[ekancler]

Originally Posted by amcmike

There's lots of good information throughout, but at least read posts 1 , 26, 44, 45 https://ls1tech.com/forums/generatio...-matter-2.html

That's a fascinating thread. It's got it all in there. The level of detail is way over my head. I'd love to some day be able to develop the right cam based on the valve timing numbers alone. That's a long way from today.

Post #26 seems to be most pertinent to this thread here. I also thought this from #48 was pretty interesting:

I never considered that when comparing two cams with the same amount of overlap, the one with more-aggressive ramps would have better idle/vacuum than the cam with less-aggressive ramps. "Gut feel" was that aggressive ramps give an aggressive idle - but the opposite appears to be true. That helps explains why an LS engine with roller lifters idles better than a flat-tappet SBC with similar cam specs - the roller lifters allow more aggressive ramps.

And this from post #33:

For years I have have heard wide lobe sep for good vacuum. Well how about properly cammed engine for the intended rpm range and if the LSA is a 107 then thats what it is. I've got several of you on here and many others with vacuum reports of 11 to 15 inches during idle. These are customers wanting street cars with pop. These are not drag cars. Many circle track sanctioning bodies have Vac rules. Some pull truck stuff also. I have learned over the years manipulating the valve events you can achieve vacuum. These cams aren't on 112 or 114 LSAs I can assure you.

Both tend to illustrate that LSA is useful to a point but valve event details tell you a helluva lot more.

[Formulas]

engine masters show did a test with 3 different LSA cams all else same

also a show on just same cam advanced / retarded

and another show on just split duration cams all else same

very infomative on ICE. trends

[GoreMaker]

LSA falls in with all the other aspects of the engine that determine how efficiently it behaves as an air pump at varying speeds. In this case, LSA determines how long the intake and exhaust valves are open at the same time during the transition from exhaust to intake stroke.

At high engine speeds, the exhaust is moving so quick that having both valves open at the same (narrow LSA) time allows the escaping exhaust gases to draw in more air for the intake stroke before the piston has started moving down, hence improving high-RPM cylinder filling and efficiency (ie. high-RPM horsepower). Conversely, at low RPM, the same conditions just cause a delay in the piston being able to create the necessary vacuum to draw air in, thereby hurting low-RPM cylinder filling and efficiency. In extreme cases, the piston might even be drawing exhaust back into the combustion chamber before the exhaust valve fully closes, thereby diluting the mixture. These are the reasons for a "lopey" idle.

Looking at it the other way, a wide LSA means there's less overlap, so the piston's downward motion creates more vacuum in the intake manifold, thereby speeding up cylinder filling and improving efficiency. This generates more low-end torque. But as the engine speeds up, the lack of overlap means that the transition between exhaust and intake is more abrupt, the flow of air is interrupted, and cylinder filling isn't as optimal. It's more "pulse-y", if you will. That means less potential horsepower at higher RPM.

Everything is a compromise. The solution is variable valve timing, which every modern engine has. But in our case, the real solution is just to pick a power band for our purposes and build for that. In general, the more vacuum a cam produces (wide LSA), the more power it will make below 5500rpm and the smoother it will idle. I like a cam with an LSA of 114. I don't much care for a lopey idle, and I don't need high-RPM horsepower on the street.

[ekancler]

Originally Posted by GoreMaker

LSA falls in with all the other aspects of the engine that determine how efficiently it behaves as an air pump at varying speeds. In this case, LSA determines how long the intake and exhaust valves are open at the same time during the transition from exhaust to intake stroke.

At high engine speeds, the exhaust is moving so quick that having both valves open at the same (narrow LSA) time allows the escaping exhaust gases to draw in more air for the intake stroke before the piston has started moving down, hence improving high-RPM cylinder filling and efficiency (ie. high-RPM horsepower). Conversely, at low RPM, the same conditions just cause a delay in the piston being able to create the necessary vacuum to draw air in, thereby hurting low-RPM cylinder filling and efficiency. In extreme cases, the piston might even be drawing exhaust back into the combustion chamber before the exhaust valve fully closes, thereby diluting the mixture. These are the reasons for a "lopey" idle.

Looking at it the other way, a wide LSA means there's less overlap, so the piston's downward motion creates more vacuum in the intake manifold, thereby speeding up cylinder filling and improving efficiency. This generates more low-end torque. But as the engine speeds up, the lack of overlap means that the transition between exhaust and intake is more abrupt, the flow of air is interrupted, and cylinder filling isn't as optimal. It's more "pulse-y", if you will. That means less potential horsepower at higher RPM.

