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Camshaft Theory

Camshaft theory questions and answers

Q1.  Should one always try to pick a cam that has as much lift as possible in relation to duration? In other words, given two cams with the same duration, should you always go with the cam that has the most lift per degree of duration? Or is it possible to have too much lift?

A.  Designing a cam profile with increased lift should result in increased duration in the high lift regions where cylinder heads flow the most air.  Short duration cams with relatively high valve lift can provide excellent responsiveness, great torque, and very good power; resulting in a very fun combination to drive.  The main drawbacks to high lift have to do with dependability.   One must chose the proper valve springs to handle the increased lift, and the heads must be set up to accommodate the extra clearance needed.  Valve to piston clearance is dependent on duration and ramp design, not maximum lift; but you should check clearance when making substantial changes to the cam, heads, or pistons.  There are a few examples where increased lift will not improve performance due to decreased velocity through the port, but these are typically in the race engine world (0.650” to 1.000” valve lift.)

Q2.  When comparing similar application cams, is the cam that has its 0.050-inch duration numbers closest to its advertised duration numbers the better cam because it is “quicker?” In other words, can you determine the “better cam” by subtracting 0.050-numbers from advertised numbers; i.e., the smallest remainder after subtracting indicates a better cam because it’s more aggressive?

A.   If the cams being compared rate the advertised duration at the same lift, the cam with the shorter advertised duration compared to the 0.050” duration should be the cam with the more aggressive ramps.  However, manufacturers disagree about where to rate the advertised duration.  Also, there seems to be instances where other manufactures will take liberties with the advertised duration.  Therefore you can not be confident that the difference between advertised duration and 0.050” duration is always a good way to rate the relative aggressiveness of different cams. 

As with higher lift cams, aggressive cams do not always make more power.   If it results in stable valve motion, the more aggressive cam will give better vacuum, better responsiveness, and a broader torque range, along with other improvements in driveability.   This results having the valve opening and closing events of a smaller cam with the area under the lift curve of a larger cam.  Additionally, engines with significant airflow or compression restrictions seem to love aggressive profiles.  This is likely due to the increased signal to get more of the charge through the restriction and/or the decreased seat timing that results in earlier intake closing and more cylinder pressure.  Aggressive ramps allow the valve to reach maximum velocity sooner, allowing more area for a given duration. 

Regardless of the aggressiveness of the profile, the designers must apply much thought and testing to improve engine performance.  The cam is sometimes called the heart or the mind of a performance engine.  Subtle characteristics of the profile shape can be critical to performance.  Both the dynamic behavior of the valve train and the air flow through the engine must be optimized to result in increased performance.  Special proprietary computer software is used to work out ideas, then the prototypes are taken to the lab for dynamic testing and dynamometer verification.  Then we will further optimize the product, using what was learned during testing, until it is ready for in vehicle testing and then released into our product line.

Q3.  What is the difference between advertised duration and “duration at 0.050?” Why measure duration at 0.050 inch? When choosing a cam, should one compare advertised duration, 0.050-inch duration, or both?

A.   Advertised duration is the measure used by manufactures to rate how large a cam will act. However, because of the problems with how advertised duration is rated that are covered in answer 2, it can be very difficult to compare advertised duration numbers between manufacturers.  Typically,the advertised duration for hydraulic cams is rated at .004” or .006” tappet lift.  Some hydraulic grinds are rated at .001” to .002”, and some may be found to be rated as high .010”.  With solids, advertised duration is typically rated at .020” tappet lift, but due to differences in intended lash setting, there are  reasons for manufacturers to vary the rating system on these.  It would not be surprising to find solid cams rated everywhere from .006” to .030” tappet lift.

As with many standards, the “standard” of measuring cams at 0.050” tappet lift came about because it provides useful information and is relatively easy to measure.  Most engine builders feel that the 0.050” duration number is the most closely related to the RPM range where the engine will make its best power.  Also, it is easier to measure the 0.050” duration than the advertised duration because the tappet velocity is much higher after it has had some time to accelerate.  When using a cam degree wheel and a dial indicator there is far less uncertainty about where the degree wheel is oriented when the dial indicator reads exactly 0.050” lift than with lifts in the .004” to .020” range.

Hence, when comparing cams from various manufactures, the .050” duration is likely to be the best measure to assure that your cam will provide good performance in its intended operating range.

