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Tony,
I think you have oversimplified the design of a tuned exhaust system. The part you missed was the 'required' length of the exhaust runners, which typically fall in the range of 24" to 36" ... depending on the rpm range of the torque peak of the engine, which needs to match the camshaft and intake tuning!
If you have Bruce Anderson's Porsche 911 Performance Handbook, take a look at the factory racing headers available around 1980-83 for the SC on p. 144. The problem with the factory heat exchanger tubes is that they are far too short to be effective, even if equal length. If you want to build your own alternatives to SSI or early '67-'74 heat exchangers, they would need to come very close to those factory headers for the individual runner lengths and tube diameter.
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Warren Hall
1973 911S Targa
[This message has been edited by Early_S_Man (edited 06-06-2001).]
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