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Old 05-03-2011, 08:26 AM
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How to calculate cam overlap ??

I've got a cam with a lobe separation of 108 deg. 202 & 213 @ .050"
.267/.273" lift at the cam. Clevite #2291880. I don't know what the adv. dur. looks like. I'm thinking of a truck engine with 9.75/1 scr.

Is there a formula that I can go by after I measure overlap in degrees? And are these measurements seat to seat, .001" off the seat. or at .050" lift?
What kind of drivability could I expect with 1.65 rockers .437/.450" valve lift.
At a cruise rpm of 1600. hwy.?

Where should I put this cam in for intake lobe center line ?

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Old 05-03-2011, 08:42 AM
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with .050" numbers that small overlap is of no consequence to you. If you're looking at driving a truck at 1600 RPM I would advise AGAINST it though, regardless of the cam you put in there. 2,000 RPM is about the minimum you want unless you want more issues down the road. 2300 RPM would be even better.

A truck has a high load even at cruise an 1600 RPM just isn't enough on a sbc.

Also, I would avoid using a clevite cam, they have nothing great about them other than price, and many better cams can be had at about the same low price.
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Old 05-03-2011, 09:58 AM
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Quote:
Originally Posted by mumbles
I've got a cam with a lobe separation of 108 deg. 202 & 213 @ .050"
.267/.273" lift at the cam. Clevite #2291880. I don't know what the adv. dur. looks like. I'm thinking of a truck engine with 9.75/1 scr.

Is there a formula that I can go by after I measure overlap in degrees? And are these measurements seat to seat, .001" off the seat. or at .050" lift?
What kind of drivability could I expect with 1.65 rockers .437/.450" valve lift.
At a cruise rpm of 1600. hwy.?

Where should I put this cam in for intake lobe center line ?
Add intake opening to exhaust closing. If this isn't known, you can estimate it by using the advertised duration and the lobe separation angle.

Add the intake and exhaust adv durations, then
divide the results by 4, then
subtract the lobe separation angle, then
multiply the results by 2.

Never had a problem w/these cams. Was before the oil went "bad", though. Should be fine as long as the correct oil supplements/cam break in lube and procedures are uses, IMO.

Last edited by cobalt327; 05-03-2011 at 10:04 AM.
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Old 05-03-2011, 07:34 PM
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I have four different camshafts on the shelf, a GM929, the Clevite, a Comp roller 12-411-8, and a GM 395 ramjet 350 roller. And a Lunati that is installed. They have nothing consistent with one another when it comes to duration except @ .050" advertised duration measurements vary from absolute seat to seat. .001" off seat, to comps .006" off seat. These differences make for wildly different advertised duration and overlap figures. And so LCA, LSA, and lift are seemingly the only constants. To roll the cam and take measurements is all I can do. But then what? What is the industry standard?
Vazard has an overlap chart for everything from trucks to race engines, but I didn't see what seat to seat measurement he uses.
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Old 05-03-2011, 08:17 PM
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Quote:
Originally Posted by mumbles
They have nothing consistent with one another when it comes to duration except @ .050"
Then use duration @ 0.050" as the point of measurement of overlap instead of the advertised duration. As long as you have a constant, it will serve the purpose for comparing the overlap between these cams, or to other cams using the 0.050" duration figures.

Depending on the individual cams, you might get a negative number for the overlap. That matters not- it still will be a valid comparison as long as you can visualize it.
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