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81 elky
10-22-2007, 10:19 PM
I was looking for an opinion regarding exhaust. I'm not an engineer, but it seems to me that the rate of exhaust gas flow at any point in the pipe would only equal the rate at the smallest restricition. More specifically, I need to fabricate an exhaust for my G body hybrid. Why would one need cat back duals with a volume of say 5 inches if the downpipe is three inches, or for that matter the three inch MAF sensor? Wouldn't a three inch single exhaust flow just s much as a dual since the volume is the same as the downpipe?

81 elky
10-22-2007, 10:22 PM
I forgot to mention I can make the GN dual kit work in my application, I just wonder if it would be a smart thing to do. This car is just for fun, built to drive on the street.

kenmosher
10-22-2007, 11:21 PM
Well, it's not quite that simple ... temperature and density will affect flow (and flow velocity). Not to mention pulses from individual cylinders (i.e pressure waves)

In fact, some exhaust designs can use "steps" or expansion/reduction of the pipes to speed up or slow down exhaust and affect scavenging in nonturbo (usually race oriented) exhaust systems.

With a turbo system, the more gentle the bends and the less restriction close to the turbo, the better. Further downstream, as the exhaust cools and loses energy, it's good to keep things as open a possible but it's less critical.

The turbo is a big "potato" in the pipe and smooths out a lot pulses and resonances, which makes it more about flow than managing reversion or pulses.

81 elky
10-23-2007, 10:24 PM
Thanks for the reply Ken. I'm still not quite sure I understand why more volume is needed downstream though. Would a single 3 inch work better, worse, or the same in a street application as the dual system? I suppose I should add that it is exempt from emissions so no catalytic converter is necessary.

kenmosher
10-24-2007, 09:24 AM
The exhaust has less energy as it cools (and slows) ... more volume gives it a chance to expand and keep a bit more flow going and gives more area for the cooler gases.

Single 3" will work fine, but personally I don't care for them ... most have much more resonance/drone and I think it looks odd.