Motorcycle Cam Selection
Thinking about doing a STAGE 2 upgrade for your motorcycle? If so than one of the things you’ll probably be upgrading are your camshafts. There are a lot of options out there and it can be difficult to decide exactly what is best for your particular application. This article will help guide you in making that selection.
One of the most common errors when selecting cams is to over cam the engine. When selecting a cam you want to make sure it is ideal for the RPM range that you plan on operating your engine in.
The basic function of cams is to open and close your intake and exhaust valves. This may seem like a fairly straight forward process, but when dealing with the dynamics of a high speed engine even a minor change can make big differences in performance.
In this article we will break the cam cycle down into 4 parts: Intake Opening, Intake Closing, Exhaust Opening, and finally Exhaust Closing. We will also discuss several cam options such as: Lobe Centerline, Lobe Separation Angle, Overlap, Duration, Lift, Symmetrical vs. Asymmetrical Cams and Compression Ratio.
One way to think about your engine is as a continually flowing process from the intake where air and fuel are brought in, through combustion, and finally exhaust. Improving flow at any stage in the process will improve your engines performance and make it more efficient. During the intake opening phase you’re bringing air into the combustion chamber so to get the best performance out of your engine you’ll want to have a high performance air filter to allow the air to flow more easily.
The point at which your intake valve opens is critical to throttle response, emissions, and gas mileage. At low speeds, and high vacuum conditions, premature intake opening during the exhaust stroke can allow exhaust gasses to flow back into the intake manifold, contaminating the fresh intake charge. A late opening intake gives smooth engine operation at idle and low RPM, and it ensures adequate manifold vacuum assuming the other three valve opening and closing points remain reasonable. As RPM increase, air demand is greater. To supply additional air and fuel, the intake valve should open sooner, which allows more time for the intake charge to fill the cylinder. With an early opening intake valve, at high rpm, the exiting exhaust gas helps draw the intake charge thru the combustion chamber purging the cylinder of residual gas, but it also increases fuel consumption by allowing part of the intake charge to escape before combustion and can make for a rough idle.
Early intake opening usually means overlap, less throttle response at low to mid RPMs, rough idle, more emissions, poor fuel economy. However, by opening the intake valve early, we can slightly increase the volumetric efficiency of the engine assuming better flow from the heads. This is where stock heads fall short compared with ported heads. However - like cams, bigger is not always better when it comes to ported heads. Big ports and big valves will drop the intake and exhaust velocities, which can cause a host of problems, as well as a loss of volumetric efficiency. Most ported Twin Cam heads with stock diameter valves/seats, used with stock intakes and high performance air filters usually flow their max CFM (cubic feet per minute) near 0.350" - 0.450" valve lift. Using a cam that has the intake valve open that far by the time the piston is at max velocity maintains the max intake charge velocity - which makes the best use of the momentum supercharging effect between idle and 3500 rpm. Using a cam with even more lift (+0.500") only reduces this effect - and power (along with adding more unnecessary wear and tear to the valve train). A stock, un-ported head has a very restrictive exhaust port, and therefore limits volumetric efficiency even further - making a cam with high lift even less effective. The thing to remember with cam timing is that the intake valve opens before TDC (top dead center) and closes after BDC (bottom dead center).
The intake closing point has more effect on engine operating characteristics than any of the other three opening and closing points. The earlier it occurs, the greater the cranking pressure. Early intake closing is critical for low-end torque and responsiveness and provides a broad power curve. It also reduces exhaust emissions while enhancing fuel economy. As RPM increases, intake charge momentum increases. This results in the intake charge continuing to flow into the combustion chamber against the rising far past BDC. The higher the engine's operating RPM, the later the intake closing should be to ensure all the charge possible makes it into the combustion chamber. Of course, closing the valve too late will create significant reversion. It is a fine balancing act. In a perfect world, the optimum intake closing point would occur just as the air stops flowing into the chamber. It would get the valve seated quickly and not waste time in the low lift regions where airflow is minimal and there is no compression building in the cylinder. It wouldn't be so fast the valve bounces as it closes, allowing the charge to escape back into the intake port and disturb the next charge. And in hydraulic street cam applications, it would insure that the closing ramps are not so fast that they result in noisy operation.