Q4.  What effect does advancing & retarding the intake and exhaust lobes accomplish? Why are most cams ground advanced from the factory? And being that they are ground advanced, why do many installers “automatically” install the cam 4 degrees advanced during the initial installation process?

A.  We will first define some terms so we all know what we are discussing.  There are some mutually exclusive definitions for these terms, so we are sure to have guys that disagree with these definitions.  However, we believe these to be the most accepted way to define cam lobe angular locations:

Lobe Separation is defined as ½ the angle in crankshaft degrees of rotation between the event of maximum exhaust valve lift and maximum intake lift.  This equals the angle between the exhaust lobe and the intake lobe on the cam if the intake and exhaust lifter bores are oriented at the same angle from vertical in the engine block.

Exhaust Centerline is defined as the angle in crankshaft degrees between the event of a specific cylinder’s exhaust valve being at maximum lift and that cylinder’s piston reaching top dead center  during overlap.

Intake Centerline is defined as the angle in crankshaft degrees between the event of the specific

cylinder’s piston being at top dead center during overlap and the event of maximum intake valve lift for that cylinder.

Advance is defined as the rotation in crankshaft degrees of the cam lobe centerlines as oriented in the motor from the orientation where both exhaust and intake centerlines would be the same. The resulting relationship between these four definitions is:

Advance = (Exhaust Centerline – Intake Centerline) / 2

Lobe Separation = (Exhaust Centerline + Intake Centerline) / 2

Exhaust Centerline = Lobe Separation + Advance

Intake Centerline = Lobe Separation – Advance

Retard = -- Advance

From these definitions we see that advancing the camshaft moves both the intake and exhaust centerlines resulting in earlier valve timing events during the engine’s cycle.  Typically, engines respond better with a few degrees advance.  This is likely due to the importance of the intake closing point on performance.  Earlier intake closing leads to increased cylinder pressure and better responsiveness.  For this reason COMP Cams and other cam companies grind their street cams advanced.  This allows our customers to install the cam with the standard timing setting and receive the benefits of increased cylinder pressure.

Some engine builders assume the timing chains will stretch or they may prefer even more advance and will therefore install the cams with additional advance.   However, we recommend that our customers install our street cams with about four degrees advance as they are ground.  More simply put, when installing a cam ground with advance, you should probably use the zero advance marks on your timing set.  You can later tailor engine performance by advancing the cam to increase low RPM torque or retarding the cam to increase top end power.

Q5.  What effect do changes in lobe separation (lobe displacement angle) have? What does narrow LDA give you? Wide lobe centers? Is there an “ideal” LDA for a particular engine operating range, or is it more a function of the efficiency of the cylinder head design?

A.  These are generalities with exceptions, but narrow lobe separations tend to increase midrange torque and result in faster revving engines, while wide separation result in wider power bands and more peak power but somewhat lazier responding engines.  However, the engine configuration plays an important role as to what is wide and what is narrow.  Longer stroke engines may need the wider separation to maintain good

power output at high rpm, while shorter stroke engines may respond better to tighter separations to accentuate the faster revving ability and add needed torque.

Longer rod to stroke ratios dwell the piston longer near top dead center and hence tend to make the engine act like it would if overlap was increased. With increased rod length, some engine builders prefer one or two degree wider separation combined with slightly shorter duration to maintain similar overlap characteristics.  However, as RPM has increased over the years, duration and lobe separation has crept up in general.

For street applications wider separations give smoother idle, more vacuum, and increased efficiency, while narrower separations give more midrange and faster acceleration.  The best compromise may depend on the induction system.  Typical dual plane manifold and carburetor equipped engines tend to want separations between 110 and 112 degrees, and fuel injected applications tend to want slightly wider separations from 112 to 114 degrees.  Fuel injection does not require the signal during overlap that carburetors need to provide correct fuel atomization, and most computer controllers require the increased vacuum at idle resulting from decreased overlap.  Bracket race type combinations with higher stall speed converters, high compression, single plane manifolds, and large carburetors usually want separations between 106 and 110 degrees.  Engines equipped with Blowers, Turbochargers, or that are used primarily with nitrous typically will work best with wider separations in the 110 to 116 degree range.  Some air cooled applications such as Harley’s, and some Porsche’s and VW’s need very tight separations in the 98 to 106 degree range for increased overlap that provides more fuel to cool the engine.

