2.2S piston & 2.4/2.7 crank

[preS]

Does any 2.2S piston go w/ the 2.4/2.7 crank or are there more than one style of 2.2S pistons?

Thanks

RIchard

[304065]

Richard, as far as I know there is only one 2,2S piston. Here is a link to Tom Butler's excellent list:

http://www.early911sregistry.org/for...highlight=list

Be careful pairing the 2,2S pistons with the 70,4mm crank-- Bruce Anderson warns that the piston skirts can hit the crankcase webs at the bottom of the stroke.

Also, by my rough calculation the 2,2S compression ratio is 9,8 to 1, that implies a dome volume of 32cc. With a 70.4 crank, the same dome volume gets you 10,6 to 1, enough to require twin plugs on pump gasoline! If you use S cams you should be OK I would think, but if you use T or E cams, watch out!

[preS]

Bruce Anderson warns that the piston skirts can hit the crankcase webs at the bottom of the stroke.

John,

This is what originates my question. I will check Bruce his book and see what he writes about it.

BTW Wayne Demsey gives a 9.6:1 compression ratio for this combination (pag 112). Is this incorrect?

Richard

[berettafan]

i have read that actual c/r on this combo is less than theoretical.

[304065]

I read that also. . . and I'm still waiting for someone to explain it to me. I know there is always a difference between the theoretical compression ratio by the calculation and the actual ratio once things are measured, but nobody's ever been able to tell me why.

For my engine build I cc'd the heads, had the rods rebuilt to reestablish center to center distance, checked the stroke of the crank, measured the pistons and bore and set the deck height exactly. So I'm not sure how it could deviate from theoretical compression. Maybe the factory pistons aren't as advertised?

[kenikh]

I've confirmed from 2 separarte Porsche wrenches that the factory numbers on Mahle pistons for CR are indeed "optimistic". They are abour .5 pts lower than advertised for the early cars.

[tfiv]

My 2.0L S motor ended up at 9 to 1 from the advertised 9.8 to 1. Gordon Ledbetter did the work. When I asked him why, he mentioned the speculation was that the factory did not take deck height into account in the theoretical calculation.

[kenikh]

My 2.0L S motor ended up at 9 to 1 from the advertised 9.8 to 1. Gordon Ledbetter did the work. When I asked him why, he mentioned the speculation was that the factory did not take deck height into account in the theoretical calculation.

If you calc CR w/ zero deck, it is pretty much dead nuts.

[Allen Henderson]

Richard et al,

Here is what I got from Bruce Anderson today on this (btw, he has a new updated book out, lots more pics and information).

"Gosh I have already said all I have to say on that in my book: To raise the compression in 2.4-liter engines, people have been replacing the 2.4-liter pistons with pistons from the 2.2-liter engines. This modification is usually performed on the 2.4-liter 911S to take advantage of various club rules that allow cars a compression increase and still let them run in the stock class. When Porsche went to the longer-stroke 2.4-liter engines, one of the things it did was lower the compression ratio of all of the models so they could run on regular gasoline. Where the 2.2-liter 911S had compression of 9.8:1, the 2.4-liter 911S had its compression reduced to 8.5:1. Using the higher-compression pistons from the shorter-stroke 2.2-liter 911S engine on one of the 2.4 9111S engines would significantly improve the engine’s performance. 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. But don’t be alarmed, for some reason it comes out to only 9.66 when measured and calculated out, not the expected 10.35:1, which still makes a fine-running engine of the 2.4 911S.

Just to show nobody has a good answer, Here was the answer we got when I wrote the development engineer for the 964 engine when we questioned what they specified it:"

Next letter....

August 25, 1994





Mr. Heinz Dorsch
Dr. Ing.hc.F. Porsche AG
Leitung Entwicklung Antriebe
Postfach 1140
7251 Weissach
Germany

Dear Mr. Dorsch,

I have measured the compression ration of many Porsche engines over the years and seem to always come up with numbers that are little bit lower than what Porsche has specified for the cars. I recently have had an opportunity to check several 964 engines and have found them to vary from 10.66:1 to 10.88:1 instead of the specified 11.3:1. Someone suggested that this might be due to the fact that some allowance has been made for thermal expansion of the piston dome at operating temperature. I was wondering if this was the case or if I have just been running into to production tolerances.

Thank you for any help that you can provide with this mystery.

Sincerely,





Bruce Anderson...

Next letter.....




November 13, 1994

Mr. Jerry Woods.

Hi Jerry,

I wrote to Heinz Dorsch at Porsche, who was the project leader for the 964 engine project about compression ratios. He is the guy that came and visited us at Garretsons. He answerd my letter (see inclusion) and I guess that it explains why we are seeing compression ratios in the 10.66:1 to 10.88:1 range instead of the specified 11.3:1.

Sincerely,
Bruce

Letter from Porsche (as a digital .gif file)

-Allen-
PS I told Bruce I was going to post this stuff and he said OK.

[Mr9146]

I'm not gonna read the thread.

Cut the skirts or plunge the case.

Done.