911 SC upgrades at teardown

JBO · 10-13-2004, 06:34 AM

My engine is being torn down to repair broken headstuds. From reading the archives it appears the best upgrades for CIS are 964 cams and higher (such as JE sold by our host) compression pistons. Cam regrind to 964 specs is about $400. The higher compression pistons are about $1,000. Not sure I can afford either at this point, but if I could only do one of the two, which would provide the biggest increase in low end torque? The regrind is obviously a lot cheaper, but from what I've read it mainly improves things in the higher rev range.

emcon5 · emcon5's Garage

What year is your car? Higher compression pistons will do less on a late SC, as the compression is higher to begin with.

Tom

JBO · 10-13-2004, 07:31 AM

1981, which is 9.3 to 1. I think the JE's are 10.5 to 1.

Superman · 10-13-2004, 07:35 AM

I have the 20/21 cams, which are AKA "964+" cams, and they most definitely do improve low end torque. If I were to put new pistons in my engine, they'd be 98mm units, raising displacement to 3.2 liters.

You should know more once the teardown is complete. Your $1000 piston option becomes basically closed if your cylinders are Alusil. Too complex for a "what if" analysis here, so let's see what you've got once the cylinders are removed, or at least exposed so we know what they are.

ChrisBennet · ChrisBennet's Garage

Neither one is going to make a big enough difference to justify the price IMO.
The 964 cams are going to let the valve linger open longer and thus closer to piston - making bent valves a little more likely if you miss a shift. That shouldn't be a problem on the street but it would be a consideration if you intend to track the car.
If I was going to track the car, I'd invest $4-500 in AASCO valve springs and get the spring seats machined to fit. They probably rob a little power and wear things out a little sooner but they could easily make the difference between a missed shift being an "oops" and going home on a trailer with bent valves.
-Chris

emcon5 · emcon5's Garage

Quote:

Originally posted by JBO
1981, which is 9.3 to 1. I think the JE's are 10.5 to 1.

You planning on running race gas?

JBO · 10-13-2004, 07:54 AM

Chevron 93 octane.

KobaltBlau · 10-13-2004, 12:04 PM

10.5:1 JEs need twin plug on street gas. You can then go to hot cams, which require a different intake arrangement (6 throttle bodies) if your emissions/inspection allows it. This makes a neat engine like a size large early S motor, but is quite a bit more expensive to do than a stock rebuild.

Lots of people stay under 9.8:1 max and use 964 cams, retaining DME or CIS. That's a nice compromise; you could keep your stock pistons and the 964 cams provide a little boost, early exhaust is a good choice to go with these.