cam timing .............again

[75-911S]

I've read every thread I could find and still have a quick question about cam timing.

Should you be able to obtain the proper overlap reading within one 360 degree rotation of the crankshaft?

Mine takes just over two rotations (720 degrees) to reach the right reading on the dial guage.

Am I doing something wrong?

Jack

[Jim Williams]

Once you reach the point rotating the crank where the intake valve begins to lift, you would have to keep rotating the crank through 720 degrees to reach this same point again. Does that make sense? There are probably other ways to describe this, but I can't think of another quick way to answer what I think your question is.

Rotating the crank only 360 degrees would get you to the same point on the opposite cylinder bank. Each cam takes two crank revolutions to make one complete cam revolution.

[911pcars]

One four stroke cycle = 720º rotation of the crank (two revs).

If you start on TDC, end of compression and rotate one rev., you arrive at TDC, end of exhaust.

Sherwood

[75-911S]

So, it's okay if the overlap reading I need to acheive for my engine (.47mm) doesn't come until just past the the end of the 720 degree cycle? (That's Z1 twice plus about 5-10 degrees)

Wayne's book says "It is important to note that you will not turn the cranksahft a full 360 degrees before reaching the proper value on the dial indicator'"

Jack

[Jim Williams]

Jack,

You have 2 Z1's to consider. One is where your cam for one cylinder bank is located such that both the intake valve and the exhaust valve rockers are on the heel of the cam for that side, that is at the point where you would have the cam to adjust the proper valve lash. Neither rocker is on a lobe at this point. At that same Z1, with out moving the crank, the opposite cam ( the cam on the opposite bank), will be on overlap, or where the exhaust valve is closing, and the intake valve is opening. It is this Z1 where you are setting the cam on the side that is on overlap.

When you turn the crank to the next Z1, or 360 degrees, the sides swap. Now you will be adjusting the overlap on the opposite bank from the one you just did.

I don't have Wayne's book, so I don't know the exact context of what he is saying. But I think he means that the intake valve will begin to open before you reach Z1 on the side where you are setting the lift of the intake valve.

HTH,

[2.7RACER]

Jack,
Remember with four stroke engines the crankshaft makes TWO rotations for ONE camshaft rotation.
When you are timing a cam, you are at "crossover" TDC Z1, which is 360 degrees from "compression" TDC Z1.
Most folks are familiar with compression TDC Z1 because that is near the point the sparkplug is fired. In your case 5 degrees after TDC.
Cam timing is done at "crossover" TDC, 360 crankshaft degrees after "compression" TDC, because the exhuast valve has just closed and the intake valve is just opening.
Since the intake valve is just beginning to move open, it allows you to use a dial indicator to measure a point on the leading side of the intake cam lobe. This dimension (0.47mm) relates to the actual degree spec for the CIS cam intake lobe (TDC).
Actually the cam has begun to open the intake valve before TDC.
The 0.47mm spec refers to the movement of the valve at TDC Z1.
At 6 degrees after TDC (just past the 5 degree mark) the indicator should read 1.0mm.
1.0mm (0.039") is the standard for Porsches. American cam makers use 0.050".
Good luck,

[75-911S]

Great information guys. I think I'm beginning to understand what's happening with the valves and cam lobes. I got a much better dial indicator and will give it another try.

Jack

[Eagledriver]

Doug said

"At 6 degrees after TDC (just past the 5 degree mark) the indicator should read 1.0mm.
1.0mm (0.039") is the standard for Porsches. American cam makers use 0.050".
Good luck,"

I don't know what this means but it makes no sense to me. The reading at 6 degrees ATDC will be different for every cam and every timing set-up for every car. There is no meaning to what the lift is at 6 degrees ATDC.

If you are getting to your number (.47mm) somewhere other that at TDC then your cams are mistimed. That is how you check the timing. You adjust them again until you get to your reading at TDC after 2 revolutions. The 2 revolutions is just to make sure there's been no movement of the parts while you were tightening the cam bolts/nuts and that you had all the slack out when you did the adjustment.

-Andy

[Jim Williams]

Andy,

I'm not sure what Doug intended, but as a guess, he has assumed that the .47 mm figure is for a certain cam, and that this particular cam is one for a 911/81 engine. The recommended setting would be halfway between .4 mm and .54 mm or .47 mm, and a setting there (at TDC) would result in the intake opening at 6 deg ATDC. By definition, Porsche specs their openings and closings at 0.040" or (roughly) 1 mm. So if one keeps rotating the crank past TDC after setting the overlap at .47 mm, one should read 1 mm of lift at 6 degrees past TDC.

Just my guess...

[2.7RACER]

Eagledriver,
You missed the line above the sentence. I was using Jack's 0.47mm spec he quoted along with my own information for HIS CIS cams.

"Actually the cam has begun to open the intake valve before TDC.
The 0.47mm spec refers to the movement of the valve at TDC Z1.
At 6 degrees after TDC (just past the 5 degree mark) the indicator should read 1.0mm.
1.0mm (0.039") is the standard for Porsches. American cam makers use 0.050".

My comment regarding 1.0mm for Porsches as compared to 0.050" for American makers is useful when comparing cam specs.
As I said for Jacks cam, he would set the timing at TDC Z1, (crossover), at 0.47mm. From Bruce Andersons Cam specs for this motor, the intake opens at 6 degees after TDC. I merely pointed out at that point, (6 degees after TDC) the dial indicator would read 1.0mm.
Basically explaining the relationship of the .47mm at TDC and 6 degrees after TDC, whch would be 1.0mm.
I'm afraid explaining cam timing without pictures is similar to writing instruction on how to tie your shoe laces.
Jim Williams surmised exactly what I was saying.
Ultimately, more pictures along with a descripition would have worked better. Thanks for your interest and comments.

[Eagledriver]

Sorry about my mis understanding. So what you're saying is that of the 911s CIS camshaft the intake valve opens at 6ATDC and the definition of open is 1mm.

-Andy

[75-911S]

Again. let me say how much I appreciate the information you guys took the time to provide. I had built enough engines in the past to give me the confidence to tackle my 2.7 CIS. But without your coaching, and that from the past threads, I don't think I could have gotten the cams right.

Did them both a couple time yesterday and got the proper readings. Slept on it, and will check them again today for peace of mind.

Thank you very much guys.

Jack

[2.7RACER]

Eagledriver,
You've got it right. Open begins at 1.0mm.