Question about valve overlap

[supersport24]

I race in a division where we're limited to a 360 cu in engine, 10.5 compression, 500 cfm carb and 7400 rpm's. I am looking at cams that I've purchased over the past few years and I seem to always be in the 246-254 duration at .050 range but the valve overlap on some of these cams range from 73 degrees overlap on the high end (Reed cam) to 34 overlap on the low end (bullet cam). That seems to be a lot discrepancy between these cams that were designed with the same rules in mind. Why would this be? I have did a lot of reading on this and thought someone on here could explain it to me. Thanks in advance.

[John Wallace]

Which made more HP?

[hoffman900]

Overlap is measured from the seat to seat duration (at hot lash), not at .050" valve lift. Likely doing so will tell a totally separate story.

[MadBill]

Two reasons for such discrepancies could be the cam supplier's assumptions re exhaust system specs and low lift flow. Better match of the former and/or more of the latter significantly reduce the required overlap.

You could try comparing the candidates to David Vizard's duration recommendations and his '128' LCA calculator, which can be found in other S/T posts.

[digger]

are both overlaps specs at 0.050"? i doubt any of them are 73* overlap @ 0.050".

my guess is the 73* is a advertised overlap where as 35* is overlap at 0.050" noting the durations in question and resulting LSA (using some assumptions)

[Geoff2]

If, when comparing the cams, the same method is being used to measure overlap [ ie, from adv duration figures or from 050 figures ], then the LSA changes the overlap. A 2* change in LSA changes overlap 4*, with tighter LSA increasing overlap.

[BigBro74]

It is all about the LASH and WHERE you measure the open/close points.

Put the cams in a block with a head , lifter and pushrod etc. - and measure the valve events yourself with a dial indicator at the valve and a degree wheel-
THen you will know the differences in the valve motion created by each (take good notes)



This exercise will solve SOME of your curiosity and create some NEW questions.................................
Jason

[digger]

It is all about the LASH and WHERE you measure the open/close points.

Put the cams in a block with a head , lifter and pushrod etc. - and measure the valve events yourself with a dial indicator at the valve and a degree wheel-
THen you will know the differences in the valve motion created by each (take good notes)



This exercise will solve SOME of your curiosity and create some NEW questions.................................
Jason

This is the only way to properly compare cams IMO, I always measure out the valve motion. The catalogue is a guide only at best. I've heard people say cam A has more duration than cam B and it does when you look at the seat-seat value, but it's mostly useless duration as it's just due to the difference clearance ramp and when you work out the lobe area the bigger duration cam has less area and the 0.050" duration is the same and 0.200" duration or any other valve is smaller. It doesn't make a good performance cam