Marty,
Cam lobes are generally ground with a gentle ramp leading up to the actual
lobe so that the valve clearance is taken up at a fairly low velocity. If
you slowly turn the cam while watching a dial indicator on the push rod, you
should see the dial indicator start to move slowly as the lobe approaches
and then speed up rather abruptly. The point when it starts to speed up is
when the valve should start to open. To accomplish this you should set the
valve clearance at whatever the tappet lift was at that point times the
rocker ratio. So if the tappet was moving slowly up to .010" then started
accelerating, and your rocker ratio is 1.45:1, you would set the valve
clearance at .0145". Check the degrees at that point then continue on past
full lift until the indicator is back down to the same point. The number of
crankshaft degrees between points is your seat to seat duration. If you
were checking degrees on the cam itself multiply times two.
If it is hard to determine the point where the velocity starts to increase
rapidly, plot the lift at each degree of rotation on a graph.
Cam timing specs are generally given at .020" or .050" tappet lift just
because timing can be more accurate at a point where the tappet velocity is
higher.
Regards,
Richard
----- Original Message -----
From: "marty sukey" <trmarty@hotmail.com>
To: "FOT" <fot@autox.team.net>
Sent: Wednesday, January 25, 2012 10:29 AM
Subject: [Fot] Camshaft Duration
> Question, at what lift is camshaft duration measured at for the values
> typically published? I am currently checking the cam that came in the
> Spit,
> no outside info available on it so I am trying to get an idea what I am
> dealing with. It is a Reed re-grind and they are out of business. Lobe
> lift is
> .290, duration is where ever I measure at :)
>
> Thanks,
> Marty
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