How to figure cam overlap?

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econo racer
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How to figure cam overlap?

Post by econo racer » Tue Nov 13, 2018 2:53 pm

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

F-BIRD'88
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Re: How to figure cam overlap?

Post by F-BIRD'88 » Tue Nov 13, 2018 3:24 pm

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
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Re: How to figure cam overlap?

Post by BradH » Tue Nov 13, 2018 3:28 pm

# 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
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Re: How to figure cam overlap?

Post by Newold1 » Tue Nov 13, 2018 5:15 pm

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 ??
The Older I Get, The Dumber I Get :wink:

PRH
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Re: How to figure cam overlap?

Post by PRH » Tue Nov 13, 2018 5:35 pm

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
Somewhat handy with a die grinder.

Newold1
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Re: How to figure cam overlap?

Post by Newold1 » Tue Nov 13, 2018 7:32 pm

Got it, forgot to think advertised versus @50.
The Older I Get, The Dumber I Get :wink:

07GTS
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Re: How to figure cam overlap?

Post by 07GTS » Wed Nov 14, 2018 4:07 am

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

CGT
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Re: How to figure cam overlap?

Post by CGT » Wed Nov 14, 2018 8:58 am

:lol:
"How or Y do you think I get all this $hit, I read all of you all builds and desktop dyno them with all the correct info and I've got your build."

pastry_chef
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Re: How to figure cam overlap?

Post by pastry_chef » Wed Nov 14, 2018 10:35 am

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

http://speedtalk.com/forum/viewtopic.php?f=1&t=54556
Mike R

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