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Originally Posted by matt_l
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Thanks for the info. What I have read about the down tube, bigger is better. This is obviously a tradeoff of packaging and power. How do you know what is big enough for the down pipe. I am planning on running a 76mm turbo, specifically the setup sloppy mechanic has on his site? https://sites.google.com/site/sloppywiki/home |
I'm assuming you are running the On3 76mm with the .96 a/r exhaust housing Matt has used on quite a few builds.
If that is the case the general rule of thumb for the exhaust is 1.25 x turbine exducer. That turbo has a 65mm turbine exducer so 1.25 x 65 gives you an optimal exhaust size of 81.25mm (3.2 inches). This is just a little over 3" but not quite 3.5". If it was me I would run the 3.5 and call it a day.
Sorry for all the math but this question gets asked a lot and I figured I would share an easy way of figuring it out and make it useful for others.