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Join Date: Nov 2004
Location: Redmond, WA
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CR of a '70 911T?

The tech specs say a '70 911T has a CR of 9.1:1, but after adding a 2.4 crank I measure it to be 8.27:1.

I just came across some random web page that listed the '71 T as 7.5:1, which would make more sense.

I quadruple checked all the measurements. The pistons are so flat that even with a deck height of 0 the CR only goes to 9.3.

Everything is stock except for the crank, so I'd expect the CR to be a bit higher than from the factory. Is the 9.1 value wrong?

Jeff
Old 03-15-2005, 04:30 PM
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8.6 is what my Porsche 911 Red Book says. Es had 9.1, Ss 9.9.
Old 03-15-2005, 06:05 PM
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Ok, that's closer. I still don't get how a longer crank lowered the CR

Could you check what the stock deck height was?
Old 03-15-2005, 06:53 PM
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A 2.2T is 8.6/1

A 2.4T is 7.5/1

Theoretically, you should end up with 9.15/1 when you put that carnk in your engine. But Porsche has apparently over-stated their compression ratios. Pretty much every engine is at least a half point lower than advertised CR.
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Old 03-15-2005, 07:47 PM
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Thanks, that's really helpful.

Right now my deck height is .050", and the CR is 8.27. Reducing the deck height or cutting the heads will increase that.

Think the increase in CR is worth it?

Jeff
Old 03-15-2005, 08:04 PM
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