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Bill Verburg Bill Verburg is offline
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Join Date: Dec 2000
Posts: 23,234
Originally Posted by Jonny042 View Post
Makes sense when you think of exhaust pulses - one at a time into (and out of) that pipe.

How does this translate to the tailpipe? In other words, will a single 60mm outlet on the muffler be big enough not to cause undue restriction?

I want to keep a stock look to the car and not go with additional pipes (and noise), hence the single outlet.

Perhaps a 2 in, 1 out, with a 70mm tip would be best? And pair well with the 42mm heat exchangers.
The Porsche H6 has one of the best firing patterns on any engine, it is an even fire design w/ each pulse 240 apart in each collector, when the sides are merged the pulses are still 120 apart, there is some congestion in most muffler designs, but that isn't the issue w/ MFI. The issue is that MFI is fairly tightly and rigidly constrained wrt operating parameters. Deviate from those parameters and the system just doesn't respond well. Minor changes to a stock engine such as muffler changes alone move the operating envelope uncomfortably close to the edge, bigger changes such as cam or displacement changes will expand the envelop even further, this is where places like Pacific become invaluable. An owner just can't fiddle the knobs far enough, internal changes to the MFI pump become mandatory for a good running engine.

Pipe size is correlated to displacement and rpm range, it's like the story of Goldilocks and the porridge, not too small and not too big is what is desired and what's right for Baby Bear isn't the same as what's right for Papa Bear. SSI primaries are 35mm(same as oe was)this is fine for up too ~3 liter & ~6800rpm, you could use these on a 3.6 but above ~5500rpm the small pipes will be choking the engine. On a 2.4 SSI are probable ya tad big at the lowest rpm and just right at the top, as the displacement increases the sweetest spot moves down the rev range for a given pipe size. By the time you get to 3.2 and especially for 7k+/- power range you need to increase the pipe size over what is found in SSI the 42mm pipes discussed here are great for that app, as you go to 3.6 and 3.8 and 4.0 or as the revs go to 7.5k or 8k or more the pipe size again needs to go up, these engines want 45-47mm pipes. This is only for max power at max rpm, some motors are tuned for more midrange and can profit from somewhat smaller pipes.

No matter what the intake and exhaust need to be in harmony for best results.
Bill Verburg
'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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