Q6.  How does lobe separation affect camshaft overlap, and how does overlap effect overall engine performance?

A.   Given the same duration, separation and overlap are inversely proportional.  Hence, as we increase the lobe separation, we decrease overlap.  More overlap decreases low RPM vacuum and response, but overlap will improve the signal provided by the fast moving exhaust to the incoming intake charge in the midrange.  This increased signal probably results in the improvement in engine acceleration, which drivers often notice.  Less overlap will increase efficiency by reducing raw fuel escaping into the exhaust and improve low-end response due to less reversion of the exhaust gasses into the intake port.  Due to the differences in cylinder head, intake, and exhaust configuration, there is a great deal of sensitivity to the overlap region of the camshaft.  Not only is the duration and area of the overlap triangle important, but the shape of the triangle can also be critical.   The details for optimizing the shape of the overlap triangle can get fairly deep and speculative, but test results show that engines can be very sensitive to this shape.  The most critical factors to understand for optimizing this region are the following:

1.  How well the heads flow from the intake toward the exhaust with both valves opened slightly.

2.  The efficiency of the intake system

3.  The efficiency exhaust system.

Through thorough testing, we have made many advances in this region.

    See answer 5 for more on the effect of lobe separation.

Q7.  What affect does an early opening intake lobe have on performance? A late closing intake lobe?        

A.  The intake opening point is critical to vacuum, throttle response, emissions and gas mileage.  Premature intake opening can allow exhaust gasses back into the intake manifold,  thereby hurting the intake pulse velocity and contaminating the charge.  As RPM increases, the exhaust scavenging and air demand should be greater.  This results in the intake needing to be opened earlier for high rpm applications, allowing more time for the intake charge to fill the cylinder.  Additionally, the earlier opening, larger duration cam designs allow for more area under the curve than similar profiles of lesser duration.

The intake closing point starts the compression stroke.  The earlier it occurs, the greater the cranking compression.  Early intake closing is critical for low-end torque and responsiveness.  As RPM increases, the intake charge momentum increases.  This results in the intake charge continuing to flow into the combustion chamber against the onrushing piston far past bottom dead center.  Hence, the higher the RPM of operation, the later the intake closing should be to ensure all the charge possible makes it to the combustion chamber.  What we really desire for proper intake closing is the following:

1. To occur just as the air stops flowing into the chamber.

2. To get the valve seated quickly and not waste time in the low lift regions, where very little airflow can occur, and no compression is building in the cylinder.

3. Not to be so fast that the valve bounces as it closes, allowing the charge to escape back into the intake port and disturb the next charge.

4. In hydraulic street applications, it is required that that the cam’s closing ramps are not so fast that they result in noisy operation that might annoy occupants of the vehicle.

Working towards those goals for many differing applications has required the large number of lobe profiles we have developed.  We are always trying to get as much charge into the cylinder as possible and keep it there for combustion.

Q8.  What affect does an early opening exhaust lobe have on performance?

And a late closing exhaust lobe?

A. Opening the exhaust valve too early will decrease torque by bleeding off the cylinder pressure from combustion that pushes the piston down.  But, the exhaust has to open soon enough to provide enough time to properly scavenge the cylinder.  Additionally, the earlier the exhaust opens the more pronounced the resulting engine sound, due to the continued combustion of the passing mixture.

Closing the exhaust too late is very similar to opening the intake too soon in that it leads to increased overlap, allowing reversion into the intake system.  Later exhaust closing events can help clean the combustion chamber from spent gasses and provides more signal to the intake system at high rpm.  However, late exhaust closing will allow some of the new intake charge to escape out the exhaust system, decreasing fuel mileage and increasing emissions.

Q9.  Putting questions 7 and 8 together, what is the overall desired relationship between the initial intake/exhaust opening and closing points as far as their total performance effects go?

A.  Combining what we have stated about the opening and closing events individually, we clearly see that as the intended RPM range increases, the seat timing points need to be moved out for earlier opening and later closings.  Aggressive cams can provide the area of larger duration profiles without the increased seat timing.  This can provide more low-end responsiveness while still providing the increases in power of the large cam due to the increased airflow capacity.  Looking back to question 2, this may provide more insight.

Q10.  What is the purpose of an asymmetric lobe profile? When is it needed? How do you know if the lobe is asymmetric (so you can use the appropriate cam degreeing method)?

