source for ratio of the rocker-valve arm to the rocker-cam arm lengths = 1.4 to 1.0

[oly]

Doug Riley's Guest Technical Article, 911 Valve Adjustment, The Backside Method says:

"The ratio of the rocker as it sits against the base circle of the cam is 1.4 to1."

Does anybody have a Porsche reference for the source of this information?

Does anybody have a picture that shows the measurement for the two lever arms that give this ratio?

[Grady Clay]

Art,

That ratio is not a fixed number. The ratio changes as the cam lobe ‘wipes’ over the flat on the rocker. The changing ratio is compensated for in the cam profile.

Best,
Grady

[oly]

Thanks for the post Grady. I thought that when the rocker is on the base circle of the cam, that the ratio is constant at 1.4 to 1. Or did I miss something?

[Grady Clay]

Art,

I agree but I wouldn’t rely on any given number given differences in base circle, wear and bent rockers.

As you know, I’m not a fan of ‘back-side’ valve adjustment. It isn’t sufficiently accurate. Sure, it can check for clearance.


Somewhere around here is a fixture I made almost 40 years ago to measure rocker arms. It has a rocker arm shaft and a stop for a fixed adjuster (no elephant’s foot). A dial indicator can sweep the cam surface. There are no Factory specifications for this. You can only compare to new and known-good rocker arms.

Best,
Grady

[oly]

Okay, thanks Grady.

Does anybody else know where 1.4 comes from?

[Bill Verburg]

Originally Posted by Grady Clay

Art,

That ratio is not a fixed number. The ratio changes as the cam lobe ‘wipes’ over the flat on the rocker. The changing ratio is compensated for in the cam profile.

Best,
Grady

I'd have to disagree w/ that. The rocker ratio is a fixed mechanical advantage. The relevant inputs are the length of the input arm and the length of the output arm. It's no different than a playground teeter totter.

The effective lift on the other hand depends on the cam ramp geometry which multiplies the rocker ratio to move the valve. The result is non linear acceleration rate of the valve which can be mapped w/ a cam doctor

[Eagledriver]

Sorry Bill but Grady is right. If you look at the face of the rocker arm you'll notice that it has a wear area that rubs the cam over about a 1 cm length. The ratio changes depending on where the cam is hitting that area. As the cam turns it hits various areas of this wear area, hence a different ratio. The base circles are not always the same either, if someone has had the cam reground it will be smaller and will make a slight difference on where the rocker arm rubs.

-Andy

[Eagledriver]

Don't know where the 1.4 comes from but you could measure it yourself with a set of calipers.

-Andy

[BURN-BROS]

The rocker arm is indeed variable ratio. It is not like a pushrod rocker that has a relatively fixed lifting point. The advertised ratio from the cam grinders use is 1.5/1 . I suspect that this ratio is taken at max lift. I have learned thru a few conversations that it is more like 1.45/1@ max lift for stock rockers and a little more for the non adjustable racing rockers.