By David Reher, Reher-Morrison Racing Engines
“In my experience, one of the best indicators of valvetrain stability is also the simplest.”
My longtime friend, teammate, and business partner, the late Buddy Morrison, had a direct approach to problem solving. When Buddy wanted to learn about oil control and crankcase windage in a racing engine, he convinced me that it was a good idea to make a dyno run on an engine without an oil pan.
Well, that test made quite a mess in the dyno cell, but it also taught us how to make a more efficient oil pan.
Buddy was always the adventurous one, and I was the conservative one. That’s probably why we got along and achieved some success together – each of us needed the other one for balance.
Around the time we decided to race in Pro Stock in 1976, Buddy decided that we needed to investigate valvetrain dynamics, so he designed and built a spin fixture. Today’s commercially available spin fixtures are high-tech devices that use lasers, computers, and software to evaluate valvetrain components. We weren’t that sophisticated, but we sure learned a lot with Buddy’s homebuilt spin fixture.
The Reher-Morrison spin fixture certainly wasn’t elegant, but it was effective. We had an old 460ci Ford engine mounted on a couple of steel beams, hooked up to a four-speed transmission that we’d turned around backward so that it worked as an overdrive to increase the speed of the output shaft. The test engine was attached to other end of the transmission by a dummy crankshaft without pistons and rods. We’d install the cylinder heads and the complete valvetrain on the test engine. Then we’d fire up that big old Ford, engage the transmission, and spin up the valvetrain to see what was really happening at 9,000 rpm.
Buddy rigged up an adjustable strobe light to “freeze” the motion of whatever valvetrain component we wanted to look at. What we saw was always amazing and occasionally alarming. When things weren’t right, we’d see valve springs bouncing off their seats and the pushrods smoking. When the valvetrain was operating smoothly, that Ford would rev up to whatever rpm we wanted – but when the valves went into float, the engine bogged down immediately. That was a very graphic demonstration of how much power is consumed by an out-of-control valvetrain.
The computerized spin fixtures that are used today by camshaft manufacturers and racing teams are more sophisticated, but the principles and physics of valvetrain dynamics haven’t changed. I’m not a camshaft designer, and I can’t give you a super-technical explanation laced with ten-digit polynomials or analyze amplitude, velocity, and jerk curves. However, decades of experience with racing engines have taught me that it’s the combination of components that makes a valvetrain stable or unstable – and that the parts can interact in unexpected ways.
There’s no recipe on how to assemble a stable valvetrain for any racing engine with part numbers for every component. You might think that the stiffest springs, the biggest, strongest pushrods, and the most rigid rocker arms would produce a perfect valvetrain – and you’d be wrong in many instances. Each component has a distinctive harmonic resonance, and when these parts interact over a range of loads and speeds, they can behave in unexpected ways.
I’ve seen engine performance improve dramatically by replacing super-stiff springs with lower rate springs. I’ve seen super-strong pushrods deflect so much under load that they leave witness marks on the cylinder heads. Do you set up the valve springs so that they are close to coil bind at maximum lift to prevent surge, or do you set them up “loose” to take advantage of controlled loft as the lifter goes over the nose of the cam lobe? Engines respond positively to both approaches, but I can’t predict with certainty which method is going to produce the maximum power in any given engine.
If computer simulations were always accurate, aircraft manufacturers wouldn’t need to do test flights. They’d just “fly” an airplane in the computer and start production. But anyone who has followed the misfortunes of the F-35 fighter or the Boeing 787 airliner knows that what works in a computer simulation doesn’t always translate to the real world. It’s the same with valvetrain software and spin fixtures: We can’t determine the precise effects of cylinder pressure, vibration, and other unknown factors without testing the parts in a running engine. I’ve seen valvetrain setups that looked absolutely perfect on the computer, but wouldn’t run at all on a dyno or a drag strip.
In my experience, one of the best indicators of valvetrain stability is also the simplest. If the valvetrain is stable, you can usually release the valve locks from the retainers with a spring compressor. Sometimes it may take a light tap with a hammer to break them loose, but if you have to beat on the retainers to break them loose from the locks, that’s a sure sign of an unhappy valvetrain. When I see racers pounding on retainers with three-pound sledge hammers trying to get the locks to let go, I know that there is a major problem in that combination of valvetrain components.
A racing engine is always telling you something, if you are willing to listen. There is always a reason when parts fail. If pieces look marginal during a teardown, something is usually wrong. If the bearings look bad, there is an oiling problem. When an engine locks up its valve keepers or beats up the ends of the pushrods, it’s time to make a change.
There are five basic elements that affect valvetrain dynamics: valve springs, pushrods, camshaft profile, valve weight, and rocker arm geometry. Optimizing power sometimes takes more than just maximizing the area under the lift curve. An engine that consistently experiences valvetrain problems is telling you that it needs a more compatible combination of these components.