How to figure cam overlap?

[econo racer]

When reading a cam card how do I figure this out? Thanks!

[F-BIRD'88]

Add up in,ake and ex duration and divide by 2.
Divide that result by 2 again and subtract lsa.

Multiply that x2. = overlap.

[BradH]

# of degrees the intake opens before TDC + # of degrees the exhaust closes after TDC

Example:
Intake opens 30* before TDC
Exhaust closes 30* after TDC
Overlap = 60*

Keep in mind that it's going to depend on where on the lobe you're measuring the duration, too. The overlap measured at effective seat duration is going to look much larger than if measured at .050" lobe lift.

I hope this helps. - Brad

[Newold1]

F-BIRD88 :

I am having issues trying to make your formula work?

A cam has 232 @ 50 intake duration and a 242 @ 50 exhaust duration with a 112 LSA

Using your formula as given - 232 + 242 divide by 2= 237 divide by 2 = 118.5 - 112 = 6.5 x 2 = 13 degrees overlap ?? seems a bit small ??

[PRH]

Overlap is “usually” associated with the advertised duration.

In the above example, 13 is correct for overlap @.050.

I do it similarly to the formula you used if I don’t know(or want to calculate) the open and close points.

Duration for intake/exhaust added together, divide by two, minus two times the lsa angle.

232 + 242 = 474
474/2= 237
237 - (112 x 2) = 13

[Newold1]

Got it, forgot to think advertised versus @50.

[07GTS]

can be a noticeable difference mine was about 5 deg at .050 but at .006 its 57 deg overlap

[CGT]

[pastry_chef]

Some time back I composed an excel, it backwards calculates from overlap and LSA to int and exh duration.

https://www.speed-talk.com/forum/viewtopic.php?f=1&t=54556