2.2S P/C on 2.4 motor: Will the compression ratio be too high?

If I put 2.2S pistons and cylinders on a 2.4 motor, I know that the compression will be bumped up quite a bit. The question is, will it be too high for the 92 octane gas?
Thanks

Clint,

Do you have Bruce Anderson's Porsche 911 Performance Handbook?

The short answer is yes, on paper, but maybe not in the real world of measured piston dome volume, deck height, and combustion chamber volume! On p. 139 of 2nd ed. Bruce addresses your exact question and says on paper it produces 10.35:1 ratio, but measured it only goes to 9.66:1 ... but he doesn't explain how that happens on a real engine!

I would suggest measuring your combustion chamber volume on your six heads to see how much they vary from the 'nominal' 68 cc's.

There are things you can do to lower the compression. To lower the compression ratio, there needs to be additional combustion chamber volume added, or less piston dome volume, and here are some things that can be done:

1. Mill or flycut 0.060" off the top of the 2.2 'S' pistons.

2. Use die grinder to 'knock off' some of the raised area on the pistons immediately around the deep pockets for valve clearance, thereby reducing some of the 'pop-up' dome volume on the 'S' piston. Dome volume on all pistons would have to bemeasured (melted wax impression would be easiest) then all with lower number would need to be touched-up, measured again, etc. until all six domes are within 0.5 cc or so, then all would need to be balanced to within 0.5 gram or so.

3. Either have the heads machined for dual plugs, or use a die grinder to open up a Pseudo-dual plug well in the head where the 2nd plug would go. All of the heads will need to be cc'd afterwards and corrections on the heads with lower volume so as to bring all to some 'targeted' number computed to get the compression ratio down below 10:1, maybe in the 9.5 to 9.8 range, if possible.

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Warren Hall
1973 911S Targa

Could shims be used at the bottom of the cylinders to increase the relative volume at the top?

[This message has been edited by mackgoo (edited 12-12-2000).]