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ChrisBennet's Avatar
 
Join Date: Aug 2001
Location: Nashua, NH USA
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Adventures in cam measurement

Hi guys,
A friend and I are trying to figure out what sort of cam he has in his 2.8L (?) motor that used to have carbs. It was originally a 1972 911E motor. We put a dial indicator on the valves for #1 so we could see what the were doing and this is what we got:

(Rough approximate, degrees of crank rotation)
0 deg Both valves closed, crank at TDC
123 deg Exhaust Opens
322 deg Intake Opens
386 deg Exhaust Closes
604 deg Intake Closes

Based on those figures I get an intake duration of 282 degrees and an exhaust duration of 263 degrees.

I'm embarassed to say why I didn't measure the lift.

We looked in Waynes book but either it's not listed in there or (more likely) we're eligible for a "deminished capacity" defense.

Help us out?
-Chris
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Old 03-31-2005, 05:36 PM
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Maybe a custom grind.....

We have a 4 cam 3.8 with our own duration and lift, and a 3.6 2 cam engine with our own spec cams in there...

you wont find these grinds in any book.
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Old 03-31-2005, 06:36 PM
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If the cam is still in the engine what is the cam timing set at? It may give another clue as to what they are or at least close to.
Old 03-31-2005, 07:33 PM
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The duration should be checked at 1mm lift and or 0.050" lift. American cams are checked at 0.010" lift or 0.050" lift. Lift and lobe centers are also very important. With that info you should be able to tell whose cam it is or at least what is happening in the engine. Thats CAM lift not valve lift by the way. Also relative to TDC where are the lobe centers?

A street engine in say a hot corvett or Porsche S engine will have something like 220 degrees duration at 0.050" lift. Never more than 230 degrees for a steet engine. An all out race engine may have something like 285 degrees at 0.050" lift.

Numbers at less than 0.050" lift are for marketing people, real people use 0.050" numbers as they are meaningful. ( Thats right marketing people are not REAL)

The best guess on the numbers that you found, assuming you used something like 0,010" not 1mm for starting and stopping is that you have a stock Porsche cam.

My personal opinion is that anything over 0.4" lift on a Porsche cam is a waste of money, cams, and dosen't add one cent to the performance.

Last edited by snowman; 04-01-2005 at 03:18 PM..
Old 03-31-2005, 07:46 PM
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The lobe centerlines are 103 intake and 105.5 exhaust. The lobe centers are 104.25 as measured.
If you measured the duration at 1mm you probably have a wide lobe center ge80 style cam. If you measured the duration at the lash point the numbers are close to a E cam but the lobe centers are about 2 degrees off.
Mark is right, the cam timing setting is another good clue to determining the profile.
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Old 03-31-2005, 08:36 PM
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Chris, My question for you is similar to what snowman brought up. Did you note the opening and closing as soon as the valve began to move? Or did you wait until the valve moved 1.0mm?
Porsche cams are specified with 0.1mm lash and 1.0mm initial lift.
My experience has shown me the difference in method from just begin to move to 1.0mm lift is approx. 40 to 44 degrees total duration.
Assuming you noted intake just beginning to lift at 322*=38*BTDC, then subtract 20*/22* and you get 16*/18* intake opening. For intake closing, 604*=64*ABDC minus 20*/22* nets 42*/44*ABDC. This gets you a intake duration of 240*/244*.
The same system works for your exhaust. 263*-42*/44*=219*/221*.
If you did not use the 1.0mm opening before noting the degrees, these corrected numbers will be in the ballpark. This would be a warm street cam.
Your original numbers would be, as I'm sure you know, a hot racing cam.
I would remeasure the cam using the 0.1mm lash and 1.0mm opening and check the total lift for intake and exhaust. While you are there I would measure the set-up dimension at cross-over as well. With this info you'll know for sure what you have.
John Dougherty could take a quick look at your numbers and I'm sure tell you what you have.
Paul Frere's book used the raw opening numbers when he quoted early cams and then changed to the 1.0mm system for later cams, which confused me for a while. Bruce Anderson uses 1.0mm. American cam makers use 0.050". All these different methods have made it difficult to really compare cams.
Let us know what you come up with. Good Luck!
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Old 03-31-2005, 09:03 PM
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John Dougherty answered before I finished my dissertation. Nice huh!
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DOUG
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Old 03-31-2005, 09:09 PM
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Thanks for the education everyone! The numbers were with .1mm lash (just like you were timing the cams).
John, you are the man! It would make sense that the cam is an "E" cam because the motor is an "E".
-Chirs
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Old 04-01-2005, 02:19 AM
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Yes, the valve lash inclusion makes the measurement of camshafts very difficult sometimes. To confuse matters, there's a European and American way...

-Wayne
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Old 04-02-2005, 01:46 AM
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