At what duration # do you use to calculate overlap of a cam?
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Joined: Oct 2002
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From: Upstate NY
Short answer:
Advertized duration (and therefore advertised overlap) is measured slightly differently by different cam companies, but .006 tappet (lifter) lift is common, like with Comp Cams. That's probably what most folks use for their overlap numbers.
If you have accurate lobe lift data (like Cam Dr. numbers), you can figure duration and overlap at any given tappet (or valve) lift.
Longer answer:
Example: a certain Comp hydraulic grind with 236* duration (@ .050 lobe lift) used on both intake and exhaust and ground on 112* lobe separation angle, has a 288* rated duration (@.006 lobe lift) by Comp, but checks at about 299*[intake] and 296*[exhaust] (@.003 lobe lift as installed in the engine). This gave 72* overlap, or what might be called "advertised overlap".
However, at .050 lobe lift (.080 valve lift with 1.6 rockers) the overlap was 10*. This is a very aggressive lobe, but idle quality is still fairly good.
Using numbers we have for the hydraulic LS-1 Hot Cam (or similar), 219* (IN) 228*(ex) on 112* LSA, we got the same 72* advertised overlap", but the .050 lobe lift overlap was -1*, so idle should be better than the Comp cam noted above. Of course, lift on the aggressive Comp was tons more and with the correct heads, intake, etc, so was the power.
My answer:
You need to look at at least two points on the lift curve for overlap to make much sense. This gets very exciting/frustrating if you are trying to compare a hydraulic and solid cam. This is where the better engine simulator programs like "Engine Enalyzer Pro" help a lot.
My $.02
Advertized duration (and therefore advertised overlap) is measured slightly differently by different cam companies, but .006 tappet (lifter) lift is common, like with Comp Cams. That's probably what most folks use for their overlap numbers.
If you have accurate lobe lift data (like Cam Dr. numbers), you can figure duration and overlap at any given tappet (or valve) lift.
Longer answer:
Example: a certain Comp hydraulic grind with 236* duration (@ .050 lobe lift) used on both intake and exhaust and ground on 112* lobe separation angle, has a 288* rated duration (@.006 lobe lift) by Comp, but checks at about 299*[intake] and 296*[exhaust] (@.003 lobe lift as installed in the engine). This gave 72* overlap, or what might be called "advertised overlap".
However, at .050 lobe lift (.080 valve lift with 1.6 rockers) the overlap was 10*. This is a very aggressive lobe, but idle quality is still fairly good.
Using numbers we have for the hydraulic LS-1 Hot Cam (or similar), 219* (IN) 228*(ex) on 112* LSA, we got the same 72* advertised overlap", but the .050 lobe lift overlap was -1*, so idle should be better than the Comp cam noted above. Of course, lift on the aggressive Comp was tons more and with the correct heads, intake, etc, so was the power.
My answer:
You need to look at at least two points on the lift curve for overlap to make much sense. This gets very exciting/frustrating if you are trying to compare a hydraulic and solid cam. This is where the better engine simulator programs like "Engine Enalyzer Pro" help a lot.
My $.02
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