Mid-Pipes and Torque Loss

[MadonnaRioRedGT]

I have heard that when off-road H- or X-Pipes are put on a Mustang, that the torque goes down. Is this true?

If it is true, can anyone explain the physics behind this? All the comments that I have heard say the reason torque goes down is because backpressure is lost. This would not decrease the torque, but it would actually increase it.

Does anyone know the real answer?

- Brian

P.S.
By the way, this is for a physics project.

 

[blackGT25]

for some reason or another... i dont kow for sure but im sure somebody that does will reply... the 4.6L mod engine likes backpressure... the engine has to work harder to create torque when it loses it.. those midpipes will give you a nice horespower gain, but a loss in torque... now if you have a power adder (SC, TC, NO2), no torque will be lost because the engine is trying to get rid of as much gas as possible to keep up with the incoming air/fuel mixture... thats how i understand it works... sorry i cant be of anymore help but this is what i know..

 

[01mgvert]

When I added LT headers and o/r x-pipe, I gained overall horsepower and torque. I did, however, notice a slight loss in low-end power. I noticed another loss in low-end power when I added Magnaflow catbacks. I do think that the 4.6 likes backpressure, at least on the low-end. The exception, of course, would be with a power adder.

 

[twogts4us]

It's really a myth, inasmuch that the midpipe itself causes the loss of power. What really occurs is a lean condition, due to increased air flow thru the cylinder. Lean is good to a point, but too lean will cause a loss of power (or worse!) But, properly tuned (adding more fuel), you won't lose power. Pure myth.

 

[Jackie Chan]

just get a midpipe

 

[Gearbanger 101]

Your loss of torque is only at the very bottom end. Nothing to worry about. once you climb about 500rpm above idle you'll start making more horsepower and torque than you would with the stocker anyway.

 

[MadonnaRioRedGT]

From a purely physical standpoint, I don't see how an engine "likes" backpressure. It can be understood that Ford designed the Modular motors to produce a certain amount of power based on a certain exhaust system with a certain amount of backpressure.

However, if you increase the size of the mid-pipe or some other aftermarket modification, horsepower will be gained because the exhaust gases can exit faster. Still, why does torque go down?

I don't think that exhaust pipes make a car run leaner. The same amount of air is coming into the cylinder from the intake valve. The pipes just make the air leave faster. Therefore, there is not an increase in the total amount of air in the cylinder and no lean condition.

What do you guys think?

 

[twogts4us]

I don't think that exhaust pipes make a car run leaner. The same amount of air is coming into the cylinder from the intake valve. The pipes just make the air leave faster. Therefore, there is not an increase in the total amount of air in the cylinder and no lean condition.
What do you guys think?

I disagree - it's not the same amount of air.

 

[Jay99]

here is one long thread about your question.
http://www.pistonheads.com/gassing/topic.asp?f=66&h=0&t=124140

finding your exact answer is hard
http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

http://home.att.net/~jroal/perftheory.htm

http://www.saabscene.co.uk/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=23;t=001091;p=0

more info on exhaust
http://www.musclecarclub.com/library/tech/exhaust.shtml

I wish I found a more solid answer without you having to do alot of reading, but heh its your project.