Compression Ratio vs BOOST - Knock Control

[Ken911]

i just switched to a 3.4 with 7.5/1 JE pistons and a 1 bar spring. this mean i have a ticking time bomb? I still am breaking it in and havent re adjusted the WUR for the new compression and displacement.

[cole930]

These threads always turn into dooms day scenarios after a while.


When you mod a 930 to produce more power it's real simple you either blow it up or you don't. You find out what measures you can take to increasing your chances of it not blowing up and you do them, and hope for the best.

If you can't afford to rebuild an engine, then, just don't go there.

If you are modding a 930 for the street there are several measures you can take to increase your chances of beating it.

1. Do not increase compression over factory spec.

2. Increase intercooler size and cooling efficiency

3. Install an aftermarket O2 sensor with a readout and know where
your afr's are at all times.

4. Install at least euro heat exchangers and a good quality free flow
muffler.

5. ALWAYS run the highest octane fuel available.

6. Run retard timing when under boost.

7. Add increased cooling.

8. Never exceed .9 - 1.0 bar of boost

9. Never run higher boost for extended periods of time.

Cole

[copbait73]

Originally Posted by Ken911

i just switched to a 3.4 with 7.5/1 JE pistons and a 1 bar spring. this mean i have a ticking time bomb? I still am breaking it in and havent re adjusted the WUR for the new compression and displacement.

I’d forgotten about this thread until your posting brought up a prompt.

Are you living on the edge? Possibly. Don, Les and Voitureltd are at 12.3:1 and I suspect these points are in fact very good guidance. If you expand the chart adding every turbocharged air-cooled Porsche street motor you will find that even the last of them stayed below 12:1 and that is with every electronic technology in existence - knock listening - pressure, temperature and flow measuring - then adjusting fuel, spark and boost.

Your increased compression against 1Bar boost places you with these three however they are twin plugged and you are not. Also, I don’t know of your charge cooler, as your banner reflects none, but the best of the best would also be needed. Lastly, southern FL is not a mean ambient condition.

I read the chart and suggest lowering your boost to .8 bar. After all, that small amount of extra boost is not giving your motor configuration a large power boost for the potential risk it presents. Effective 11.5:1 on pump gas running at elevated temperatures still presents risk if that is how you roll.