Is it necessary to re-shim valve springs when going to SC cams

[flightlead404]

I'm just getting ready to put my engine back in after a bunch of stuff including SC cams.

I just came across a note somewhere on the innerwebz that says "you must re-shim the valves to obtain the correct valve height to prevent binding due to the increased lift of the SC cams".

I have not seen that requirement previously.

Is it normal or expected practice to do this when switching from stock 930 to SC cams in an otherwise unmolested 930?

Yes, I said "molested" just for you Mr. Knees.

[Alan L]

I changed my cams (to 'super SC') and had no such instructions. I used the normal shim specs. If that actually was the case, there would be a bunch of different shim heights per cams?
According to the specs I have there is less than a mm difference in installed height between a 930 and a 911 up to '89. The 911 height is slightly larger.
Good question. An engine builder will know the exact answer. All I can say is mine is functioning fine on the old specs.
Alan

[flightlead404]

Originally Posted by Alan L

I changed my cams (to 'super SC') and had no such instructions. I used the normal shim specs. If that actually was the case, there would be a bunch of different shim heights per cams?
According to the specs I have there is less than a mm difference in installed height between a 930 and a 911 up to '89. The 911 height is slightly larger.
Good question. An engine builder will know the exact answer. All I can say is mine is functioning fine on the old specs.
Alan

The reason I ask is I see lots of discussion of cams on here, and none on this topic. But, I'd hate to put the engine back in and have to try and do it in the car, or have to take it back out again. Or bind valve springs and have a valve/piston crash at high RPM

[911tracker85]

hmmm... Steve Weiner has suggested SC cams for my build. sure he will know.

[1979-930]

Shimming a spring would not increase it's travel and prevent binding.
The only time I have heard of shimming springs is to increase the resting seat pressure when reusing older springs.
I think you stumbled across some bad internet info.

[Alan L]

Originally Posted by 1979-930

Shimming a spring would not increase it's travel and prevent binding.
The only time I have heard of shimming springs is to increase the resting seat pressure when reusing older springs.
I think you stumbled across some bad internet info.

I would have to disagree. Yes, the shims will set the opening and closing pressures. But it also sets the length the valve can travel before spring bind (in the extreme case). But generally the springs are no where near binding in normal conditions. So I suspect a variance of <1mm is not critical.
You may get a more qualified answer on the 911 Tech forum?
All I can say is with my Super SC cams, I set the 930 specs (which is a slightly tighter -more shimmed height than the SC) and have no issues.
Alan

[RarlyL8]

The only change I make in valve springs when adding SC cams is to go with stiffer springs that would help out when you bounce off the rev limiter after the tires break loose.

[flightlead404]

Originally Posted by RarlyL8

The only change I make in valve springs when adding SC cams is to go with stiffer springs that would help out when you bounce off the rev limiter after the tires break loose.

Thanks all, I'm reassured.

I'm guessing (based on what I know about my lack of ability to leave things alone) I'll have this motor out again in a couple years to do P's&C's, porting, twin plugs, and whatever other nonsense I can think of, maybe even a full EFI rather than the half-EFI I'm doing now. I'll use new springs when I do that so maybe I'll re-address the issue then.