[GhostRL]

A post of mine from shortbus. Good info.

The long and short of it is, you want an exhaust that flows enough to support your engine.

A good article in the May 2005 issue of Popular Hot Rodding Magazine. The article was written by David Vizard who seems to be one of the most accredited and respected people in the automotive performance world. In the article he stated that CFM is a great way to help develop a zero loss exhaust system. For zero loss a exhaust must flow 2.2 CFM per horsepower(This means less than 1% of total power produced by the engine is lost due to back pressure.) David stated that per square inch of exhaust tubing there is 115 CFM of flow. So plugging some numbers into the good ole TI-89….. ((3.14 * radius^2)*115[*2 for a dual exhaust system])/2.2 = Max hp supported with zero loss

So, in theory, assuming the muffler flows as much as the pipe, we have (at the flywheel and with 12% drivetrain loss)

A 2.75” single system is good for a 310hp engine with zero loss… (272.8 WHP)
A 3” Single system is good for a 370hp engine with zero loss… (325.6 WHP)
A 3.5” Single system is good for a 503hp engine with zero loss… (442 WHP)
A 4” Single system is good for a 657hp engine with zero loss… (578 WHP)

Now these numbers are assuming that everything else is set up perfectly. The muffler must flow as much as the open pipe or more to get zero loss at the listed hp levels. He also states that using a muffler with a larger inlet/outlet diameter than your exhaust pipe is a great way to get more out of a smaller diameter system since the muffler flow will be able to match the straight pipe flow. Now there are many other things to consider when designing an exhaust system. Lets consider the flipside.

The other side of the equation: scavenging. This is the name of the effect a proper exhaust system should induce. Basically, your exhaust is flowing at a high enough velocity that it creates a vacuum at the exhaust manifold. You want this because, that small amount of vacuum helps facilitate pulling the spent exhaust gases out of the cylinder when your exhaust valve opens on that particular stroke of the camshaft. Specifically tuned length header tubes/collecters can help amplify this effect. Oversized exhausts will lessen/nullify this effect, and that is why they MAY lose power compared to a slightly smaller size, expecially at lower rpms where total flow and velocity is low(er). Remember, you only need enough flow to support the amount of air in/out of your motor with zero or negligible loss due to backpressure. Any more then that, and you lose your scavenging effect. Hope this clears a bit up.


I have two mufflers and one resonator on my 3" exhaust. Each of them is roughly 4-4.5" OD and all are 3" striaght through. Dont forget muffler choice is critical to maintain flow (and velocity).