Inliners International Forums General Discussion Engines GMC rockerarm ratio

I'm building up a 302 GMC,everyone ,including cam grinders told me the rocker ratio is 1.5.You can carefully measure the rocker pivot points, seems like 1.5
So here I am checking cam timing,and I notice the valve is missing .040 lift when fully open.I set up the indicator on the tappet,.285 just like it's supposed to be.But up at the valve retainer,it's .395. Should be .435 ish.Now nothing is flexing and these measurements are at zero lash. .285 x 1.4 is .399. I compared the sheetmetal GMC rockers to 235/261 Chevy rockers,the same thing.
We are all getting cheated on lift, must be a plot by V-8 guys

Tony, You are not necessarily being cheated. I don't know how GM measures rocker ratio. But I worked as an engine development engineer at Ford and the way it was measured there was to publish the INSTANTANEOUS rocker arm ratio at the midpoint of the production cam's lift. That is also usually where the rocker arm contacts the valve tip at a right angle, giving the greatest mechanical advantage. So it is very likely that the lobe lift multiplied by the rocker ratio does not give the true lift at the valve. That's why light checking springs are handy for installing the valve train and measuring the true events of the cam, both duration AND lift. Good luck with your project.

By the way, aftermarket rocker arms I have tested usually will give greater lift than the production ones they replace, closer to the published ratio. Clever people, those cam grinders.

Yes,I even juggled the rockerarm geometry on way or another,but it only made a difference in lift of a few thousands.Generally, the rocker should be over center the samw amount when the valve is closed and when fully opened and the rocker tip should contact the middle of the valve stem as much as possible.
This is something to keep in mind when camming an engine,you aren't getting what you think you're getting.
Be best to check the lift at the lifter,then at the valve with the stock cam or any cam for that matter to see what the actual ratio is before ordering a new cam.Not only is the lift affected,but also the duration.

Tony, you are slightly ahead of me on fixing up a 302, could you keep us posted on your progress and the results? Let us know if you get higher ratio rockwers and how they work out, etc.

Thanks,

Ed Pruss
Longmont, CO

Ed,I looked into roller rockers,ouch,I looked away. I'm having another cam ground to my own lift and duration specs.This guy ,at least on the phone, really knows his stuff.I'll see how his work is when I get the cam in a few weeks

Hi Tony . . .

What you're measuring is rocker arm inefficency . . . and is yet another reason why we have over head cam designs today.

Essentially the problem is that rocker arm motion is an arc - not a straight line. This means that the rocker arm ratio changes as lift increases.

As I understand it, on these old engines the 'advertised' rocker arm ratio is measured at rest. So you start off at approximately 1.5:1 and as you move through the the lift the ratio degrades to 1.4 or worse. Roller rockers do a somewhat better job of maintaining a more constant ratio - but the motion is still an arc so the tip of the rocker is moving away from the valve stem toward the fulcrum:

and since only the valve side distance gets shorter - because the pushrod tip follows the arc on the lifter side by moving off perpendicular - the ratio fades the higher you lift.

Here's a link to a tech article on the Chevy Performance site that describes the problem - Rocker Arm Geometry. It inlcudes kudos for Crane stamped rockers for v-thingies. These are advertised at 1.6:1 but actually start off at a higher ratio and stay above 1.6:1 throughout the arc.

stock49

Exactly,now just what is the published rocker arm ratio for a GMC? Anyone ever see it in print?I never have.
I tried a variety of rockerarm angles to see how it affects lift.Having the rocker parallel to head surface at mid travel,goes over center the same distance when opened or closed.And the more stock approach,where the rocker is fairly parallel to the head surface when the valve is closed and goes over center when opening.Little difference in lift, about .005 or so.More important is to keep the rocker tip centered on the valve stem for least wear.

Tony,

Do you ever wonder how many people just slap this stuff together and never ever check to see if the lift numbers on the cam match the cam card? I bet 95% of the folks never check.

Everything I checked when putting my head together was just a little bit screwed up. Had I used the springs that the shop installed, 4 of them would have bound up and broke something.

Keep double and triple checking stuff, when you get it all together you will have a balanced motor that runs all the better compared to the wham-bam-thankyou-maam motors that alot of people run.

Hi to you all,
New guy #4140 here.

May I suggest you go to>
http://mid-lift.com/Choosing_EB.htm and navagate
the many pages of info at Miller Engeneeering,
for a very interesting point of view on the
rocker and its geometry. I get some of what Mr.Miller is saying but its a reach and maybe you guys will "get it".
..bye for now, Bob P.