A late closing intake valve will yield poor compression and will cause poor performance over most of the entire RPM range.
A semi-late closing intake will have a good mid range and pretty good top end but not the best.
An early closing intake (30-35 degrees) is what we like for a heavy bike because it will give an excellent bottom end performance and a good midrange.
The intake valve closing point is intimately related to an engine's dynamic or "effective" compression ratio. Compression ratio is also dependent on cam duration.
A mild cam with an early intake valve closing point will work well at low RPM. However, at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. As a result, performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed.” This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
An aggressive cam with a late intake valve closing point will work well at high RPM, however, at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. As a result, torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed.” Thus, a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
An early closing (30 - 38 ABDC) = high dynamic compression, great low to mid rpm torque for a very broad power band, requires lower static compression which means less stress and strain on the engine, less risk of heat damage and detonation, and more reliability, but engine RPMs are limited. The engine will "quit pulling" around 4800 rpm. As intake valve closing gets later (40-45) the power band moves up about 250 -300 rpm, narrows slightly unless more static compression is built in (e.g. thinner head gasket). Torque remains about the same, but due to higher RPMs, HP increases slightly. Throttle response from idle drops slightly, head temperatures increase slightly, making detonation a realistic risk, fuel management/tuning becomes even more critical and exhaust pipe diameter, length, backpressure designs become more influential. The engine will pull thru 5000 rpm. Closing the valve even later (+45 ABDC) shifts the power band way up the rpm scale. Increased static compression is necessary to achieve any TQ/HP typically exceeding 12:1. Fuel management/tuning are very critical to reduce detonation and the risk of heat damage. Higher compression shortens the engine's life. Because this cam only functions well at higher RPMs, the other cam specifications can take advantage of this and be optimized for more power. What's lost is smooth idling and some usable power/torque at low to mid RPMs, crisp throttle responses from idle, engine heat issues become critical. Essentially ideal for a drag bike: doesn’t idle well, pops & snorts until the throttle is twisted, but when it finally begins to roar the engine is barely manageable.
The most important timing event is the intake valve closing angle. The intake closing point determines the minimum rpm at which the engine begins to do its best work. The later the intake valves close, the higher the rpm must be before the engine gets "on the cam."
If you have one of the late closing cam designs installed, say one that closes the intake valves later than 40 degrees, then you cannot expect excellent performance at 2000 rpm. No carburetor adjustment, ignition adjustment or exhaust system can change this.
Overall, the exhaust valve opening point has the least effect on engine performance of any of the four opening and closing points. Opening the exhaust valve to early decreases torque by bleeding off cylinder pressure from the combustion that is used to push the piston down, yet the exhaust has to open early enough to provide enough time to properly scavenge the cylinder. An early opening exhaust valve may benefit scavenging on high-rpm engines because most useful cylinder pressure is used up anyway by the time the piston hits 90-degrees before BDC on the power stroke.
Later exhaust valve opening helps low rpm performance by keeping pressure on the piston longer, and it reduces emissions.
With an early opening exhaust you lose the entire bottom end and the mid range will be lazy.
Semi-early opening exhaust will give you good cylinder scavenging which results in a cleaner cylinder mixture at high rpm the low end will suffer some but the mid range will be very good.
Late closing exhaust will give you a narrow RPM band. The low end will be good as well as the midrange but we will have an engine difficult to use.
Stock cams typically open the exhaust valve late (36 BBDC) to maximize the burn time and pass emission tests easier...but suffer from pumping losses because the piston has to work harder to mechanically push out the burnt gases. If the cam opens the exhaust valve a little sooner (40-43 BBDC), we can use blow-down (the expansion of burning A/F) to help scavenge the cylinder. This gets the burnt gases moving, reduces the piston effort, and decreases pumping losses up too about 4000 rpm. However if the cam opens the exhaust valve too soon ( 45+ BBDC) the blow-down will bleed off much of the expansion pressure of the power stroke from idle thru about 2500 RPMs. The RPMs must be higher to overcome the time available for blow-down.