Everything is a compromise. The solution is variable valve timing, which every modern engine has. But in our case, the real solution is just to pick a power band for our purposes and build for that. In general, the more vacuum a cam produces (wide LSA), the more power it will make below 5500rpm and the smoother it will idle. I like a cam with an LSA of 114. I don't much care for a lopey idle, and I don't need high-RPM horsepower on the street.

Alas, no variable valve timing for us. Not to get sidetracked... but Rhoads lifters are a clever solution, and I gather very well made, but apparently tricky to tune just right to minimize clicking. I don't think I'm likely to go there unless it's with a builder who really knows that business inside and out. Even then, probably not for me. And some others make variable duration lifters as well.

Question - A tighter Lsa cam is going to result in reduced fuel economy and increased emissions at idle and lower RPMs. But can one improve fuel economy and emissions at mid and higher RPMs?

[Formulas]

Originally Posted by GoreMaker

LSA falls in with all the other aspects of the engine that determine how efficiently it behaves as an air pump at varying speeds. In this case, LSA determines how long the intake and exhaust valves are open at the same time during the transition from exhaust to intake stroke.

At high engine speeds, the exhaust is moving so quick that having both valves open at the same (narrow LSA) time allows the escaping exhaust gases to draw in more air for the intake stroke before the piston has started moving down, hence improving high-RPM cylinder filling and efficiency (ie. high-RPM horsepower). Conversely, at low RPM, the same conditions just cause a delay in the piston being able to create the necessary vacuum to draw air in, thereby hurting low-RPM cylinder filling and efficiency. In extreme cases, the piston might even be drawing exhaust back into the combustion chamber before the exhaust valve fully closes, thereby diluting the mixture. These are the reasons for a "lopey" idle.

Looking at it the other way, a wide LSA means there's less overlap, so the piston's downward motion creates more vacuum in the intake manifold, thereby speeding up cylinder filling and improving efficiency. This generates more low-end torque. But as the engine speeds up, the lack of overlap means that the transition between exhaust and intake is more abrupt, the flow of air is interrupted, and cylinder filling isn't as optimal. It's more "pulse-y", if you will. That means less potential horsepower at higher RPM.

Everything is a compromise. The solution is variable valve timing, which every modern engine has. But in our case, the real solution is just to pick a power band for our purposes and build for that. In general, the more vacuum a cam produces (wide LSA), the more power it will make below 5500rpm and the smoother it will idle. I like a cam with an LSA of 114. I don't much care for a lopey idle, and I don't need high-RPM horsepower on the street.

you should watch the engine masters results ..... to see the trends ... when they used a wide lobe sep the ONLY place it made more power was a small blip at the last 200 rpm in the run which started at 3000

yes a wide sep has a cleaner idle to 2000 ish rpm which is not captured on a dyno

iam not going to argue or re'count the entire show its easy enough to find watch and understand

[ekancler]

There seems to be differing opinions/experiences on whether a wider Lsa, all else equal, will start to outcompete a tighter Lsa cam at the highest RPMs.

Yes, wider Lsa is more efficient and powerful at idle and very low RPMs. Yes, tighter Lsa cams outcompete in the mid-band and high RPMs due to better scavenging. But at the highest RPMs, and this holds true in articles and videos I've read/seen, sometimes the tighter Lsa cam maintains an advantage, sometimes it loses its advantage at around peak HP.

What are the competing phenenoma at play here?

[Cliff R]

Rhoads lifters have been providing variable valve timing for our older engines for decades. Combining them with high ratio rocker arms and well chosen duration/LSA for the engine CID and compression ratio and one can very quickly mimic the power of a very well chosen HR cam at much less cost..

All hydraulic lifters provide some variable valve timing based on the design (leak down rates) but for the most part those with very tight plunger to body tolerances act pretty much like solid lifters.

Overlap plays a big role in engine function and it is a product of the LSA and duration of the intake/exhaust lobes.

Cam companies push tight LSA for the most part. This is done more for the "bling" factor than anything else as it makes the engine less efficient at idle speed so we get "low" vacuum and some "lope" in the exhaust note.

I've installed/tested a lot of camshafts in these engines. In the 455 engine really tight LSA makes for "explosive" mid-range power as it enhances what they are already very good at. Wide LSA smooths them out some and pushes peak power higher in the RPM range.

What I have done and do here is to use higher compression than most, more duration, relative wide LSA and end up with engines that are user friendly with good idle quality and street manners, but still run very strong at the track.

The WORST 455's I've built, worked on or tuned have been lower compression with smaller cams on tight LSA's. Can't say "turd" loud enough here and that is not a good direction to go with them......IMHO.

When you get into moderate compression 455's and bigger camshafts LSA really matters a lot less as they are going to make great power either way, so it becomes more of a matter of personal choice at the point. Some folks like "menacing" idle quality and stinky exhaust, others prefer their engines to be a bit more tame at a glance. Either way it's hard to kill one off provided you have put a big enough cam in it. Here the smallest cam I'd ever use in a 455 build would be 230 @ .050" and a least 280 @ .006".............

