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Book One of Tech Tips 2000TM features The Modern Myths of High PerformanceTM. This first installment (in no particular order of magnitude) will contain the most common misconceptions in High Performance engine building today. We'll be discussing subjects you won't read about in the magazines, but are nevertheless in great need of clarification for the racing community at large. And just so there is no misunderstanding, I will limit the subject matter to those issues with which I have significant personal knowledge and experience. Unlike some others in the racing industry, I will make no assumptions, nor will I postulate any theory unless I have clearly labeled it as such. I welcome your comments and suggestions.
I am, for your higher performance
- Ron Iskenderian
Compression Rules: "The Joker in the Deck"
In order to hold costs down in oval track racing today we have numerous "RULE" classes. Cam "Lift" rules, engine "Vacuum" rules, "Carburation" rules and of course "Compression" rules. I for one, certainly have no quarrel with compression Ratio rules, because if you are talking true compression ratio, and it's properly measured and enforced, it is absolute. On the other hand, if it's a "compression effect" rule or the measurement of cranking compression in the cylinder as read on a compression gauge, then you have just pushed one of my buttons! However, before I begin my tirade, let us review why we have such rules as these in the first place. Do they exist so that we can go faster or slower; to make racing more or less competitive? And, are those who break such rules punished or rewarded? Well you say, the obvious answers to these questions are "slower", "more competitive" & "punished". Wrong! Not when it comes to compression "effect" rules!
Very simply, with a carburetor rule, he who installs a "bigger" one usually goes faster, can be detected and penalized. Likewise with a "cam lift" or "engine" vacuum rule. You cheat by installing a bigger cam and although you go faster, we can catch you and "you're outta there". Even with a compression ratio rule, you can have your "15 minutes of fame" if you like, but eventually you're going down because we can detect your indiscretion.
However, guess what happens to those who comply with a "compression effect" rule? Why, they are virtually always running a larger camshaft, go faster and of course can never be punished because they are within the rules!
Impossible you say? Quite possible I contend and in fact this is actually what happens virtually 100% of the time! I know, I'm in the camshaft business! Simply put, I believe compression effect rules are the single most ridiculous rules in existence. Rules that guarantee to keep speeds up, not down and actually penalize those with weaker engines and fewer resources. How can that be? Please read on...
With a given static compression ratio, you will always have a higher reading on your compression tester gauge with a stock or low duration cam, because you will be closing the intake valve earlier on the compression stroke. The resultant longer effective compression stroke always delivers a higher gauge reading. Now switch to a longer duration cam. Your intake valve will close later, lowering your gauge reading because of the shorter effective compression stroke. Some people feel this is impossible, insisting that if it were true, why will you go faster with the bigger cam? The answer is, the bigger cam will have higher compression effect in the cylinder at higher engine speeds (where all that extra valve timing can do you some good), however at lower speeds and especially at starter-cranking speeds, the effect will be lower.
If you wish to prove this to yourself, simply recall how in the past you may have noticed losing bottom end torque when installing a longer duration cam. Do you think you lost that torque because of higher cylinder pressure? Of course not, but that is only the logic of deductive reasoning. Lord only knows what the "compression effect" rules people were thinking of when they came up with this stuff. It doesn't make sense and just about any cam grinder I know of, could have told them as much. On the other hand, there are some cam grinders who are confused about "compression effect" and cylinder flow dynamics. (Perhaps they consulted one of them.)Back To Top
What Causes Intake Reversion? Once and for all, let us have the TRUTH!
With the proliferation of the Motorsports Industry over the years, many new faces have come on the scene. In the cam grinding business today, there are many younger, less experienced companies struggling for recognition of their talents and a few have turned to postulating new theories in order to attract attention. However, they are I believe unfortunately, too often guilty of shooting from the hip.
Two in particular are responsible for perpetuating the "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!
Reversion, carburetor/Injector "stand-off" or the general effect of the backing up of the intake Fuel/Air charge normally associated with longer duration high-performance camshafts is actually caused by a Later Intake Closing! How do we know this to be true? The answer lies in the basic principles of physics. For as with geometry and trigonometry, these sacred truths do not change simply because someone chooses to ignore them in an attempt to garner a reputation.