A.  There are three reasons designers choose to make asymmetrical lobe designs.  The first is to try to optimize the dynamics of the valve train system.  The second is to optimize the airflow of the engine.  The third is to reduce noise in a hydraulic system while minimizing seat timing.

Optimizing the dynamics of the valve train system is necessary when one wants to design 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 adsorb the kinetic energy of the valve train, then close the valve as fast as possible without resulting in valve bounce.   Because the valve train is a complex system, there are many different theories about how to design the most aggressive, stable profile. However, very few believe that the opening and closing sides should be exactly the same.

Optimizing airflow can be even more complex.  Some cylinder head configurations may favor a slow opening intake or may need a slower closing exhaust to optimize airflow.  In many cases the cam designers may want to skew the timing points around to try to increase the power potential of the engine, thus requiring a asymmetric design. Lastly, hydraulic lifters will provide quiet operation if the closing velocity is below a certain threshold.  However the opening velocity can be higher and still provide quiet operation.  Hence, almost all modern hydraulic profiles are asymmetric.

At COMP Cams, we have over three thousand designs.  A vast majority of these are asymmetric.  However, the asymmetry is usually slight enough that any method of degreeing the cam should be effective.  We recommend the intake centerline method because it is simple, very repeatable, and not effected by the asymmetry of the lobes.

Q11.  What is the purpose of a dual-pattern cam? How do you know if you need a dual-pattern cam?

A.  Until very recently, dual-pattern street cams used the same family of profiles on both the intake and the exhaust, with a slightly larger profile on one than the other.  Typically, dual pattern cams are ground 6 to 12 degrees larger on the exhaust than the intake.  The theory is that the larger exhaust will allow the dual pattern cam to make similar high RPM horsepower to a somewhat larger single pattern cam.  The smaller duration intake should also result in better off idle response than with a single pattern cam.  However, the single pattern cam should make more midrange torque and power than the dual pattern.  As for which is better, it is more a matter of opinion than fact for most applications.  Some applications with sufficient restrictions will definitely favor the dual pattern.

Now we have rethought the whole question of what we can change with dual pattern cams.  Not only can we change the duration between intake and exhaust, but we can also optimize each profile for the task at hand.  The new Xtreme Energy series by COMP Cams has individually optimized profiles for use on the intake and exhaust. The intake profiles minimize seat timing and maximize area.  The exhaust profiles promote excellent scavenging, increased signal, and maximum airflow.  By focusing on the requirements of each lobe individually instead of just playing with duration we can develop real gains with dual pattern cam designs.

Q12.  Some engines have a larger lifter foot diameter than others. For example, most GM applications have a 0.842-inch foot diameter, Fords use 0.875-inch, and Chryslers use 0.904-inch. How is having a larger foot diameter an advantage? If limited to a flat-tappet cam, do engines with the bigger lifter foot have an inherent advantage?

A.  With flat-tappet camshafts, the maximum velocity allowed by the tappet, before the contact point between the tappet and cam goes off the edge and causes failure, is directly proportional to the tappet diameter.  Hence, a larger tappet will allow the use of a profile with higher maximum velocity.  A profile that is designed with higher maximum velocity can have more area and more lift for a given duration than a similar profile with less maximum velocity.  However, that higher velocity profile will require a better valve spring than the lower velocity design to maintain stability at the same RPM.  Also, by designing the valve motion around the rocker ratio, the same valve motion can be accomplished with a smaller lifter and more rocker ratio as can be accomplished with a larger lifter and a standard ratio.  Hence, the larger lifter foot diameter can be an advantage, but only if the profile is designed for that lifter diameter, the correct valve springs are used, and the smaller lifter can not be used with more rocker ratio.  There are also considerations of weight and wear resistance that come into play when considering what is the optimum lifter diameter.

Q13.  When is a hydraulic lifter preferred? A mechanical lifter? A solid roller lifter? A hydraulic roller lifter? What are the advantages and disadvantages of each?

A.  Hydraulic flat tappet camshaft and lifter systems are the most popular configuration for street applications.  They provide quiet operation, low maintenance, easy installation, great response and good power.  This wins out for ease and economy with very little compromise.

