cbracerx

So I'm in the middle of rebuilding a 1980 SC engine with the stock CIS pistons (yes, small port heads). My machine shop sent me a set of Web Cam Mechanical Injection S cams instead of stock SC cams. I didn't notice this until after I installed the cams and started timing them. The engine has Crank Fire Ignition, SSI's, Sport Muffler and Weber 46's ready to bolt on.

Am I crazy to consider actually using these cams? The overlap is breathtaking, but the intake valve lift is not much different than 964 cams which are a popular upgrade with stock pistons.

I/E Lift, Duration
S Cam - .455".399", 263/235
SC Cam - .450".395", 228/218
964 Cam - .470"/.430", 238/226


See http://www.drcamshafts.com/911profiles.htm for a nice chart comparing these profiles.

Is this a valve to piston no go? I'm not going to machine the pistons - that ship has sailed with the heads bolted on

NineMeister

Send the cams back and fit 964 cams instead. The S cam has over 5mm of lift at TDC compared to the SC with 1.0mm. Since the pistons have enough clearance for around 2mm of intake lift they would definitely have to be pocketed to run the mechanical S cams.

sithot

I guess you know CIS doesn't like a lot of overlap. Reversion wave. The 964's are about as far as you can go. I've driven a car like yours with 964 cams and it pulled very evenly to redline without any peakiness.

Steve Weiner-Rennsport Systems

Some really gold advice here,...solid gold.

Steam Driver

I noticed the specific comment "small port heads." Does this info apply to the later large port heads as well?

NineMeister

We like small port heads......

Steam Driver

Ah, but the big port heads are what I have!

whalebird

You may want to investigate the piston choice you've made. Aside of cams, CIS pistons are different than carb/mfi pistons. I'm not sure you will be getting the most from your investment thus far using the CIS pistons and webers together. S cams would be glorious in your engine with compatible pistons, and 964 cams would be fine if you were still running CIS, but you may want to turn that ship around.
I dont want to be misleading so do defer to other qualified opinions.

NineMeister

Here's the power graph of a 911SC rally engine that we built with 964 cams, PMO carbs & SSI exhaust. Single plug engine with 98mm Mahle P&C kit on stock bottom end, the only difference from stock specification was that we modelled the engine on the computer then opened up the ports in a set of 930 turbo heads to the optimum diameter predicted by the model, giving the maximum torque throughout the range as well as decent top end power.

Interestingly the customer installed the engine and "ran it in" on fully synthetic oil, then complained about the performance. The dyno curve shows the before & after we de-glazed the bores and ran it in properly on break-in mineral oil.

gvr

Dear Colin

What size PMO Carbs were you using and what compression ratio was pistons,
what was the Torque in N.M. on dyno test

Thanks
George