964 cam in 81 US 3.0

[aigel]

I am playing with the idea of putting 964 cams in place of the SC cams. Actually welding and regrinding my cams to 964 profile.

1. Will I have any valve to piston clearance problems with the stock 81 US pistons (CR: 9.3:1)?

2. Will I be fine with the stock SC valve springs using the 964 cam?

I have no time to take the heads off or do anything else that 's more involved. Please let me know if I can do this without valve to piston clearance problems, before I haul the bumpsticks off to the cam grinder!

Thanks!

George

[dd74]

I think Jim (Superman) did the same mod as what you're thinking of. He used 20/21s, which are a bit more radical than 964 cams. If memory serves me, he has a stock valvetrain, so if 20/21s will be fine, 964s should work, too.

911 valvetrains are fairly substantial. They can withstand 7,000 + RPMs. You won't rev that high with your engine unless you miss a shift.

...so don't miss a shift.

[mtelliott]

You might want to get your rockers reconditioned (or new ones). That's what was suggested to me when I was thinking about it. I didn't end up doing it because I wasn't sure what it would do to emissions. I heard conflicting advice so I went conservative.

[oneblueyedog]

I got new rockers after my regrinds. I've always heard it was essential since the cam and rockers must bed together on their hardend surfaces. Worn rockers have less of a hardend surface since they are already worn to the old cam profile.

Lee 78sc
Elgin 330 cis cams (cheater cam)
New Stock Mahle cis pistons 8.5:1
Boat tailed webs.
Much more fun to drive now.

[aigel]

I'll get new ones. I learned that you would want new bushings too, if you have them refurbished. The cost will be similar to just getting new ones.

EBS says I should have no clearance problems with the 964 cam. Of course I will check, but want to make sure that I am 99% confident. Would shuck to have the 964 grind ground down again to the SC for clearance reasons.

Thanks, George

[Tinker]

You need to reset that value spring sims to get the correct valve spring height for the cams. Otherwise you risk coil bind if you ever miss a shift or slightly over rev the motor.

Tinker

[aigel]

Uh-oh, now it is getting interesting. I am not prepared for that. Better go check out the lift difference betwen the 964 and the SC cam. Thanks for the pointer!

George

[aigel]

3.0 CIS cam grinds, 964 vs. web 20/21

According to this thread I am looking at about 200 and 600 micron of lift difference on the intake and exhaust side between the SC and the 964 grind. I doubt that this will present a problem. What do you guys think?

George