Cam Duration
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
Cam Duration
Does cam duration affect the need for initial timing increase or decrease?
For example would I need more initial timing on this cam
Intake Exhaust
Duration At 0.05 230 236
Lobe Lift 0.325 0.327
Lobe Separation 110
Vice this cam
Intake Exhaust
Duration At 0.05 212 212
Lobe Lift 0.2795 0.2795
Lobe Separation 110
For example would I need more initial timing on this cam
Intake Exhaust
Duration At 0.05 230 236
Lobe Lift 0.325 0.327
Lobe Separation 110
Vice this cam
Intake Exhaust
Duration At 0.05 212 212
Lobe Lift 0.2795 0.2795
Lobe Separation 110
Drifting
Joined: Oct 2004
Posts: 1,496
Likes: 1
From: Lafayette Louisiana
I have read more knowledgeable people here say that performance cams will need more initial advance. My car likes 17 degrees initial with a corrected duration(1.6 rockers) of 238 intake 249 exhaust. I think stock spec engine are around 8-10?
Melting Slicks
Joined: May 2001
Posts: 2,131
Likes: 4
From: May help you? You can sure as hell try!
Increased cam durations generally increase valve overlap during the exhaust/intake strokes. This increases residual exhaust gas recirculation because the intake valve opens earlier into the end of the exhaust stroke. At idle, this extra residual exhaust gas slows combustion, requiring an increase in spark advance in order to maintain a stable idle combustion process. This is one reason why performance cams generally have a rougher idle than stock cams. Another reason is due to the reduced vacuum at idle caused by the late intake valve closing which blows some of the charge back into the intake manifold.
It's pretty safe to say that you'll want some additional spark advance at idle with the increased duration of the cam you're looking at. The catch here is you'll need to keep about the same maximum mechanical advance, which is around 36-38'. So, you'll want to rotate the distributor a bit to increase the spark advance at idle and limit the advance travel to maintain youir max advance by modifying your advance machanism with a bushing or some other type of stop.
It's pretty safe to say that you'll want some additional spark advance at idle with the increased duration of the cam you're looking at. The catch here is you'll need to keep about the same maximum mechanical advance, which is around 36-38'. So, you'll want to rotate the distributor a bit to increase the spark advance at idle and limit the advance travel to maintain youir max advance by modifying your advance machanism with a bushing or some other type of stop.
Last edited by LemansBlue68; Nov 30, 2006 at 05:20 PM.
