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kenrinc's Avatar
 
Join Date: Jan 2003
Location: San Jose, CA
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high-lift timing

Could someone clarify the high-lift cam timing for me. Engine is 901/14, 68' 911L with smog pump. All original. Wayne says to turn through 360 deg to Z1 then "at this point the camshaft should be rotated 180deg and the dots face downward"

I'm assuming I can just turn the crank another 180 degrees which will do the same thing?

Then I loosen the nut, remove the pin and move to the overlap value and retighten.

Thanks

Ken-camshaft
Old 02-09-2006, 11:38 AM
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Ken,
Wayne is saying At TDC, COMPRESSION, the dots will be UP.
Then to get to the point where you actually time the cams; You then turn the crank shaft 360 degrees.
At this point although the crank has made one complete rotation, the camshaft has only made one-half rotation (180 degrees).
This is TDC, CROSS-OVER point and the dots would be DOWN.
The significance of TDC crossover is the exhaust valve has just closed and the intake valve has just begun to open up.
In your case engine 901/14 the intake valve has opened 1.0mm at 20 degrees before TDC, so at TDC crossover it will be open more, just guessing somewhere around 3mm to 4mm. I don't have the exact spec.
To set the cam correctly you will need a dial indicator and the factory spec for your cam.
Or a degree wheel and the actual open and close specs for your cam.
Once you set the cam timing, a rough check will be to look at the dots at cross-over, the dots will be down.
At TDC compression the dots will be up.
So the dots are a great reference, but not enough resolution to actually time the cams.
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'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
'85 911 Carrera, stock, just painted, Orient Red

Last edited by 2.7RACER; 02-09-2006 at 07:43 PM..
Old 02-09-2006, 07:39 PM
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That's how I timed them so I must of done it right. I guess where I'm confused is how this differs from the low lift cams. Every other text I have (5) on timing cams does not differentiate between the two. Anderson's is a perfect example. He explains the timing quite simply in one paragraph.

Ken
Old 02-09-2006, 09:47 PM
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Ken,
When you are timing cams you don't really care whether it is a high or low lift cam.
What you are interested in is one of two things.
1. What is the intake valve opening dimension at TDC-crossover?
In your case somewhere between 3mm and 4mm.
2. At what crankshaft degree does the intake valve open 1.0mm?
With your motor a stock cam opens the intake 1.0mm at 20 degrees before TDC-crossover.
If you know either one of the above, you can successfully time your cams.
Further a high lift cam as opposed to a lower lift cam would likely be open slightly more at TDC-crossover if the opening spec was the same.
In other words, if both cams began to open at 20 degrees before TDC-crossover, The high lift would likely ramp open a bit faster because it must open the valve more than the lower lift cam.
Therefore the dimension at TDC-crossover for a higher lift cam, opening at the SAME DEGREES as the lower lift cam would be greater by a small amount.
Bottom line sounds like you got your cams time correctly.
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'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
'85 911 Carrera, stock, just painted, Orient Red

Last edited by 2.7RACER; 02-15-2006 at 08:07 PM..
Old 02-10-2006, 07:23 AM
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I went back and checked this last night and we have both sides within spec (assuming were lined up with the case) I love Wayne's book but the timing section seemed a bit overly dramatic. The factory manual was much more clear in this respect. But I know Wayne has to cater to the entire list of options that someone could be facing when building a motor.

I don't see how this could be a precision process. The book may emphasize the importance of getting the overlap "exactly" right but the bottom line is, the case seam can't be called a constant. A human being lining up a pulley to a case seam? The same exact place everytime? Maybe I'm missing something?

You could be .010 off on either side of that mark and never detect it (you can't tell the difference). Neither can you bring the pulley back to the same exact spot at the case seam on subsequent rotations.

$.02

Ken-

Last edited by kenrinc; 02-13-2006 at 01:48 PM..
Old 02-13-2006, 01:42 PM
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You may be right about repeatabilty on getting TDC exactly right on.
However I don't believe you or even a dyno could detect a .010mm or .010" error.
Porsche moved the SC/Carrera cam timing six degrees back and forth. I doubt many people could tell the difference.
When cam timing chains wear the cam timing can get retarded as much as 2 or 3 degrees. Tough to see that difference in performance.
I think we end up believing since we are working with thousandths of an inch or tenths of a millimeter this is rocket science. It isn't.
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DOUG
'76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's.
'85 911 Carrera, stock, just painted, Orient Red
Old 02-13-2006, 02:49 PM
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Quote:
Originally posted by kenrinc
I went back and checked this last night and we have both sides within spec (assuming were lined up with the case) I love Wayne's book but the timing section seemed a bit overly dramatic. The factory manual was much more clear in this respect. But I know Wayne has to cater to the entire list of options that someone could be facing when building a motor.

I don't see how this could be a precision process. The book may emphasize the importance of getting the overlap "exactly" right but the bottom line is, the case seam can't be called a constant. A human being lining up a pulley to a case seam? The same exact place everytime? Maybe I'm missing something?

You could be .010 off on either side of that mark and never detect it (you can't tell the difference). Neither can you bring the pulley back to the same exact spot at the case seam on subsequent rotations.

$.02

Ken-
When timing the cams I always mount the fan for this reason - the case seam is far away and wide compared to the fan housing notch.
It can be very precise and repeatable (+/- .02mm or less on a 3.2 or 964 cam) with some care.
-Chris
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Old 02-13-2006, 03:14 PM
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