Quote:
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