Cam timing overlap value +/- ??

[yelcab1]

Anybody knows what the tolerance range for the cam timing over lap values on an 80 SC is?

The left side is dead nuts on at 1.55mm, but the right side can only get to 1.45mm no matter what I do. So, there must be a good enough range. What is it?

[DonE]

Can you get the left side to 1.45mm? Its much more important to be matched than to be dead on a number (like 1.55mm). Or maybe both to 1.6?

[yelcab1]

No way to get the right side any higher than 1.45. I could get the left side to 1.45 as well, but there must be a tolerance.

[Craig Stevens]

The Spec book shows a 1.4 to 1.7 value. Good Luck, Craig

[ChrisBennet]

If you treat the holes as small slots you can get a finer adjustment. In other words, use the "slop" between the pin and hole to get a finer adjustment before tightinging the big nut.
Sometime removing the sprocked, rotating it some and putting it back on will get you a different setting.
-Chris

[Nitrometano]

Some times exist a play between the chain sprocket and the cam sprocket were the pin fits.

[yelcab1]

I turned the crankshaft 1 whole revolution and only got to 1.3. So I turned the cam shaft to get to 1.55mm. But even after I did that, and rechecked it with an additional 2 full turns of the crank, I only got to 1.45. There is just enough slop that getting the additional 0.1mm is not repeatable.

I think I'll settle for 1.55 on the left and 1.45 on the right and call that good enough, especially if the tolerance is 1.4 - 1.7mm.

Thanks all.

[aigel]

yelcab1:

Yes, you are fine with what you are doing. The engine doesn't respond to slight advance / retarding of cams anyway, especially if you are within spec! I also found that you may spend half a day getting it "perfect" only to come back the next day to check it again to be back to square one with a not so perfect reading.

George

[ChrisBennet]

Originally posted by yelcab1
I turned the crankshaft 1 whole revolution and only got to 1.3. So I turned the cam shaft to get to 1.55mm. But even after I did that, and rechecked it with an additional 2 full turns of the crank, I only got to 1.45. There is just enough slop that getting the additional 0.1mm is not repeatable.

I think I'll settle for 1.55 on the left and 1.45 on the right and call that good enough, especially if the tolerance is 1.4 - 1.7mm.

Thanks all.

I know you're probably a little frustrated but don't settle for it like that. Your readings should be repeatable (on one side) within a range of say .01-.03mm I think.
See my posts in this thread esp. the part about how tension effects consistency and how you need to check it multiple times:
Cam timing and chain tension

Cam timing isn't that hard but until you get to the "Aha! I get it!" point it can be a little frustating. Don't let frustration ruin one of the most satifying parts of a motor build. If you find yourself frowning PM me and I'll give you my number and we'll get you squared away in no time.
-Chris

[2.7RACER]

yelcab,
At 1.55mm for your '80SC you are setting the intake to open 1.0 mm, at 7 degrees before TDC.
For later SC's ('81-'83) Porsche specified .9mm to 1.1mm, which equaled 1 degree before TDC. 1.0mm=1 deg.
In 1984 Porsche split the difference at 4 degrees before TDC, or 1.1mm to 1.4mm. 1.25mm=4 deg.
All these specs are with a 0.1mm valve clearance.
So if you interpolate just a little each 0.09mm is one degree.
1.54mm=7 deg. BTDC
1.45mm=6 deg. BTDC
1.36mm=5 deg. BTDC
1.27mm=4 deg. BTDC
1.18mm=3 deg. BTDC
1.09mm=2 deg. BTDC
1.00mm=1 deg. BTDC
These aren't down to the exact decimal points or to 5 places, but you get the idea.
If you figure each .1mm is approximately 1 degree you won't go wrong.
I don't think you or your motor will notice a one degree difference in cam timing.
But now you know what it all means.