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Discussion Starter #1
Anyone know how to determine overlap from cam specs? I'm trying to find out what the overlap on a RPM cam is. Edelbrock doesn't give it in the specs. I have the timing events but come up with 103degs but I think that is incorrect.

I added the exh opening event BBDC to the int closing event ABDC (59+44 @.050). Is this the correct way to do this?

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Pat Kelley
66 & 67 El Caminos

[This message has been edited by Pat Kelley (edited 08-15-99).]
 

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You're close, but have it backwards.
Start with intake opening degrees,and add
exhaust closing degrees. Total is the overlap.
 

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Discussion Starter #3
Thanks, that make more sense. 69 degs seems closer than 103. Although, I did have a 301 with a cam that had 108 degs of overlap.

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Pat Kelley
66 & 67 El Caminos

[This message has been edited by Pat Kelley (edited 08-15-99).]
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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Pat, makes a big difference here whether you're using the advertised timing numbers or the .050 numbers. I think overlap traditionally used the advertised numbers. If you use the .050 numbers the overlap will really shrink since most of the overlap occurs at low lift.

Tom
 

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Discussion Starter #5
Tom, I did think about that, but I don't have the advertised timing numbers.

What I'm trying to determine is; can I run heads giving me 10.63:1 CR with this cam and have the engine live.

Here are the cam specs:

Duration at .006 Lift: Intake 308° Exhaust 318° Centerlines
Duration at .050 Lift: Intake 234° Exhaust 244° Lobe Separation: 112°
Lift at cam: Intake .325 Exhaust .340 Intake Centerline: 107°
Lift at valve: Intake .488 Exhaust .510
Timing at .050 lift: Open Close
Intake 10° BTDC 44° ABDC
Exhaust 59° BBDC 5° ATDC

Engine is:
355ci
Flat top forged TRW pistons
RPM cam

Heads I want to run are 461's with 60cc chambers. With a shim gasket, for a quench of .035-.040", CR is around 10.63.

Thoughts?

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Pat Kelley
66 & 67 El Caminos
 

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That lobe seperation of 112° will keep in a lot of cylinder pressure. A cam with a 108° lobe seperation will bleed off a lot more. You may want to get rid of the shim gaskets and run a standard Fel-Pro perma-torque at about .042" thickness this will drop your compression to about 10.3:1.
 
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