Will 2.2T pistons raise compression in a 2.4T motor?
 

I cannot seem to get a straight answer on this question. I have a 2.4T motor, and good set of 2.2T pistons. The 2.4 does not need a rebuild, but it's out of the car, and about to go into what used to be a 912 (perhaps I should call it a 912-6). The question: Will the 2.2T pistons raise the compression of the motor? If so, how much? Is is enough to merit the work to put them into the 2.4T? I would also like to swap in E cams. I have a set of 40IDA Webers, which I intend to use.

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I believe Bruce Anderson addresses this question in his Performance Handbook. If you don't have or can't find a copy, I will dig out my copy and see what he says. I seem to recall it relates to the "S" piston, and the length difference in the rods for the 2.2 and 2.4... But I could be wrong........

Lemme know.

Don: The 2.2 has a 66mm stroke and the 2.4 has a 70.4 stroke. The dome on the 2.2 piston is larger to compensate for the shorter stroke and reduced swept volume of the cylinder due to the shorter stroke. As a result if you put the 2.2 pistons in a engine with a longer stroke the compression will go up. I can't answer how much without doing a calculation of dome volume on the 2.2 piston but my guess would be at least one point of compression. (9.0 to one in a 2.2 with 66mm stroke to 10.0 to one in a 2.4 with 70.4 stroke) I have heard of people using E cams with T pistons by enlarging the valve pockets on the T pistons so I think it can be done but I am not sure how the cast T pistons will like 7000RPM on a regular basis. I am sure the S cams have too much lift and there is not enough material in the piston dome for the valve relief. A safer bet would be a set of forged JE pistons for $800.

AWS

When I stroked my 2.2E to 2.4, I was repeatedly told by several reputable builders +.5 compression ratio if i used one base gasket. ie 9.1-> 9.6

This info is also in Wayne's book. unfortunately, it's in storage right now...

IMO, I wouldn't tear down the motor. i'm sure you can find a better place to throw $1K...

You are in luck. I have the book out right now for my own project

Page 139 Porsche 911 Performance Handbook by Bruce Anderson.

"To raise the compression in 2.4-liter engines, people have been replacing the 2.4-liter pistions with pistons from the 2.2-liter engines. This modification is usually performed on the 2.4-liter 911 S to take advantage of various club rules that allow cars a compression increase and still let them run in the stock class..........yadda.....yadda.....yadda.......The change in stroke will increase the compression ratio by about 0.55 points above what it would have been when used on the shorter-stroke engine."

I'll paraphrase the rest about you had better double check the CR by cc'ing the heads, the dome, and measuring deck height before you go slapping it together. Make sure you have at least the factory recommended 0.8mm (0.031") between piston dome and head, but he recommends 1.5mm (0.060"). You also must check valve piston clearance to make sure you still have 1.5mm (0.060") min.

If you don't have a copy of his book, I highly recommend.

Kelly