I was gonna do a top end swap on the '97 GP GT I was driving so when I put the TOGs on, I went ahead and ran 3" back to where the tailpipes split. Well, that caused a bad low pressure issue which continually tripped the rear sensor. The low pressure/air flow across it made it think the cat was stopped up upstream. When I just had the exhaust redone to put in a new CAT, I had it knocked down to 2.5" since the top swap was off and the difference in the low end alone was amazing. It went from screeching the tires a bit on launch to totally busting them loose and second was a loud bark without a shift kit. Fuel mileage appears to be about 1MPG higher in town and 2MPG on the highway. The proper pipe size means so much ........

it sounds more like it was a tuning/fueling issue than exhaust, when you have the o2/pcm messing with the fueling its going to hurt more than help. if you have the proper ign and fuel it will make more power open. i run open headers on my firebird at the track, even when i just had the stock 305 it was a couple tenths off vs the dual glasspacks i ran on the street

Moved to performance section.

Not a tuning and fueling issue at all HOWEVER the efficiency of an engine is dependent on exhaust gas velocity and the "pulses" in the exhaust system. These vacuum pulses created by each cylinder'* exhaust are what help scavenge the exhaust of the next firing cylinder and so forth and so on. The pulses are more important on the bottom end and if the exhaust pipe is too large, the pulses are weakened to the point they no longer help pull ALL the exhaust gas out. The next time the intake valve opens, there is less room for fresh fuel/air to enter and burn which means less power and causes a loss of torque. As the engine reaches higher RPM ranges, the pulses become less of a factor and that is why running open headers on a high RPM only engine is technically a moot point when discussing a street engine that will never see that type of constant high RPM.

In the case of the vehicle in this post and the fact it doesn't run a turbo, too large a pipe will cost you the absolute peak performance your set-up is capable of. I would refer you back to the link I posted from one of the largest exhaust component companies around that has researched the pipe size question for ages.

i never believe the info of places that sell what you are reading about. i do believe the fact that i was at the track most every week for many years and real people with real cars and trucks of all sorts go faster with the mufflers off or cutouts un-done

Has anybody ever seen headers 4 a Pontiac 3.1 V6? I have only come across headers 4 the 3.8

Milzy Motorsports