Back Pressure: Facts, Fiction, Myths, Rumors, and Lies
 

That's it. I can't take it anymore. Everyone state your case if you think engine back pressure IS or IS NOT good (i.e., desirable). Qualify your assertion with details, context, quantification, etc. Links to actual studies and engineering docs would be nice. There seem to be roughly two schools on this:

A: It's all crap. You don't ever want engine back pressure. Headers exist to create a scavenging effect, not back pressure. You want as little as possible to keep the "air pump" (a.k.a., engine) flowing as efficiently as possible.

B: Some back pressure is necessary. Not sure why, something to due with cylinder pressures, and therefore torque/power. The most typical case sited is small import engines that LOSE power when big exhaust systems are put on them.

A) that is it--no back pressure more power. End of story.:drink::pot_stir:

B) over scavenging can effect power--but no back pressure makes more power than any amount of back pressure.:lhdevil:

My answer is B and I found this http://www.custom-car.us/exhaust/default.aspx to back it up. I don't believe you need back pressure as it is a sign of a restriction but it's my opinion that you need a small enough pipe to keep up the velocity without restricting it. If you go too big then you lose velocity and therefore the scavenging effect.

Haven't read the article yet, but is this part of the reason for the stepped headers (smaller off the heads stepping up to a slightly larger size) on dragsters and other mega-power vehicles?

That would be my guess. It's also why guys that run open headers add an extention pipe that's 18-24 inches long after the collector.

Increasing exhaust gas velocity reduces back pressure. The faster the spent gases move away from the exhaust port the more the intake change can fill the cylinder. Yes a smaller to larger pipe (stepped) can increase velocity. Increased velocity makes less back pressure.:drink:

I'm with Glenn, before ordering parts for my stroker build I read a "how to build high performance engines" book. It states, that if an engine is tuned properly, it will always make more hp. But having back pressure can help tune the torque curve, so you might opt for manifolds in a motorhome engine over headers, get it? So for this reason I vote B.

The only exception I would say, is that heads seem to flow slightly better with short primary tubes attached. Then again, I wouldn't consider a 4" primary tube a backpressure creating restriction. It probably just acts like an extension of the port and somehow smooths the air flow to increase velocity? :huh:

So once again, talking horsepower, and not developing torque at a certain rpm, the engine will always make more hp with less back pressure. Assuming of course the engine will be tuned to run correctly in that state.

I remember reading an article in MMFF a while back about exhaust and how bigger is not always better, but that article talked about the exhaust pulse and how headers can play a role in them (so this article does not support me thinking but does talk about the effect of the exhaust as a whole). The exhaust was more complicated than I once thought and based on the article what pretty much everyone thought. I think that eliminating back presure has a negative affect on the velocity and as a result possibly the scavenging. i think like everything pretty much in your car it is a delicate balance of the entire set-up of the eninge as to what ammount of back presure would be ideal. If straight pipes were the best then everyone would do it. I had a buddy years ago who put a stright pipe on his 4.0L ranger and said the car did not seem to have as much power and went back to a mufler.

What he was missing was the low end torque, especially because his intake manifold/ cam were probably tuned for low end also. But I'll bet that he made more hp at 5500 rpm.

It appears that the term "back pressure" is not being understood here.

What are you guys thinking back pressure is??