In last month's Motor Series installment, we discussed maximizing engine efficiency by optimizing an engine's compression ratio, combustion process, and cylinder sealing. This month, we'll focus on camshafts. What does a camshaft do? Camshafts are a fundamental ingredient to engine performance for two reasons: First, they improve airflow through the engine by optimizing the induction and exhaust cycles. Second, cams act as the engine's brain and personality because they control the timing and duration of most major events during engine operation. Regardless of whether your engine is small or large in displacement, camshafts are the principal means by which you can tune the engine's horsepower and torque curve to give it the personality you desire. If you want to increase low-end torque to launch a heavy bike from a stop, or top-end horsepower for greater top speed, choosing the appropriate cam configuration will help you achieve the desired performance results. But keep in mind that the bigger-is-better theory does not apply to camshafts as it typically does to displacement.
The primary factors for cam selection are engine displacement, mechanical compression ratio, and the most important: rpm range. However, other factors such as induction and exhaust airflow, stroke, rod length, rod-to-stroke ratio, gearing and total bike weight also come into play. Once these factors are known, camshafts with appropriate variables can be selected. Common camshaft variables include valve lift, duration, overlap, and the timing of the valve events (points at which the valves open and close). In the case of racing camshafts, rate of lift is also a factor. Nevertheless, these variables are critical in determining not only an engine's maximum power output but also the shape of its power curve.
Each camshaft includes one or more elliptical lobes welded or machined to the shaft. The purpose of a lobe is to open and close an intake or exhaust valve. The shape and location of the lobe on the shaft determines when the valve opens, how high it lifts, when it closes, and how rapidly the valve movement takes place. For Harley V-Twin engines, which have four valves (two per cylinder), there are four cam lobes: two intakes and two exhausts. The Sportster engine has four camshafts, each with one lobe, while the Twin Cam engine has two camshafts with two lobes each. Conversely, the Big Twin Evo, Shovelhead, Panhead, and Knucklehead engines have one camshaft with four lobes. Although the V-Rod engine is somewhat similar to the Sportster in that it uses four camshafts, it requires eight lobes (two per shaft) because it has four valves per cylinder or eight valves total.
As an aside to this discussion, it is interesting to note that the V-Rod engine uses overhead camshafts, which places the cams in the rocker box area located above the head and valves. In contrast, all other previously mentioned Harley V-Twin engines are pushrod engines where the camshaft(s) is placed near the crankcase area, which is below the valves and cylinder head. Pushrod engines include a valvetrain, consisting of tappet (also called lifter), pushrod, and rocker arm, to transmit camshaft lobe action to each valve.
Although true performance differences between two cams can only be determined by dyno or trackside testing, a cam's duration and valve opening and closing points can offer some hints about its performance characteristics. With that thought in mind, let's review some key cam terminology along with some supporting information you should know when selecting a cam. To simplify the following discussion, we will assume we have a pushrod engine with one camshaft.
* Valve lift
* Lobe Separation Angle (LSA)
* Single or Dual Pattern
* TDC Lift
* Rocker Arm Ratio
* Opening/Closing Ramp Aggressiveness
* Solid or Hydraulic Lifters
**Table 1 **A single-pattern cam has the same amount of duration and lift for the intake and exhaust valves. In contrast, a dual-pattern cam has a different amount of duration and/or lift for the intake and exhaust sides.
Duration is the amount of time a valve is opened and is measured in crankshaft degrees. You can determine the duration for the intake or exhaust valve by adding together the opening and closing values, and then adding another 180 degrees for the distance the crank moves between TDC and BDC. For example, if a cam lobe opens an intake valve at 26 degrees and closes it at 52 degrees, the valve would be opened a total of 258 degrees (26 + 52 + 180 = 258).
Keep in mind, however, to be consistent and accurate in calculating valve timing, you have to use the same measuring point in all calculations. Unfortunately, some cam manufactures use different measuring points, so accurate comparisons may be difficult. Here's what you need to know to keep out of trouble when comparing cam timings: To minimize cam lobe wear and noise, subtle opening and closing ramps are ground onto the lobes to slowly and gently start the opening and closing process. Valve opening starts slowly when the tappet contacts the ramp, and then accelerates. Movement slows again as the valve reaches full open then accelerates again before the closing ramp slows movement down.
