Quote:
Originally posted by mikequig
Ok, I have done some research on the 3 engines that I have access to and here are the findings:
. . .
Footnote: The ultimate "cheater motor" would presumably be to marry the biggers port to the large runners and Euro P/C's. The dyno would tell, but then why did Porsche abandon the larger ports after 78-9?
-Mike
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Great thread, guys.
As to "why did Porsche abandon the larger ports" I'll bet it was about
Mixing!
It is easy to model an engine as an "air-pump" where bigger valves would make for a more efficient air pump.
However, it far too simple to say bigger valves give more performance when it comes to
combustion.
Smaller diameter ports -> Higher intake velocities -> better mixing.
=> Porsche found the "sweet spot" between mixing and flow, in ~1979. (for the 3.0)
An engine with better mixing on the intake and compression stroke, will deliver a more uniform and complete power stroke.
If the engine does not have good mixing of air & fuel, it will have more problems of detonation (esp when matched to higher compression pistons) unless higher octane rated fuels are used.
. . .Or you can run them extra rich, or pull back the timing. . .or some other inefficeint solution used back in the "bad ol' days" of constantly fouling sparkplugs.