Specifically, when the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. The piston is in the vicinity of T.D.C. (only .425" down the hole @40o BTDC - on a typical 350" Chevy with 5.700" rods) and no appreciable threat is posed to the forthcoming intake charge. The "False Reversion Hypothesis" taken to an extreme would lead one to the equally false conclusion that any overlapping of the intake and exhaust valves is totally undesirable. Automotive engineers of the late 1800's and early 1900's used to think this way. They were deathly afraid of overlap, so much so they actually employed "Negative" overlap (minus 5 or 10 degrees) to be absolutely sure none would occur. What was the result? These engines were severely "throttled back" or limited to low speeds and mediocre output. [ Reference: Iskenderian's Tech Article "Cam Degreeing is Simple"] But, more progressive engineers of the early 1920's who performed "brazen experiments" with longer duration cams proved these overlap fears to be only so much "stuff and nonsense", as both power, rpm and performance were actually improved. These engineers demonstrated that overlap did not cause engines to quiver, backfire or lock-up on the spot! Although, the ignorance displayed by their predecessors is easily explained by their lack of experience, (internal combustion engine design being in it's infancy) it was none the less the result of an incorrect hypothesis.
Should you need further persuasion that reversion is not caused by earlier intake opening and the resulting extension of valve overlap, consider this: What happens when you advance any camshaft? The intake as well as the exhaust valves open earlier. Does this advancing of the cam cause more reversion? Of course not. Throttle response and torque are enhanced. Yet, if these theories were correct wouldn't the engine run more poorly, especially at lower RPM? The answer is obviously yes, and because so, these theories are invalid. A brief look at what's happening on the other end of the valve-timing diagram will tell you why.
For when a camshaft is advanced, not only do both valves open earlier but they of course also close earlier - and here in lies the key to reducing Intake Reversion. Close your intake valves earlier and any tendency for the occurrence of Reversion or the backing up of the intake charge as the piston rises on the compression stroke will be reduced. It's not complex, nor is it a mystery. And the circumstances surrounding it's occurrence have not changed. In fact any experienced mechanic could tell you as much, for, as Ed's good friend the legendary Smokey Yunick might say, "Only country smarts are required to solve the problem."Back To Top
Longer Exhaust Duration: Is this really necessary?
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.Back To Top
Intake Restriction and Over Scavenging: "Waste not...Want not!"
It is certainly an over simplification to make the statement "that which is not wasted, should be inducted". However, in the case of restricted intake systems and in particular 2-BBL carb rules, it is not far off the mark. Engines with such restrictions are "choked off" to the point where they will not run much past 6500 RPM (if even that high) without dropping off sharply in power. You might have trouble running very fast yourself if someone had your windpipe choked down to say 50 or 60% of it's normal capacity. Under such conditions, would you volunteer to give blood at the Red Cross? Of course not, but without knowing so, racers often do the equivalent with their engines by running a camshaft better suited for a 4-BBL class! How So?
If you'll recall in last months tech tip: "Longer Exhaust Duration: Is This Really Necessary?" I discussed how, through habit, many racers and cam grinders alike are predisposed to running camshafts with longer exhaust durations, whether they need to or not! Well, in the case of restricted intake applications, if there was ever a situation in which you'd want to avoid the longer exhaust "trap" it's here! Especially the 8, 10, 12 or even longer degree spreads, I often discover people employing.
Use such a cam at you own risk - and don't be surprised to find that your exhaust temperatures are unusually high. Your headers in fact may even glow cherry red. There is a very good reason for this. Raw (unburned) fuel is burning "late" or in the pipe (header/manifold). You may have a good equilibrium of flow going here but there is just one problem. Much of what should be inducted into the cylinder is being scavenged out the exhaust! You see, although back pressure in an exhaust system can be restrictive, the only thing that could be even worse is a reduction of it to the point where you are now, in effect pulling a vacuum. In the case of an intake restriction, very slight back pressure is preferable to avoid "over scavenging".
