Piston weight trade-off...What to do?

[M491]

OK...In rebuilding my 2.4 911T engine, I had to decide between 2.4 911S pistons and 2.2 911E pistons. I finally decided on the 2.2 911E pistons paired with 911E cams. Now I am in doubt again. The 2.2 911E pistons weigh a full 53 grams more than the 2.4 911S pistons. Keep in mind that the 2.2 911E pistons offer about 9.6:1 CR vs 8.5:1 with the 2.4 911S pistons.

So here is the question: Am I better off using lighter weight 2.4 911S pistons even if the compression ratio is lower, or am I better off using the heavier 2.2 911E pistons which would give me a higher CR? I don't know what the real trade-off is! Please offer all the adivice or comments you can think of. Thanks in advance for your assistance! M

[ajwans]

How fast do you want to rev it? The lighter the rotating mass the quicker
you will be able to spin it.

andy

[M491]

As fast as possible. I don't know. The real question is, am I better off having lighter pistons with a lower compression ratio, or pistons weighing 53 grams more with a higher compression ratio. Certainly, one of these must be better than the other. No? Thanks! M

[edmayo]

The weight difference is due to E pistons are cast and S pistons are forged. Also S pistons use a shorter piston pin, less weight. Tough call, never heard of E piston breaking, but there was a reason the S pistons were forged. Since you're using the lower RPM E cams I guess I'd go with E pistons, you won't be revving that high with those cams anyway.

[rick2]

S pistons and e cams is my vote. Shave the heads and you've got more compression.

[M491]

Ed Mayo, thanks. I may end up with S cams in the engine since I have a set of them too. I cannot make up my mind. Just in case I do decide to go that way, I am going to go with the forged S pistons. My E pistons are for sale.

Rick2, brilliant idea! Is there a formula to be used to inform how much should be shaved off of heads to raise compression ratio? In other words, how much should be shaved from the heads to raise the compression ratio from 8.5:1 to 9.5:1? Also, it scares me to shave the heads because of valve to piston clearance. If I shave, say, a millimeter from the heads, wont that cause my valves to be that much closer to my pistons? How do I get around that?

Thanks very much for your help!

[66S]

Leave the base gaskets off the cylinders.

[ajwans]

I would rather offset bore the conrod bushings than shave the
heads. Easier to reverse and you're not moving the camshaft
closer to the counter shaft. JMHO.

andy

[rick2]

There are a couple of ways to get the comp up. The heads get shaved, .010 or .020 thousands to get half a point if I remember correctly. Like Andy said the cams are closer. Usually not a problem with the chains. I use solid tensioners as well. You can offset the rods or leave the base gaskets off. All are options. My machinist usually cleans up the bases of the heads anyhow that's why I go that route. You'll have to check your clearances no matter what. If you go the piston pin route the clearance isn't as close. If your worried about the weight like you originally posted you can always lighten the piston pins or even the pistons as well to get back the weight. Like I said there are different approaches and everyone has their favorite way.