valve timing

[MrScott]

Lets say 'someone' finished their first valve adjustment and adjusted #1 and #4 at 5 deg after TDC rather than TDC.
Other than embarrassment are there any negative consequences? Aren't both valves closed for some time before and after TDC?

[mpetry]

Well the valve timing should always be set with the cylinder in question set to top dead center as marked on the pulley, with Z1 and the hash marks at 120 and 240 degrees.

However, when the cylinder at TDC, the rocker arms are not on the "lobe" of the cam - they are on the base circle, where there is no tension on the rocker arms. The base circle extends for (at least) 60 degrees on either side of TDC.

A good check for yourself is to "wiggle" the rocker arms before you adjust the valve. If you can feel a little bit of lash under the rocker arm - then you are on the base circle part of the cam. If you can't move the rocker arms, then check everything and make sure you're in the right place.

So to answer your question, there should not be any problem if you adjusted the valves a few degrees on either side of TDC. But there is'nt a 5 degree mark on the pulley at the 120 and 240 degree marks - so are you sure you did the valve adjustment correctly ?

Suggest you check everything again.

Mark Petry
Bainbridge Island, WA

[MrScott]

All rockers passed the wiggle test and I got all the marks exactly, just mixed up z1 and 5 after, so cylinders 1 and 4 were in question.

Re: base circle, makes sense. I didn't realize it could be so far off. I wondered how the guys who used a remote starter could time it so precisely When I did the adjustment, if I passed the mark by even 1mm I would go around again. Lesson learned, thanks!

EDIT: So if I understand correctly, in theory one could turn the crank halfway between 2 120 deg hash marks and adjust two cylinders?

[2.70Racer]

MrScott,
It is true the valves are both closed during the COMPRESSION stroke of TDC.
However as the Porsche motor is a 4 CYCLE engine, you must remember the pistons reach TDC twice during one complete 4 cycle.
At TDC OVERLAP both valves can be open slightly. The exhaust valve is closing and the intake valve is opening.
If you have mis-timed the camshafts by 5 crankshaft degrees after TDC the exhaust valve will be significantly more open than it should be when the piston reaches TDC at the overlap stroke.
You may get away with it and maybe not.
Best to reopen the engine and time the cams correctly.

[tharbert]

I'm just trying to clarify... Does there seem to be a mis-use of terms? Are you trying to say you are using the timing marks to place the engine in the "correct" time in the stroke so the rocker is off the lobe (on the circle) and thus available for adjustment? That's waht Mark answered. Or did you actually fiddle with the cam timing? I think 2.70Racer was addressing that.

[rllevin]

Assume correct term is "valve lash adjustment" ?

[MrScott]

Right, I was referring to valve lash adjustment. I did not adjust the camshafts in any way. Sorry for the confusion.

[2.70Racer]

Yes, I mis-understood your question. You are correct, you can adjust the valve lash once the follower is off the lobe and on the base circle as pointed out by Mark Petry.
In fact it is possible to adjust several valves as long as the followers are on the base circle.