Back to my post, you don't need any backpressure, however, it is a natural result from a properly designed exhaust that will keep the velocity up, and prevent the valves from being thermal shocked, and since it is unavoidable, as long as you are maximizing the exhaust velocity at the RPM range that you want to optimize [usually peak HP], you will have an acceptable backpressure in the other ranges [ignoring other issues such as valve timing, reversion from poor header design, etc, etc...].

 

That was one really long messed up sentence. So you're saying an engine does need back pressure or does not? what if you took the exhaust manifold off and everything after it. Would the engine run? And I'm talking about Honda engines specifically (aka economy car engines). Just wondering because when you see high hp drag engines or whatever, Ferrari F1 engines, they just have pipes coming right off the head into the air, no header, etc. So could you do that with a Honda engine?

 

Running open head is bad, bad juju.

https://www.honda-acura.net/forums/s...ight=open+head

You at least need a manifold of some form. Many of the faster all motor drag Hondas use a side exit Hytech.

 

OK lets have a scenario . . . imagine you are drinking from a cup using a straw . . u suck from the straw to 'pull' up the water correct ? ? ? now imagine the water are the INTAKE air/fuel mixture and your mouth as the exhaust gas . . you need the exhaust system and some back pressure (but dunnoe how much) to pull the intake mixture in from the intake vale during high RPM cam overlap . . . so a good exhaust system not only helps to expel exhaust gas . . it may help in your intake too . . or you can also see it in the way that . . each pulse of exhaust gas from a cylinder . . will help to pull out another pulse of exhaust from the next cylinder . . soO i guess with a well selected exhaust . . you will make more power then without anything and also with lesser noise and pollution

heehee please do not blame me for my rubbish this is how i feel

 

That is the scavenging effect, which is exactly what you're after.

 

so basically you need a somewhat restrictive exhaust. That's why there's a 3-inch "turbo" exhaust and a 2.5-inch "n/a" exhaust right? You guys are confusing me. First you say only riceboys think cars need backpressure, then you say you need a restrictive exhaust to cause a scavenging effect because it makes more power.

Who's got the truth?

 

If there was a way to have no backpressure, scavenge, and have good exhaust velocity, we'd be all over it like a highschooler on his passed out prom date.

You don't need backpressure, you need a properly sized exhaust. Backpressure is just an unavoidable result of a good exhaust system.

 

its all about no backpressure in the rpm range you are driving in.

a 3 inch exhaust will work good at 9000 rpms but suck ass at 4000

a 2 inch exhuast will work good at 3000 rpms but not 7000.

 

What size engine is that rule of thumb for? Like a regular Civic engine 1.6? Or Teg, Lude engine. Just curious, I know my corvette has Dual 2-3/4" exhaust and has good torque

 

After doing a little reading I learned a few things:
Backpressure typically comes from things getting in the way of exhaust flow in the muffler to quiet the exhaust. The less backpressure the better from the muffler. The pipe diameter is somewhat independent of backpressure and should be kept as large as possible without slowing down exhaust velocity which usually means 2.25" or 2.5" unless you have FI. Although these things were basically already said, they didn't really click until I knew where backpressure really came from.