[ekancler]

Originally Posted by Cliff R

Rhoads lifters have been providing variable valve timing for our older engines for decades. Combining them with high ratio rocker arms and well chosen duration/LSA for the engine CID and compression ratio and one can very quickly mimic the power of a very well chosen HR cam at much less cost..

All hydraulic lifters provide some variable valve timing based on the design (leak down rates) but for the most part those with very tight plunger to body tolerances act pretty much like solid lifters.

Overlap plays a big role in engine function and it is a product of the LSA and duration of the intake/exhaust lobes.

Cam companies push tight LSA for the most part. This is done more for the "bling" factor than anything else as it makes the engine less efficient at idle speed so we get "low" vacuum and some "lope" in the exhaust note.

I've installed/tested a lot of camshafts in these engines. In the 455 engine really tight LSA makes for "explosive" mid-range power as it enhances what they are already very good at. Wide LSA smooths them out some and pushes peak power higher in the RPM range.

What I have done and do here is to use higher compression than most, more duration, relative wide LSA and end up with engines that are user friendly with good idle quality and street manners, but still run very strong at the track.

The WORST 455's I've built, worked on or tuned have been lower compression with smaller cams on tight LSA's. Can't say "turd" loud enough here and that is not a good direction to go with them......IMHO.

When you get into moderate compression 455's and bigger camshafts LSA really matters a lot less as they are going to make great power either way, so it becomes more of a matter of personal choice at the point. Some folks like "menacing" idle quality and stinky exhaust, others prefer their engines to be a bit more tame at a glance. Either way it's hard to kill one off provided you have put a big enough cam in it. Here the smallest cam I'd ever use in a 455 build would be 230 @ .050" and a least 280 @ .006".............

Thanks Cliff. What's your experience with 400 engines in this regard?

[Stan Weiss]

Originally Posted by Formulas

you should watch the engine masters results ..... to see the trends ... when they used a wide lobe sep the ONLY place it made more power was a small blip at the last 200 rpm in the run which started at 3000

yes a wide sep has a cleaner idle to 2000 ish rpm which is not captured on a dyno

iam not going to argue or re'count the entire show its easy enough to find watch and understand

Back when they had the Engine Master comp. Not only were they running tight LSA but if they had the PTV clearance the ICL was in the high 90's

Stan

[Half-Inch Stud]

Post 13 Note 2: Tight LSA vs Reversion;

Well, my exhaust study showed the need for large long Collectors to mitigate Reversion, yet ironically enable an "EGR" during cruise rpm. So i built 5" dia, 37" long Collectors that bolt onto the Header' existing Collectors. Mechanically clean, and the Reversion seems quite mitigated ( been over a decade so i'm keeping them on..). Such collector extensions time the "boost from overlap" to kick in at 3900 rpm, and supposing the 1/4 mile results indicate the long collectors do no harm.

Highway cruise in OD lacks lugging. Pull-aways in any gear are possible and practical, except in OD being possible but not practical. I pull away in OD at times out of laziness to move the shifter or see how she runs.

9.00:1 with LSA per Sig. I will read/see me Sig when this post shows. Says 112! Hmmm, gonna need to review some cam cards. The 108 LSA was designated for the Spare engine.

[Cliff R]

I would NOT use, recommend or even play around with super tight LSA camshafts with the ICL down in the 90's or even low 100's. I've had quite a few customers venture in that direction following magazine articles showing "amazing" dyno results from that deal. One of those experiences was recently where a customer bought some carb rebuild and tuning parts for his 454 Big Block Chevy. During the conversation I asked about the CID, compression ratio, cam specs etc. He very quickly told me that the engine was 9 to 1 compression, and cam was ground on a really tight LSA with the ICL down around 100. I remember the seat timing was also pretty short, 262 degrees @ .006".

I did my part and supplied what was needed to get the carb rebuilt and up to par. Once placed in service the phone calls started coming in, NOTHING with that engine was working well or making the grade, and no tuning efforts with the carb and distributor were making it any better. After quite a few nearly endless phone calls things went silent so I don't know what they ended up doing with it. I do know that everyone involved was pretty disappointed in it.

Just a guess on my part but the cam wasn't big enough for a moderate compression 454 build right to start with, and would only get worse closing the intake really early and not much seat timing as well................

[Stan Weiss]

Clearly he had not enough duration.

Stan

[johnta1]

Here is some text I have saved that may help some on LSA:

Resolving the Mysteries of Lobe Center Angles

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.

