Long but worth reading;
A successful cam design must take into account two major factors: the mechanical dynamics of the system, and the desired optimal gas dynamics. In this feature we are going to deal with the gas dynamics as precise valve train motion means nothing unless the valves are opened and closed at the appropriate moments. This means selecting or having a cam ground with the right event timing for your engine. Initially, at least, this may appear something of a black art known only to a select few cam designers. But this is most certainly not the case as I intend to show.
Looking solely at gas dynamics, we find once a cam opening duration has been decided the next most important consideration is the lobe centerline angle (LCA). This as much as duration dictates the cam's "character." In spite of that significance, its complex nature makes LCA one of the least explained aspects of cam specifications.
First let us define the lobe centerline angle. In simplest terms it is the angle between the intake and exhaust lobe peaks. Notably, it is the only cam attribute described in camshaft degrees rather than crankshaft degrees. Remember, the cam runs at half engine speed, and a cam producing 300 crank degrees of "off the seat" timing has a lobe which occupies 150 degrees of cam angle.
OVERLAP AND DELAY
The LCA dictates two important valve timing attributes: valve overlap around TDC, and how much intake or exhaust valve closure delay there is past the end of the relevant stroke.
When discussing LCA’s we talk in terms of "tight" or "wide." Tight LCA’s have the lobes closer together, making the angle between them smaller, wider LCA’s have wider angles. Generally speaking, the majority of cams fall between 98 and 120 degrees LCA.
Let's hold cam advance in the motor constant and look what happens to valve events with LCA changes. Tightening the LCA produces more valve overlap around TDC, while wider equates to less. At the other end of the induction stroke, a wide LCA produces a longer delay to valve closure after the piston has passed BDC. Tight LCA’s produce earlier intake closure after BDC.
Most of us are aware that extending cam duration moves the usable rpm range up. If increased duration is the only change, then the longer cam normally robs power from the bottom end of the rpm range and adds to the top. When only cam duration changes there is usually little change in peak torque. All the longer period does is move the point of peak torque up the rpm range. Most of the increase in horsepower occurs in the upper 30-40% of the rpm range. Changing LCA’s has a different but equally significant effect on the power curve. Without a working understanding of this, you cannot hope to effectively spec out your own cams, so here's what you need to know.
Because of its significance we will deal first with that very important race engine event, the overlap period. By tightening the LCA, the amount of valve overlap for a given duration is increased. For the first and most important half of the induction stroke the intake valve is opened farther by a cam with a tight LCA than one with a wide LCA. This produces a greater flow area as the piston starts to pull in a fresh charge.
Increased valve flow area in the first half of the induction stroke has significant importance for many reasons. The principal one is that a typical production-based 2-valve race engine inevitably lacks adequate valve area in relation to its displacement. Starting the valve motion sooner means more velocity and lift before the beginning of the induction stroke. It is often argued that opening duration after BDC is more effective at producing power than opening before the induction stroke starts. In reality a cam for maximum output for a given duration must have a good balance of opening at both ends of the induction stroke.
If a valve is opened at a suitably early point, the intake port velocity tends, later in the induction stroke, to increase enough to offset any negative effects of a marginally earlier closing. This early opening can be vitally important, especially for an engine having effectively tuned intake and exhaust lengths. In addition, data from 'in cylinder' pressure measurements throw yet more light on the matter. For commonly used rod/stroke ratios, peak flow demand by the piston motion down the bore normally occurs between about 72-78 degrees. However at lower RPM the greatest pressure difference between cylinder and intake port may occur as little as 20-30 degrees after TDC. As RPM reaches peak power level so the point of greatest pressure difference moves back to 90-100 degrees ATDC. For a small-block Chevy, if that pressure point moves back much past about 115 degrees then no further power with increasing RPM will be seen. In other words the engine has, in no uncertain terms, hit its peak. By having the intake farther open during the first half of the induction stroke we can, to a certain extent, delay the retardation of the maximum port to cylinder pressure difference.
Looking at peak intake port demand, which is also peak velocity, we find it tends mostly to occur over a relatively narrow part of the induction stroke. It mostly takes place between peak piston velocity and peak valve lift that follows some 25-35 degrees later. This, and the effect of pressure wave tuning in the intake and exhaust, are important reasons why the initial opening point of the intake valve can be so critical.
Promoting good cylinder filling early on in the induction stroke allows a beneficially earlier closing of the intake. If practical, this increases the amount of charge trapped at valve closure and results in an increase in torque output. A late valve closure from a wide LCA decreases torque.
