Backpressure

[SeanOO13]

I have heard many rumors around this forum and other places that an engine needs a certain amount of backpressure to run. I have also heard from this forum and many other places that an engine does not need backpreasure to run. I would like to know the truth about this subject. Does a 4 stroke engine need backpressure to run? If you do not know the answer, DO NOT post what you think is the answer and claim it to be fact. Thank you for your help.

[zeppelin]

these two quotes will tell you everything you need to know

QUOTE]Originally posted by GPracer2500
This is a somewhat complicated subject. The short of it goes something like this (to my understanding):

Backpressure does does not help produce torque or HP. What DOES help produce torque and HP at lower RPMs is high exhaust gas velocity. Exhuast velocity and backpressure are sometimes confused with one another.

Let's say you put a 4 inch straight pipe on your ATV engine. That exhaust would be capable of flowing a huge amount of air with basically zero backpressure. But without a huge engine to push out a huge amount of exhaust the velocity of the exhaust as it moves through the pipe will be low.

Low exhaust velocity is not good for performance. High exhaust velocity creates a scaveging effect that actually pulls the exhaust out of the engine.

So, a (comparitively) restrictive exhaust that produces high exhaust gas velocities at lower RPM will add performance to an engine--but only in low RPM ranges where the gas velocity would be low without the restrictions.

Reducing backpressure does nothing or my hurt performance unless reducing the backpressure raises the exhaust gas velocity.

Hope that made some sense. [/QUOTE]



Originally posted by wilkin250r
GPracer has my vote!!!

Excellent post. I LOVE technical threads! Let me further complicate matters by saying that backpressure DOES have an effect.

Confused? Let me explain.

GPracer had it 75% correct. Exhaust velocity is the real key, and he mentions the reason. High exhaust velocity will produce a scavenging effect, which will increase the efficiency of an engine. (this thread is about backpressure, not scavenging. If you want to know scavenging, do a search, or start a new thread).

Camshafts are specifically designed with overlap to take advantage of the scavenging effect. However, what GPracer failed to mention was that scanvenging can actually hurt low RPM performance. It can actually pull fresh fuel/air out of the cylinder during the valve overlap. Loss of fuel obviously also means loss of power.

Backpressure is the killer of the scavenging effect IF that backpressure is caused by muffler restrictions. Increasing the backpressure will reduce exhaust velocity, and thus reduce the problems that scavenging causes at low RPMs. However, the door swings both ways. It will also reduce the benefits at high RPMs.



So, you can indeed see that backpressure CAN indeed boost low-end power and torque, but only if it lowers exhaust velocity. A small headpipe diameter would have high backpressure AND higher exhaust velocity, resulting in the worst of both worlds.