I'll take a stab at this.....

Consider the motor as being an air pump. Restrictions at either end (intake or exhaust) limit output as well as efficiency. So, in order to improve efficiency on the exhaust side, most folks go with a larger and lower restriction system. I believe you can reach a point of diminishing returns when the exhaust system is too large a diameter pipe and the scavenging effects are reduced (might reduce the exhaust pulse)...
Anyone out there, please correct me if I'm wrong on this....

 

Reducing backpressure increases effiency to a point (basically because you are pumping more air). You reach a point, though, where there is not enough back pressure, and effiency and thus power starts to go back down. Not sure exactly why that happens, though.

 

That happens because of exhaust velocity. Notice how a header is all bent funny? Wouldn't straight tubing be simpler?

Yes.

But, the tubes are made from exact lengths for a reason... when each exhaust pulse hits the collector at the end of the header, the pulses are timed in such a way that it creates a small vacuum (ie: Scavenging) that increases power by reducing pumping losses from the exhaust stroke, and helping to suck in the new charge of fresh air during valve overlap.

When your exhaust is TOO large, then the speed at which the pulses hit the collector goes down, reducing the scavenging effect. This is why some old farts say when they tell you that "Backpressure is NEEDED by the engine to make sure you valves work properly, blah, blah, blah". Truth is, most of them just don't understand the purpose of an exhaust manifold (header).

Turbocharged (but not supercharged) engines don't count, as the location of the turbo prevents any sort of scavenging from taking place. However, super/turbocharged engines are designed with more valve overlap, so that the pressurized charge of fresh air blows out the exhaust during the cycle, having a similar effect.

And, of course, a smaller exhaust is undesirable because of restriction (much like an intake). The bigger the straw, the more milkshake you get (intake), and the bigger bubbles you can blow in your soda (exhaust).

 

Great explinations and theorys.
Thanks to all----

 

I don't know what the theory is or why, All I can tell you is my 72 duster on the chassic dyno was 267 at the rear wheels without the mufflers on the headers, with muffers it dynoed at 282 and torque increase by 20 ft lbs at a lower rpm.
I still miss that car.

 

Old fart here... Think of it this way, breathing. If you breathe too fast, you hyperventilate. If the intake charge comes in and exits to the exhaust, it is wasted. That happens at low RPM with a large exhaust. At High RPM, the loss is minimal (remember the fast breathing?). At low RPM though, we only need to take in what we need. Something needs to stop the flow, thus restriction. Let's move from point A to B. Fuel/Air (FA) moves from the intake into the combustion chamber (cc). Due to the scavenging effect of the camshaft, a portion is exited through the exhaust. At lower speeds, one could see how fuel AND air are lost, thus decreasing power. This is why the grind of cam for your application is so important. Variable valve timing is on the board for many engine companies, I will definitely adapt......