the IDEAL cam LCA

[grumpyvette]

http://www.enginebuildermag.com/Article ... teria.aspx

http://www.circletrack.com/techarticles ... index.html

viewtopic.php?f=52&t=2782&p=7214#p7214

viewtopic.php?f=52&t=112&p=139#p139

viewtopic.php?f=52&t=324

in case you don,t understand the chart, you take the engine displacement PER CYLINDER divided by the valve diam. then you use that on the chart to locate the lsa
example
lets assume youve got a 383sbc, 383/8=47.88
divide that by intake diam., lets say 2.02 and you get 23.7 youll see the ideal is near 105 lsa, but then you ask,WHY are most cams ground with a 110-112 LSA its because that tight lsa may maximize the peak volumetric efficiency,and peak hp/torque, it will also be far from ideal at low speed, with a lopey idle, or for emission testing or for ease of low and mid rpm ease of tuning or for sensors to read because of low rpm reversion in the intake runners.so a compromise with a wider LSA is used, sacrificing a bit of peak hp for better average drive ability and lower emissions and better mileage
http://www.herbertcams.com/560-580-lift-250-255-dur-050-110-lobe-center/

theres some rather significant advantages in ordering a cam that closely matches the engines
requirements, and after building dozens of similar engines youll notice some trends and you may want to tweak the cam timing, this of course requires both a good understanding of what your trying to change or accomplish and what the changes you intend to make are likely to do to engine performance.

heres a cam thats rather well known for producing good power in a 10.7:1 compression 406 SBC combo.
(use mostly for racing in a light weight car like a cobra replica)
Ive built several times in the past, it really requires the following matched components
210-230cc port heads, 5" scat rods on a forged crank, a 3600-3800rpm stall converter, a 4.11:1-4.56:1 rear gear and an automatic trans that shifts at 6300rpm under full load race acceleration.
a good single plane intake like the edelbrock VIC series, an 850 cfm carb, long tube 1 3/4" headers 1.6:1 roller rockers and a rocker stud girdle

heres the came basic cam, but with a few mods that provide about 20 extra horse power, but at the cost of a noticeably rougher lope in the idle
notice the tighter LSA and its a 4/7 swap cam version on a smaller base circle

look at the chart,

Critical Cam Event Criteria


By David Vizard


I just know someone is going to ask what qualifies me to write on this subject so let’s get that out of the way up front. During a large portion of 1985 I dyno tested more than 4,000 cam and rocker combinations in three differently spec’d SB Chevys for Crane.

The following year I did a similar but larger exercise on 2.0L Pinto motors and the "A" series Mini Cooper motors for Kent Cams in England. Between 1987 and now I have done a number of similar, if somewhat smaller, exercises for other well-known cam companies including Isky and Comp Cams.

All this amounts to a stack of dyno sheets that I would estimate at over six feet high! Now that’s out of the way let’s get down to business.

None of us who are in business for ourselves has to be reminded that time is money and in the high-performance engine building business that can be a sensitive subject. Anyone intent on building a customer base understands that quality, reliability and results at a reasonable cost to the customer are the key factors toward success. The problem is, if you spend the time to do the job to your very best the customer probably can’t afford it.

The trick here is to understand where the best compromises lay. If you can get 98 percent of the results in 25-50 percent of the time then chances are you will have satisfied customers.

There are probably very few places where it is more important to strike the right compromises than in the area of cam selection and installation. Let’s set the scene here. You have a customer with a moderate budget who wants a performance street motor built and part of the deal is that it’s up to you to select the cam for the job. Without spending forever researching it, and nearly as long installing and timing it, what cam will be the best for the job? Here are some pointers that will help you make better choices in a number of areas for a better end result.

Duration
All too often, cams are selected on the basis of duration. This is not the best criterion, however. Overlap is a significantly more influential factor. The key here is not to go overboard on this.

The principle parameter dictating how much can be used is strongly influenced by the ratio of displacement relative to cylinder head airflow. This means large motors with less than adequate heads not only tolerate more overlap without losing streetable qualities but also need more to produce best output.

When the head flow is high in relation to the engine size overlap needs to be moderated. The key issue then is to strike a good compromise between overlap and duration. My advice here is to be conservative on duration and moderate on overlap, but try to go for as much lift as can be achieved without mechanically compromising valve train reliability.

Getting back to duration and overlap, we find that in considering these two factors together we are dictating what the cams Lobe Centerline Angle (LCA) will be.

LCAs
There is much said about LCAs that needs to be thrown out with the garbage. First let me make one major point quite clear. For a given spec of engine there is only one LCA that will give optimum torque and horsepower. It is not, as many cam company techs would have you believe, a variable parameter.