The exhaust opening stage is where a good set of exhaust pipes are important. The exhaust pipe must not only flow enough, but they must also be designed so that the reversion pulse is compatible with camshaft timing.
Excessively late exhaust valve closing is similar to opening the intake too soon- it leads to increased overlap, allowing either reversion back up the intake, or the intake mixture to keep right on going out the exhaust. On the other hand, late closing events can help purge spent gasses from the combustion chamber and provide more vacuum signal to the intake at high rpm. Early exhaust valve closing yields a smoother operating engine. It does not necessarily hurt the top-end, particularly if it is combined with a later intake valve opening. As engine operating range increases, designers must move all the opening and closing points out to achieve earlier openings and later closings, or design a more aggressive profile to provide increased area under the curve without seat timing increases. Exhaust Valve Closing - usually between 4 (early) and 20 (late) deg ATDC. An early closing = less overlap, late closing = large overlap. Less overlap (exhaust valve closes at 4) makes it easier to pass a smog test, smooth idle and great fuel economy. A mild overlap (exhaust valve closes at 8-12) makes good low to mid rpm range power, better throttle response, fair fuel economy, slightly more emissions. And large overlap (exhaust valve closes at 13-20) allows a lot of intake charge dilution/loss (bad emissions), poorer fuel economy, rough idle, less throttle response from idle, and makes most of the power at higher RPMs. Note: the amount of overlap also depends on the cam's intake valve opening specifications.
Lobe Center lines give you a relative perspective of how advanced or retarded a cam is in relation to top dead center (TDC). Harley cam profiles typically have an intake centerline from 98 to 108 degrees. An intake centerline of 98 is considered to be the most advanced and generally gives the most torque. A centerline of 108 will give power in the upper rpm range.
An exhaust centerline of 112 is the most advanced while the 102 is the most retarded. Again an advanced lobe will give power in the lower rpm range while the retarded lobe will have its power range extended in the rpm range. For practical terms, most cams for Harley are in the range of 96-108 on intake and 112-102 on the exhaust.
Tailoring the valve opening and closing points on an actual camshaft is accomplished by varying the lobe centerline locations, changing the LSA, and refining the profile shape itself. Advancing the cam moves both the intake and exhaust in an equal amount, resulting in earlier valve timing events. Engines typically respond better with a few degrees of advance, probably due to the importance of the intake closing point on performance. For racing, advanced cams benefit torque converter stall, improve off-the-line drag race launches, and help circle-track cars come off the corner. Cam companies often grind their street cams advanced (4 degrees is typical), which allows the end-user to receive the benefits of increased cylinder pressure yet still install the cam using the standard timing marks. Increasing the intake lobe center line from 104 to 106 degrees is considered retarding. All events will take place later in the engine cycle. Retarding the cam causes the intake valve to open and close later. This will reduce cylinder pressure which reduces the low speed performance of the engine.
Advancing the intake and retarding the exhaust (“closing up the centers”) increases overlap and should move the power up in the RPM range, usually at the sacrifice of bottom end power. The result would be lower numerical values on both intake and exhaust lobe centers.
Retarding the intake and advancing the exhaust (“spreading the centers”) decreases overlap and should result in a wider power band at the sacrifice of some top end power. This condition would be indicated by higher numerical values on both intake and exhaust lobe centers. By moving only one cam the results are less predictable, but usually it is the intake that is moved to change power characteristics since small changes here seem to have a greater effect.
Lobe Separation Angle:
Lobe separation is the angle between the center bump of the intake lobe and its counterpart on the exhaust lobe. Think of it like the two points on a pair of scissors relative to the hinge in the middle. If the scissors are nearly closed, you can cut well as long as what you are cutting is thin. To cut thick stuff, you open wider, but have less leverage, so it can be harder to get the done. The same principle applies with separation on cam lobes. Typically, lobe separation for street cams runs between 97 and 108 (camshaft) degrees. The relationship between intake and exhaust is ground into the cam and can’t be altered by advancing or retarding the overall cam timing.
