Which pistons for 2.5 rebuild?
 

I am in the process of rebulding my motor, which is a 69 case with counterweighted crank, 85mm RS pistons and barrels, 2.7 heads, E cams and 40 Webers. Because of the low compession ratio of this combo, I am wondering if the performance gain would justify the expense of swapping the pistons for a set of 10.3:1 JE's which should yield a C.R in the low 9's with this combo?
Anbody running a motor like this with the higher C.R and if so what is it putting out? A previous motor I had similar to the above with RS pistons, 2.2 heads and S cams was just om 200 hp at the flywheel.

Robert

Sorry - meant 90mm RS pistons and sleeves.

Robert

Robert,

I think 10.3:1 JEs ending up in low 9s would be a substandard combination, with a high dome piston and no squish. You could, however, have JE make you up a set of pistons or they may have done a combination like this before and have something on file.

I know 'Minkoff' on this board built a 2.5, but he used 89mm mahles:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/152863-2-5-l-race-photo.html?highlight=2.5
http://forums.pelicanparts.com/showthread.php?s=&threadid=151159&perpage=20&highl ight=2\.5&pagenumber=2

Robert-
I'd talk to John at EBS. He assured me that the JEs I bought would produce about 9.75:1 in my short-stroke motor. I believe that these are the standard pistons for 2.7 twin plug applications. Those RS pistons are going to result in a CR of about 7.5:1.

Don't forget that you will need to install piston squirters in your early case if you plan on using nikasil cylinders.
-Scott

Thanks Scott. I assume from your signature that you havn't run you motor yet. What type of numbers are you expecting from it?
How important is it to fit piston squirters? Both my previous motor
which I put about 20 000 kms on with lots of track work and then sold with the car and is still running strongly and my current motor which I havn't run and was fairly tired on strip down but with no obvious problems are running Nickasils with no squirters.
Also how much more stress will the bump in C.R from 7.5 to 9.5
put on the mag case?

Robert

Robert-
I gather that your '69 case is already modified to accept the 90mm cylinders. Have you also provided for the second intermediate shaft bearing? How about the oil bypass modification? It's possible to find a late 7R case that has all of the above modifications listed above, plus piston squirters and beefed-up webbing aroud the flywheel seal and additional material around the cylinder spigots. It's great that you have had such good luck with your previoius builds, but at the least a 7R case is cheap insurance. They are, in Porsche terms, a dime a dozen. A 7R case would require that you find a 2.4 or 2.7 distributor.

I don't know how much stress the added compression will generate. It seems like it wil add heat. Pehaps your previous results have been partially possible due to low compression ratios.

You are correct in that my motor is not yet together. I'm still wrestling with decisions concerning ports and cams. If it remains "E" spec, with 32mm ports and MFI, it should produce 170hp. With S cams and ports 205?

-Scott