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Re: [Fot] Camshaft Duration

To: "marty sukey" <trmarty@hotmail.com>, "FOT" <fot@autox.team.net>
Subject: Re: [Fot] Camshaft Duration
From: "Richard Good" <goodparts@verizon.net>
Date: Wed, 25 Jan 2012 13:45:39 -0500
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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