Bill,
Cam timing specs given at .050" lift are generally measured at .050"
tappet lift so you would not want to divide by the rocker ratio.
Richard Good
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?
|