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hey guys, i'm looking at either getting 2.25 and 2.5 inch complete exhaust, from headers to the tip and was wondering if anyone has had backpressure problems with 2.5 inch and is there much of a power gain difference between 2.25 and 2.5inch systems? I can't seem to find any threads to do with backpressure kicking in at certain sizes, just threads about what it is.

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You should do some more reading. Try using Google and search exhaust myths

2.5 isnt needed for a stock, or lightly modified engine.

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You have a stock-ish NB, right? 2.25" is the generally accepted size, though 2" may actually be better. 2.5" is definitely too big for a non-turbo MX-5.

thanks for the reply paul. plhol, i have researched this for over a month now. So 2.5 inch is really just for turbo? i intend to get the most of out of my car with NA so 2.25inch is the ideal size for modified engine and aftermarket ecu?

Depends on how modified. 2 inch is good up to about 150 hp / 120 kW at the wheels in mandrel mends (for memory) . Bigger is worse not better.

Back pressure doesn't just kick in at a certain size. It's about flow restrictions and gas velocity

Plohls advice may help if you want to know more. Ie use Google not forum search and search myth and back pressure.

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ok that answered my question. thank you very much and yes i've tried google, other car forums and anything that had "backpressure" in the thread, but never really answered my question.

read this:-
http://www.thumpertalk.com/topic/571594 ... -the-myth/

one thing i have come you realize with modding exhausts. is there is a pretty simple theory i work on. and it seems to work fine for small capacity lightly modified cars. whatever exhaust you think you need. remove 1/4 of an inch, and that will be the better of the two.