Thread: FMS Roller Rockers/ Pushrod length

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Originally Posted by techinspector1

Maybe you should call up CompCams and tell them that this article they wrote is all wrong.
http://www.compcams.com/Products/CC-'Pushrods'-0.aspx

There are many different theories as to what is correct valve train geometry, such as mid-lift geometry, jesel geometry, the roller-tip-in-the-middle-of-the-valve theory (R.T.I.M.O.V.), etc. Similarly, there are as many different beliefs, or final conclusions, by the end user as to which one is best to adhere.

Choose whichever you want. Just remember that where the roller tip rides on the valve has literally nothing to do with valve train geometry. In fact, giving priority to "putting the roller tip in the middle of the valve" in order to establish good geometry makes all kinds of dangerous presumptions about the combination of valve train components being used, and actually can (and usally does) throw off geometry and thereby cost horsepower because the valves don't receive the linear information correctly translated by the radial sweep of the (incorrectly positioned) rocker arm. So instead of having a given amount of valve lift for the 30*, 40*, 50* of crankshaft rotation, instead you have less, and/or the valve train is imposing greater frictional loads than it should be, etcetera, all of which costs horsepower.

In a properly engineered valve train, made of valve train components which are properly engineered for the specific engine appplication, and when set up with properly evaluated and optimized valve train geometry, will usually allow the roller tip to fall in the "correct" location. But trying to get there from the reverse approach (which the practice of R.T.I.M.O.V. theory is) may or may not get you in the ballpark...depending on the combination of parts used since all rocker arm manufacturers make rocker arms as they see fit (there is no standard for rocker arm design).

Making sure the roller tip is on the valve stem where you want it basically just locates the roller tip on the valve where you want it to be, making optimized geometry secondary. But with excellent valve train geometry the roller tip may be positioned just about anywhere on the tip of the stem that you want and the engine will run fine, even if outboard or inboard of the center of the stem. Why? Because contrary to popular belief the rollerized tip's primary purpose is not friction reduction as most people think (the needle rollers in the fulcrum provide the big friction reduction). The purpose of the roller tip is to provide a second pivot point that is constantly aiming the loads directly in-line with the valve tip's contact surface so as to maintain direct force in line with the valve stem over the course of the rocker arm's radial sweep.

The best example of this that I can give is this: imagine extending your arm out straight as though your shoulder is the pivot point like a rocker arm trunnion and your closed fist is the roller tip's axle. Now, from your closed fist hang a weight from a piece of string and consider the weight at the end of the string to represent the roller's diameter contacting the valve tip. As you move your arm up and down radially, the weight and string remains vertical and unchanged in vertical positioning (and force) even though the radial motion of the rocker arm is changing direction.

I recently rebuilt a sportsman drag race engine that had a 7600 rpm shift point. The engine had been together for 7 years before it kicked a rod out of the pan. We gave it a new shortblock. On this engine, the rollers were on the absolute ousde edge of the stem tips. How much guide wear after 7 years of roller cam spring pressures? ZERO, ZIP ZILCH, NADA. Why? Because the combination of valve train components were prepped with optimized valve train geometry which correctly loaded the valve train.

Paul