Solid lifters provide for a stiff system that can more easily maintain control at high RPM. With hydraulic lifters there is a designed lack of stiffness.  At high RPM this can lead to rapid loss of power due to valve bounce.  Below 6000 RPM there is very little advantage to solid designs.  Above 7000 RPM it is much easier to design around solid lifters.  As designs for hydraulic tappets and lobes evolve, the line between where hydraulics and solids have the advantage becomes wider and more blurred, but solid profiles should perform better at high RPM.  There is also an issue about the maintenance of checking the lash settings for solid lifter street applications, but that can be minimized with good rocker adjustment locking devices.

Solid roller lifters allow much higher tappet velocities than typical flat tappet lifters.  The maximum safe tappet velocity for an 0.842” flat tappet is less than 0.007” lift per cam degree of rotation.  Roller lifters can allow tappet velocities over 0.010” lift per cam degree of rotation.  Solid rollers also allow for the spring forces necessary to maintain control with these very aggressive designs.  Solid rollers are limited by how quickly they can get the valve off the seat because of geometrical limitations.  As they are designed for quicker acceleration off the seat, the ramps become inverted.  There is a limit to how inverted the ramps (a.k.a. flanks) can be before they become too difficult to grind.  However, the lack of acceleration is more than made up for by the increased area allowed by the much higher velocities.  Lifter wear used to be the main drawback, but new lifters are being introduced that will provide greatly increased durability.  Now cost is the main drawback.

Hydraulic roller lifters provide many of the same advantages as solid roller lifters.  However, they are limited by RPM even more significantly than hydraulic flat tappet designs.  It is very difficult to get the advantages of the more aggressive potential of hydraulic roller designs and maintain stability over 6500 RPM without relying on very high spring forces.  As the tappet designs improve, this line will move up and broaden just as it has with hydraulic flat tappet designs.  However, cost will still be a drawback unless the engine came with hydraulic roller lifters.

Q14.  Changing the rocker ratio affects lift at the valve. If the cam with the most total lift is generally preferred, why not always use the highest ratio rocker available? Are there any disadvantages to high-ratio rockers?

A.  There is a limit to how fast you can move the valve and how quickly you can accelerate it before the valve spring cannot control the system.  If a profile was a good design for 1.6:1 rockers, chances are it will be unstable with 1.8:1 rockers.  We use high rocker ratios very effectively with profiles designed around them, but they are not an effective cure-all for every profile.  The correct procedure is to design the desired valve motion, then back out the required tappet motion.  From there, one may use subtle changes in rocker ratio to optimize the system, but going up more than a point in rocker ratio will usually be unsuccessful.

Q15.  Cylinder pressure versus octane versus compression ratio: Is it possible to quantify or know in advance how high a compression ratio you can run based on cranking cylinder pressure? Along the same lines, will installation of a radical cam allow you to run higher compression on pump gas because with greater overlap, the radical cam has less cranking compression pressure? If all this is true, why not list the typical cranking compression pressure developed by your profiles?

A.  Unfortunately, I do not have a good answer to this question.  As you are probably aware, octane requirement has a great deal to do with combustion chamber shape, temperature, fuel mixture, spark advance, rod length, piston configuration, the cam profiles, and the air flow potential of the engine.

To get a little deeper into your question, a more aggressive cam will result in more cylinder pressure for a given duration at 0.050” because of the reduced seat timing and increased airflow.  Hence, if one had an 11:1 compression 350 Chevy with a soft ramp 240 @ .050” lift cam and had detonation problems, a more aggressive 240 @ .050” cam would only aggravate the problem.  However, a larger duration cam should reduce the knock under a load in the low to mid-range operating range.

This leads to the question, “why do cam manufacturers often advertise a certain minimum compression for a given camshaft?”  The answer has to do with responsiveness.  In the NASCAR Craftsman Truck Series, the rules limit compression to 9.5:1.  However, the typical cams are over 260° @ .050” and can be as large as 280 degrees @ .050”.  Most cam catalogs would not recommend a cam much larger than 240 degrees @ .050” with that low of compression for street use.  The discrepancy lies in the intended operating range of the two vehicles.  Most street vehicles need to provide decent responsiveness by 3000 RPM.  However, the Super Trucks operate between 6000 and 9000 RPM.  At high RPM the momentum of the air column can make up for some of the losses due to reduced compression, but at low rpm, the late closing of the intake can only be offset by increasing the compression.  Hence, to provide decent responsiveness with a large cam it is necessary to increase the compression.

