I got a word of advice from a friend who's had a number of 2nd gens and has done some work on the engines. He said unless I doing heavy mods to the engine like street porting, I shouldn't put money into a highflow catback system.

He said this would actually rob the engine of power due to lower back pressure.

This threw me for a loop. I like the sound but asthetics aside, I don't want to pony up 300+ for that's not going to result in performance gains.

So, what's the deal?

 

It's not backpressure, it's velocity. If the exhaust piping is too large, the exhaust gas velocity is reduced, which in turn reduces the performance of the engine because the exhaust gas is just dribbling out of the pipe. If the exhaust piping is too small, the velocity is increased to the point that excess drag hinders the flow.

The exhaust pipe diameter can increase as it extends further from the engine.

A catback exhaust will not improve performance as much as a full performance exhaust system.

 

It also depends if you have a turbo. With a turbo there really isnt such a thing as too big, only a point at which bigger means extra weight and $$ but no gain in power.

 

Evil Aviator has you on the right track. I've heard so many people that think they know something try to tell me that internal combustion engines need backpressure. Or "a certain amount of back pressure" Either statement is just not true. They need TUNING which has more to do with sound waves and exhaust temperature (exhaust gas density) than resistance to flow. The hotter the gas the smaller the pipe can be because the gas is less dense. Exhaust gasses drop 100's of degrees as they go through just a couple feet of a header. That is one of the reasons exhaust is so tricky to get right, that and about a million other variables.

 

For a turbo rx7, 3 inch exhaust is usually the optimum size. It will support almost any flow you send to it (unless you are making 800hp) and increases power whether you are a stock car or a highly modded car.

 

Thanks for the advice.

So, for my stock na, it seems I should keep the stock piping and maybe just get some nice sounding mufflers?

On my old '86 na I had installed a Greddy Trust 3" catback system. Didn't notice a huge improvement. Then I got rid of two of the three cats and boy did it move after that!

 

if you're looking just to change the sound, then sure. if you want gains, then you're going to need a header, too.

 

...Those cats were probably clogged anyways. Sound, buy new mufflers, power on the exhaust side...High flow cat should net like..1 hp..lmfao but get a complete perf. exhaust to get it right.