Hi all, im just poring on behalf of a friend of mine, to see weather anyone has done extensive testing with GX exhaust manifold header pipe diameter and length etc etc. It is a race car so there are rules and regulations to abide by. It runs an a12 with h89 head on unleaded, standard a12 carby, 60thou oversize bore and a pretty tame cam. Im sure someone out there has done the dyno ground work and hopefully they are able to help out with some specifications. Im mainly after what length is ideal for the twin pipes and what diameter pipe, then what size do the collect back to, and for how long, and when in the equation will the hotdog muffler come (also regulation).

If there is any more info i can provide to help get a better answer ill try my best to do so. Thanks in advance, GG.

Datsun did the dyno work. For a street car, the stock length is optimum.

For drag racing, spray paint the pipe, do a 1/4 mile pass, then note where the paint is burnt off. Where it stops is the optimum length. End the collector right there.

For scavenging effect, have the collector end in a muffler, just as the factory GX exhaust does.




After this, adding another length of exhaust pipe to the back and with a larger muffler will keep it quiet without changing the scavenging effect.

See Exhaust Sizing

Thanks for the info DD. Its for oval track racing, being quiet isnt an issue. a typical hotdog muffler does quite enough.
What i still dont know is how long to make the dual pipes and what diameter pipe do they dump back to? The idea with the paint sounds great, similar to a lipstick idea that i have previously heard from a mate.

If you feed them into a pipe, you lose much of the scavenging effect. Length around 26 inches from the exhaust valve and either:
* end the exhaust system
or
* feed them into a large chamber (a resonator or freeflow muffler, not a hot-dog muffler) - then if you wish, a pipe can exit to the rear of the chassis

The exact length needed depends on your engine build, state of tuning, and racing conditions. Using the paint or lipstick is one method of letting the engine tell you the length it wants.