Q16.  How do changes in connecting rod length (rod ratio) affect camshaft selection?

A.  Longer rods increase the amount of time the piston dwells near top dead center.  It also can increase the signal by accelerating the piston faster after it moves away from top dead center.  The increase in effective overlap and the increase in signal to the intake charge may result in a slightly shorter duration cam with a slightly wider lobe separation being optimum.  If a circle track racer goes from a 6.000” rod to a 6.125” rod, he might want to try stepping down 2 to 6 degrees on duration and going out a degree or two on lobe separation.

Q17.  How do changes in stroke affect camshaft selection?

A.  Longer stroke engines create more torque and do not typically operate at as high of RPM.  Hence, to operate at the same engine speed with two engines of the same displacement and different strokes, the longer stoke engine should require a larger duration camshaft. Slightly wider lobe separations will also help longer stroke engines at high RPM by broadening their operating range.

Q18.   How do changes in engine displacement (within the same engine family) affect camshaft


A.   Larger displacement engines will need larger duration cams to provide airflow for the increased displacement for operation at the same engine speed.  Also, the stroke is often increased, resulting in the larger duration and wider separation from answer 17.

Q19.  How does a street exhaust system impact proper camshaft selection?

A.  With the advances in exhaust system technology, the detrimental effect of a good street exhaust system is secondary at worst.  If there were anything to change, it would be to be further concerned with not over camming the engine.  Larger cams can provide more power through open exhaust that may not be supported by the exhaust system.  However, the exhaust system will generally support, and may even enhance, the increased torque of a smaller cam.  Hence, in high horsepower street applications, one should not trade 60 ft lbs of torque in the midrange (that the exhaust will not diminish) for 50 HP at high RPM (that may not be supported by the exhaust system.)

Many people will increase the exhaust duration, but the restriction is far enough downstream that the exhaust pulses may be stacked up enough that increasing the pulse width may do very little toward increasing the total flow.  When the restriction is closer to the exhaust valve, increasing the exhaust duration may be much more effective.

Q20.  How does a catalytic converter and/or computerized engine management impact proper cam selection? How radical a cam can you put in a computer car without screwing up the computer? 

A.  Converter considerations are similar to typical exhaust considerations except too much exhaust duration may lead to raw fuel burning out the converter.

Computer considerations depend highly on the computer and the modifications.  You will be safe if you keep the valve events close to their stock locations but increase the area substantially.  Most computers will just be happy that the air happens to be extraordinarily good and operate accordingly.  Some OEM computers (like on GM LT1’s, LT4’s and Mass Air 5.0L Mustangs) will accommodate substantial changes, but it is best to discuss your application with a reputable manufacturer in detail.

Aftermarket fuel injection systems vary as to how much cam they can handle.  Some are fully programmable and can be used with any camshaft.  With these systems, we have seen that it may

be better to decrease the duration slightly and increase the separation slightly when swapping over from a carburated system to a fuel injected system.  This may be because we were using the extra overlap to band-aid some of the problems inherent in the carburated system with additional signal from increased overlap.

Q21.  Mechanical versus hydraulic profiles: It is my understanding that you cannot directly compare duration and lift of a mechanical and hydraulic profile. The mechanical cam’s required lash clear­ance actually means that real-world performance of the mechanical cam is less than its “paper” specs would indicate. Therefore, how would one compare these two dissimilar types of cams?

A.  Typically one would not use a solid cam in place of a hydraulic, or visa versa.  However, the solid should act a bit smaller as you stated in the question.  Unfortunately, how much is a difficult question because you are never exactly sure how much the hydraulic lifter compresses when loaded by the push rod.  If we assume a hydraulic tappet compresses .004”, then compare aggressive 240 duration @ 0.050” solid and hydraulic designs, we see the differences in figure 21.

Q22.  Installing high-ratio rockers obviously changes (increases) valve lift. But does it also change the cam’s effective duration. If so, how much (is there a simple formula or equation for this as there is for figuring the total valve lift)?

A. There is no real formula, but you would be safe to assume that you increase the duration about two degrees for each point of rocker ratio. However, with the increase in area, it may act as if the cam grows four degrees in duration per point of rocker.  Hence, we often find people dropping back as much as ten degrees in duration when going from 1.6:1 to 1.8:1 rockers; still maintaining their horsepower while dramatically increasing torque.

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