CAM Timing question

[hobieboy]

Just reading up on the factory timing spec for the 930, and it says that the intake port opens at 3 degree ATDC.

But, the valve lift @ TDC overlap is supposed to be .65-.80 mm.

Now, if the intake port opens After TDC, why would valve lift at TDC overlap greater than 0?

I probably misunderstood something, but where's my error here?

Thanks...

[stlrj]

The 3 degree ATDC is with the valve lash set at 1.0 mm. for the standard way of measuring the degrees of camshat duration. I don't know who deceided that this is the way to measure duration, but it seems to be an industry standard.

In reality, however, we adjust the lash on our rockers to 0.1 mm which yields the .65-.80 mm lift that you will see on you gage.

Cheers,

Joe Garcia
86 Carrera
Redwood PCA since 1976

[hobieboy]

Do you mind explaining setting the valve lash to 1.0mm for measurement of duration?

If I'm setting the CAM timing, I'd set the lift to be .65-.80mm based on 0.1mm valve clearance. So where does the 1.0mm come from?

[stlrj]

I'm not certain if the valve lash is actually set to 1.0 mm. Instead, degrees of duration are not counted until 1.0 mm of lift is achieved since the valve is not effectively opened until 1.0 mm of lift. This would compensate for differing cam profiles and be a more accurate measure of effective valve duration.

Joe