Exhaust Theory Demystified, by David Vizard

[Joe C.]

Here is an excerpt:

"Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power."

[Alan Drake]

Re: Exhaust Theory Demystified, by David Vizard

Is there an simple way to size them? Have used a vac guage on pipe, can this be used as a tool? And if so where on pipe and what values are we looking for?

[Duke W.]

Re: Exhaust Theory Demystified, by David Vizard

He's talking about header primary pipes, not exhaust pipes after the collector.

Duke

[Joe C.]

Re: Exhaust Theory Demystified, by David Vizard

You're right, it does say "primary pipes". That passage doesn't relate to exhaust after the collector.

Joe

[Duke W.]

Re: Exhaust Theory Demystified, by David Vizard

For a high performance road engine that needs good torque bandwidth across the rev range, 1 5/8" x 34" primary header pipes are about right, but I would never put headers on a road engine because they don't really make significantly more top end power, but do increase the midrange if properly designed and fabricted, which is often not the case with off-the-shelf headers. Also, a relatively high overlap cam is required - something on the order of a SHP cam, not a base engine cam. The key for a high output road engine is a low backpressure exhaust system, not headers.

For a (legal) FI vintage racing 327 making around 475 HP SAE gross at around 6500-7000 and where maximum average power from 5000 to 7200 is the objective, it's a toss-up between 1 3/4" and 1 7/8", same 34" length, and fabrication has to be "race quality".

It always comes down to what the objective is for the engine - road car or (real) race car.

Dukue