Consumers are often given the impression that cams need to be wider for street use than performance/race use so as to preserve idle quality. If a cam has to be ground on a wider LCA to preserve idle quality it is because the duration selected was too much to begin with. Fixing one cam selection mistake with another is not the way to go!

Here is something that, even as a professional engine builder, you may not be aware of. Some cam companies deliberately grind their off-the-shelf cams on wider-than-optimal LCAs. Why? Because they understand their customers.

Most performance orientated buyers a) are duration fixated and b) subconsciously or otherwise, adhere to the Stroker McGurk principle. This principle holds that "if some is good, more must be better and too much is just right." Although that may be true if we are talking bank balances it is most certainly not the case with cams.

By spreading the LCAs and reducing the overlap the cam grinders can give their customers a bigger cam to enhance bragging rights while still preserving some streetability for that same over-exuberant customer. Just how much this policy, consciously or otherwise, is adopted varies from company to company.

Being aware of this will go a long way to explaining why, when you look through different company catalogs, the cams for a given application differ by so much. If you ever thought that they can’t all be right then you are already a rung or two further up the ladder than the rest of the crowd.

For a truly impressive performance engine, its all about having lots of torque throughout the rpm range used. When considering valve events it pays to remember that, especially at low speed, torque is greatly influenced by the closing point of the intake valve. Tighter LCAs close the intake sooner as do cams timed in with more (and hopefully correct) advance.

So what is the price paid for a LCA that is artificially spread from optimum? Answer: a loss of ft.lbs. throughout the rpm range. Obviously the best situation is to have the optimum LCA but in the absence of this what are the best compromises?

The best way to cover this is to start from an optimum and see what we lose by first going too tight (a smaller number) then too wide (a larger number). If we have an optimally spec’d LCA for a race engine then we find that as the LCA is widened torque over the entire rpm band used drops off quite rapidly. For example, in a 350 SB Chevy, two degrees too wide can mean a loss of 20 or more ft.lbs. throughout the rpm range. On the other hand 2° too tight will have almost no effect on the output over the rpm range used while racing. If the LCA is too tight the motor needs to turn higher rpms before it comes on-the-cam and the idle is rougher. For a race engine then it is best to err on the tight side rather than the wide side. Because the idle quality goes as overlap is increased it is better to err slightly on the wide side if we are talking street usage. Here I am talking 2° off optimum, not the 4° to 6° and even 8° that is often seen. If you are installing such cams in commonly modified engines then you might want to consider upgrading your source of cam advice.

Cam Timing
If we are dealing with most domestic V8s then we have three options when it comes to what can be a time-consuming job, namely installing and timing the cam. The first and fastest option is to just install the cam and not bother to check to see where it is. This amounts to taking on blind faith the cam grinder’s ability to grind the cam right and the accuracy of the drive train from crank to cam. Granted, much of the time this will work but the other side of the coin is that a lesser, though still significant, number will not.

The second option is to use the commonly available multi-keyway gears. These are normally cut so that you can move in a 4° increment either way.

Lastly there is the "do it right by whatever means it takes" method. This, in its most convenient form, can involve adjustable timing gears; less conveniently, offset dowel pins; and the worst-case and most time-consuming scenario, shims and files.

Going with option number one is fine if you know that the type of engine you are working on has consistent timing gears and that the cam you are using is also ground right (usually the case if it is a quantity-produced off-the-shelf item). If this does work for you I don’t have anything further to say on the subject but if options two or three are it, then the following guidelines might prove useful.

If you are using a 4° increment multi-keyway gear then the chances of getting the timing within +/- one-half a degree are 4 to 1.

Let us consider the situation when one keyway puts the cam 2° retarded and the other 2° advance. Which do you go for? Assuming the cam has optimal valve opening events to start with then, in terms of power production, the output drops off faster if the cam is retarded than if it is advanced.

In the case in question, 2° too much advance will still typically deliver 99.7 percent of the engine’s capability whereas 2° retarded can drop output by as much as 3 percent. Also it is worth taking into account that as the valve train drive wears the cam becomes retarded. In the first 5,000 miles cam and timing chain wear can bring about a half-degree of retard.

The worst-case scenario is when the choice is between 1° retard and 3° of advance. The difference between optimal timing and one degree retarded is barely measurable on the dyno and, if the drive system is unlikely to change that situation, then go with the one degree retard. If the initial timing chain/gear wear is a factor in the equation then go with the 3° too far advance, especially if your customer is likely to shoot some nitrous into the motor. After 5,000 miles the advance is likely to be only 2-1/2° too much and that is still marginally better than 1-1/2° retarded.

