Why do 4 cylinder engines need backpressure?

[Aahz]

I have always heard that four bangers need back pressure for optimum performance unless they are turbo charged. I have been arguing with my roommate that they do need back pressure and he says that it is only some senser crap that makes later model four cylinders lose power when the exhaust is removed and that older models run fine and with better power with an open exhaust.
Is this true???

 

[Justin66]

All engines have back pressure. Generally back pressure is better reduced, but in most cases having a little helps with low end torque and to return the valves better.... i believe

 

[LS_VTEC]

All engines will have backpressure. Its a natural occurrence in engines and how they flow. Excessive back pressure will damage exhaust valves with time.

 

[Luis998]

Honda engines do not use back pressure, they use scavenging which relies on exhaust velocity to create a low pressure area that literally sucks the spent charge out of the cylinder while simultaneously pulling the fresh charge in. In other words, back pressure's bad, m'kay. Don't do back pressure. Look at it this way, if I put a slight amount of pressure on the back side of a door while you try to open it, is that back pressure going to make it easier or more difficult for you to open the door?

This is a pretty good read: Exhaust Backpressure

Good read indeed, Scott. Thanks. ^_^

 

[K2e2vin]

Honda engines do not use back pressure, they use scavenging which relies on exhaust velocity to create a low pressure area that literally sucks the spent charge out of the cylinder while simultaneously pulling the fresh charge in. In other words, back pressure's bad, m'kay. Don't do back pressure. Look at it this way, if I put a slight amount of pressure on the back side of a door while you try to open it, is that back pressure going to make it easier or more difficult for you to open the door?

This is a pretty good read: Exhaust Backpressure

I was about to say something along those lines. It seems that many confuse backpressure with velocity.

 

[BrokenRex]

the whole "backpressure = torque" thing stems from old hot rodders using different primary piping sizes for their headers. they found that smaller pipes made more low rpm torque, and larger pipes made more higher rpm HP. with the smaller diameter pipe, the header produced better low rpm scavenging due to the higher velocities. as soon as the rpm got past that peak however, excessive backpressure was produced. the larger pipes took higher rpm to produce enough velocity (because of the larger diameter, it takes more exhaust to keep the velocity up.) so while at lower rpm, that large space didn't scavenge as well because the velocity was down. so naturally they associated backpressure with torque.

the above only applies to header design however, after the header (or exhaust manifold) bigger is better for any rpm.