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Cams

john998

Well Known and Active Forum Website User
VOC Member
Hello,
Having made a simple rig to measure the lift on my spare cams, I have found that all 3 sets show a large difference in lift between the long and the short cam.
The Mk3 short gives 115 thou lift at 40 degrees, the long gives 82 thou at 40 degrees, this is turning both cams clockwise.
Am I missing the obvious? is it because the cam followers are laid out at different angles.
Also is there an easy way to identify an unmarked cam, to determine if it is Mk1 or M3
Now awaiting the brickbats.
Regards John.
 

bmetcalf

Well Known and Active Forum Website User
VOC Member
I have Andrews cams with quieting ramps and I was told to time them using .050" as the starting point to allow for variances with the ramps. You may be in the "ramp zone."
 

timetraveller

Well Known and Active Forum Website User
VOC Member
Unless your jig mimics the flat based lever followers then your results will be meaningless. As the cam slides along the base of the follower it has more or less leverage on the pushrod and hence the valve. When it is touching close to the pivot of the follower the cam lift is geared up more on the hollow end of the cam follower and vice versa. If you have built a correct jig and you are still getting different lifts at the same cam rotation then either the cams are different e.g. Mk1, II or IIIs, or they are badly made. Many Vincent cams were badly made!!
 

john998

Well Known and Active Forum Website User
VOC Member
Hello,
Thanks for that, I had not thought about the flat followers. I have just tried a fourth set of cams and got the same difference, I understand that in absolute terms the readings are meaningless, as far as timing an engine, but as the readings are all off the same small jig I would have expected similar readings on short and long cams.
Regards John.
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Hi John,

Good idea to have a jig made but as Timetravellers stated, it really should mimic the original configuration. I can understand your perspective that although the jig is consistent for all cams tested, in reality, the line of contact between the cam surface and follower moves a considerable distance along the follower (as witnessed by the wear marks) in a very short angular rotation of the cam. Having a test configuation that is different to that used in the engine may infact disproportionately amplify a small and perhaps trivial difference that exists between each cam profile. I too have made a cam jig as well as a computational simulation and can only say that flat followers give rise to rapidly changing contact positions (read higher rubbing velocities and wear) which require asymmetric cam profiles to correct. I've found many different cam profiles in Vincents, including symmetrical ones, but I've not really detected much difference in the seat of the pants response in all but propper race engines. Good luck with your investigations.
 
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