Lotus Elan

Timing in cams

PostPost by: jono » Wed Feb 07, 2018 3:18 pm

...lift at TDC or maximum lift method?

Discuss
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PostPost by: rviani » Wed Feb 07, 2018 6:44 pm

Probably lots of opinions on this subject. However at the recommendation of an engine builder I respect and who specializes in twin cams and other "race engines" I have always used the maximum lift method. The reason given by him is that the gradual change in lift near where the valves are closed makes crankshaft position harder to measure accurately than crankshaft position at the centerline of maximum lift which can be "easily" determined using a method similar to that used in determining TDC. Interested to hear what others think.
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PostPost by: promotor » Wed Feb 07, 2018 8:06 pm

Shims need to be absolutely spot-on for the lift @ TDC method as wrong clearance will give different timing events, but with the max lift method it doesn't matter as long as the max lift is set exactly to the degree figure: you could have no valve clearance whatsoever and still get the max llift at the same spot for any base setting of a cams timing.
Not that there'd be too much error but it's the most sensible way in my eyes if wanting it to be exact.
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PostPost by: Craven » Wed Feb 07, 2018 8:50 pm

Lift at TDC is a tricky one as you need to know the cam manufactures specified tappet clearance, not always readily found.
As an example, lift at TDC may be stated with a tappet clearance of 1mm (0.039) As we use 0.008 or 0.010 inch means this lift data is pretty much useless.
This stated tappet clearance is to overcome the vagaries of cam lobe ramps.
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PostPost by: elanman999 » Wed Feb 07, 2018 10:35 pm

I always use the maximum lift method but you do need a fine probe on the clock.
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PostPost by: nmauduit » Thu Feb 08, 2018 9:00 am

The camshaft manufacturer would probably advise how to proceed with his profile...

If the cam profile is standard and symmetrical, I would use the method that consist of finding the centerline like one finds TDC on the piston (measuring a given amount, say 1 mm, of lift before and after the top).

Modern profiles often being not symmetrical (acceleration being higher on the way up than down, that is max up slope higher than max down slope), I would put the extra care of finding the 0.050 up and down to refer to the manufacturer data.

I would not just try to find the max lift one shot even with a micrometer dial gauge on an engine to be assembled carefully.
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PostPost by: rgh0 » Thu Feb 08, 2018 10:30 am

I always time on MOP as that is what I am interested in achieving and thus i like to measure it directly.

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