I am building my '71 911T engine out to a 2.4, somewhere between E and S. No mods to heads and stock exhaust. 2.4 crank and rods. Cams are Web Cams 05-241. Lift 450/.400, duration 250°/240°...lift is like an S but duration closer to E.
I'm having pistons custom made at Arias.
My question is: How high can I safely go on compression? I told them 9.5, will this work well with pump gas? Does anyone have experience with a build like this?
Doug Bender
twin-plugged or not?
will you be driving where the pump gas may not be so great (rural areas)?
you will want a nice safety margin too...
No, just single plug. I live in the Los Angeles area. 91 octane easily available.
Maybe back off to 9.2 for some margin.
The actual compression ratio will be relatively higher with the E cam duration . I'd be inclined to go with 9 to 1 .
No matter what it says on the piston box you don't really know the comp.ration unless you measure the head and piston dome cc's at your deck height, and then do the math. Short of that I'd recommend really staying on the conservative side in case the C.R. is higher than you expect. As Richy said, the dynamic C.R. will be higher with the short duration 'E' cams. I've had no trouble with 9.5 so far, but you have to know exactly what you have when you start pushing boundaries.
Early S Registry member #90
R Gruppe member #138
Fort Worth Tx.
Aftermarket pistons usually have extra material on the piston crown to custom-machine to the customer's preferred C.R., oil clearance, etc. That said, perhaps Arias would be open to building a set with say 9.5-10.00:1 CR, then send to you for initial build and to confirm actual CR (your engine and heads, sealing plate, graduated burett, etc.), then send back to Arias for final machining. Assuming all cylinders/combustion chambers are equal, you only need to measure one cylinder to arrive at the needed correction calcs. Remember; measure twice (or three or more times) and cut once.
Desired CR is a result of your choice of camshaft, altitude, available fuel octane and other operating conditions.
Sherwood
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