Quote:
Originally Posted by Bruno49
I'm currently rebuilding my Porsche 911 engine, 1976 type 911/81 with K-Jetronic, and I'm in the process of getting the camshaft timing right. I'm puzzled by the data some books give for the valve timing in degrees. For example Bruce Anderson gives in his book "Porsche 911 Performance Handbook" a table where he lists valve timing in degrees for various engine types.
Firstly, the camshaft ID numbers of my engine are:
Camshaft Left: 911.143.00.
Camshaft Right: 911.144.00
For these camshafts and engines the intake valve stroke in overlapping TDC with 0.1mm valve clearance should be set to: 0.4 – 0.54mm. To get this 0.54 mm valve stroke, the crankshaft pulley moves for 25mm at its circumference. The diameter of the crankshaft pulley is 130.5mm. This roughly means that a move of one millimeter at the circumference of this pulley approximates one degree, i.e.25mm equals 25degrees. This means for the intake valve timing in degrees:
"In opens at 25 degrees BTDC (BTDC=Before Top Dead Center)".
However, Bruce Anderson gives in his book for the engine type 911/81 (same as 911/42 engine) the following vale timing in degrees :
"In opens 6 degree ATDC (ATDC=After Top Dead Center)".
Can anybody explain how a valve that opens at 6 degrees ATDC with camshaft 911.143.00 result in a stroke of 0.54mm in overlapping TDC? In other words, how does this camshaft timing in degrees translate to the desired valve stroke?
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That's a good question
here are the published specs for that cam
the specs say that there is 3° of overlap, but when I map it out the exhaust closes @ 2° BTDC and the intake doesn't open till 6° after so according to that there is no overlap
the duration's check out fine, as stated overlap is fixed when the cam is made so whether it is installed advanced /retarded or on spec there is supposed to be 3° of overlap and on spec the intake is supposed to be open .5mm
I suspect that the answer is in the advance/or retard spec but I can't get there