Quick Timing Q

1972_911T · 07-11-2006, 02:16 AM

Ive just been setting my cam timing after fixing an oil leak from the cam thrust plate to cam housing gasket. The timing for my engine is 1.25 range 1.1 - 1.4 mm. To get in this range I had an option of two holes one was just above 1.1 and the other was just below the 1.4 there was no inbetween. Is this right? when I did the timing the first time im sure I had better adjustability and could get closer to the 1.25 median.

Thanks Steve

cstreit · cstreit's Garage

Most gears will allow a slight rocking back and forth between the flange gear and the chain gear. You can get the last bit that way... Though I've found that there is around ( I think) .003" between pins? I'd try again...

MBEngineering · 07-12-2006, 11:06 AM

HI steve take the cam gear off the cam and turn the gear in the chain 90 deg and you will find the setting will come back, if you do not under stand ring me and i will exp'

regards mike

2.7RACER · 09:56 PM

Steve,
Mike is correct. Essentially jumping the cam sprocket 5 or 6 teeth will reindex things so you can then get the holes closer to the 1.25mm setting you are looking for.
If you jump the sprocket by 10 teeth you won't see much difference from what you currently have.
Go for 5 or 6 cam sprocket teeth.
It all involves 17 holes, 16 dwells and 27 sprocket teeth; driven by a 24 tooth sprocket on the intermediate shaft with a 48 tooth gear, driven by a 27 tooth gear on the crank.
Similar to indexing your rear torsion bars with different numbers of teeth at each end of the bar.
This is an interesting engineering exercise. Once I study it more, I may be able to explain it without too many numbers.
I'm still wrapping my brain around it looking for the light.
So far I do know that jumping the cam sprocket 10 teeth will get you back close, but not exactly, to where is was. Thus my advice to jump 5 or 6 teeth.
Be careful not to jump the drive sprocket inside on the intermediate shaft.
Mike at MBEngineering understands the issue. Hopefully he can explain it without too much math.
I spent a couple hours today with paper and calculator trying to understand this. At this point, I think there are over 7,000 different positions you can put the cam sprocket in relation to the crankshaft. This is not counting jumping the drive sprocket on the intermediate shaft or jumping the intermediate shaft itself.
So divide 360 degrees by 7000 and you can get something close to 1/20th degree accuracy.
Your current problem is one hole is moving the timing over 2 crankshaft degrees, so you end up straddling the prime setting.
It's all gobbly-gook. If you like playing with numbers, it does get interesting.
My soon to be obsolete 2 cents.

1972_911T · 07-13-2006, 12:58 AM

Hi guys thanks for the responces, I think I understand what your saying so what I want to do is pull off the cam/chain wheel from the cam lift off the chain and turn it about 5 holes and refit. This should reindex the chain wheel allowing me to get closer to the median value of 1.25.

Thanks Steve