CLASS RACER FORUM - Rocker Arm Ratio

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Re: Rocker Arm Ratio

Please forgive my ignorance on this. Where is lift checked in teardown ? The pushrod, lifter or valve ? But if it's @ the valve I don't get why all of this matters. Again, new to stock but not racing.

Re: Rocker Arm Ratio

Dave,
The specs you see in the NHRA Blueprint guide list it as "Valve Lift".
Valve lift is measured at the valve. I have never seen a lobe lift listing in the NHRA Blueprint specs that I usually deal with. That is not to say they(lobe lift specs) may be there for some XXX engine that is or could be under the watchful eye of Billy Nees. (wink,wink)
Since you are new to this I would suggest you visit with a NHRA Tech inspector and ask questions while watching what goes on. It might help you and keep you from a costly mistake or two. See it never hurts or is bad to ask questions.

Re: Rocker Arm Ratio

In your example, you used a SBC with .410 net lift & I assume thats what the spec sheet calls for @ the valve. If so, why is there a slippery slope with changing pushrod length to get .418 lift @ the valve. Its over spec. I understand whats being said about different angles affecting lift. Was this some older school stuff trying to get more from the stock rocker before the rollers were legal ? Just asking.

Re: Rocker Arm Ratio

Dave,
It is about the geometry. You can do exactly the same thing with the new Roller rockers as with the older stamped steel rockers when it comes to making more or less lift to meet the specs. How can a rule for ratio be enforced when you can move the relationship of the rocker to other parts and have the ratio numbers change?

Re: Rocker Arm Ratio

It sounds like this could get into a maddening cycle but ultimately since these can be manipulated it sounds like its the job of the engine builder to make sure all this checks to spec.

Re: Rocker Arm Ratio

Quote:

Originally Posted by Adger Smith (Post 413782)

.
A couple of years ago he and I had this discussion about rocker ratio. The paygrade above us at EMC was wanting us to limit Rocker Ratio.
According to him there is no real way to measure rocker ratio correctly.
Too many variables.
Example: In a Small Block Chevy you are dealing with three distinct intersecting angles. Pushrod, Rocker Stud, Valve angles. Yes, if extended far enough those angles will intersect. Where you locate the rocker on any of the various planes between those angles influences the effective ratio of the rocker.
Say you have a cam lift of .275 and you are looking for a 1.5 ratio to give you a net lift of .410. You set your valve length and pushrod length to get that .410 net lift. OK Presto we have 1.5 Rocker ratio. Does that make the rocker a 1.5 ratio rocker? Probably not. Now you drop the pushrod length by .100 and reposition the rocker and woops the valve lift is now .418.
OOPS, have we changesd the ratio of the rocker, what ratio was it and what is it now. Humm, lets see we lower the length of the valve tip and, oops again the measured valve lift goes to .412. Then if you want to get serious you can mill the stud bosses at an angle and tilt the stud back and the measured lift will change again, all with the same rocker. Wesley told me that measuring Rocker ratio on an engine is not a place you want to go.
Danny must know rocker ratio is a slippery slope. Wesley does.

Adger -
While I agree with you in theory that you cannot measure the "exact" ratio of a rocker arm, you can determine the difference between a rocker that was intended to be used in the "1.5" range as opposed to one that is designed to be in the "1.8" range or the "1.3" range, especially when there is already a sub-industry that is based on the history of production rocker arms. And that is the purpose of restricting the engine to be built within the same parameters as it was engineered. Nothing wrong with that. Now there will always be those bit-twiddling brainiacs who will figure out how to squeeze the mechanical motion of a fixed rotating lever into a ellipse for their advantage, but it would be to the benefit of all to not reduce the technical acceptance of camshaft checking to what is measured at the valve. It introduces another plateau of expense to an already prohibitively expensive endeavor.

There is a decided difference between somebody fiddling with the production tolerances of rocker arms to change characteristics by a .01 of a ratio or so to attain a bit more lift at the valve and somebody designing a camshaft lobe be used with a rocker arm that is .2-.4 of a ratio more or less than the engine's original design spec to gain horsepower.

Re: Rocker Arm Ratio

As long as it does not exceed what the spec says at the valve....I don't see a problem.

Of course one could manipulate the valvetrain.....larger cam diameter, offset lifters, change the phasing, relocate the attachment point of the rocker, change the ratio, longer/shorter valve stem, ect. all in a quest to build a better mousetrap.

But, at the end of the day ,if the lift at the valve SEAT does not exceed the spec called for...who cares?

Re: Rocker Arm Ratio

Quote:

Originally Posted by Bill Diehl (Post 413959)

It is a problem. You explained the considerations well. I do care.

Re: Rocker Arm Ratio

2013 Indy Tear Down - They checked total lift at the valve

Re: Rocker Arm Ratio

Quote:

Originally Posted by Dwight Southerland (Post 413968)

It is a problem. You explained the considerations well. I do care.

Correct, Dwight. All sorts of manipulations are possible if you only check lift at the valve.