Posted: 02/12/13 09:37 AM
Resolving the Mysteries of Lobe Center Angles
Hot Rod Magazine, February, 2009
The LCA (lobe centerline angle) is the angle between the intake and exhaust lobe peaks.
Driving the cam is a chain and sprocket system connected to the crankshaft.
Degreeing a camshaft means you are comparing the position of the No. 1 intake lobe with true TDC.
Camshaft overlap is the triangular shaped area that forms as the exhaust valve closes and and the intake valve opens.
Introduction by Scooter Brothers, R&D Director, Competition Cams:
In spite of all the material published about cams, cam design, applications and the like, our experience at Competition Cams indicates there still exists much mystique concerning cam timing and valve events. We know this because of our cam help hot line (800/999-0853), which answers as many as 2500 technical calls a day. Because we repeatedly hear the same questions and because it's a subject many don't really know, including amateur and professional engine builders alike, we felt a technically sound primer on one of the most often asked and least written about subjects would be a great help to many. The subject: camshaft lobe centerline angles, or LCAs.
I put this idea to performance consultant and technical writer David Vizard. In recent years he has personally tested over 600 cam combinations on his own dyno, and designed some potent race-winning, best selling cams as a result of his work. With a 30-year background in explaining complex automotive subjects to performance enthusiasts from firsthand experience, he's in a strong position to write authoritatively about the subject. If this feature doesn't answer your questions, then by all means call one of our technicians on our Competition Cams hotline-we'll be glad to help.
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 valvetrain 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 we shall see.
Looking solely at gas dynamics, we find that 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 LCAs we talk in terms of "tight" or "wide." Tight LCAs have the lobes closer together, making the angle between them smaller; wider LCAs 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 LCAs 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 to 40 percent of the rpm range. Changing LCAs 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 to 78 degrees. However, at lower RPM the greatest pressure difference between cylinder and intake port may occur as little as 20 to 30 degrees after TDC. As RPM reaches peak power level so the point of greatest pressure difference moves back to 90 to 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 to 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 percent 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 to70 percent 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 cylinder's demand in the first half of the stroke. If it doesn't then, short of some very good shockwave 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 LCAs, 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 backpressure. Remember, during the overlap period both valves are open. If there's any exhaust backpressure or if the exhaust port velocities are too low it will encourage exhaust reversion. The tighter LCAs 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
Even though this Web site focuses on high-performance cars, it's worth taking a look at cams for street in general and trucks in particular. For a given type of engine the range of LCAs 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 LCAs 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 LCAs typically ranging from 110 to 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 LCAs 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 LCAs 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. Also, 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 LCAs
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 LCAs 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 non restricted 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 effect changes in bore and stroke have on the optimum LCA.
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.
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 a 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 airflow 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 LCAs 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 to 70 percent 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.
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 work 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 two-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 LCAs. 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 specing 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.
When The Flag Drops.,.
The Bull ***t Stops.,.
P. Engineer, Engine Builder