I want to put headers and a bigger exhaust in my 78 mini camper. Has anyone done this with bad results? Will the decreased backpressure screw anything up. Someone told me a horror story about gas mileage plummeting with a header. How big is too big on the exhaust? 3 inches? Thanks for the advice.

 

2" - 2 1/2" pipe is plenty for the little 8 valve motrs we got. Even moreso for the 20r. 3" is retarted over kill for an N/A setup especially one for a 33 year old drivetrain. You are gonna want to keep some back pressure for scavenging effect to have enough bottom end to push your boat around of course. Gas mileage wouldn't plummit from a header install either. A tired motor with excessive blow by and lower compression will cause bad gas mileage. That equals more fuel being dumped to make power at highr rev. All the header is doing is expelling the motors spent gases a litlle more efficiantly. Look into making a system that will free up some top end power without killing your torque. Proper sized piping with less bends and a good free flowing muff will get you there.