How does valve timing in degrees work?
 

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?

not sure of question, but maybe this will help.

since our cams have the exhaust lobes and intake lobes on the same cam, valve overlap is machined into the cam and cant be changed no matter where you time them. the CENTER can happen anywhere NEAR TDC. IE, intake could open 12 BTDC and exhaust could close 6 BTDC for 6 degrees over lap. that same cam can be timed so the intake opens at 0 TDC and exhaust closes 6 ATDC for the same overlap. a good example are the SC engines, which i think has you confused. porsche used the same cam, but just timed them differently.

i suspected my cams were not timed correctly by the PO, so i "reverse timed" my cams. i rotated the crank until i had the amount of lift for my car. then i mark the pulley at the seam in the case. (saving some math, i measured my 5 degree mark and my 15 degree mark in relation to TDC) this came out to 1mm per degree. the mark i put on my pulley was 15 degrees advanced.

back to you.
although exhaust valve closing is the critical timing event, the cams have to be measured at the intake valve and how much it has opened at TDC. (you need a reference on the crank pulley so instead of porsche adding MORE marks for each engine, they used TDC.

so in the end, you CANT change the AMOUNT of valve overlap, just where it happens. the ONLY way to change overlap is to get different cams.

now, on other cars with dual over head cams, one for in and one for out, you CAN change overlap. but that is a how other issue.

if your serious about doing it by degrees get a timing wheel jegs has them for not too bad. screws into the front of the crank twist turn tune.

If no timing wheel, you can divide 360 by the number of teeth on the flywheel and calculate rotation per tooth. You can get within a degree, which is well within the tolerance in the manuals (at least for the old cars).

Get a degree wheel, it is $25.

If you use the Bentley, it's very easy to do.

That's a good question

here are the published specs for that cam

http://forums.pelicanparts.com/uploa...1355837141.gif


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