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Re: Truth about 3" Down Pipes
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Posted by Dean (more from Dean) on Fri, 21 Nov 2003 09:05:09 Share Post by Email
In Reply to: Truth about 3" Down Pipes, Jeff in ATL, Fri, 21 Nov 2003 06:29:35
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It is not quite that. But there are laws against replacing a functional cat converter that is less than 8 years old. Federal, not state law. It is also not permitted to change the location of a cat, has to be in the same location. They don't want cats down stream where they do not 'light off' as quickly. You don't hear about that very often.

From Corky Bell's 'Maximun Boost', an essential read at some point if you are into serous engine power mods, a 2" single pipe down stream of the turbo should be good for 100HP, 200HP needs 2.25", 300 needs 2.75. For a 3" pipe Figure 11.4 suggests 700 HP. But he suggests that zero back pressure is also the ideal. To get low back pressure you need a larger cat and a larger cat will work better in a larger pipe to reduce dynamic losses. He also suggests a pipe 10% bigger than the turbine exit diameter.

His pipe (I assume internal IDs) recommendations are based on getting the gas velocities below 250 ft/sec. I am guessing that these rules might also need to be bent to deal with the higher boost pressures on a small engine vs a larger engine with lower boost producing the same HP. On the next page, P134, he states that the postition of the cat is "locked in by law." He also suggests that most cats will not be a problem and will produce a back pressure of 2 PSI or less which he considers acceptable. The overall backpressure of the total exhaust system would be higher than that 2 PSI figure. Overall, the closer a cat is to the turbo, the better it will work.

Some have stated on this BB, that the pipe after the downpipe, can be a bit smaller to prevent the exhaust from making too much gargling noise. Corky does not address that issue that I can see. Corky's rule of thumb suggests that a 2.5" exhaust system is good for 400HP.

He does not discuss the effects of down pipes that are larger than the exhaust system.

Hugh MacInnes, "Turbochargers" 1978, (I have had that since 1979!), suggests an abrupt and large transition to a larger diameter after the turbine, to cause the turbine's exit swirl flow to degrade to turbulant flow. He also then suggests that the dimater can be slowly reduced after that with little pressure loss. the 3" DP will certaily do that.

posted by 65.68.10...
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