quote:Originally posted by Rocky:
NOTE: These specs are so different than what is shown in the link below, I am wondering if there is some "reference difference" that I am not comprehending...
The valve event timing on the cam card is based on duration at 0.050"; the valve event timing in my post is based on advertised duration. I calculated the specs for the cam on the cam card, based on advertised duration:
Intake lobe mathematic centerline = 108° ATDC
Exhaust lobe mathematic centerline = 116° BTDC
112° lobe separation angle (camshaft degrees)
Lobes: Symmetric
---------------------------------
Exhaust valve opening = 76.5° BBDC
Intake valve opening = 29.5° BTDC
54° overlap
Exhaust valve closing = 24.5° ATDC
Intake valve closing = 65.5° ABDC
When discussing valve event timing its customary to discuss "seated" valve events, which is why I use the advertised duration figures (0.006" tappet lift). However, there is merit in considering events at 0.050" when evaluating a camshaft because it takes into account how aggressive the cam lobes are, this is more important as the hydraulic intensity of cam lobes drops below about 60°.
For good drivability with 4V heads I like to see 0° overlap based on duration at 0.050, with the overlap centered around TDC. That means intake valve opens at 0° or later and the exhaust valve closes at 0° or earlier based on duration at 0.050. I also like to see the intake valve closing at 40° ABDC or earlier based on duration at 0.050.
The hydraulic intensity of the cam lobes used on the cam you've been recommended is 51°. Those have got to be Comp Cams Xtreme Energy lobes. I'd recommend using Comp Cam's Magnum lobes instead.