I was having a discussion with some friends about valve trains and rocker ratio. I seem to remember that the GM engineers used 1.7 ratio (I think) rockers on the LS1 for some reason. Same thing with the Ford 302. They used factory 1.6 ratio rockers.

I thought that if the lobe was smaller it would be easier for the cam to move the valve, but the hi ratio rocker would nix that. Another thought was that if the lobe was smaller, you could design it so the valve could be kept at max lift for alot longer because the ramp on the lobe would be tiny, and the lift would be made up at the rocker.

Is there some property of mechanics that makes a low, lobe lift cam with hi ratio rockers work better?

 

Cap'n,

Well, there are two theories at play. Generally speaking, a larger lobe diameter is more desirable (bigger is better).

With a flat tappet design, the larger diameter lobe actually provides better valve control. Since the contact area on the bottom of a flat tappet is relatively large, the larger the cam lobe, the better the ability to manipulate the lifter more accurately, allowing more controlled ramps at the same net lifts. If you sketch or visualize the contact of a lobe ramp with a flat lifter, you can see the limited possibilities for ramp control at a defined maximum lift.

With a roller lifter, the phenomenon is less pronounced, and unless you want to split hairs and consifer the arc of contact of the lifter roller. With the relatively small radial contact area of a roller lifter, the ability to control the valve is much greater and the lobe diameter is less relevant. Still, with a larger diameter, the available profile variatiojn is greater. Maybe that's why even though the LS1s use higher ratio rockers, they have larger camshaft diameters. Another clever design feature of the LS1 camshafts is that they are made tubular (hollow) for reduced mass.