A cam ground on a wide LCA has less intake valve opening at TDC, so reaches peak opening later in the induction stroke. This means as the piston accelerates down the bore it creates a greater discrepancy between the flow delivered by the valve and the flow required by the cylinder. Put simply, this is because during the first half of the induction stroke the valve is not as far open when a wide LCA is used as it is with a tight one.
POST BDC FILLING
When using a wide as opposed to tight LCA, the intake valve stays open longer after BDC. Because of this, it can be argued that if the cylinder wasn't filled by the time the piston reached BDC or thereabouts, there's time for it to go on filling. Here's some numbers to make the point. At peak power, the cylinder of a typical race engine receives as much as 20% of its charge after the piston has passed BDC. This technique to gain cylinder filling becomes self- limiting because of increasing piston velocity up the bore. Too much delay means a reversion process begins to expel some of the intake charge. This intake charge reversion (not to be confused with exhaust reversion) reduces torque and is most prevalent at 60-70% of peak power rpm.
Of the two techniques, earlier intake valve opening, as produced by the tighter LCA, produces best results. High rpm cylinder pressure measurements suggest that the port/valve combination needs to substantially satisfy the cylinders demand in the first half of the stroke. If it doesn't then, short of some very good shock wave tuning on the intake, it is unlikely to make up for it in the second half.
WHICH WAY TO GO?
So far the case looks good for tight LCA's, and so it is, but there are tradeoffs. Increased overlap equates to reduced idle quality, vacuum, and harsher running prior to coming up on the cam. Probably the most significant factor to the engine tuner though is a tight LCA's intolerance of exhaust system back pressure. Remember, during the overlap period both valves are open. If there's any exhaust back pressure or if the exhaust port velocities are too low it will encourage exhaust reversion. The tighter LCA’s are, the more likely problematical exhaust reversion into the intake will occur. Put simply, we can say that a tight LCA cam produces a power curve that is, for want of a better description, more "punchy". At low rpm when off the cam, it runs rougher, and it comes on the cam with more of a "bang".
A cam on wide centerlines produces a wider power band. It will idle smoother and produce better vacuum, but the price paid is a reduction in output throughout the working rpm range.
THE STREET MARKET
Granted, this magazine is about racecars, but we have to tow them to the track, so this justifies a look at cams for street in general and trucks in particular. For a given type of engine the range of LCA’s offered by different cam companies is surprisingly wide. If you've had in mind that they can't all be right, score yourself 10 points.
Deciding LCA’s for a popular line of street cams is, apart from engineering requirements, a question of market perception. Corporate marketing policies dictate as much as anything what will be used. For instance, some companies tend to grind their performance street profiles on wide LCA’s typically ranging from 110-116 degrees. This produces what these companies feel to be the most marketable balance between idle quality, vacuum, economy and horsepower. Very often the choice of wide LCA’s is made knowing that some of the potential power increase will be sacrificed for idle quality and high vacuum for any accessories requiring it.
Wide LCA’s are not the only way to go. Not everyone wants the smoothest idle and the highest intake manifold vacuum possible. Many, building even the mildest tow vehicle engine, are more interested in maximizing torque. To satisfy this market, some companies will grind their popular short duration profiles on a tighter LCA. Such cams, though less civilized when longer street duration is used, tend to produce more torque. However, it is important to realize that a tighter LCA is totally acceptable if the overlap developed by the LCA and duration combination isn't excessive
We've already looked at the test results of extending duration and holding the LCA constant, now let's try the reverse.
The 355 cubic inch small-block Chevy dyno mule motor for our LCA tests was essentially the same as used for the duration tests.
It utilized 10.25:1 Sealed Power pistons, together with mildly ported straight plug 186 head castings. On the induction side, an Edelbrock Victor Jr. intake was used, along with a 650 cfm double pumper electronic Quarter Mile-O-Dial Holley for rapid mixture optimization. Exhaust dumped through 1 3/4-inch headers, and from there through the dyno cell's 'zero restriction' muffler system. The dyno used was at Advanced Performance Technology's (APT) Riverside facility. Being a SuperFlow computerized dyno, it was set to take measurements while the engine was accelerating at the commonly used 300 rpm per second rate. Chevron Unleaded Premium gas was used throughout our tests, and by optimizing the engine's quench clearances, mean best torque timing could be achieved without detonation.
The cams tested were hydraulic and measured out at 292 degrees at the SAE standard of 0.006 inches valve lift. The graph, Fig. 6, gives pertinent cam details and dyno results. Curve #1 is for a cam on 105-degree LCA, Curve #2 on 108, and Curve #3 on 111. On the face of it, Curve #1 looks marginally superior as it produced the best peak torque, horsepower and volumetric efficiency.
