Rocker arm questions

So what affect does going from a 1.73 to a 1.8 rocker have on duration with the cam?
Ive had people tell me you loose duration to gain lift and other people vise versa So im allittle beyond confused.
Also what brands of rocker arms should i be looking at?

Is there any way to figure the duration increase?

I know valve lift is lobe lift x rocker ratio = gross valve lift.
ex 356 x 1.73 = 615 lift before lash setting then you would subtract x amount of valve lash for final take home lift

I don't know of any set way of figuring this. You could call a cam grinder and they could help you.

Maddmattmustangs wrote:Is there any way to figure the duration increase?

More specifically, seat-to-seat duration does not change per se, however area under the curve changes considerably. Whether or not this helps or hurts power depends on what your engine may or may not need valve timing event-wise.

In order to plot this out on paper ("figure the duration increase"), you need to know your timing events at 0.100", 0.200", 0.300", etc. and factor by 1.73 and then 1.8, and then overlay the products calculated between the two rocker arm ratios.

Paul

Paul Kane wrote:

Maddmattmustangs wrote:Is there any way to figure the duration increase?

More specifically, seat-to-seat duration does not change per se, however area under the curve changes considerably. Whether or not this helps or hurts power depends on what your engine may or may not need valve timing event-wise.

In order to plot this out on paper ("figure the duration increase"), you need to know your timing events at 0.100", 0.200", 0.300", etc. and factor by 1.73 and then 1.8, and then overlay the products calculated between the two rocker arm ratios.

Paul

what would i need to figure my timming events at those points?

What you could do would be to use your timing points at .050 CAM lift, then multiply that figure by 1.73 to see what VALVE lift you have at that position. Then divide that VALVE lift by 1.8 to see what CAM lift you now have to use when you check your duration using a degree wheel. This will tell you the effective change in duration. It'll be a LOT less of a change than you think at .020, .050 and .100 valve lift and a lot more of a change higher up the scale, .200, .300 etc. This is why changing to higher ratio rockers changes max lift somewhat but does not decrease valve to piston clearance all that much.