"Even though he has decades of experience building race-wining engines at every level, engine builder Keith Dorton of Automotive Specialists says he still works closely with the camshaft manufactures he trusts to help improve performance. Here, Dorton mock up an engine with a new roller cam from Crane Cams."
If you have ever flipped through a camshaft catalog it can be easy to get overwhelmed by the tables of data provided for each cam family. There are numbers for lift, duration, lobe split, and on and on. And if you don’t have an idea what you’re looking for, it’s easy to get lost.
What’s funny is that for an experienced engine builder or cam designer, all that information doesn’t even begin to tell the story. In today’s ultra-competitive world of stock car racing, there is a lot more that goes into race-winning camshaft design than just dialing in X amount of lift and Y duration. For example, did you know that most leading-edge cam designers break the cam lobe down not into simply the opening and closing faces, but they actually look at the opening face as at least eight separate zones? And, of course, they put just as much thought into the closing ramp as well. Outsiders might sneer at what they consider our ’50s-era carbureted race engines, but there is a lot of top-shelf science going on here, and you can take advantage of your competition by making use of it.
For this article we picked out three of the top camshaft manufacturers in our sport and asked them a very simple question, “Beyond basic lift and duration, what goes into building a camshaft capable of winning races?” And, as you might expect, representatives from all three manufacturers said that there was so much going on that explaining it all would not simply fill a book, but volumes of books.
We spoke to the experts at three leading valvetrain manufacturers to find out just what goes into a camshaft capable of taking ...
Still, all three were very happy to spend some time talking about advanced camshaft design, and we compiled the very best nuggets here for you to digest and use to advance your racing program. There is no way you can take this and go out and grind your own cams, but even if you aren’t building your own engines this information can help you ask the right questions when working with your engine builder or camshaft manufacturer for your next circle track racing engine.
Communication is Key
Just like a good marriage, the key to finding the absolute best camshaft for the engine in your race car is good communication. After all, who wants to waste the time and money cycling through three or four camshafts and paying for expensive dyno time trying to figure out which one is best. Instead, leaning on the experience of a quality cam designer/manufacturer can help you avoid wasting your precious time and money.
“There is a lot we can learn from a racer or an engine builder just over the phone,” Allan Bechtloff of Crane Cams says. “And that is way beyond the simple things like engine size and the limitations imposed by the rulebook. We will usually want to know what cam and engine combination you have run before and how well that worked for you. What do you need more of, low end torque or more pull in the high rpm range? What have you tried already, and have you opened the lash up or closed it down? Have you tried advancing your current cam? What about different rocker ratios? The type of cylinder head you are running is also important, especially the flow rates. Even whether or not you are having trouble spinning your tires off the corner is important for us to know. Sometimes it may not seem like much, but every little bit of information can help us tailor the cam to fit your needs.”
Lose on the Dyno, Win on the Track
There’s an old adage that says we don’t race engine dyno’s, we race on the track. That’s definitely true and a response to engine builders who try to claim that their product is better by simply throwing out a peak horsepower number. But smart engine builders and cam designers know that a big peak number is often fool’s gold when it comes to actual performance on the track.
Instead, the cam should be used to mold the power band to the race car and the track. If you are on a short, tight racetrack running hard compound tires, there is often little use putting a ton of torque to the wheels low in the rpm range. It may feel like the engine has lots of power when the driver spins the rear tires every time he gets on the gas, but that’s probably not helping him lower his lap times.
But, however, if you are racing a Sprint Car or a Dirt Late Model on a track that stays tacky with lots of grip, you probably want a ton of torque down low so that you can set up a pass coming out of turns.
“A lot of times if a driver is spinning the rear tires, we’ll find that we need to bleed off some torque,” explains Ron Iskenderian of Isky Racing Cams. “You want to make the cam worse on the dyno in order to make the car faster on the track. But how do you do it? Well, the trend would be toward a little wider lobe separation. You can also use a little longer exhaust duration. So you would tend toward making the camshaft a little longer or bigger overall, except in the case of a restricted intake class where you have to be careful about that. Also, if you are trying to bleed off to torque, you want to make sure not to advance the camshaft.”
