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copbait73 copbait73 is offline
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Join Date: Nov 2003
Location: Chesterfield IN
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Actually the 911 engine is very robust. Except for possibly the Cadillac North Star, NO water-cooled motor can run long after blowing a headgasket. You will break rings, torch or ventilate pistons with high boost. Catastrophic failures are usually high revving induced. In this regard the 911 is better than most engines at tolerating extremes.

Regarding magnesium case the 73.5T (CIS) has the latest and best – 7R. There are many discussions placing limits on it by HP and to the extreme this is true however for most applications the limit is thermal. Let the oil temperature get high, magnesium loses it strength quickly and studs pull out. They really got a bad rap when emissions hit causing these cases to cook.

As for boost I use the chart Porsche Engineering published. The chart places the 73.5T with 8:1C.R and 6psi boost at 10.5:1 effective C.R. The same as the level the factory sent the 3.0L TURBO out with. This gives similar power but more torque than the 1973 2.7LRS (MFI) engine.

There will be many opinions regarding boost levels. A couple of years ago I surveyed owners with purpose built motors and some were running 12.5:1 effective boost with limited success. These motors were unique in having twin plugs, modern turbos, serious charge air cooling and better than pump premium fuel. As this applies to the 73.5T, 12.5:1 effective C.R. is .8 bar (11.6 psi).
Old 09-01-2011, 05:27 PM
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