Owner, B³ Racing Engines
Now, as if the out-of-control parts-wrecking accelerations and decelerations weren’t bad enough, some real performance is lost. How, you ask? Simply put, if the valvetrain is not set to the 1/2-lift method, there is inefficiency and power is lost. If the fulcrum point (shaft) is too low, the valve will accelerate slowly off the seat and require more degrees of crankshaft rotation to reach a given lift, up to the point where it reaches a perpendicular plane to the valve stem, where it finally catches up. For example, a fulcrum point that is .250" too low, will reach the perpendicular plane at .500" lift. If your valve lift is around .500", the valve is at maximum velocity and the crankshaft degrees needed to reach .100", .200", .300" lift etc. will be greater, because the roller is sweeping sideways more than it is opening the valve. That means that the valve is dwelling longer at lower lifts. With lower lift is less curtain area (area between the valve face and seat), reducing the volume of air the heads move per crankshaft degree.
So what, you say. It will flow better when the valve gets to higher lifts. Sure it will, but, and you knew there would be a "but," as the valve reaches the perpendicular plane and maximum velocity, it is moving rapidly through the higher lift ranges where the airflow is best. That means less dwell time in the higher lifts, reducing the amount of time the engine has to breathe deeply, and fill the cylinders efficiently. Imagine you were out for a run and you had to breathe through a large straw. Every time you need to take a breath, someone has the straw pinched shut, and opens it very slowly, gradually opening it quicker until it is all the way open, and then quickly pinching it down to a smaller size, and then finally closed. Do you think you could get enough air to fill your lungs completely? Probably not, and your engine is no different. Each cylinder is like a lung that needs to be filled completely and efficiently to perform at its best.
Ok, what if we look at the same .500" lift scenario with the fulcrum point at the correct location? Now the action at the valve will change dramatically. Because the fulcrum is higher, there will be less sideways motion at lower lifts, so the valve will accelerate quicker off the seat, and reach the perpendicular plane at .250" lift. At this point, the valve will start to decelerate until it reaches .500" lift, where it will dwell briefly, and then reverse the cycle. Let’s see, get the valve to .250" quicker, slow it down from .250" to .500", dwell briefly at .500”, accelerate back to .250”, and slow the valve to gently set it down on the seat. Sounds kind of like how the cam lobe is designed, doesn’t it? That really is the idea here. Get the most accurate cam motion possible to the valve, through the various links involved in an overhead valve, pushrod type engine. Any other way is wasting that motion, and costing power.
Here is a great opportunity to address the misconception that peak lift flow numbers aren’t as important as the mid lift numbers, because the valve only sees peak lift once, and the rest of the lift numbers twice; once opening and once closing. As you can see from the previous example, the valve will stall at full lift as the lifter goes over the nose of the cam. There are far more degrees of crankshaft rotation at full lift than at any point in the lift cycle, even with the opening and closing points combined, i.e. .400" opening and .400" closing. Here’s an analogy for you. Would you rather stay at a five star resort for a while, or just drive by it twice?
I hope this has been informative, and most of all, thought provoking. I could have gone on with a lot more detail, but I don’t want either of us to suffer from "tech overload." There will be more geometry tech to come as time allows. We still haven’t addressed the pushrod side of the rocker arm, and some of the related components that also play an important role in valvetrain efficiency. Hopefully, we can get into that shortly. Until then, enjoy!