Rocker Arms

[Jeff Lee]

Originally Posted by J Giermanski

Just a novice here asking a stupid question and trying to learn something. What differance does it make if the spec is correct at the retainer? The valve will not open any more or less. Am I wrong on this? Thanks for your help.

Rate of lift is affected. Could be good, could be bad.

[Travis Miller]

To Bruce, Alan, and anyone else concerned with this rocker arm discussion,

I got in this discussion when the statement was incorrectly made that any rocker arm ratio could be used. That led people to believe that it was legal to use 1.6 or 1.7 rockers when the specs call for 1.5 rockers. Then it expanded to therotical measurement using off the car tools. We could not care less what the rocker arm ratio is when it is laying on the workbench. We are concerned with the "as run on the engine ratio". That ratio is figured by taking the lift at the retainer and the lift at the lifter then doing the math. As long as that ratio does not exceed the spec in the Engine Blueprint Specification for an OEM or accepted replacement rocker arm, it will pass.

We know that the ratio of stamped steel rocker arms can vary, usually to the short side. That is why the spec given is the max allowed. However an OEM rocker arm that the factory says is 1.5 is not going to figure out to be 1.6 or 1.7. That ratio comes from using the wrong rocker arm. If the spec says that the ratio is 1.5, then that is the max the ratio can be. If anyone wants to play with having a cam ground to make up for factory shortcomings, fine. Just be sure you meet the lift specs at the valve retainer and you do not exceed the rocker arm ratio specs. If you do happen to run across a rocker that exceeds the spec for the ratio, then its simple...do not use it.

Now you know how we check ratio....lift at the valve retainer and lift at the lifter, then do the math.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

[Jeff Lee]

Originally Posted by Alan Roehrich

Travis,
In the interest of trying to be legal, and trying to get along with the Tech officials, exactly what is the tolerance here.

Lets be brutally honest about this stuff.

First off, stamped steel ball and stud rocker ratios vary, in no small amount, when checking real factory stock pieces. A stock GM 7/16" stud 1.7:1 big block Chevy rocker may check anywhere between 1.65:1 and 1.74:1 or so, and that's out of maybe 30 pieces. That's JUST the rockers themselves, with NO other changes. You might not find a set of 16 that are exactly 1.7:1 out of 100 rockers. I know, I've sorted them that way. And that was genuine GM stuff, not aftermarket stuff.

Second, we all know that rocker arms are NOT a truly constant ratio device. Change ANY of the three points of the fulcrum, and the ratio changes. This means that as the rocker travels from valve closed to maximum valve lift and back, the actual ratio of the rocker changes. This also means that changing pushrod length changes rocker ratio.

Third, we also know that valvespring pressure changes the amount of lift generated. I can increase my seat and open pressure by 50 pounds or so and my lift may change 0.020".

Also, we had an engine at one point that actually had a cam with extra lobe lift ground in because they could not get to the factory valve lift.

So exactly what tolerance exists? If I put a short pushrod in so that I'm 0.010" under the valve lift spec and have enough piston to valve clearance, and that reduces my rocker ratio slightly, have I made the engine illegal? If an engine has a geometry problem, and I need an extra 0.005" lobe lift to get within 0.010" or so of the correct valve lift, is that illegal?

All correct and good points.
I would ad that since NHRA allows for an adjustable pushrod (no spec given), or adjustable rocker (but not both), and doesn't check lobe lift (as the spec is not given), and clearly indicates that lift is measured at the retainer, that it doesn't really matter what the RR is.
I wont pretend that I've checked even a small percentage of the millions of stamped rockers out there, but the ones I have checked, buckets full, have NEVER checked exactly as advertised. I don't recall ever checking a 1.5 SBC or 1.6 SBC or 1.6 SBF or 1.6 SBF that ever showed a higher than advertised RR. But I'm sure some are out there.
So from there, I must conclude there has to be a tolerance. And I would bet the SBC racer in Gainesville had an advertised rocker that in reality was 1.5something. Do we round up or do we round down?
Tech can bust any racer out there at anytime for any reason. I just can't believe with no tolerance given in such a sloppy atmosphere of undefined specs (the valve train) that this is an area of concern.
Yes, It's amazing what an open forum will bring to light.

[Jeff Lee]

Looks like I was typing at the same time Travis was typing.

Now wouldn't this be a lot easier if NHRA allowed accurate $279 Gold roller rockers in stock?
Oh yea, that will ruin the sport...

[Pat Cook]

So, if we have a cam that is suppose to have .400 lift, but the closest rocker we can find figures out to 1. 46 ratio. it would be ok to have a cam ground with a.273 lobe to make up for the rocker ratio, but if we have a rocker that actually has a ratio of 1.56 you can not have a cam ground that has a .256 lobe lift to get the lift to check legal?

Just curious.....

[GTX JOHN]

You are correct Pat as tech has explained to us..... on our Mopars we bush and have ratio corrected and equalized before anything else.

[Travis Miller]

Originally Posted by Pat Cook

So, if we have a cam that is suppose to have .400 lift, but the closest rocker we can find figures out to 1. 46 ratio. it would be ok to have a cam ground with a.273 lobe to make up for the rocker ratio, but if we have a rocker that actually has a ratio of 1.56 you can not have a cam ground that has a .256 lobe lift to get the lift to check legal?

Just curious.....

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

[Tom keedle]

Originally Posted by Travis Miller

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

how would that be any different from a cam being ground different?

[Alan Roehrich]

With a given tappet diameter, especially a flat tappet, there is an absolute limit as to how fast you can accelerate that tappet. So, given the tappet diameter, and a factory rocker ratio, there is a design limit on how fast you can accelerate the valve. Increasing the rocker ratio allows you to accelerate the valve faster. So, if you reduced your maximum lobe lift in order to use a higher ratio rocker, you'd be disguising a method of exceeding the design limit of the original valvetrain with regards to valve acceleration.

When you have a limited amount of lift, the secret to power, within reason, is how fast you can get to that lift limit from the valve being on the seat, how long you can hold it there, and how quickly you can get it closed. It's called, in general terms, "area under the curve". Again, this is a generalization, not an exact rule. For a given combination, there is an ideal, that combination may or may not be able to achieve that ideal. And that ideal may not always be the maximum amount of "area under the curve".

It is really hard to explain this, especially using generalizations as opposed to actual examples and the math involved. That's about as clear as I can make it here.

[Pat Cook]

Originally Posted by Travis Miller

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

I am no "engine master" by any means, but is there any real power by doing that? and isn't it the same thing as the way stocker cams are ground anyway? the dwell cam verse the rpm cam?

Is there any dyno proof? just curious.....