The idea is to build a cam that is a little more gentle on the torque curve at the low-end rpm range but then comes on strong around the flag stand and pulls all the way to corner entry at the end of the straight. If you get it right, a well-designed cam can function as a poor man’s very legal traction control. It may not win on the dyno, but it can definitely help a low-traction race car get around the track faster, and that is what racing is all about.
The Cam Doesn’t Matter
OK, that’s really not true, but we did at least get your attention with that headline, didn’t we? But we still have a point, and that is the engine doesn’t really care what the camshaft is doing. All the engine cares about is the motion of the intake and exhaust valves -- specifically, when they open and close and how far they open to allow air and fuel to move into an out of combustion chambers.
That’s why Billy Godbold of Comp Cams says that when he is designing a camshaft for any application one of the first things he does is determine when he wants the intake valve to close on the compression stroke. Godbold was even kind enough to share with us his own checklist he begins with when designing a cam. He says that generally he starts with these reference points and then works back to determine the ideal duration and lift.
1. Rpm (What limit do I need to set?)
2. Valvetrain mass and stiffness
3. Cylinder head flow relative to engine displacement (cfm/cubic inches)
5. Required durability (Pro Stock drag racing vs circle track vs Le Mans)
6. Valve sizes
7. Valvesprings available and lift rating
“Basically,” he explains, “you want to figure out what lobe families would work best before you try to nail down the duration. This is in part because an aggressive lobe with shorter seat timing (duration) will support more airflow and rpm than if you have to go to a smoother family. However, you always need to select a lobe family that will meet both your rpm limit and service interval requirements that are dictated by questions one, two, and five.” In other words, Godbold visualizes the valve opening and closing events that the engine wants, makes sure the valvesprings can handle that loading and then selects the cam shape that will make it happen.
“Questions three, six, and seven are more about lift. After three and six let you figure out where you ‘want’ to run the lift, question seven sets where you ‘can’ run the lift with available componets,” he adds. “This often shows you how much emphasis you need to place on finding or developing new valve springs and assorted bits.
“After you have a grasp of the lobe families you can safely run and the general lift range, then you go back and start to nail down duration and centerlines to optimize the valve timing points for a given application. That brings you back mainly to questions three and four. Always remember, the greater the cylinder head flow relative to displacement, the less valve duration you need. Also, it’s important to note the relationship between rocker ratio and valve duration. A general rule of thumb is that point of rocker ratio added results in about +2 degrees duration at the valve. Hence, a 260 at 0.050 tappet duration cam acts about 4 degrees larger with a 1.8:1 ratio rocker arm than it would with a 1.6:1 ratio. Lash goes the opposite direction. Every time you loosen the lash 0.004-inch, the duration will shrink about 2 to 3 degrees, depending on how quick the cam is off the seat -- more aggressive cams change less with lash because they have steeper ramps.”
Rocker Ratio and Lobe Design
Speaking of rocker arm ratios, there is a lot that can be done there to manipulate how the camshaft activates the valves.
“The valve doesn’t know what caused the motion,” Bechtloff says. “It opens and closes, but it doesn’t know what the rocker arm is responsible for versus the cam. In many cases you can use either or both to accomplish what you’re wanting to do, but if you are trying to increase valve movement by increasing the rocker arm ratio, it speeds the valve up out-of-the-way real fast and then closes it fast. The motor usually likes that; it wants the valve to be closed and then when it’s time for it to open to disappear up out-of-the-way instantly so it can flow more air. Then it wants to have the valve magically appear back on the seat when it’s time for it to be closed. It’s that time where the valve is moving up or down from full lift to on the seat that flow through the ports is not as efficient as possible. Using a high rocker arm ratio is good in that it makes the valve move very fast, but whatever it gives you in terms of speed on the opening side, it also does the same thing on the closing side. That means it slams the valves shut which dynamically can cause a lot of issues. It puts a lot of jerk in the spring and can bounce the valve on the seat.
