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(Once again)
Backpressure is NOT good for any engine! The reason for the smaller diameter piping for a naturally aspirated engine is to help keep up exhaust velocity which helps evacuate the exhaust gases in the low RPM's but in the higher RPM's bigger is better cause you have enough exhaust flow to make use of the extra room, so for N/A you kinda have to make a comprimise but for turbo the bigger the better. But yeah 2 1/4- 2 1/2 is a good choice.
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