OVERLAP---two ways to figure on a cam?
 

I have a pair of BBC 540 cubed short deck engines. I have called Crower and asked how much overlap one of their cams had---it was a hyd roller advertised duration of 300*/307*(236*/244* @ .050) on a 114* LS. Their answer was; 12* of overlap.

I called Ultradyne Cams and talked to Harold about one of his cams that is almost identical on the "@ .050" numbers as the Crower, but the advertised duration numbers were a little different; 288*/296* (236*/244* @ .050) on a 114* LS. His answer was; 64* of overlap.

I know the both cams have different advertised numbers and I have somewhat learned the difference between the two cams on that (ramp opening/closing speeds). BUT I don't understand how two almost identical cams can be so far apart in OVERLAP unless there is two ways of figuring overlap.

Can anyone explain that in layman terms please? Thanks

Mark/KAAMA

The Crower is at .050 or more lift, and the Ultradyne is looking at all of the overlap from seat to seat duration. I verified this on my desktop dyno and they come out exactly at the numbers they gave you.

Vette is correct.

One is measured at .050, the other is measured at seat to seat openings.

DAVE

Okay, well let me ask you guys this then. Is 12* of overlap considered A LOT of overlap? Or are there other factors involved like cubic inches, etc. Sounds like the Crower probably has more than the Ultradyne then right? Thanks again.

KAAMA, the Crower will have more overlap overall, but the same overlap at .050" lift and over. Either case is a mild case of valve overlap. We just ran a 228/235 duration at .050" hydraulic flat tappet cam in a 454 Mag. with stock exhaust and had no reversion problem. This cam had 7.5 degrees of overlap at .050". My 272/280 at .050 solid roller has 52 degrees of overlap at .050". Cylinder pressures at cranking will still be interesting on your motors. If they are in the 150 to 185 psi range, that would be great. If the cam is to small or on the wrong LSA, they may be 225 psi, and if the 114 LSA is to wide they may be 120 PSI. My 13:1 496 with the above 272/280 solid roller on a 112+4 reads 185 to 190 PSI across the board. As you know I run 110 race fuel.

Well, 12* of overlap is not excessive with aftermarket exhaust that will guard you against reversion.
If you have stock Merc pipes then yes it's too much and you will almost definatly get reversion.

If one cam has more over lap than the other with the same duration and same lsa, the intake centerline is different.

Cubic inches will change a cams rpm range. It tends to "tame" the cam.
Example:
If you put a 236/244 cam in a 502 your hp will peak in the 5500 rpm range and the torque around 4-4500.
If you put it in a 598 the hp will most likely peak around 5-5200 and the torque will peak around 3-3500 rpm. Because the 598 will have the need to move more air and fuel.

DAVE

Yep, I guess I'm going to have to find out what the cylinder pressures are for sure---should be interesting enough to find out. Thanks guys