[Stan Weiss]

Thanks John

Generally speaking, the majority of cams fall between 98 and 120 degrees LCA.

Stan

[Cliff R]

Not nearly enough duration for sure Stan. Keep in mind that magazine articles don't always explain to the readers that following a course of action using a 102LSA cam with the ICL at 98, for example, doesn't work for chit if you try using a cam with 50-60 degrees LESS seat timing at .050" than the one the "professional" engine builder spec'd out for his custom "max-effort" deal.

For sure targeting specific engines with those sort of cams may help win a dyno contest for the Engine Masters articles, but it's the WRONG direction to go with your tiny little street cam chosen to build "more low end torque", which are EXACTLY the words my last horribly disappointed customer told me when I asked about his engine combo and choices he made for it.......FWIW.....

[Half-Inch Stud]

i loath and despise the Scooter Borther explanation: too longk, many useless scenarios, and LCA in place of LSA.

Oh heck; Pick you idea intake lobe duration and ILC ( Intake Lobe Center ). The pick you mandatory Exhaust closing for best performance. Then I bet you can jerk the Exhaust Opening (and thus EXH Lobe duration) between 90* ATDC thru 120* ATDC and have only minor performance difference (yet a loud exhaust note for 90*ATDC) And the resulting LSA will jerk a 30 degrees spread too. So here, a few words were used to make clear my concern for LSA specing as be-all end-all.

[Stan Weiss]

Originally Posted by Cliff R

Not nearly enough duration for sure Stan. Keep in mind that magazine articles don't always explain to the readers that following a course of action using a 102LSA cam with the ICL at 98, for example, doesn't work for chit if you try using a cam with 50-60 degrees LESS seat timing at .050" than the one the "professional" engine builder spec'd out for his custom "max-effort" deal.

For sure targeting specific engines with those sort of cams may help win a dyno contest for the Engine Masters articles, but it's the WRONG direction to go with your tiny little street cam chosen to build "more low end torque", which are EXACTLY the words my last horribly disappointed customer told me when I asked about his engine combo and choices he made for it.......FWIW.....

Cliff,
Please don't miss quote me. I did not said "tiny little"!!!! Even if he ran 110 octane to handle the cylinder pressure and could run full timing. He would be lucky to see peak HP at 4400 RPM. He had the wrong duration

Stan

[Formulas]

the engine masters dyno show is not a show of extremes or targeting a specific winner based on any wonky parimiters to pick a winner

they "USUALLY" take a brand X engine that can be catagorized as a healthy street engine and explore different engine building myths and tecniques

on cams they took an engine and ran it on the dyno just changing where the cam was installed at =ICL and examined the power curve

on another episode they used same exact lobes on 3 different cams only change was lobe centers and examined dyno chart results


another episode they examined camshafts main difference was single duration and split duration and examined results

its all pretty usefull information if you would give it a chance

they also did back to back cast iron and aluminum heads comparison same ports same chambers same company, and no you dont have to have more compression with aluminum or more timing, alum came out the winner slight edge on power and less weight

i must be the only viewer but alot of what you think may not be right per test results

[amcmike]

Originally Posted by Cliff R

Not nearly enough duration for sure Stan. Keep in mind that magazine articles don't always explain to the readers that following a course of action using a 102LSA cam with the ICL at 98, for example, doesn't work for chit if you try using a cam with 50-60 degrees LESS seat timing at .050" than the one the "professional" engine builder spec'd out for his custom "max-effort" deal.

For sure targeting specific engines with those sort of cams may help win a dyno contest for the Engine Masters articles, but it's the WRONG direction to go with your tiny little street cam chosen to build "more low end torque", which are EXACTLY the words my last horribly disappointed customer told me when I asked about his engine combo and choices he made for it.......FWIW.....

Hence why it's best to choose a camshaft based on valve opening/closing events. LSA (aka LCA) and advance are just reference to calculating those events.

[amcmike]

Originally Posted by Formulas

the engine masters dyno show is not a show of extremes or targeting a specific winner based on any wonky parimiters to pick a winner

they "USUALLY" take a brand X engine that can be catagorized as a healthy street engine and explore different engine building myths and tecniques

on cams they took an engine and ran it on the dyno just changing where the cam was installed at =ICL and examined the power curve

on another episode they used same exact lobes on 3 different cams only change was lobe centers and examined dyno chart results


another episode they examined camshafts main difference was single duration and split duration and examined results

its all pretty usefull information if you would give it a chance

they also did back to back cast iron and aluminum heads comparison same ports same chambers same company, and no you dont have to have more compression with aluminum or more timing, alum came out the winner slight edge on power and less weight

i must be the only viewer but alot of what you think may not be right per test results

I believe he was referring back to the contest series that PHR ran years ago, not the show.