It is commonly accepted that if a cam is too advanced it enhances low-end output because the intake closes earlier. It is also generally held that if the cam is retarded it will enhance top-end output at the expense of low end mainly because the intake closes later.

In reality the complete picture is a little more complex. If you find that retarding the cam makes a worthwhile difference in top-end output then part of the reason is that the cam’s basic valve events are not that close to optimal. The same goes if low end increases by any significant amount.

If cam events are virtually optimal on the cam then timing the cam correctly becomes more important. If events are not "right on" then we find that retiming the cam may put one event nearer optimal while others move away from optimal. Under these circumstances retiming the cam only produces marginal changes. These may show a certain advantage in one part of the rpm range at the expense of another.

However if the cam is capable of delivering optimal valve events in every respect we find the cam becomes much more sensitive to timing. An error in timing means that all events are now displaced from optimum. The result is that the engine can lose output everywhere over its entire working rpm band.

The bottom line here is if you know the cam is well speced for the application it becomes more important to time it in as the chance of impaired results is greater. "

many guys don,t understand that on these multi key timing sets there areTHREE different letters,
on the crank gear and three matching keyway slots.

0 on the cam gear gets lined up with 0 on the crank gear, if the 0 crank key slot is used to index the cam at TDC

0 on the cam gear gets lined up with R on the crank gear, if the R crank key slot is used to index the cam at 4 degrees retarded from TDC

0 on the cam gear gets lined up with A on the crank gear, if the A crank key slot is used to index the cam at 4 degrees ADVANCED from TDC


BTW, read thru these

http://www.webcamshafts.com/pages/cam_glossary.html

viewtopic.php?f=52&t=480

viewtopic.php?f=52&t=322

http://www.thirdgen.org/sbc-camshafts-primer

viewtopic.php?f=52&t=90

viewtopic.php?f=52&t=727

viewtopic.php?f=52&t=181

http://www.popularhotrodding.com/tech/0 ... index.html

http://www.compcams.com/Community/Artic ... 2026144213

http://www.compcams.com/CAMQUEST/

http://www.crower.com/valve-timing-chart

http://cochise.uia.net/pkelley2/Overlap.html

http://www.compcams.com/Technical/TimingTutorial/

 

[grumpyvette]

Cam Science
Resolving the Mysteries of Lobe Center Angles
By David Vizard

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. -SB)


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.


GAS DYNAMICS
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.


TDC THROUGH-FLOW
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 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 effect changes in bore and stroke have on the optimum LCA.


CR EFFECT
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
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.
VALVETRAIN DESIGN
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.


CONCLUSIONS
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.

 

[anderson3754]

I've read this several times today and now and I'm really perplexed and confused more then ever.

This term LCA to me is Lobe Centerline Angle as in crankshaft degrees ground into the cam. LSA is Lobe Seperation Angle as in camshaft degrees.

Or can LCA and LSA be used interchangeably depending on the context of the conversation. :?: :?

 

[grumpyvette]

almost everyone (INCLUDING MYSELF AT TIMES) tends to use the terms almost interchangeably ,SOMETIMES IN ERROR,because while the LSA and LCA can be the same the cam can be installed retarded or advances which has no effect on the LSA but will change the LCA, but they are NOT the same thing, the LSA LOBE SEPARATION ANGLE, is ground in to the cam and does not change
LSA is calculated by adding the center-lines of the intake and exhaust lobes, adding them together, and then dividing by two. For example, a cam with a 108 intake center-line and a 116 exhaust center-line would have an LSA of 112. The LSA is ground into the cam and cannot be changed without grinding a whole new cam, as it necessitates spreading the lobes or tightening the lobes.

the LCA LOBE CENTER ANGLE, is changeable when you degree in the cam and it does change, in relation to the cranks rotation as you advance or retard the cams location.
quote
"Intake Centerline
The intake centerline (ICL) of a cam is the most overlooked spec. The ICL can have nearly as much of an effect on peak power and the RPM at which peak power occurs as duration. The intake centerline is the position, in crank degrees, that the intake lobe hits peak lift (the center of the lobe). Most small block Chevrolet cams seem to come ground on a 108 ICL, meaning the intake valve reaches peak lift at 108 degrees. ICL is inter-related with LSA, in that the LSA of a given cam is typically considered the "straight up" point for that cam. For example, a cam with a 114 LSA, if ground with an ICL of 114, would be considered "straight up". However, the same cam ground on a 110 ICL is considered to be 4 degrees advanced."