Curve #2 is slightly down at the low end. In a couple of places further up the rpm range it marginally exceeded the output of the 105-degree LCA cam. However, these curves alone don't tell all. The 105 cam had only just come on the cam at the first reading of 2250 rpm. You'll note that it produced less torque here than the 108 cam. The 108-degree LCA cam would pull right down to about 2000 rpm and run satisfactorily. At 2250 rpm it was positively on the cam, and at this point it produced noticeably more torque than the 105 cam. Curve #3 represents a cam on 111-degree LCA. As can be seen, it is consistently 10-20 hp down at the top end over cams #1 and #2.
So far tests don't show too well for 111-degree LCA cam, but this angle cam did idle almost like a stocker, and produced a higher vacuum than the other two cams. It would be easy to assume that cams on such wide centerlines don't work, but this is not so. They just have different characteristics and you, as the end user must decide where your priorities lie. Remember that good vacuum is an important factor for a vehicle that has vacuum accessories such as power brakes, vacuum operated air conditioning controls, etc. The tighter the LCA you choose, the shorter the cam must be to preserve vacuum and idle. This is so because the overlap comes back to roughly the same as that given by a longer duration, wider LCA cam. Obviously a shorter cam on a tighter LCA won't make as much top end horsepower, so again there is a balance of tradeoffs to consider.
RACE ENGINE LCA’s
Choosing the LCA for a race engine becomes simplified because compromises are virtually nonexistent. We are no longer concerned with anything other than maximizing engine output over the RPM range used. That's good, but to be successful it's necessary to make a better job of maximizing output than the next guy. To do that you need to understand those factors affecting the optimum LCA for the job.
The easiest way to explain how optimum LCA’s can change is to use a base spec engine which has been dyno-optimized as a starting point. By making hypothetical changes to this engine it becomes easier to see how the optimum LCA is affected. Let us assume the following: 355 CID from 4.03 inches x 3.48 inch bore/stroke combination, a set of reasonably well ported heads, 12.5:1 compression ratio, a nonrestricted exhaust, a single 4-barrel carb on a race manifold, a single pattern flat tappet cam at 310 degrees seat duration and about 265 at 0.50-inch lift, and 1.5:1 rockers. Such a combination usually produces the best all around results at about 107-degree LCA.
To better understand how the required LCA changes, always consider that it is strongly tied in with the cylinder heads' flow capability and the displacement the head must supply. In its simplest form, this equates to a ratio of cfm per cubic inch. With that in mind, let's start with the affect changes in bore and stroke have on the optimum LCA.
Okay, here we go-pin your ears back and pay attention! Assuming no change in head flow efficiency, we find that any increase in the displacement requires an decrease in the LCA. For a typical 350, every additional 15 CID increase requires a reduction of one degree LCA, and vice versa.
Now let's fix the displacement and see how head flow affects the optimum LCA. The same air flow to displacement trend also holds true here. If flow capability over a large part of the valve lift curve increases, the optimum LCA will spread, and if it decreases the reverse is true. If a dramatic increase in intake low lift flow is achieved, the tendency is to require less overlap. This means the LCA spreads, and this may have to be used with shorter intake duration. However, the reduced overlap is the most critical aspect. An increase in low lift flow without a compensating reduction in the overlap area can reduce output right up until very high rpm is reached. The intent here is to restore the overlap triangle, in terms of cfm /degrees, back to its original optimum value. Sure, it's tempting to analyze thousandths of valve lift and degrees around TDC, but the engine does not recognize valve lift as measured by a dial indicator-only flow capability. This means all overlap characteristics should be related in terms of cfm/degrees not inch/degrees. Achieving an exceptionally high flow at low lift on the intake can cause the engine to react as if it has 20 or so degrees additional overlap. This often proves way over the top for an engine with previously optimum valve events. An increase in low lift flow is potentially good for added power but, if substantial, usually requires a revision of the valve opening and closure points.
BORE & STROKE CHANGES
If head flow is reduced, the LCA needs to tighten up. Now why would anyone want to use a head with less flow? Well, no one wants to, but a long stroke/small bore combination may force the situation. A long stroke engine has less room for valves than a short stroke, so may have less breathing capability on that score. This causes a long stroke engine to need tighter LCA's than a short stroke.
High and low lift flow capability can also affect the picture. We have already discussed what can happen when low lift flow is increased, now let's look at high lift flow. An increase in high lift flow only, during the last 60-70% of the valve lift envelope used, requires a slightly tighter LCA. This only comes about because it allows the intake valve to be closed a few degrees earlier for the same peak power rpm. However, for most practical purposes we can ignore its effect without incurring a performance loss. By leaving the cam timing unchanged, a slightly higher rpm capability is produced along with some extra power.