As a guideline, if the rest of the numbers are comparable, a cam with a lobe that is less separate (for example, 98 to 103 degrees) will offer a broader spread of power and tend to produce power at the low end, while wide lobes make for a more “cammy” cam, coming on harder and later in the game. Lobe Separation Angles (LSA) of 100-103 degrees tends to produce power at the low end.
LSA and Lift affect the "sound" and idle quality. Generally, smaller lobe separation angles cause an engine to produce more midrange torque and high rpm power, and be more responsive, while larger lobe separation angles result in broader torque, improved idle characteristics, and more peak horsepower.
A “tight” lobe separation angle of 103 degrees or less creates more valve overlap, which helps create that lumpy idle characteristic of big camshafts. The tighter LSA’s are, the more likely problematic exhaust reversion into the intake will occur. Put simply, we can say that a tight LSA 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." Narrow LSA’s tend to increase midrange torque and result in faster revving engines. Generally, smaller lobe separation angles cause an engine to produce more midrange torque and high rpm power, and be more responsive. Typically, however, small lobe center numbers (more overlap) equates to more midrange power at the expense of top-end power. Probably the most significant factor to the engine tuner though is a tight LSA’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. A cam with 102 degrees of lobe separation angle will have more overlap and a rougher idle than one with 108 degrees, but it'll usually make more midrange power. A tighter lobe has more overlap. A tighter centerline starts the torque curve sooner, and doesn't give as wide of a power band. A wider lobe doesn't start the torque curve as soon, but it continues to make torque longer and has a broader power band.
A street engine with a wide LSA has higher vacuum and a smoother idle. Big numbers (less overlap) will give more top end, sacrificing midrange. 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.
Narrow LSA (98-103)
Moves Torque to Lower RPM
Increase midrange Torque
Increases Maximum Torque
Faster revving engine and more responsive
Narrow Power band
Builds Higher Cylinder Pressure
Increase Chance of Engine Knock
Increase Cranking Compression
Increase Effective Compression
Idle Vacuum is Reduced
Idle Quality Suffers (lumpy idle characteristic)
Open Valve-Overlap Increases
Closed Valve-Overlap Increases
Decreases Piston-to-Valve Clearance
Wide LSA (104-108)
Raise Torque to Higher RPM
Reduces Maximum Torque
Broadens Power Band
Lazier initial response
More peak Horsepower
Reduce Maximum Cylinder Pressure
Decrease Chance of Engine Knock
Decrease Cranking Compression
Decrease Effective Compression
Idle Vacuum is Increased
Idle Quality Improves
Open Valve-Overlap Decreases
Closed Valve-Overlap Decreases
Increases Piston-to-Valve Clearance
The objective of overlap is for the exhaust gases which are already running down the exhaust pipe to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. Duration, lift and LSA combine to produce an "overlap triangle". The greater the duration and lift, the more overlap area, LSA’s remaining equal. Given the same duration, LSA and overlap are inversely proportional: Increased LSA decreases overlap (and vice versa). More overlap decreases low RPM vacuum and response, but in the midrange, overlap improves the signal provided by the fast moving exhaust to the incoming intake charge. This increased signal typically provides a noticeable engine acceleration improvement.
Less overlap increases efficiency by reducing the amount of raw fuel that escapes thru the exhaust, while improving low-end response due to less reversion of the exhaust gasses back up the intake port; the result is better idle, stronger vacuum signal and improved fuel economy. Due to the differences in the cylinder head, intake and exhaust configuration, different engine combos are extremely sensitive to the camshaft's overlap region. Not only is the duration and area of the overlap important but also its overall shape. Much recent progress in cam design has been due to careful tailoring of the shape of the overlap triangle. According to Comp Cams, the most critical engine factors for optimizing overlap include intake system efficiency, exhaust system efficiency, and how well the heads flow from the intake toward the exhaust with both valves slightly open.
Camshaft overlap duration less than 30 degrees tends to produce good low end power.
Increased overlap equates to reduced idle quality, vacuum, and harsher running prior to coming up on the cam. Lots of overlap works great at high rpm because more intake charge manages to cram itself into the cylinder, but lots of overlap will also make the engine run badly at low rpm, as exhaust gas manages to make its way back up the intake manifold, diluting the incoming air/fuel charge, and depositing soot on the intake runners, carburetor, etc. Cams with a lot of overlap tend to cause rougher idling because of the lack of vacuum they create in the manifold.
