Duration change ?

[MONGO510]

Hey fella's, Will a cam have more duration with 1.6 ratio rockers than 1.5?

I know the lift changes but what effect does this have on duration?

Thanks,

John

[grumpyvette]

while it depends on the ramp design on the cam lobes you could usually depend on getting an insignificant EFFECTIVE increase in durration in the 3-4 degree range ,swapping from a 1.5 to a 1.6 rocker ratio, the durration does not change but the RATE at which the valve accellerates away from its seat DOES change enough to change the EFFECTIVE flow thru the port marginally,but your not likely to see any real benefit untill the higher rpms are reached simply because the cylinders don,t (SEE ) a big differance in flow at low rpms due to the time available for flow into the cylinder. look at it this way,if the cylinders can fill ,any increase in potential flow is just that (POTENTIAL, but UNUSED FLOW.) untill the ports restricted DUE to durration,lift or AVAILABLE TIME, if you have a 7% increase in flow( the approximate differance in peak lift) it will only make a differance if that 7% becomes an advantage in efficiency. at 1000rpm theres a flow pulse thru a port 8.3 times a second, at 7000rpm theres 58.3 pulses per second and the extra 7% potential might help

 

read

http://www.gregsengine.com/rocratio.htm

 

http://www.compcams.com/Community/Articles/Details.asp?ID=2132654598

 

 

notice theres little change in effective flow into the ports due to the higher lift untill about 3000rpm

 

swapping rocker ratios is a good tunning tool, but if you need a serious change in cam timing a new cam, not rocker ratio changes are needed, you may also want to keep in mind that that increase in rate of accelleration and higher total lift CAN get you into clearance issues and unstable valve train harmonics if your already running close to your limits.

swapping to those 1.6:1 ratio rockers on a mild cam usually helps, but once your running at high rpms with a serious cam, its usually better to use a longer durration,higher lift cam with its slower accelleration rates and the lower ratio,rockers to increase valve train stability,or the valve spring pressures required get higher than ideal real fast

[MONGO510]

Thanks Grumpy!