I called Accufab today with the same question (70vs75) Accufab told me that there 70mm flows almost the same as the 75mm and they recommend that if you have a stock car with just a few bolt on's such as mufflers and air filter go with the 70mm and If you have a very free breathing engine such as headers and o/r pipe and perhaps cams and or ported heads go with the 75mm. I also asked them about idle quality and throtle response differences and they said there wouldn't be a noticeable difference between the two. I ordered The 75mm just so I don't regret it later if I ever go ahead with cams and headwork.
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