[VWsuperhero]

How exactly does backpressure work? The only thing I can imagine it being is some way of the exhaust gases "pushing" off of the air currently in the exhaust tubing to create a bit of resistance in some way. Completely wrong?

I know that you can't just take off everything on your exhaust system including the muffler and expect it to run great, but how come a high flow muffler is that much better than stock? Wouldn't it need to be more restrictive to get the necessary backpressure?
When does the system start to lack the necessary pressure... after both resonators are off, cat if off, or both?
Questions I've always wondered .....

 

[Elwood]

Re: How much backpressure is needed? (VWsuperhero)

There is not an easy answer to your question. Most exhaust systems are designed to flow gases at 300 ft/sec, as I recall. The rate at which the gases flow affects the scavenging effect of the header/exhaust manifold. It also effects the position of the power in the torque curve. Slow moving gases tend to cause the power in the torque curve to move way up the power band (high RPM). So…small diameter tubing in the header tends to cause the gases to move faster. This increases your low to mid-range torque, but without some sort of a restriction, the gases will slow down and everything you’ve worked for is lost. If you put a 2.5” exhaust system on the car, all your power moves way up the power band. You may realize more horsepower due to fewer restrictions, but your torque peak might end up at 5000 RPM as well. This discussion doesn’t even address the acoustic tuning of the exhaust. Sound waves bouncing up and down the exhaust pipe influence performance as well. The first restriction (like the catalytic converter) sends a sound wave back up the exhaust to the engine. The distance from the restriction to the engine is critical to keep from adversely effecting performance as well.
So how does Joe Blow keep from wasting money and destroying performance? Dyno plots. Don’t buy any kind of an aftermarket exhaust without looking at a Dyno plot to see what it may do to your performance. Talking to guys that have good results (like here on the Vortex) is always a good idea.
Some real engineer (I am a blatant amateur with limited knowledge) may chime in and expound on my note, but I think I’ve addressed most of the issues.

 

[ValveFloat]

Re: How much backpressure is needed? (VWhombre)

Yea VWhombre I think you've got it. I think backpressure is basically a myth. The important things are gas velocity and pressure waves. I think that the sound waves and the pressure waves are the same thing but I'm not sure. Every place in the exhaust that has a change in cross section smaller or bigger, will cause a reversion wave back towards the engine. Reversion waves can help or hurt depending on their timing.