Longer Exhaust Duration: Is this really necessary? More Tech Tips
Most stock camshafts from American production V8, V6 and 4 cylinder engines manufactured today are ground with the longer exhaust lobe duration. Or, another way of looking at this is that they are ground with shorter intake durations! The former embraces the viewpoint that either the Exhaust Ports or Exhaust Pipe system is somewhat restrictive, and is in need of an assist. The latter suggests that the intake system is rather efficient and cam timing can be trimmed back a bit with out much sacrifice in power, in order to maximize throttle response and cruising efficiency.
Take your pick here. There is no absolutely correct viewpoint - because both are probably true! In a stock engine running at conservative RPM levels, for the sake of overall efficiency, fuel economy and a quiet smooth running engine, this staggering of intake and exhaust duration is quite common and appropriate.
However, High Performance is another thing entirely. Change one factor, let's say in this case, the exhaust system (installing headers and larger pipes) and you have just negated in most cases, the need for that longer exhaust lobe. Now couple this change with a different intake system and camshaft and you have really scrambled the equation. But, wait just a moment. Why is it that so many people (racers & cam grinders alike) insist on running a cam with longer exhaust duration regardless of what equipment is employed? The answer is "habit". Most of them have been somewhat successful in doing it their way and will probably never change unless virtually forced by circumstances to do so.
Before we go any further however let's review what it actually is we are trying to do with an engine when we attempt to make more power. Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!
Example #1 (Oval track racing) Here, I have often observed that the most experienced drivers are those who are most likely to run a single pattern (equal on intake and exhaust duration) cam. Why? Because such cams always, I repeat always make more torque! These veterans have a more educated foot and greater experience in feathering the throttle in the corners. They can therefore, utilize the benefit of added torque, in the lower to mid RPM range, to their advantage.
Their counterparts, the younger drivers on the circuit, generally are not as experienced and may at times actually get "crossed up" in the corners especially with a lighter car or when they are learning the ropes. In their case, a longer exhaust duration is often the more appropriate choice. It will often help them to drive better, more "flat footed" if you will, without consequence. But please for the sake of accuracy, let us be truthful. The benefit comes from an actual bleeding off of low to mid range torque, which is always what happens when Exh. Duration is lengthened, not from any improvement. The improvement, (if any) would come because of an improvement in scavenging at the extreme upper end of the power curve and would usually be marginal at best. Yet the so-called "extra power" potential of a longer Exh. Duration cam is most often why they are touted - power most people are backing away from at the end of the strait away!
Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well.
One of my favorite expressions is how "The Drag Racing mentality has infiltrated the ranks of Oval Track". Many have crossed over and made the switch in the past 10-15 years and some have brought their preconceived notions about how to cam an engine with them. A few may actually read these concepts and if they do so will at least come away with a better understanding of what they are doing. On the other hand they also could find that this information might actually help their cars to run just a bit faster!
Note: Readers may find Camfather Ed Iskenderian's Top Tuners Tip #33 "Can an Exhaust System Over-Scavenge the Combustion Chambers" to be a relevant precursor
Inverse Radius Cams: Just Say No!
These days everybody wants more: Bigger, "Fatter Intake Profiles, yielding more "Area Under the Lift Curve" and Higher V.E. (Volumetric Efficiency). Some Racers mistakenly believe that a more aggressive roller camshaft requires a visit to one of the cam companies struggling for recognition, who push alternative cams with cute names like "Inverted Radius" (Actually a Re-entrant, Concave or Hollow-Flank profile Cam). Unfortunately these cams are not a wise choice considering their major drawback: the undesirable hidden side effect of reduced valve train life expectancy! Subsequently, many who purchase these cams learn to live with problems like broken or prematurely worn-out valve springs, "dropped" valves, bent pushrods, etc., unaware their valve train has been "Jerked" around by a camshaft of reduced stability at higher RPM!
These cam grinders are not however solely responsible for promoting misinformation about these so called "Inverse Radius" Cams. With the aid of the computer and the addition of new tools of the last decade or so such as the "Cam Doctor" "Audi Cam Pro" etc., Cam Profile Dynamics which were once the privy of cam designers alone can now be viewed by many others. With this new technology, there has been a new crop of "experts" who love to talk Cam Profile/Valve "Dynamics".
I am often amused by the cavalier attitude of some of these neophytes who seem to have this view when it comes to Valve Acceleration & Jerk Curves: "The more radical looking they are the better". Well, pardon me for saying so but to paraphrase conservative columnist William F. Buckley, "Ignorance Is Not A Virtue". The only time you should want your 2nd & 3rd differential curves (Acceleration & Jerk) to have greater amplitude or to have the "look and feel" of "Radical" is when you're dealing with limited RPM levels.
