View Full Version : Using a longer duration on exhaust lobe to use a smaller primary manifold?

01-13-2011, 05:21 AM
I was talking with a guy that I work with about some guys using a longer exhaust duration on motors with stock exhaust manifolds or "shortie" style headers that have smaller primaries. Is there any truth to this?
Could using a longer exhaust duration on the exhaust side to keep the air flowing for a longer period of time allow you to not see as much of a drawback from using a shortie style header? Could this make the "most" out of a small primary tubed manifold set-up by NOT relying on a LOT of exhaust gas velocity, and just using the longer duration to make up for the short length of manifold tubing and the "cat" being so close to the exhaust side of the head?

The guy also told me that using a smaller primary tubing like that keeps the velocity at lower rpm's and doesn't "rob" SOME low end torque that a long tube design would/could....

On vehicles that are forced to not be able to use long tube style headers that have better scavaging (like california smog legal exhaust manifolds) would this be worth running in a car or is this just hearsay from an old man?

01-21-2011, 04:37 PM
This is how one compensates (to a certain degree) for the exhaust restriction.
Yes it is worthwile in a car that has to be emission legit. 400>420rwhp achievable.

The Alchemist
01-31-2011, 05:29 PM
What do you think GM does? That's why the factory cams have at least 7-10 more degrees of exhaust duration. That's why guys with the ls7's don't see much gains by just bolting on longtubes.