The effect of changes in compression ratio used on the optimum LCA is rarely dealt with, but it can be significant. The first step towards understanding why the CR affects the LCA is to appreciate the difference between the cylinder pressure plot of a high and low compression engine.
In a low compression engine, peak combustion pressures are lower than in a high compression unit. But percentage-wise, the pressure doesn't drop off as fast as it does in a high compression unit as the power stroke progresses. At the higher rpm a high compression motor is likely to run at, it needs a little more time to blow down the cylinder. This we can do by opening the exhaust valve earlier than with a low compression engine. This proves possible with little or no penalty because a high compression means more work on the piston at the beginning of the stroke and less towards the end. So the higher the CR, the wider the LCA can be made by virtue of extended duration by opening the exhaust valve earlier. A rough rule of thumb is to open the exhaust valve 1-2 degrees earlier for every point of compression increase from a previously optimally timed cam. Opening the exhaust valve 2 degrees earlier means the LCA has spread by half a degree.
Engine geometry other than the bore and stroke also influences the most favorable LCA. The connecting rod length to stroke ratio has a measurable effect on the position of the piston in the bore at any point of crankshaft rotation.
It is important to understand that the induction system does not know how far around the crank has turned. It only recognizes piston position and velocity, and it's subsequent effect on gas speed throughout the valve lift cycle. If the LCA and valve events were optimal then changing the rod/stroke ratio a significant amount will require a new cam profile to restore the original event timing.
Take, for example, the rod length tests done for a well-known tech magazine a couple of years ago on a 330-inch engine. For the experiment, the connecting rod length was changed by a whole inch, from 5.5 inches to 6.5. What effect would this have had on the required cam event timing? If the original cam were a 280-degree piece on a 110 LCA, then to restore the original parameters the new cam would have to be 279 degrees with a LCA of 109. These changes in the required cam spec, especially the LCA, would have measurably affected the results this test produced; though the trends would still have been the same.
The rocker ratio used can have a strong influence on the LCA. We've seen, like the rod length test, back-to-back dyno tests of various rocker ratios that have indicated a far more complex picture than is actually the case. Such tests showed that on occasion, high lift rockers don't work yet offered no reason why. From the point of view of the gas dynamics in an under-valved 2-valve engine, high lift rockers up to ratios of 1.8-1.9:1 always works if used correctly! The most likely reason for negative results when switching to higher ratio rocker is because the overlap triangle on an optimized engine was already as big as the combination would tolerate. If the LCA is already optimal on a big camed race engine, changing to high lift rockers will usually reduce the output, especially if used on the exhaust.
For a 2-valve engine, possible power reduction from high lift rockers becomes less likely and of lesser proportions when cylinder head flow per cubic inch drops. That's the situation for bigger inch small-blocks or really big-inch big-blocks. To make the most of high lift rockers, the reoptimization of the LCA is necessary. This means spreading the LCA’s. By how much depends on the head flow to cubic inch ratio. Generally, large engines require little or no change, whereas small engines may need as much as 2-3 degrees greater spread.
In the same way, a change from a flat tappet to a roller cam can affect the LCA required. To avoid a very lengthy valvetrain dynamics discussion to explain why, it is suggested you read the book "How To Build & Modify Small-Block Chevy Valvetrains," published by and available through MotorBooks International, and Competition Cams or any good bookstore.
For cams under about 270 degrees, changing from a flat tappet to a roller will need a slight tightening of the LCA, about 1-2 degrees. From 270 to about 285 it holds constant, but over 285 the LCA will need spreading a degree or two.
All you have read so far might indicate there is a lot to this area of cam design. However if you absorb this, then as an aid to specking out and building a high performance engine, it will prove a valuable tool. In a sport that puts so much emphasis on technical capability, knowledge of camshaft lobe center angles can make the difference between winning and losing.
Most people when deciding to upgrade their engines look to change the camshaft. The cam is probably the least understood component in the engine. Hydraulic, mechanical, roller and hydraulic roller are the 4 basic types of cams in use today. There are IR designs as well, which only add to the confusion.
For the majority of people will just pick a cam out of the parts book, or worse still let the local counter boy make the choice for them, there is very little hope. For the people who don't want to settle, we present the following article.
The terms associated with camshafts are also not always understood, or more often misunderstood. Practically everyone understands what valve lift is, but the terms centerline and lobe separation and base circle are terms which will get different explanations from different people. Let's get the terms straight first;
Valve Lift Well this is the easy one, The distance that the lobe of the cam lifts the tappet multiplied by the rocker arm ratio determines the valve lift. Everyone knows this one!
