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Bill Verburg Bill Verburg is offline
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Join Date: Dec 2000
Posts: 23,138
One conclusion that I've drawn from exhaust design is that there are two distinct zones that need to be considered for exhaust design. Specifically they are what I would call "low speed" performance which is from idle up to a maybe 1000 RPM above the torque peak, and "high speed" performance which goes from there up to the red-line
thats true, when I raced single cylinder motorcycles we tuned for the track. On the tight low speed and hilly circuits we used smaller longer pipes for low speed torque optmization, the faster flatter tracks we used bigger shorter pipes to enhance top end.

The trick is always to keep the gas velocity high. This is done by keeping heat in the pipe, and having the pipe no bigger than absolutely necessary for the highest rpm that is commonly used.

W/ a single cylinder m/c it's easy and cheap to experiment and swap, w/ a 911 it's not.

But the basic principles are always the same[list=1][*]keep heat in to keep gas velocity up, these days Jet Hot coating is the way to go[*]longer pipes enhance torque across the rev range[*]shorter pipes enhance hp at the top end[*]use the smallest pipes that cary the full unrestricted flow at the highest rev point, again to keep gas velocity as high as possible for the longest period of time[*]tune is relatively insensitive to fairly large differences in primary pipe length[*]tune is relatively sensitive to collector configuration, size and termination[*]acoustic effects are only significant when there is a relatively large amount of valve overlap available[*]even fire engines/exhaust configurations do not benefit from x-overs[*]odd fire engines do benefit from x-overs because they prevent the overload condition where 2 consecutive gas slugs try to exit through the same collector(slowing gas velocity)[/list=1]
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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Last edited by Bill Verburg; 12-16-2005 at 02:38 PM..
Old 12-16-2005, 02:34 PM
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