Overlap (lots of duration and tight lobe-separation angles) decreases cylinder pressure, especially at low rpm, which allows an engine to run a higher compression ratio and still work on pump gas. High cylinder pressure, which is caused partly by a high compression ratio, is what makes an engine detonate on pump gas. Decreasing the cylinder pressure by adding duration is just like taking compression out of the engine, but mostly only at low rpm.
Duration has a marked affect on a cam's power band and drive-ability. Higher durations increase the top-end at the expense of the low end. A cam's "advertised duration" has been a popular sales tool, but to compare two different cams using these numbers is dicey because there's no set tappet rise for measuring advertised duration. Measuring duration at 0.053-inch tappet lift has become standard with most high-performance cams. Most engine builders feel that 0.053” duration is closely related to the RPM range where the engine makes its best power. When comparing two cams, if both profiles rate the advertised duration at the same lift, the cam with the shorter advertised duration in comparison to the 0.053” duration has a more aggressive ramp. Providing it maintains stable valve motion, the aggressive profile yields better vacuum, increased responsiveness, a broader torque range, and drivability improvements because it effectively has the opening and closing points of a smaller cam combined with the area under the lift curve of a larger cam. Engines with significant airflow or compression restrictions like aggressive profiles. This is due to the increased signal that gets more of the charge through the restriction and/or the decreased seat timing that results in earlier intake closing and more cylinder pressure. Big cams with more duration and overlap allow octane-limited engines to run higher compression without detonating in the low to midrange. Conversely, running too big a cam, with too low a compression ratio leads to a sluggish response below 3,000 rpm.
Duration generally ranges from 220 degrees for a torquey bottom-end cam all the way to 295 degrees for a “top end rush,” typically measured at 0.053 inch lift.
As a general rule, lower-duration cams in the neighborhood of 210 to 200 degrees at 0.053 work best for stock-type replacement cams. Stepping past 220 degrees of duration (at 0.053) places the cam into the bolt-on, mid-range style category. These cams work well with the stock compression, intake and exhaust. Cams with 240-plus degrees of duration or more are beginning to step into the performance arena and generally work better with other induction, compression, and exhaust modifications. Duration has a marked affect on the cams power band and drivability.
Higher durations increase the top-end at the expense of the low end. As a general rule, cams with 220-235 degrees of duration tend to produce good low end torque. Cams with 235-250 degrees of duration tend to work best in the mid-ranges and cams over 260 degrees work best for top end power.
It is important to remember here that the duration values given are to be used as a general rule and that increasing the duration will have an effect on the idle characteristics and overall drivability.
Long duration, late intake closing cam designs are necessary to drag the last bit of power out of an engine. Unfortunately, these same cams can perform poorly under more normal riding conditions. In the quest for maximum power output, often times Harley owners choose a late closing, high-rpm cam for their engine. The problem with such choices is that the engine seldom spends time in the rpm range favored by such cams.
Cam (or lobe) lift is the maximum height or distance that the lifter or follower is raised off the cam. More lift generally means better top-end power, but you’ll sacrifice bottom-end response. In addition, cams with high lift typical put more wear and tear on the valve train.
Designing a cam profile with more lobe lift results in increased duration in the high-lift regions where cylinder heads flow the most air. Short duration cams with relatively high lift can provide excellent responsiveness, great torque, and good power. But high lift cams are less dependable. You need the right valve springs to handle the increased lift, and the heads must be set up to accommodate the extra lift. There are a few examples where increased lift won't improve performance due to decreased velocity through the port; these typically occur in the race engine world (0.650- to 1.00-inch valve lift). Some late model engines with restrictive throttle-body, intake, cylinder head runner and exhaust flow simply can't flow enough air to support higher lift.
For street bikes, lift figures are best kept at or below 0.500 inch, simply because, with the right cam, you can still get all the power you can use, but you won’t need a new valve train every 20,000 miles. With the right cylinder head/piston combination, lifts in the mid 0.500 inch range, even perhaps encroaching on 0.600 inch can work, but pushrods flex, and the extra benefits of the lift are quashed by the limits of flow through the ports (particularly the exhaust port), so why bother? Mega lift is more valuable to drag racers who re-engineer the whole plot.