“The rocker arm is just like a teeter totter. It’s a lever, so whatever speed you get opening the valve up you have to live with on the closing side as well. But the cam lobe can be designed in such a way, and most of them are, so that it’s asymmetrical. This means that it can have a quick opening and a gentle closing. If the designer of the cam knows that the intention of the engine builder is to run a rocker with a really high ratio he can design around that so that it sets the valve down very gently. The net speed is a combination of the cam and the rocker arm ratio.”
Running high ratio rocker arms works to achieve the concept Godbold spoke about earlier for maximizing engine performance by minimizing the amount of time the valve is off the seat. At first, that seems counterintuitive, but a camshaft that slowly opens the valves means that the intake closing event has to be backed up all the way well into the compression stroke. Being able to open and close the valves very quickly means you can minimize reversion as well as lost air and fuel to overlap. But ripping the valves open and slamming them closed is extremely hard on the valvetrain and you soon find yourself trying to reach a very delicate balance between power and longevity.
The Camshaft as a System
It’s no secret that in most applications you can make more power by dialing up a more aggressive camshaft, but the downside is you will quickly reach a point where things like springs and valves start breaking. Valve bounce, which releases valuable compression back into the ports, is also an issue when the cam becomes more aggressive than the spring can handle. This is why every cam designer we talked to said you cannot build a great cam in a vacuum.
Instead, you must consider the cam as part of the entire valvetrain system. If the cam is going to chew through valvesprings like a puppy with a new toy, then it isn’t the springs that are bad, you just designed a camshaft that is too strong for the application. Now you need to either dial back on the cam’s aggressiveness or -- ideally -- find a spring that can hold up to the punishment.
Finding or developing new components that can stand up to the pounding delivered by an ultra-aggressive camshaft has become a big part of the winning-camshaft equation. For example, Isky has developed and patented its own high-performance roller lifter that completely does away with needle bearings. Instead, Isky’s EZ-Roll lifters use a solid one-piece bearing that significantly increases the size of the loading area between the roller axle and the bearing. With a more traditional needle bearing lifter Isky was having trouble with the needle bearings giving up when the lifters were mated with very aggressive roller camshafts. But they say the issue is now a nonfactor with their new lifter technology, allowing super aggressive cams be used in many more applications.
The same holds true for Comp Cams’ new 5/16-inch pushrods with a thicker 0.105-inch wall. Thanks to Spintron testing we’ve known for years that stiffer pushrods can improve valve control in practically any application. But the answer has usually been to go with a bigger pushrod. But for many stock car racing classes that require an untouched head or even stock diameter pushrods, a 5/16 stick was as big as you could get. Comp’s new 0.105 thick wall pushrods are nearly 20 percent stiffer than the standard 0.080 wall pushrods allowing much greater options when it comes to camshaft choice.
“When I recommend a stiffer pushrod I’ve had engine builders resist and tell me, ‘Well, I’ve never failed a pushrod,’” Godbold says with a laugh. “But that’s not the issue. The stiffer pushrod cuts down on valvetrain deflection and helps us be more accurate controlling the valve timing events.”
Godbold says that when Comp began encouraging engine builders to use stiffer pushrods they actually got reports back that the engine builders didn’t like the pushrods because they were losing power on the engine dyno. After doing some research they discovered that over the years those engine builders had actually found cams that masked the effects of pushrod flex almost by accident. When a pushrod bends or flexes it delays valve opening which effectively shortens the cam’s duration. So when stiffer pushrods were used with the same cam, the engine saw for the first time cam’s true duration, which turned out to be too much for the application. The engine builders were able to gain back all the power they lost -- and actually make more on top of that -- by switching to a smaller camshaft.
“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.”