revisited ??? how to figure how rocker ratio effects duration?????

[oldsman71]

switching to 1.6's from 1.5' s lift is easy to figure but how does it change the duration?? is there a way to figure this?

[oldsman71]

the 1.6's never seemed to make the difference a cam change did

[Ron Ward]

After scratching my head for a minute, it hit me. Duration will not be affected by the rocker ratio change. Duration is based on a point on the camshaft lobe where the lifter starts it's upward movement and finishes its downward movement. Rocker arm ratio has no bearing on this point, thus it does not change and the lifter and pushrod will perform as they had with the 1.5:1 ratio rockers.


Ron

[oldsman71]

thats what I thought but some comp cams guy claimed something different, all I plan on is lift, not sure what kind of power
you get with an extra .030 of lift.

[jcharliem]

The rocker ratio will change the "effective" duration at 0.050" lift (the common measurement for cam duration). This is because the valve action will be different at that measurement point with the change in rocker ratio. The 1.6 rocker will get the valve off the seat and to 0.050" lift quicker (the reverse when the valve closes)...thus, increasing effective duration. Therefore, you've extended the period the valve is open...although slight.

Rule of thumb...switching from a 1.5 to 1.6 ratio will increase your effective duration by about 3 degrees.

[oldsman71]

I heard something like that before, I didnt think it gained much, thanks

[Beagle]

3* of duration @ .050 is quite a bit when you think about it. Very small numbers make very big differences in this game.

[boxer3main]

3* of duration @ .050 is quite a bit when you think about it. Very small numbers make very big differences in this game.

if not noticable, it was low compression engine.
3 degrees is big.

if to wonder about the ratios, they are in everything .. time is distance. I interpret all kinds of things that way.

[oldsman71]

would make my cam 207/217 at .050 and .450/.471 stiil too small

[oldsman71]

3* of duration @ .050 is quite a bit when you think about it. Very small numbers make very big differences in this game.

it might help a bit, got some old beater 462 heads with2.02/1.60's thought about putting them on with some 1.6 rockers to get a few more rpm's out of my poopy old 355, it now has old 76cc 1.74 intake valve truck heads. running a few hundred rpm's short of making it threw the 1/4 mile in my powerband now.