Symmetrical cams: This simply means that the cam lobe is the same on both sides. This means that the valve opens and closes at the same rate.
Asymmetric Lobes: In the past, both opening and closing sides of a cam lobe were identical. Most recently, designers developed asymmetrical lobes, wherein the shape of the opening and closing sides differ. Asymmetry helps optimize the dynamics of a valve train system by producing a lobe with the shortest seat timing and the most area. The designer wants to open the valve as fast as possible without overcoming the spring's ability to absorb the valve train's kinetic energy, and then close the valve as fast as possible without resulting in valve bounce. There are many different theories about how to design the most aggressive, stable profile. Hydraulic lifters can provide quiet valve train operation only if the closing velocity is kept below a certain threshold. However, the opening velocity can be higher and still provide quiet operation. Almost all modern hydraulic profiles have some symmetry.
Asymmetric cams allow the valve to open at one speed and close at another. If the cam designer has chosen to set the valve down slowly on the seat it will be a quitter cam.
In the case of single pattern cams both the intake and exhaust lobe are the same. A cam can be asymmetrical and single pattern or symmetrical and single pattern. Dual pattern cams have different profiles on the intake and exhaust lobes. A cam of this type can be any combination of asymmetrical or symmetrical of profiles.
The static compression ratio that your engine displays on paper does not translate directly to higher cylinder pressures. The cylinder pressure (prior to ignition) during engine operation is dependent on what can loosely be called "dynamic or effective compression ratio". The pressure is greatly affected by the timing of your valve events (i.e. cam duration and timing). Specifically, the intake valve closing point is intimately related to an engine's dynamic or "effective" compression ratio.
In principle the piston cannot compress the mixture until the intake valve closes. Thus if the intake valve closes when the piston has already moved quite some distance up the bore, then the amount that the intake charge will be compressed is reduced. The "effective compression stroke" has been reduced.
An engine with a performance cam operating at low RPM will suffer a loss of torque due to the fact that the effective compression ratio is reduced by the late intake valve closing point. However, as the RPM increases "inertia supercharging" becomes important. At high RPMs the intake charge is moving into the cylinder at high velocity. As such it has a lot of inertia and will continue moving into the cylinder past BDC, even though the piston has changed direction and is now moving up the bore (towards the incoming charge). Ideally the intake valve will close just before the incoming air stops and reverses direction. This guarantees that the maximum amount of air/fuel mixture has been drawn into the cylinder prior to ignition. When this happens an engine is said to have "come on the cam". In order to ensure that the mixture is still compressed sufficiently over the reduced effective compression stroke it is necessary to increase the static compression ratio. This is why high performance engines with aggressive camshafts also tend to have high static compression ratios.
Bottom line: Static compression ratio and cam choice should be considered as a system.
A mild cam with an early intake valve closing point will work well at low RPM. But at high RPM the intake valve will close before the maximum amount of air/fuel mixture has been drawn into the cylinder. As a result performance at high RPM will suffer. If a high static compression ratio is used with a mild cam (i.e. and early intake valve closing point) then the mixture may end up being "over-compressed". This will lead to excessive compression losses, detonation and could even lead to head gasket or piston failure.
On the other hand, an aggressive cam with a late intake valve closing point will work well at high RPM. But at low RPM the intake valve will close too late for sufficient compression of the intake charge to occur. As a result torque and performance will suffer. If a low static compression ratio is used with an aggressive cam (i.e. a late intake valve closing point) then the mixture may end up being "under-compressed". Thus a high performance cam with long duration should ideally be combined with a higher static compression ratio. That way the engine can benefit at high RPM from the maximized amount of intake charge afforded by the late intake valve closing, and still achieve sufficient compression of the mixture as a by-product of the dynamic compression ratio.
Note: Parts of this article were taken from the V-Twin Forum posted by user Totenkopf dated 02-29-2008 entitled "Explaining How Cams Work and What the Numbers Mean"