ShadowWulf

We all know how improving exhaust flow will give us gains in performance. I know the idea is to relieve exhaust backpressure.

My question is: Why is some backpressure still needed? I ask this because I've seen people ask, "Well, why not take the entire exhaust headers off of my engine for max flow?" and getting the response that the appropriate amount of backpressure is needed to get the best performance.

Here's an example:

I have a 4.0L V6 S197 Mustang. I plan on doing N/A mods. I know that I want to make the exhaust as free flowing as possible (but still street legal). My first instinct is to go with true-duals. But I keep hearing how, for N/A applications, there are better gains by sticking to a single Y-pipeexhaust.

My overall question is: Why is some backpressure still needed?

GreyStang

Because with no backpressure some of the fuel/air intake charge would rush right through the cylinder & into the exhaust manifold. Ever seen a car running with just header tubes? Seen the fire coming out the pipe? That's unused fuel burning and is losing a lot ofpower like that.

So running with either open exhaust or an exhaust system with tubes too large & power goes DOWN [:@]

Carfanatic

So why do some of the old hot rodders use cut outs when drag racing?

8barrell460

Ever feel how dead a car with a decomposed or weak exahaust runs, and the exhaust fart grumble noises it makes?

RJW

Because the affects of exhaust gas pulses are pretty much over by the time you get to cut outs.

golden_eye

I also heard something about a low pressure area just at the exhaust valves that helps pull the exhaust gases out of the cylinder. Not enough back pressure can cause this to disappear and leave already used exhaust gas in the cylinder. I'm still a little weak on the physics of engines so this may not be entirely true.

67mustang302

You don't actually want any back PRESSURE per se, what you do want is proper power pulse tuning, which is a shockwave that's generated based on the length/diameter of the pipes, that travels back up the length of the primaries to the exhaust valves. When an exhaust is properly set up there should be no restriction to flow, and shock waves moving throughout the pipes assist in properly exhausting gasses.

The back pressure most people think of is actually a shockwave that's traveling from the collector up the primaries, not pressure from the exhaust gasses themselves. Gas pressure and shock waves are not the same thing.

Bobert

I always windered this myself. Good post man. Awesome responses aswell.

Kinda makes you wonder if going 3" all the way back is the best

Jfsram

There is no one answer to this thread.

In a nutshell it depends on the camshaft. More importantly, the overlap in a camshaft.

At racing rpms the exhaust pushes out the exhaust port. The tail end of the pulse, pulls the intake gases in. This occurs when both valves are open at the same time.

This was best demostrated to me on a car I no longer own.
Had a small camshaft, small overlap in a 70 Plymouth Road Runner. It ran 12.5 @ 107 with a traditional dual exhaust, Dynomax mufflers and tailpipes. With the exhaust completely disconnected 12.4 @ 108.

Later swapped to a bigger camshaft with more overlap.
Woke the car up big time. 11.4 @ 118. Problem was with the exhaust connected it was low 12's. Barely faster than the small cam.

Classic example of big overlap want free flowing exhaust. Small overlap really doesn't care.

OnyxCobra

3" all the way back is too large unless you're running a serious engine combo.