Duration The time in which the valve is off the seat during tappet lift, measured in CRANKSHAFT degrees. As there has to be some point in which you begin to measure the lift of a cam there are usually two figures given on a spec card. The Advertised Duration and the Duration at some arbitrarily chosen point (Usually .050" lift) Some manufacturers use a different amount of lift and this can cause confusion Most Cam manufacturers use the .050" figure, but it is wise to be sure when comparing different grinds. When checking a cam you should always check it at the tappet rather than the valve because of minor variations due to lash, and rocker arm ratio.
Centerline The Centerline of a Cam is the actual position or phasing of the cam in relation to the Crankshaft. To wit: The position of the center line of the #1 INTAKE Lobe of the cam in relation to the position of the #1 Piston measured in Crankshaft degrees of rotation AFTER TDC. This is the figure that is used when we talk about 'Degreeing " a cam.
Lobe Separation This is the PHYSICAL configuration of the cam in relation to the actual spacing of the intake and exhaust lobes from each other. Lobe separation is ground into the camshaft. You CANNOT change it (Unless you reground the cam). You CAN change the Centerline by degreeing. These two terms are often confused with each other.
Base Circle This is the lowest part of the cam lobe also referred to sometimes as the "Heel" of the cam. This is the fully closed position of the valve. This is also where you should make your valve adjustments. If you adjust your valve lash at any other point on the cam, you will have problems. We will discuss the proper way to adjust your valves later on.
Some stroker motors require the use of a "Reduced Base Circle cam" in order to clear the rods. These cams are ground with a smaller base diameter and require some specialized components such as longer pushrods.
Symmetrical A cam that is Symmetrical has both sides of the cam lobe exactly the same. In other words, the intake ramp of the cam lobe that accelerates the lifter to actuate the valve has the same shape as the portion of the ramp on the downside of the lobe that lowers the lifter. These designs are very easy on the valvetrain as it is a smooth transition from open to closed.
Asymmetrical An Asymmetrical cam has opening and closing ramps that are unlike and unequal. This profiles usually found on high performance cams and offers a high velocity opening and a lower velocity closing ramp in order to snap the valve open quickly and then set it back down more gently.
Dual Pattern Again, a grind that is usually found in a high performance cam. The Intake lobe configuration is different from the exhaust lobe. Usually the exhaust lobe is ground with slightly more duration that the intake lobe. Small block Chevy engines really like more duration on the exhaust in most cases. A single pattern cam (Both lobes giving the same amount of duration) work well in street engines.
Cam Walk - A phenomenon that occurs with Roller cams due to slight inaccuracies in the lifter bore spacing. Most Roller cams use a cam button to control the tendency of the cam to unscrew itself from the block. Bushing the lifter bores can control this problem but is very expensive. A cam button will work quite well in 90% of cases.
The Proper Way to Set Your Valves
Ask ten different mechanics and you will probably get ten different answers as to the proper way to set valve lash. The following is the ONLY way to do it, and I will accept no argument to the contrary!
We are dealing with mechanical cams here, If you don't know how to set up an hydraulic cam then you have no business reading this.
Aside from the obvious such as proper assembly of the engine, and proper degreeing of the cam etc. Setting the valve lash is THE MOST IMPORTANT aspect of your engine. By experimenting with various lash settings you can actually see how different grinds will behave in your engine. You can stagger lash values and find out what your engine likes. The main problem that most people have is getting the valves to stay set once you have the lash setting you want.
We check our valve lash after every pass in our dragster, we RARELY have to make any adjustments because we set it right the FIRST time.
No matter if you use regular poly locks or a shaft mounted setup such as a Jesel system the method is exactly the same. The outer nut on a poly lock system is used to adjust the lash and the set screw is used to bottom out on the top of the stud and lock the setting. The use of a stud girdle sometimes requires a slight compensation in the lash setting . You will have to check this out for yourself to find the optimal setting.
The method of setting your valve lash properly is so simple that to do it any other way makes no sense to begin with.
The lash must be adjusted when the tappet is on the heel of the cam lobe. Remember that this is the lowest part of the lobe, hence the tappet is as far down in the bore as it will go. You can't see this when the heads are on, the way that you determine when the tappet is in the proper spot to adjust is as follows;
Adjust the Intake valve lash when the EXHAUST valve begins to open, this will assure that the intake valve is on the heel of the intake lobe.
Adjust the Exhaust valve when the INTAKE valve has just closed. Your setting will not change if you do your valves this way.
Reread the above two sentences and you will see the beauty and simplicity of this method.
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