EXAMPLE heres a cam I use in lots of street strip 383 builds



if I install it , dot-to dot ITS going to be at 4 degrees advanced from the split the overlap position, its LSA is 106 degrees and that wont change
quote
"This is a good time to discuss retarding/advancing and the effects of both. Retarding or advancing a cam can be achieved in two ways. The most common is to install the cam with offset bushings in the cam gear to adjust the cam timing, but it can also done by grinding the advance/retard into the cam core itself. You might have a cam ground with a 112 LSA and a 108 ICL...that means that if installed straight up it would be 4 degrees advanced. You could chose to retard or advance the cam from that point using an adjustable timing set as well, but the changes would be + or - from the 108 degree starting point. A cam that has a 112 LSA and 112 ICL would be ground straight up, and to advance it four degrees (resulting in a 108 ICL "as installed") would require an adjustable timing set. For an LT1, any advance or retard must be ground into the cam, as using offset cam bushings would alter ignition timing since the Optispark is driven off the cam gear.

Advancing a cam means you are moving the valve events earlier in the cycle. If done when installing the cam you will be moving both the intake and exhaust events earlier in the cycle. This generally boosts low speed torque at the expense of peak HP. Retarding a cam does the opposite, it will improve peak power by delaying valve events, which will increase peak HP and move the peak HP rpm up, but at the expense of low-end power.

Grinding the advance/retard into the cam offers a couple of advantages. You can advance the intake valve while holding the exhaust valve where it is, which would increase overlap, or you could retard the intake, which would increase the LSA and reduce overlap, but it would also delay the intake valve closing which would improve top end power. Or you can move both lobes and split the difference. This is where the benefits of custom cams come in: you can tailor the cam to your specific needs."

lets assume this angle is 110 degrees
if we install the cam at split overlap or strait up the LSA and the LCA, or ICL remain the same

if we install the cam at split overlap or strait up but then I advance the cam 4 degrees in relation to the crank rotation the LSA remains the same 110 degrees and the LCA, or ICL
changes to 106 degrees
if we install the cam at split overlap or strait up but then I retard the cam 4 degrees in relation to the crank rotation the LSA remains the same 110 degrees and the LCA, or ICL changes to 114degrees


advancing the cam 4 degrees moves the whole torque curve about 200rpm lower in the rpm range
retarding the cam 4 degrees moves the whole torque curve about 200rpm higher in the rpm range

http://www.lt1howto.com/articles/camshafts.htm

http://www.cartechbooks.com/vstore/show ... 4&CATID=21

http://www.empirenet.com/pkelley2/Overlap.html



Intake Reversion - Enginology
Reversion And How It Can Affect Power
From the August, 2010 issue of Circle Track
By Jim McFarland

Intake Reversion
Depending upon who you might ask, a definition of so-called "reversion" may be compiled in multiple ways. Bottom line, its effects are not necessarily beneficial to efficient combustion, regardless of how it may be defined. Over time, various devices have been construed as helpful in containing reversion, but before we examine how it might be contained, let's review how it develops.

In very slow motion, suppose we break down a typical induction cycle (in a normally aspirated engine) into segments that help identify how reversion is created. You may want to make periodic references to the little illustration we provided this month. It's intended to deliver a graphical image with the following text. In order to simplify an understanding of the phenomenon, we'll focus our description on a single-cylinder, four-stroke cycle engine using a carburetor. That will keep any notions of how a variety of pressure excursions are occurring in a multi-cylinder engine equipped with an intake manifold of single- or dual-plane design can affect the reversion landscape. Including these would rapidly complicate the discussion.

When the intake cycle begins, pressure in the intake manifold is less than atmospheric. At the same time, pressure in the cylinder is higher than atmospheric as exhaust residue is being "pumped" out through the exhaust passage. Now the intake valve begins to open. Since intake events begin before the piston reaches BDC on the exhaust stroke (far ahead of this point in racing engines), a "reverse flow" condition (pulse) is directed back into the intake track and against the direction of normal induction flow. While this condition is counter and disruptive to normal such flow, it also includes gaseous and generally non-combustible material we call exhaust gas.

Depending upon engine speed, the point of pre-BDC intake valve opening, exhaust system efficiency, and related variables, the distance over which this "reversion" pulse (and material) can travel back toward the carburetor will vary. Actually, the higher the rpm, the less penetration it will have into the intake manifold. Stated another way, as rpm increases, reversion activity in the intake manifold becomes contamination to combustion in the cylinders. In severe cases, you'll observe exhaust gas stains on the bottom of the carburetor base. What we do know is that the stronger the pulse, the more time is required to equalize inlet path pressure and cylinder pressure, and the more disruptive reversion becomes.

