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The SSI's are copies, or clones, albeit made of a better looking and lasting material, of the original factory 'tuned' equal-length header design heat exchangers used from 1964 thru 1974. The reason the tuning provides more power is that for rpm near the torque peak, pulses from adjacent cylinders pull, or 'extract' exhaust gases from the next cylinder whose exhaust valve opens, emptying the cylinder a little more efficiently than if using a non-tuned exhaust, so there is less waste combustion products left in the cylinder to dilute the intake charge. In addition, coupled with the more aggressive cam profiles with more overlap, the extracted exhaust charge actually pulls in more intake charge than would normally occur with out the tuned exhaust!
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Warren Hall
1973 911S Targa
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