some when you only want to go WOT, but backpressure in exhaust piping, itself, isn't a desirable trait of any exhaust system - it just causes heat buildup and pumping losses.

One reason an all out forced induction race car (ie: funny car or rail) needs pipes at all is to get the exhaust away from the car and keep the exhaust valves from being exposed to cold air. They also have so much power coming out of them that they affect steering & traction, so the direction they got them pointing is important.