At this point, cylinder pressure exceeds inlet path pressure. As time passes, cylinder pressure drops to atmospheric, essentially equal to the exhaust. Immediately thereafter, just for an instant, pressure in the inlet path, cylinder, and exhaust path are equal. The exhaust valve then closes and atmospheric pressure begins to force flow into the lower pressure cylinder, beginning the "effective" portion of the intake cycle.

Several consequences of reversion are worth noting. Among them, one is that any combustion residue left in the intake passage and cylinder as a result of reversion, isn't combustible. However, it will occupy some volume in the combustion space, thereby displacing an equal amount of fresh air/fuel charge. The degree of power loss is in direct proportion to the volume of combustion residue present during the next combustion cycle and its effectiveness in reducing flame temperature. After all, this is what EGR (exhaust gas recirculation) does for on-road engines required to meet certain emissions (NOx) standards. In such vehicles, EGR also tends to reduce fuel economy and net power. Despite what might be conventional thinking, this condition is not completely outside the realm of racing engines.

So what are the telltale signs of reversion and what can you do about mitigating the problem? Let's examine some of the more common ones. In extreme cases, a condition often called "stand-off" occurs, during which you will see fuel vapors hovering above the carburetor. Two-plane intake manifolds tend to provide less plenum "damping" of reversion pulses than single-plane versions. In fact, when an engine is fitted with a fuel injection system for which there is no union of runner stacks through a common air chamber that helps dampen such pulses (effectively a plenum function), the problem can be even more acute. If nothing else, the condition further verifies the fact intake flow is bi-directional, under certain circumstances.

By inspection of intake manifold runners at or near their interface with cylinder heads, it's possible to detect traces of exhaust soot on runner walls. If the problem is less severe, intake port coloration becomes the next indicator. Discoloration of a carburetor's base is another spot to inspect. And if you happen to be evaluating engine performance on an engine dyno, watch for the slope of mapped BSFC curves to increase above peak torque rpm. Although there can be other causes of reversion (inadequate exhaust port flow, insufficient exhaust valve duration, mechanical separation of air and fuel at high rpm, ad more), matching this condition with discolorations along the inlet path will separate reversion from other causes of BSFC disruption. At the same time, you may also discover EGTs decreasing below what you'd consider normal for upper rpm power output. Remember, exhaust gas residue tends to cool the combustion process.

Over time, various means have been employed to reduce the impact of reversion on an otherwise efficient combustion process. One particularly effective method is to focus on improving low lift exhaust port flow. This can involve general reduction in exhaust backpressure or shaping exhaust valve seats, pockets and ports that include the backside of exhaust valves. In addition, you can work on reducing airflow in a reverse direction (back toward the combustion space) by valve seat and valve head modifications that promote flow out of the cylinder and not in a back-flow direction. Reverse-flow testing exhaust ports and valves on an airflow bench can be used to determine the effectiveness of such modifications. This technique aids in decreasing cylinder pressure during the last stages of the exhaust cycle, thereby reducing reversion pressure excursions back into the intake path during early opening of the intake valve.

In particular, you may want to study the example time-pressure trace (previously mentioned) that shows positive vs. negative pressure in the inlet path, from intake opening to intake closing. This trace is an example of the pressure history in a single intake path, measured at a point near the junction between the intake manifold and cylinder head. The amplitude and duration of the initial pressure spike is a function of reversion pressure. The area bounded by the horizontal axis and negative pressure part of the trace represents cylinder-filling efficiency (volumetric efficiency), and the smaller spike at the end is created by the kinetic energy decay at the conclusion of the inlet cycle. As you might expect, any successful efforts to reduce the amplitude and duration of the reversion spike nets an increase in the volumetric efficiency area (as previously described).

One final note. Keep in mind that because inlet flow is considered elastic in nature, "negative" pressure spikes created by reversion pulses can affect fuel delivery at the carburetor. Fundamentally, carburetors will deliver fuel based on any pressure differential across its circuitry. Airflow (pulses) delivered to or pressure differentials created across a carburetor will cause a release of fuel, whether the flow is toward or away from the combustion space, thereby upsetting intended carburetor calibrations. The best bet is to address the problem of reversion as not only real but potentially of sufficient significance that it can diminish your best efforts to optimize power.

 

[anderson3754]

Thanks again for the quick response

I did not realize these terms were used at times interchangeably. I love the additional info you posted. Now I have more data to study . I only have an 8th grade understanding of cams and would like to graduate someday from the
12 grade of cam schools. :lol:

Hey thanks again