Bill, the specs on the cam I'm running indicate that the cam should be
installed with 3 degrees of advance, and that with this advance the
centerlines are Intake 106 degrees ATDC and Exhaust 112 degrees BTDC. At
20 thou checking clearance (I know this would normally be 50 thou but not
on this cam) Intake opens 37 degrees BTDC, closes 69 degrees ABDC; Exhaust
opens 75 degrees BBDC, closes 31 degrees ATDC. This is 286 degrees
duration at 20 thou.
Make whatever sense of this you can. I'm not an expert on this, but
thought these would be helpful.
I suspect that your original installation specs were pretty close...
- Tony Drews
At 07:37 PM 2/26/2005, Bill Babcock wrote:
> I think you're having a lot more fun than me. I'm struggling with cam
>timing on my motor and I need a sanity check.
>
>Here's the problem. I've got a kind of unknown cam in Peyote--the guy who
>could give me some hints on the specs is incommunicado for a few weeks. I'm
>compounding my problem by installing a vernier cam wheel, so there's no
>marks to start from. I set up a degree wheel and dial indicator, and started
>measuring. I assumed that the timing should be set with .050" lift, and
>since I'm coming straight from the cam follower and assume a 1.4 rocker
>ratio, I'm using .036" lift.
>
>I tried setting it up with 50 degree BTDC inlet open and got some very
>screwy numbers. So I checked the centerline of the intake and exhaust cam
>lobes and decided to start with 110 degrees ATDC. After a bit of fiddling I
>measure:
>Intake opens 33 degree btdc
>centerline 112 degrees ATDC
>Intake closes 65 degress ABDC
>
>which gave me
>exhaust open 72 degrees BBDC
>Exhaust CL 115 degrees BTDC
>Exhaust closes 25 degrees ATDC
>
>and that calculates to:
>Overlap 58 degrees
>Intake duration 278
>Exhaust duration 277
>calculated centerline intake: 106 ATDC
>Calculated exhaust CL: 113.5 BTDC
>
>Weird. there's no way the exhaust should close that early and the intake
>opening is about good for a street engine.
>
>So I tried again, setting the centerline of the intake cam to 100 degrees
>
>I got:
>Intake
>open 37 degres BTDC
>CL 98 degrees ATDC
>closes 57 degrees BBDC
>
>exhaust
>opens 79 degrees BBDC
>CL 120 degrees
>closes 17 degrees ATDC
>
>which calculates to
>Overlap 54 degrees
>Intake dur 274
>exhaust dur 276
>Inlet cl 100 ATDC
>exhause cl 121 BTDC
>
>I'm flummoxed. Can't figure out what I'm doing wrong. Any ideas?
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