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.
Myth 6:Limited lift cams with higher than normal lifter acceleration rates make more power but don't last long.
Response: This is right only if you don't know the fix.
Having built a few limited-lift motors that made race-winning power (and needed to be dyno mules) has meant finding a fix for potential rapid cam lobe wear on cam profiles where rapid lifter acceleration has been pushed to the limit. Traditionally, such cams can have a short life. If the break-in procedure did not quite make the grade, a lobe (or lobes) might not even make it through the first race. About eight years ago, I found the closest thing yet toward eliminating this problem. This fix comes in a bottle labeled "Oil Extreme." Not only does it cut the likelihood of a cam failure by about 75 percent (at my best guess), but also, in 99 percent of cases, it shows a power increase amounting to about 5 hp, on average. This stuff is not available in stores because most of it is used by big corporations, naval applications, and other military operations. To get it, and it's cheap enough, you have to go directly to the manufacturer-Jet Set Life Technologies in Grand Terrace, California.
Myth 7:Those restricted classes that call for hydraulic flat-tappet cams and a minimum idle vacuum need much shorter intake and exhaust duration.
Response: No vacuum is lost faster by the exhaust part of the overlap period, so shortening this will pull the vacuum up.
Most of the vacuum at idle is lost due to the intrusion of the exhaust part of the overlap into the intake part. If the exhaust part of the overlap is cut by 10 degrees, the increase in idle vacuum is much greater than if the same had been done to the intake. If a hydraulic cam and idle vacuum rule exists for your class, try using a soft (faster leakdown) lifter on the exhaust. Don't overdo things here, as some very fast lifters never recover anything near their full duration and power may be lost. The best plan is to use as tight a lifter as you can find (or a near-bottomed-out one) with a relatively soft lifter from Crane or Comp. To make sure that the exhaust lifters regain as much duration as possible, use some Oil Extreme in the oil.
Myth 8:A big intake port tends to compensate in a valve-lift-rule engine.
Response: Absolutely not.
It is so common, especially with small-block motors, to quote the flow of heads at 0.700 (700 thousandths) that if a head fails to look strong there, it is assumed to be less than satisfactory. Having good flow at such a lift value is totally academic if the class rules call for a maximum valve lift of say 0.500 (500 thousandths). In fact, if a head goes on flowing really well, about 0.050 (50 thousandths) above the required valve lift, the port volume above this to support such flow could be hurting power, not enhancing it. Why? Because such flow characteristics are indicative of a port too big for the job. Anytime the port is too big, velocity is cut and the ramming momentum that is responsible for much of the volumetric efficiency numbers over 100 is reduced. Worse yet, it's a square law. Cut the velocity by 10 percent and the ramming pressure will drop just over 20 percent.
Shown here are all the main aspects of a camshaft. The most important factors to get right are the overlap and the LCA. The overlap has to be correct in terms of area so degrees and rate of opening dictate how optimal it may be.
1. Intake lobe lift
2. Intake opening flank
3. Intake duration
4. Exhaust lift
5. Exhaust opening flank
6. Exhaust duration
7. Lobe centerline angle (LCA)
8. Cam advance and retard
Myth 9:There isn't much to be gained from hollow stem steel valves.
Response: Think again!
Bearing in mind their much...
Bearing in mind their much lower cost, these hollow stem stainless valves from Ferrea are better than a halfway house to titanium.
This is short and sweet. Our tests, which involved spinning Ferrea hollow versus solid stem valves, have shown this: In the range between 7,000 and 8,000 rpm, the lighter, hollow stem valves not only dynamically behave better on the way up, but also deliver (depending on the spring) between 200 to 400 more rpm before loss of control sets in.
Ever wonder what the difference is between valve toss and valve float? These Spintron screens provide the answer. Shown on the left screen, as indicated by the yellow arrow, is the valve being tossed (lofted or floating) due to component separation somewhere in the valvetrain. This usually occurs at the pushrod, but it could be at the lifter or the rocker. This valve toss can actually enhance power, and it is a characteristic sought after for an all-out pushrod valvetrain. Shown on the right screen, indicated by the red arrow, is the effect of valve bounce. This is where the valve's closing velocity with the seat is too high and the valve simply bounces back off the seat. When this happens by more than a few thousandths of an inch, power starts to drop off rapidly.
Myth 10:Solids always make a better curve and more power than hydraulics.
Response: Always is an overstatement.
It is easier to make horsepower from a solid because it has less lifter related issues. That stated, a well spec'd hydraulic can actually beat a solid with the same off-the-seat duration, but it takes the right profiles and lots of dyno time to sort out the right lifter characteristics and lubes. Lastly, aeration of the oil can be an issue, so a good, functional pan becomes a must.
Myth 11:Changing pushrods isn't worth a darn.
Response: Time spent testing valvetrain combinations will reveal the contrary.
There is far more to pushrod science than even many experienced and successful engine builders might imagine. Not only must a pushrod be stiff and have as high a natural lateral resonant frequency as possible, but it also must have an ability to damp out spurious vibrations developed within the valvetrain system. Stock pushrods tend to be made of materials that are better at damping than they are at avoiding flex and lateral resonance at higher rpm.
Going from a factory pushrod to a high-tech (and that term is not an overstatement) pushrod from Crane, Comp, or the like, might net you anywhere between 0 and 15 hp, depending on the dynamic characteristics of the cam and other valvetrain components. In all our tests (over a dozen to date), a good aftermarket pushrod has never lost power. Incremental gains are the norm, but gains of 9 and 15 hp were realized on two occasions.
Myth 12:A tighter intake lash gives more duration and therefore more top-end power.
Response: Not usually.
Intake valves in general like to be opened and closed rapidly, especially when they are in the vicinity of the valve seat in the head. Using the opening ramp of the cam as a means of extending duration is not a good move because the acceleration on this part of the cam's profile is slow. It amounts more to creating a temporary valve leak than a performance enhancement. Lash figures on the loose side almost always deliver better results, but don't go too loose. Doing so will upset the dynamics, and potential gains will be offset by reduced valve control and increased seat bounce at the point of closure.