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spuggy spuggy is offline
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Join Date: Jul 2003
Location: Glorious Pac NW
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Originally Posted by parasailsam View Post
If that is the case ( to high comp) what to do?
I don't think too high compression is causing it to run badly, I just wanted to point out that it running badly might be preventing you from destroying the motor...

If your SC is 8.5:1 and you're making 10 PSI of boost, that's ~0.68 bar - which looks to be putting you somewhere in the ballpark of a 12.5:1 dynamic C/R.

This is worse than running 12.5:1 on an N/A motor (which requires race gas, I believe), because the turbo not only transfers heat from the hot side, the act of compressing a gas raises its temperature (remember how bicycle pumps get hot at the end? Same thing).

If you want the motor to live after you sort out your current running problems, I think that you need to reduce static compression - e.g. by fitting flat top pistons, spacers under the barrels, or to reduce boost (weaker wastegate spring).

Either approach lowers the dynamic compression ratio.

I think an '83 SC is 8.5:1 (weren't all the 9.25:1 SC's earlier than that?). It's already marked where that is on the graph: 1 bar = 14.7 PSI, so 0.25 bar = 14.7 * 0.25 = 3.5 PSI of boost, for a dynamic compression ratio of 10.0:1.

Pat on this board was running 6 PSI (~0.4 bar) on his SC without an intercooler, but he was using water injection.
'77 S with '78 930 power and a few other things.
Old 01-25-2009, 07:05 PM
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