Yes, Yes I know. You are probably thinking "what's wrong with a little scavenging?". Well, nothing if you can afford it. But with intake restrictions (either small 2-BBL carbs and/or restrictor plates) you must be very careful. You already have reduced intake potential and therefore simply cannot be cavalier about valve overlap and scavenging or you'll be way down on power and have those nice bright cherry red pipes to show for it! Case in Point: One racer who called me was in this exact situation and was running, not surprisingly, a 14 Degree longer exhaust duration. It was Friday afternoon and he needed a cam the next day for the last "points race" of the season and UPS had already picked up at Isky. "Too Bad" I said, "You don't have a set of those low ratio break-in rocker arms because they could really help in this case". " I do have some" he said "but they are only 1.2:1 ratio - is that okay?" I told him to use them (on his exhaust valves only of course) and he finished the race 2nd having come from the back of the pack. Later we made him the right cam so he could avoid this make shift approach.
Unfortunately, the symptoms are not always as obvious as in this case to allow for a speedy diagnosis. Also, it's not only longer exhaust duration that causes the problem. Although it is usually the primary offender, it is often coupled with too close a lobe separation angle of say 104 Degrees. A widening to 106 Degrees or preferably 108 Degrees (some go even wider) is usually prudent.
I am not absolutely dead set against a slightly longer exhaust duration in these cases as a 2-4 Degree longer exhaust lobe is permissible under some circumstances (if your running a completely stock exhaust system including mufflers for example). Each case is different, depending upon the equipment employed. I might even recommend shorter exhaust duration to some; if I feel they have "overdone" their exhaust ports and or exhaust system a bit. What matters is the end result and if you're out of balance on one side simply employ what I call the "Great Law of Compensation" to bring you back to that equilibrium of flow.
So, how can you tell if you may need to make some of these changes in your camshaft? Well, short of trying a lower exhaust rocker arm ratio, you can increase exhaust valve lash .004" - .008" temporarily to see if there is any improvement. You can also try and increase restriction (smaller headers or pipes, or in the case of open headers a longer collector) and simply observe the results. Remember, "One test is worth a thousand expert opinions". Keep this old axiom in your "tool box" and you'll be ahead of the game. How do you think Smokey's shop got to be "The best Damn Garage in Town" anyway? Yes, he had those country smarts, but his experiences in racing and his willingness to test are legendary!Back To Top
Rod Lengths/Ratios: Much ado about almost nothing.
Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".
To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!
Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).
To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.
In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!
Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.
Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.
(NOTE: You will need Acrobat Reader to view the charts. It is a free utility program. Click icon to download it. .)Back To Top
Stainless Valves: Do you really need them?
I would like to preface my remarks by stating that I own no degree in metallurgy. I have, however, accumulated a great deal of knowledge in this field over the years form my experiences working with materials, sources, and heat treatment facilities designing high performance engine components.
The first thing one learns when exploring the subject is that the reason there are so many different ferrous (containing iron) and non-ferrous alloys in existence is that each one serves a specific purpose and benefits us in a particular way.
Take Stainless Steels in general for example. They offer excellent corrosion resistance and are higher in strength than conventional steels at elevated temperatures. In other words, if you run a super charged fuel dragster or funny car you had better be running valves made of one of these alloys. Todays 9:1 compression rule motors are also candidates for stainless steel valves because of the temperatures they experience.
But what about the typical S/B Chevy 350 V8 at the drag stripe or on a 1/2-mile dirt track that's normally aspirated? Should you run a stainless steel valve in these engines? The answer is, you certainly don't need to (and you actually might be better off by not doing so.) Here's why: Stainless steel is only higher in strength than most conventional alloys at elevated temperatures. It is, in fact a softer material at room temperature, and significantly lower in strength than conventional valve alloys at temperatures below 800/1000 degrees! That means for example, that those 2.02/1.600" HI-PO Chevy valves with chrome stems you took out in favor of stainless valves were probably at least as good and may have actually been stronger in this environment!
Stainless steels are generally softer and this factor combined with the higher alloy content makes them prone to "gauling." (Try and run one for example without the hardened tip or the chrome stem and see what happens!) The reason I have chosen to bring this subject to light is that racing is a very expensive proposition today and I dislike seeing so many people spending so much $$ on equipment they don't need. How many times for example, have you seen a racer running on bald tires because he ran out of $$ building the engine?
Today there is so much hoopla in the field of High Performance it is difficult to separate the wheat from the chaff. However in the Tech Tips 2000 series, I enjoy pointing out what is actually a legitimate claim as opposed to so much "stuff and nonsense." When it comes to high performance engine building, just like in other areas of life, remember the ancient roman adage "Caveat Emptor". Let the buyer beware!Back To Top
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".Back To Top
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.