“You know, you have to pay attention to every last detail,” Godbold says. “I wish I could say there is one change that will set the world on fire, but the truth is it’s more like 100 little things. Things like building a 5/16 pushrod with greater wall thickness to help out the guys that can’t run a 3/8 pushrod. Or our aluminum rocker arms where we’ve changed the profile -- now it has more arch in it -- because we found we could increase the stiffness a little bit more without increasing the mass. It’s those little changes that go right on down the line that really add up to some significant gains.”
Compression Ratio vs Dynamic Compression
Compression ratio is a simple calculation measuring the amount of volume swept by the piston as it travels from bottom dead center to top dead center divided by the volume of the combustion chamber. Dynamic compression is also often referred to as cranking pressure, and unlike the compression ratio is the compression, or cylinder pressure, the engine actually sees when it’s running. That’s because no performance oriented camshaft actually closes the valves at either precisely TDC or precisely BDC.
“When you consider valve motion, the dynamic compression becomes a much more accurate measure of engine performance,” Bechtloff says. Crane Cams has their own, very effective means of modeling valvetrain movment in a running engine, but Bechtloff says that there are website calculators on the internet that will help you determine your engine’s dynamic compression. “In a racing application the piston may be moving up the cylinder bore, but the intake hasn’t closed yet, especially on racing cam with greater duration. So you don’t start squeezing the air until that intake closes. For example, I’ve got an IMCA guy I was working with, and by the rules he is lucky to get his compression ratio up to 9.5. But if you consider the dynamic compression ratio and the cam he wants to run, it’s down to 8.2 in reality. So we moved away from the larger cam with more duration that was opening the valves more slowly and went to a shorter duration cam that was more aggressive and then advanced it to put the intake valve opening where we wanted it. By doing that we boosted the dynamic compression ratio back up to 8.6, significantly improved engine performance. Some people also call it cranking pressure. But whatever you call it, by keeping an eye on your dynamic compression you can generally move in the right direction because cylinder pressure equals torque.”
The Four Pattern Cam
Comp recently introduced Four Pattern camshafts to the Saturday night racing market with great success. Four Pattern cams have been around for a while -- Comp developed them with NASCAR engine builders to be used in Cup race engines -- but they have only now become affordable for us regular people thanks to advancements in CNC cam grinding technology.
The concept behind the Four Pattern cam is to have one set of lobes for the outer four runners and a different set of lobes for the inner four runners which are typically shorter. “The only place where the Four Pattern camshafts are really useful is where we have long and short runners,” Godbold explains. “And that is where you have a single carburetor on a V-8 engine with a common plenum. But I have been surprised at the breadth of the interest in these cams. They have really caught on in classes with cast iron OE intake manifolds and in other places where you just wouldn’t have thought. But that’s because Four Pattern cams can really be a benefit to those engines by helping maximize the power in all eight cylinders instead of making a compromise that’s not best for any of them.
“You know, just about anybody can grind a four pattern cam,” Godbold continues,”what we’ve tried to do is to provide a better camshaft with the technology we have at our disposal. We have new cores that we’ve developed for the Four Patterns, and we’ve also decided that we are only going to grind them on our CNC equipment. Plus, for every Four Pattern cam we grind, we check it on the Adcole machine and provide the customer with a full Adcole report. So you have more data on the camshaft for free than if you went out and bought a $1,300 Cam Doctor. The Adcole is a quarter-million-dollar gauge that checks every lobe and every spec on that cam. And the reason why we do that is when you start programming four different lobes on one cam it’s easy to get something wrong. If you make a mistake of a couple degrees on a standard cam, that isn’t going to affect performance that much. But if you make an error of one degree on the inboard lobe versus the outboard lobe on a four pattern cam, that’s going to make a real difference. The Adcole report helps give you confidence that your cam is right on the money.”
“We’ve seen it plenty of times where a racer thinks he’s got something wrong with his carburetor because the engine doesn’t seem to be running quite right,” Bechtloff says. “Or an engine builder notices a miss with the engine on the dyno at a certain rpm, and he assumes it must be the ignition. Then they find out later it’s the valve that’s unhappy because the spring has hit its harmonic point. You can try to change the spring out for one that has a different frequency or redesign the lobe on the camshaft so that it activates the spring differently, but generally, you are going to have to face that excitation point somewhere.
“The problem is the cam lobe wants to excite the valvespring, and every spring has a natural frequency. Where those two coincide is where you got trouble. What you generally try to do is design a system where you can avoid hitting the spring’s harmonic point at all, but on most race cars there are going to be one or two points in the rpm range where those things match up. What you want to do is manipulate things so that if you have spring harmonics it occurs early where the rpm is low so that the force they generate when they are unhappy is mild. The harmonics makes the valvetrain unhappy, but the rpm means there’s just not enough force to them to do any real damage.
“But if the spring reaches its harmonic point later on it can be very destructive. If the natural excitation point is higher in the rpm range what you have to do is try to pull through that point very rapidly so that it isn’t in that destructive stage for very long. This isn’t a big deal for drag racing because they run up through the rpm range so quickly that they really don’t have to worry about it. But for a stock car racer, you are typically pulling through your rpm range longer, plus you may be hitting that excitation point two times per lap. So harmonics can be really destructive on a circle track motor. That is something you definitely have to watch out for.”