“We learned some shocking truths about valvetrains.”
In the last two months, I have discovered a new kind of power: the power of the press. I’ve worked with mills, lathes, and hones for thirty years, so it was a real challenge for me to sit down at a word processor and begin my second career as a writer. I genuinely appreciate the comments and compliments I have received since I joined the ranks of National DRAGSTER’s back-page columnists.
My sense of humor is no match for Bob Frey’s, and I can’t offer any insights into the human condition like Rev. Owen and Dr. Torstveit. I do know a little about building engines, however, and that is going to be my subject. I promise that I won’t deluge you with theory and analysis; my intention is to offer practical advice and real-world recommendations based on my own experience.
I don’t make any claim that my word is gospel – I’ll leave that to the good reverend. Other engine builders may not agree with my opinions about particular parts and procedures, but I know what works for us at Reher-Morrison Racing Engines.
I always advise racers to consider the long-term costs and consequences when they are buying engine components. Parts that are initially inexpensive can be very costly to maintain over the course of a season. For example, a flat tappet camshaft is much cheaper than a roller cam and lifters – but in the long run, a flat tappet cam can be a black hole that sucks up dollars.
Flat tappets may be fine for street engines, but I don’t recommend them for any serious racing application. Sure, NASCAR Winston Cup engines have flat tappets, but that is because the rulebook requires them. We have built several NASCAR motors, and the extra work it takes to make flat tappets live in a racing environment is a real pain. The tappets must be precisely positioned on the tapered cam lobes in order to make the lifters rotate. If the tappets don’t spin properly, they will wear the lobes flat almost instantly. A flat tappet cam must be broken in with light-tension springs, which involves the extra expense and effort of switching to heavy springs after the break-in period. Finally, there is an absolute limit to how much spring pressure a flat tappet can withstand before it galls the camshaft and turns itself blue.
The odds are against any production block having every lifter bore in the right position and at the correct angle. NASCAR engine builders use elaborate fixtures to machine the lifter bores perpendicular to the camshaft centerline and to position the lifters accurately on the lobes. Many also install jets in the oil galleries that spray oil directly on the cam lobes.
It simply does not make economic sense for a bracket racer to prepare a block to NASCAR standards in order to use a cheap flat tappet cam. And if a flat tappet fails or the cam goes flat, the cost of repairing the engine would have paid for a roller cam and lifters in the first place. For anyone racing a hard-running bracket car every weekend, I believe that a roller cam is the only way to go. The money you save in the long run makes the roller cam an excellent investment.
The most common mistake I see in engine building is to use valve springs with inadequate pressure. Not all springs are created equal; just because a set of coils is described as “roller springs” in a catalog or advertisement does not mean that the springs will produce enough pressure to do their job.
There are several misconceptions about valve springs that influence racers to make poor decisions. A customer who says, “I don’t need good springs because I’m running stock valves,” is badly mistaken. Steel valves are heavy, and adequate spring pressure is absolutely essential to control their motion. A valve’s inertia increases with the square of the engine speed, so even a small increase in rpm requires significantly more spring pressure to maintain valvetrain stability.
It is a myth that stiff springs will pop the heads off valves or cause valve tuliping. The only time that the valve head is subject to spring tension is when the valve is closed and resting on its seat. At all other times, the valve sees only a compressive load between the tip of the valve stem and the groove for the valve locks. In our Pro Stock engines, we use 7-inch long titanium valves with tiny 7-millimeter stems and springs that exert more than 1,000 pounds of open pressure – and we’ve never broken or tuliped a valve due to high spring pressure.
In fact, too little spring pressure is almost always the root cause of valvetrain failures. We spent a year studying valve springs using an Optron, a sophisticated electronic device that can precisely record valve motion and reveal valve float. We learned some shocking truths about valvetrain behavior at high rpm. Even with a relatively mild camshaft profile, the valves bounce on their seats before they close. If the spring is too light, the valve bounces uncontrollably. The valve hits the seat, rebounds, hangs in the chamber awhile, and the bounces erratically several more times. Imagine how hard this is on the valve and the rest of the valvetrain!
Even with high-pressure springs, the valves still bounce when they close. The crucial difference is that the bounce is controlled and predictable, like dropping a basketball. The valve bounce diminishes progressively, and generally on the third bounce the valve stays closed until the next cycle.
The evidence is unmistakable when we tear down an engine that has been run with weak springs: The valve seats are usually beaten up, the valve job is wiped out, and there is fretting on the valve faces. It’s fortunate when we catch these problems early because weak springs will almost certainly cause a catastrophic failure.
Another excuse I’ve heard for not using stiff valve springs is that they take more horsepower to compress. My reply is that each spring stores energy, and for every valve that is opening another one is closing. Anyone who has been whacked by a torque wrench while turning a crankshaft can testify that the valve springs exert considerable force on the closing ramps!
have never installed stiffer valve springs on an engine and lost power; the improvement in valvetrain dynamics more than offsets whatever additional power is required to overcome the springs’ resistance.
I think that any serious big-block drag racing engine should have at least 220 pounds of seat pressure after it has been run. While 1.550-inch diameter chrome-silicon springs may have adequate pressure when they are first installed, they eventually fatigue and lose their tension. We use 1.625-inch diameter Vasco Jet springs exclusively on our Super Series engines and big-block cylinder head packages. These are the same springs that we used in Pro Stock engines just a few years ago; we have virtually eliminated valvetrain breakage in our bracket racing engines by installing these high-pressure springs.
For my money, peace of mind is worth the cost of premium valve springs in any drag racing engine.