Rocker Clarification is up!!!

[Reed Granrt]

Originally Posted by Greg Reimer 7376

Some time back on another thread under the Tech section, we discussed push rod length and how to optimize it. Quite a lot of discussion resulted in the description of the desmodronic valvetrain commonly found in the various Kettering overhead valve engines. What it boiled down to in layman's language was that push rod length could affect rocker arm ratio.My 327 has a max allowable lift on the intakes of.390". That means that lobe length at the cam has to be .260" with a perfect 1.5 ratio rocker arm. We all know in the real world that nothing is ever perfect in consistant enough quantities to ever be counted on.The trick here is to have a valvetrain where the lobe lift is 2/3rds the valve lift. We discussed how a short pushrod with the same cam lift,rocker arm and lifter could result in a different lift at the valve than a long pushrod. The means for determining rocker arm ratios is to first determine valve lift at the retainer, as it's checked by NHRA,then divide that figure by the cam lobe lift, checked at the lifter,and it should verify the ACTUAL rocker arm ratio. With a roller tip rocker arm,some of the problems associated with OEM stamped arms vanish, but it will be interesting to see what new factors creep up as we start doing this.I am about to reassemble my 327 stocker motor soon, I'll compare results obtained with it as it goes together. I bought a new Dart block,had Gregg Luneack machine it and deck it, I'll soon see what possibly changed. The discussion we had on the Tech section a few months ago got so involved that before we were done, the various compilations of info resulted in a mini-textbook on push rod length. Fun reading!!

Greg
I am playing with a couple of stocker motors at present and I am working with the cam grinder. In doing so I asked the cam grinder what ratio rocker did he develop the cam lobe around. He told me that in this case it was slightly less than the factory designed ratio because most will never be long. Almost all will be short on ratio. So I sat the rocker arm up upside down in the mill an used a laser beam to find my pivot point so that I could establish my pushrod "designed'" length. I then installed the cam between centers and plotted the area under curve. I then put the head on and lifter in with that rocker arm and using that laser determined pivot point, I adjusted a pushrod for my optimum lift. I then went thru the motion of determining area under the curve again and it followed area under the curve as a cam. Now I will move push rod lengths around a see area under the curve again and let you know what I find. But with the new rocker arm rule and the ability to make all rockers exactly the same correct ratio, I think we will see people adjusting pushrod more than ever just to get their lifts in spec and they may loose area under the curve as a result. My .02 worth
reed

[Lew Silverman]

So in the example that Greg gave with his 327, a camshaft with a lobe lift of .260 and a rocker arm ratio of 1.5 would give a theoretical lift at the spring retainer of .390, as specified. Could you not also use a camshaft with a lobe lift of .24375 and a rocker arm with a ratio of 1.6 to get the same lift at the retainer and still make spec? It would also, I think, open the valve faster and improve cylinder filling. Another reason to fabricate a head dyno!

Lew

[Alan Roehrich]

Originally Posted by Lew Silverman

So in the example that Greg gave with his 327, a camshaft with a lobe lift of .260 and a rocker arm ratio of 1.5 would give a theoretical lift at the spring retainer of .390, as specified. Could you not also use a camshaft with a lobe lift of .24375 and a rocker arm with a ratio of 1.6 to get the same lift at the retainer and still make spec? It would also, I think, open the valve faster and improve cylinder filling. Another reason to fabricate a head dyno!

Lew

Unless the rule has changed, a 1.6:1 rocker arm on an engine that came with a 1.5:1 rocker arm is illegal in Stock Eliminator.

[ss wannabee]

Don't see why ratio would matter as long as valve lift spec wasn't exceeded...Again, the spring pressure deal "opened" up this can of worms...

Would think though...that MOST stocker cam lobe profiles were created with the STOCK rocker arm ratio in mind....notice that I said MOST...but not ALL...

[Alan Roehrich]

Originally Posted by ss wannabee

Don't see why ratio would matter as long as valve lift spec wasn't exceeded...Again, the spring pressure deal "opened" up this can of worms...

Would think though...that MOST stocker cam lobe profiles were created with the STOCK rocker arm ratio in mind....notice that I said MOST...but not ALL...

It's called "area under the curve". Making the rocker move the valve faster than the lobe can with the stock rocker ratio gives you more area under the lift curve.

[Tom keedle]

Originally Posted by Alan Roehrich

Unless the rule has changed, a 1.6:1 rocker arm on an engine that came with a 1.5:1 rocker arm is illegal in Stock Eliminator.

what about vice-versa?

[X-TECH MAN]

Originally Posted by Tom keedle

what about vice-versa?

Less would gain you nothing unless you had some major piston to valve clearence or geometry problems with some odd ball engine. The OEM stock steel stamped rockers are way less to begin with.

[novassdude]

If the stock stamped rockers were less and all the cam makers know this. What are the odds that just switching to a aftermarket roller rocker will bump the lift at the retainer over the allowed limit?

[Stocker 2]

Originally Posted by novassdude

If the stock stamped rockers were less and all the cam makers know this. What are the odds that just switching to a aftermarket roller rocker will bump the lift at the retainer over the allowed limit?

That's right! Anyone making the switch to roller rockers will need a new cam ground. Sounds like the cam manufacturers were all for allowing aftermarker roller rockers.

[X-TECH MAN]

Originally Posted by novassdude

If the stock stamped rockers were less and all the cam makers know this. What are the odds that just switching to a aftermarket roller rocker will bump the lift at the retainer over the allowed limit?

Thats for the racer to check to make sure this dosent happen. Not the cam company or rocker makers.