Cam timing questions

[nreed]

I have searched the board and have not found a thread that helps.

I am new at cam timing issues so please bear with me.

1. By measuring piston travel in cyclinder #1, I have discovered on my engine that TDC lines up with the left of the two closely spaced pulley marks not the right one.

2. I used a dial gauge to complete a cam profile for both intake and exhaust for both left and right cams.

3. I am confused by the cam timing specs. For my 86 930, Wayne's book says the following:

intake opens 3 deg ATDC
intake closes 37 deg ABDC
exhaust opens 27 deg BBDC
exhaust closes 5 deg BTDC

For cylinder #1 these numbers for crank rotation work out to 363, 577, 153 and 355 respecively.

4. I do get some very nice cam lobe curves but they do not agree with the above numbers. As an example the intake valve begins to open at 340 deg not 363 deg. BTW, I adjusted the valve gap to .1mm. and I did have a running engine.

5. What does "intake open" mean? Does it really mean the position of "intake valve stroke in overlapping TDC with 0.1mm valve cearance" which is 0.7mm for my stock engine cams. This does work out to be fairly close, but for whatever reason is not exact.

6. In other words as an example. My graph shows that at 155 deg the exhaust valve is already lifted by .7mm. This value is much closer to the actual spec of 153 deg.

7. Should I use this .7 mm ("intake valve stroke in overlapping TDC with 0.1mm valve cearance") line for open and close measurements?

Thanks and sorry for the long winded question.

Norm

[Rot 911]

On my '86 3.2 TDC is also the left of two closely spaced pulley marks. Wayne's book is based on the SC and I think the timing marks are different for the SC.

[hobieboy]

I have asked a similar question recently & what I found out (Thanks Strlj) is that intake valve opens means the valve had already lifted by 1mm.

That seems to match up with all the cam spec - for the stock 930 cam, you set the valve lift @ TDV overlap to .7-.9mm, and timing is 3 degree ATDC. For a SC CAM, the spec is 1.4-1.7mm and the timing is 7 degree BTDC.

This also seems to agree with your observation.

HTH

[nreed]

Thanks Patrick...I should have read the answer to your question from Strlj more carefully. I read 1 mm as .1 mm and that was what confused me. Now I get it.

The numbers work out much better for my cams. I now know what I started out with.

Norm

p.s. hope your trip was a good one and I'll get the leak down tester and tools back to you soon. NR