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
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.