Usually, that refers to the external diameter of the pipe. I'm using 2" pipe, the internal diameter is 1.6" if I'm not mistaken.

I have a question. Certain cars have more than one, or even more than two centre box on their exhaust system. What is the purpose for that? Is it aims to achieve an optimal gas flow? It seems that more expensive cars have more than one center box.

David, when you change your exhaust manifold/extractor to a tuned one, the engine volumentric efficiency is increased. With a tuned extractor, an optimum gas flow is generated after the exhaust manifold. So, with engine that run more efficiently, the overall engine can save more fuel consumption, that is in a controlled environment, excluding the driver driving behaviour. That's why, certain people said that after they changed to a tuned extractor, e.g. 4-2-1 design extractor, they can notice an improvement in fuel consumption. That's what I know from my reading through books and automotive websites.

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Added on December 31, 2007, 1:24 pmSleep, swirling or spiral flow muffler/resonator use another method of sound cancellation. Instead of using fiber material for sound insulation, this type of muffler/resonator use spiral flow louver for sound cancellation.
I'm not sure for the effectiveness of the sound cancellation compared to fiber muffler, but according to one of the manufacturer of this type of resonator, Morosso, it "helps speed the exhaust gasses out of the exhaust system to lower the exhaust gas temperatures as quickly as possible". I've never try this type of resonator, but some of people who use this resonator said that they can feel a general improvement in car performance compared to when they are using fiber filled resonator.





This post has been edited by Malvine: Dec 31 2007, 01:24 PM

V88, I think it's normal, because I also experience that when I change to 4-2-1 extractor.