When it comes to valvetrains, nothing is simple, even though on the surface it may seem so. At trade shows like PRI and SEMA, I get to talk, but more importantly, I listen to conversations among many pro racers and engine builders. I have found that even at this level there are many misconceptions. Here are 12 that should be set straight.
Myth 1:The most critical aspect of speccing a cam is the intake and exhaust duration.
The overlap, not the duration,...
The overlap, not the duration, is the No. 1 factor to consider when speccing out a cam. Get this right, along with the LCA, and only one duration figure will fit the bill.
In reality, the most influential aspects of a performance cam are the overlap period and the LCA or Lobe Centerline Angle (see graphic, "Camshaft Attributes"). For our novices, the overlap is the time that both intake and exhaust valves are open simultaneously around TDC, prior to the start of the intake stroke proper. For a race engine, overlap scavenging is a very important part of the induction process. On a Cup motor, the exhaust's scavenging action is more influential on the induction process than the piston going down the bore.
The amount of overlap an engine needs for optimum output over a given rpm range depends on the rpm range involved as well as the ratio of low lift flow of both the intake and exhaust valves in relation to the displacement of the cylinders involved. Correctly speccing a cam (as opposed to relying on a best guess) should begin with determining the amount of overlap required. From here, the correct LCA needs to be determined. This is not an adjustable feature, as is so often thought. Within a couple of degrees, only one LCA will deliver maximum torque and horsepower over the required rpm range. From the performance point of view, it is better to be 2 degrees too tight on the LCA than 1 degree too wide.
Once the overlap and LCA have been decided, there is only one duration figure, and this can be determined by the following simple equation: [(Overlap x 0.5) + LCA] x 2. An example looks like this: 80 degrees of overlap x 0.5 = 40. 40 added to a LCA of 108 = 148. This is half the duration, so the answer, to meet the overlap and LCA criteria, works out at 296 degrees duration.
So how do we determine the overlap and LCA needed? That's a good question, and one that can only be answered at length.
This small-block Ford race...
This small-block Ford race engine has a 1.75 intake rocker ratio (black) and 1.6 exhaust (red) because the intake valve responds to high accelerations while the exhaust does not.
Myth 2:It stands to reason that the faster the valves are opened, the more power the engine will make.
Response: Not quite-this statement is nearer a half truth.
In practice, and with one proviso (See No. 5), the faster the intake is opened and closed, the more power the engine is likely to make; however, this does not apply to the exhaust. Believe it or not, a fast-opening exhaust valve is really only a benefit to a street economy engine with a short cam. For a race engine, where high rpm output is the major criteria, excessive exhaust valve acceleration can actually make the job of finding power more difficult. This becomes more so at higher CRs. A gentler but slightly earlier opening of an exhaust valve can produce better results than a faster opening occurring slightly later.
Myth 3:Aluminum rockers rev more than stainless steel rockers.
Response: This is a little like using the help menu for certain computer software. What has been said is true but almost irrelevant to what we are trying to achieve.
In reality, the rpm capability of a stainless rocker such as a Comp Cams Pro Magnum is so close to that delivered by an equivalent aluminum rocker that it makes virtually no difference. My own Spintron testing on a nominal 8,000-rpm valvetrain showed a Comp aluminum rocker to be only 25 rpm more to loss of control than a Comp Pro Magnum. The advantage of the Pro Magnums is that they have a much longer life compared to fatigue-prone aluminum rockers.
When tested on the Spintron, the budget stainless Magnum (A) from Comp Cams almost matched the same ratio aluminum rocker (C). When the rocker ratio was stepped up by 0.1, as per rocker B, the rpm to loss of control dropped 95 rpm for a conventional spring but only 65 for a beehive spring.
Myth 4:More spring means more rpm.
Response: You would think so, but definitely no.
By adopting a beehive spring...
By adopting a beehive spring (left), you can cut more mass out of the valvetrain than by substituting a stainless valve for a titanium one. It's also a lot cheaper.
These Spintron results tell...
These Spintron results tell the story. With 5 pounds less on the seat and 20 pounds less over the nose, the beehive spring hit the critical 0.015 (15 thousandths) seat bounce limit 1,000 rpm later than the conventional spring.
The monster conventional spring...
The monster conventional spring on the right lost control of the valvetrain 950 rpm sooner than its significantly lighter beehive counterpart (left).
It seems logical that the more spring poundage used, the more rpm achieved until control is lost. Unfortunately, a little factor we can label "spring efficiency" comes into play. Remember, a spring has to not only control the valvetrain's mass, but also its own. This means the lighter a spring is (in terms of its own mass), the more its delivered force becomes available to control the rest of the valvetrain. This is why an expensive Vasco Jet or Pacaloy spring outperforms its cheaper, less-exotic counterparts. However, if we are talking about the pinnacle of spring efficiency, short of air springs, then the Comp Cams beehive springs almost certainly take the top spot. Our tests over the last three years have shown that on average, these springs will deliver about 1,000 rpm more for the same poundage, or the same rpm for about 5 percent less seat poundage and about 20 percent less over the nose. There is more to it than just rpm. Because these springs are so much better behaved (in terms of resonance as the engine goes up the rpm range), output is often improved. This happens primarily at the points where the original spring has gone through resonance and some minor valve bounce has occurred. Because the beehive spring has no fixed resonant frequency, spurious valve bounce as rpm rises is all but eliminated. From our tests, we have seen a 5hp increase in a nominal 475hp engine with a solid cam, and as much as 12 hp on a hydraulic.
Myth 5:High-ratio rockers are not always good for power.
Response: Yes they are!
The Gold Race rockers from...
The Gold Race rockers from Crane Cams feature a geometry that favors faster initial lift rates than most other rockers. This and the fact they are about five hundredths of a ratio higher than claimed is a definite asset toward making power.
There have been many published dyno tests that appear to contradict what is being said here, and that's because the tests were not conducted properly. You have probably read more than once that when dyno testing, you should change one thing at a time. Well, in reality, that's not where it's at if things have progressed well beyond the rank of basic dyno testing.
Here is a perfect example of having to make two changes to get a positive result, while either change by itself drops output. Let's say your engine has 1.6:1 rockers and the cam selected is optimal in terms of the overlap, LCA, and duration used. If this is the case, and the engine is still starved of air, then increasing the intake acceleration will allow the engine to make more power (the exhaust may also respond if the setup is really down, but otherwise exhaust responds to duration rather than increased valve accelerations). However, it won't happen as a result of just bolting on a set of higher ratio intake rockers. Doing this and nothing else will upset the area of the overlap triangle in relation to the rest of the intake opening event. Anytime the intake valve acceleration is increased (by whatever means), the cam's LCA needs to be widened. For most pushrod V-8s, it amounts to about 1?2 to 3?4 degree per point of rocker ratio increase. If you take care of business like this, a higher ratio rocker on the intake will virtually always deliver.