I have a 97 Camaro z/28 i replaced the stock muffler with a regular Flowmaster Muffler when it burned out i went with a Flowmaster Cat back system and it slow the car down 4 Tenths in the 1/8 mile i know thats hard to believe, Can someone tell me the size of the pipe coming from the cat converter to the muffler and the size dia of the tailpipe on a stock z/28 i am thinking that the pipes with the cat back system might have been to big in dia also this car is stock. Thanks for the help.

 

the intermediate pipe (from cat to muffler) is a 3 inch pipe. Out of the muffler is 2.5". These cat backs should allow less restriction because of the mandral bent pipes. A completely stock car could loose 4 tenths for several reasons... if you think its the exhaust, it could have a leak (contrary to popular belief, cars NEED to have some back pressue to seal the valves). Other reasons include temperature, tire hook up, and if its a manual tranmission.... driver shifting.

 

The stock intermediate pipe is a 2.75" crimp-bent pipe, with reduced flow areas where it goes over the axle. The connection at the stock muffler appears to neck down to 2.5". The typical cat-back will include a 3.0" IM mandrel bent pipe. You can also buy catbacks with 3.5" and 4.0" IM pipes (Mufflex).

The typical 3" cat-back will pick up 10-15HP over the stock exhaust system.

 

Contrary to popular belief, cars don't "need backpressure" to make power, low-end or high end. If you were able to apply a vacuum to the exhaust side, you would make MORE power, everywhere.

How would pressure on the backside of the valve help seal it, anyway? Pressure in the exhaust would act to UNSEAT the valve.

 

I correct myself... You need VELOCITY in order for them to seal correctly.
http://www.veryuseful.com/mustang/te...Scavenging.pdf

 

And the velocity, for purposes of scavenging, is important in the primaries, not in the system downstream of the collectors. For exhaust pipes and mufflers, "bigger is better".

 

^^^ Pretty much! Of course there is a point of diminishing returns, but even a 3.5" exhaust on a modestly tuned ~165hp 2.4 liter 240Z actually helped torque everywhere, if only by a smidge...

From your link:
Kevin Davis has done extensive testing of varying
backpressure on a number of performance engines.
These range from turbocharged Subaru Legacy RS flat fours to full-house traditional pushrod V8s. In
not one case has he found any improvement in any engine performance parameter with increased
exhaust backpressure.

Backpressure is bad, always, and hurts scavenging, acting against reflected negative waves which help draw exhaust out of cylinders.