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

[M491Cabriolet]

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!

[RWebb]

how often will you be revving the motor above 9,000 rpm?

[M491Cabriolet]

Thanks Randy. So, I gather from your question that if I don't rev the engine more than the limit of 7300 RPMs, I would be better off going with the heavier 2.2 911E pistons because they have a CR of something like 9.6:1 vs the 8.5:1 of the lighter 2.4 911S pistons? Is that right? Thanks.

[RWebb]

absolutely

[304065]

The E pistons are cast, the S forged. Reducing reciprocating mass is great, but remember that the benefits are realized at higher engine speeds. The E has a 6800 RPM redline. In the 25 or so races I have done with my 2,2E, I have found that you can rev the engine to 7000 or so if you are passing somebody and need to, but the engine doesn't make more power or torque, it just makes noise. So if you are sticking with E cams (and ports etc) then the higher mass of the piston never comes into play.

[M491Cabriolet]

John, Thanks. Maybe I did not understand what you said well, but I'm wondering if it might be better, then, for me to go with the 7300 RPM redline, lighter, forged 2.4 911S pistons. I am going with E cams and a 911S 7300 RPM distributor, by the way. Is there a way to increase the CR of the 8.5:1 S pistons? Please let me know. Thanks.

[Zeke]

You need to keep in mind the difference between the cranks on the 2.2 vs. the 2.4. The holes are the same size. The trick was to use the 2.2 E piston on the the 2.4 crank, IIRC. Maybe I've forgotten all the nuances. Anyway, that set up brought about 10:1 compression.

And, where do you find decent 37 YO pistons?

[David]

Compression!

My JE's are significantly lighter than my old Mahles, but I'd rather have the Mahles if they had the compression of the JE's.

[RWebb]

- compression is where your power comes from

- the idea is to kick the connecting rod down harder. that is what turns the crank

- faster is better. more rpm means more power ---> the crank is turning faster so the motor makes more hp

that's the basics, ok?

there can be problems in getting higher rpm -- it stresses the pistons, the valvetrain & etc. those are things the engineer worries about. the hot-rodder (you) has to consider them and their design limits

there can be problems getting more compression -- the 911 combustion chamber is poorly designed (or old-timey) b/c it is an air cooled engine they could never update it. as the piston gets wider, and the comp. ratio goes up, and fuel quality goes down, you need to use twin-plugs to get the whole charge ignited "at once" -- not sure if you need twin plugs with the degree of messing around you want to do.

which brings up the issue of how much messing around you really ARE willing to do.

is it JUST a question of which pistons?

or are you willing to shovel in more $$ for more work on the motor??

[Steve@Rennsport]

Originally Posted by john_cramer

The E pistons are cast, the S forged. Reducing reciprocating mass is great, but remember that the benefits are realized at higher engine speeds. The E has a 6800 RPM redline. In the 25 or so races I have done with my 2,2E, I have found that you can rev the engine to 7000 or so if you are passing somebody and need to, but the engine doesn't make more power or torque, it just makes noise. So if you are sticking with E cams (and ports etc) then the higher mass of the piston never comes into play.

Totally concur with Professor Kramer's sound advice (again).

Cast pistons should not be taken beyond 7K otherwise you run the risk of tearing the wrist pin bosses out of them. For an E-cam motor, its nothing to worry about but if S cams are used, I would be concerned if the engine sees any track time.

E-pistons and E-cams make a happy marriage.

[911pcars]

Not sure how much high rpm power you can wring out of the reduced air flow created by E-cams. 6500 rpm might be the limit of power production using e-cams.

Sherwood

[M491Cabriolet]

Wow. Thanks everybody. Milt, if I recall correctly, the 2.2 E pistons do not actually ring the compression up to 10:1 although the math says they should I think the figure used by Bruce Anderson was 9.6:1. What does IIRC mean?

I don't think I would have to go with twin plugs with a 9.6:1 compression ratio. Is that an incorrect assumption?

Finally, I thought the E pistons were forged like the S pstons. How can one tell if pistons are cast or forged?

Thanks again everybody!

[RWebb]

IIRC = if i recall correctly