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Old 01-13-2010, 11:54 AM   #8 (permalink)
Sharif@Forged
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Good dicussion. One thing to note, is that as modern EMS and combustion chamber technologies improve we are seeing higher and higher OEM compression ratios in both NA and FI applications. NA cars in the 1970's (no I wasn't driving) would run 8.5:1 CR and that was considered cutting edge! Now we are seeing NA OEM engines in the 12.5:1 ranges and direct fuel injection. A carefully calibrated tune, can run a high compression engine with forced induction just fine.

As mentioned, most aftermarket engine builders select slightly lower compression pistons, in order to reduce the chances of detonation and give the tuner more tools to work with for max power. At the end of the day's it's all about cylinder pressure management.

With respect to spool up in practical purposes, one would be hard pressed to detect a difference in spool up characteristics between a 10.0:1 and 9.0:1 CR, for instance. Not to mention, that the turbos we run on the 350/370 are all smallish T25 flanged turbos with fairly small exhaust housing. So turbo lag it's much an issue at all.
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