It is no coincidence that industrial engine production cams such as those manufactured by John Deere, Caterpillar Tractor, Allis Chalmers etc. are designed in this manner. They are low speed engines! (Have you ever heard of anyone running one to five or six thousand RPM?) They are the cams with the familiar hollow or concave flank and whenever you see one of these cams, remember: Their physical appearance is the result of their High Acceleration & Jerk Peaks and such characteristics are always associated with more moderate (not higher) RPM levels! These inverted looking cams will give you a slight torque increase because they will usually have 2-4 degrees less seat duration for a given duration at .050" lift. The problem occurs when you try to run such cams at up to 8000 plus RPM where they do not belong. It is simply a case of "Nada Por Nada" as they say in old Mexico. You don't get something for nothing. Those who can limit such cams to say 7000 or so maximum RPM and have appropriate valve train components won't fair too badly, but those who insist on consistently running cams like these at Higher RPM levels is "Rolling the dice" every time they does so. They should therefore not be too surprised when the dice on occasion roll "Craps".
Roller Lifters: Keep 'Em Rolling Longer
Most racers are aware of the advantages of Roller Lifters. For those who are not, a brief review is in order. Roller Cams & Lifters are employed today in all-out racing engines where valve lift/area requirements preclude the possibility of employing a flat tappet (solid lifter cam). Higher Lift requires higher valve spring loads (pressures) and flat tappet cams can only handle so much. Additionally, increased rates of lift (cam lobe velocity) above .007" per degree for example on an .842" diameter G.M. lifter, would cause the lobe to reach-out over the edge of the lifters' cam face. Consequently, with either too much spring or too high a lift rate, most racers know that extremely radical flat tappet cams will eventually self-destruct.
But, what about Roller Lifters? Are they as indestructible as many believe? How do we prolong the life of their roller bearings in today's modern race only engines? Roller lifters require special care and maintenance if they are to provide good service life. Here are the 4 most important factors you should consider to insure their success.
1. AVOID DRY "START UP": Roller Lifter Bearings are assembled with a "tacky" rust-preventing grease that is not intended for lubrication. Therefore, new lifters should have their roller bearings thoroughly washed in clean solvent or acetone to completely remove this assembly grease. After air drying, premium motor-oil (non-synthetic) such as Penzoil SAE 25W50 GTP Racing Oil (The best of the mineral based oils) or Amzoil "Red" Racing Oil (synthetic) should be used to pre-lube the bearings just before installation.
2. AVOID "OVERLOAD": Increased load always means reduced service life. Want 50% more thrust from a jet engine? Ask Rolls Royce or G.E. and they'll tell you to expect about ¼th the service life between overhauls. Similarly, employing drag race valve springs in the 900, 1000 to 1100 lb. Range will reduce the life of your roller bearings between rebuilds much the same as will employing high-impact roller cam profiles.
3. EMPLOY A REV KIT WHEN POSSIBLE: The primary advantage of Camfather Ed Isky's invention of the 1950's is that by pre-loading each Roller Lifter Bearing to its respective cam lobe, you eliminate needle roller bearing "skew". Skewing (the momentary mis-alignment of the bearings' needle rollers to their respective races) is provoked by the start-stop skidding action of the roller bearings each time the lash is taken-up. Eliminate it and you extend roller bearing life dramatically! Unfortunately, many engines such as the Big Block Chevy which could use one the most, don't lend themselves to such an installation because of the severe angularity of the pushrod coming out of the lifter.
4. EMPLOY LIFTERS WITH "PRESSURE-FED" OIL TO THE NEEDLE ROLLER BEARINGS: Hope is a good thing. But hoping oil will eventually find its way to your Roller Lifter bearings is not. Unfortunately, most roller lifters on the market do not pressure feed oil to the needle rollers, depending on the "splash & a little luck" system instead. In contrast, all Isky Roller Lifters feature pressure fed oil to their roller bearings. Isky's Top of the line "Red Zone" Series lifters feature an exclusive 3-Point "Multi-Port" oiling system to constantly bathe the needle rollers with cooling lubrication. Additionally, they feature our famous Marathon Roller bearing with the toughest shock absorbing heavy duty outer bearing race on the market for the highest possible load carrying capability and sustained Hi-Rpm Endurance. And, they're fully rebuildable, making them your best long-term
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Setting The Record Straight - Exploding Valvetrain Myths
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 
Response: No.
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. If you want to know, e-mail us and ask for the story.
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
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.
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
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 9:There isn't much to be gained from hollow stem steel valves.
Response: Think again!
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
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
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.
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
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.