Since it is difficult to accurately measure valve opening and closing points when the tappet is on the slow moving ramp, manufacturers take measurements when the tappet is beyond the ramp (or most of it) and the tappet is moving quickly. In the Harley world, valve timing is most commonly taken when the cam lobe raises the tappet 0.053-inch. Some cam manufacturers measure at 0.020-inch tappet lift. Regardless, when comparing cam specs, be sure the numbers were taken at the same tappet lift.
Duration determines the location of the engine's power band and its maximum potential rpm. Generally, duration is a function of engine displacement and operating rpm. All else being equal, increasing duration moves the points at which peak torque and horsepower occur to a higher rpm. Basically, as duration is increased, high-rpm horsepower increases at the expense of low-speed torque. Of course, the reverse is also true when decreasing duration.
Lift is the distance the valve is raised off the seat and is measured in thousandths of an inch. Higher valve lift does not change the engine's power curve. Instead, it generally adds power throughout the entire curve as long as port flow increases at the higher lifts. In general terms, valve lift helps generate torque and horsepower, but the speed at which the valve is raised to full lift is also a factor. Increasing lift is a good way to increase power without significantly reducing low-speed performance. But remember that some cylinder heads stop flowing more air as lift is continually increased. In this situation, a head responds more to duration than lift. From a reliability standpoint, there needs to be some relationship between lift and duration, since only increasing lift without an accompanying increase in duration can produce high valve acceleration rates, which can increase valvetrain wear and reduce reliability. Cams with very high lift and relatively mild duration are typically used to maximize low and midrange power or used with low-compression engines.
The optimum valve lift is generally determined by the cylinder head design. This is one reason a cam should be matched to not only the displacement of the engine and its application, but also the cylinder head. When a valve lifts 25 percent of its diameter, the area of its orifice is equal to the area of its diameter. For this reason, the ideal valve lift is usually some ratio of the valve's diameter. Stock engines usually have a maximum lift of about .25 of the intake valve diameter while racing engines are usually .30 to .35 of valve diameter. Up to about .15 of valve diameter, airflow is primarily controlled by the valve and seat area. At higher lifts, however, it is usually limited by the capacity of the port and sometimes by the shape of the valve or combustion chamber.
When installing a high-lift cam, it is critical that the engine is properly set up to accommodate the increased valve lift. Cylinder heads and the right-side crankcase half may require machining. Longer valve stems, stronger valve springs, and spring spacing may be needed. Also, valve-to-valve and valve-to-piston clearances must be checked.
Overlap is the number of crankshaft degrees where both the intake and exhaust valves are opened simultaneously at the end of the exhaust stroke and beginning of the intake stroke. Overlap is calculated by adding the intake valve opening timing (before TDC) to the exhaust valve closing timing (after TDC). For example, the Andrews 64G cam listed in Table 2 has an intake opening of 30 degrees before TDC and an exhaust closing of 30 degrees after TDC. Therefore, the overlap period is 60 degrees (30 + 30 = 60).
The amount of overlap and where it occurs will have a major impact on the engine's ability to make power. If optimized to the engine combination, the overlap period can improve exhaust scavenging and improve cylinder filling with fresh fuel mixture. Overlap is important because the exhaust headers can be tuned to take advantage of the overlap period for improved scavenging and the idle quality is affected by the amount of overlap. A large overlap produces a lopey idle and works best with free-flowing open headers at high rpm. At lower engine speeds, however, it allows exhaust reversion into the intake tract, reducing idle smoothness and low rpm performance. To maintain low-rpm power, it is best to use the least amount of valve overlap consistent with high rpm power requirements and minimize overlap with a restricted exhaust system.
Cam lobe centerline is another factor in camshaft design. The lobe centerline is an imaginary line that simultaneously passes through the point of maximum lift on the lobe's nose (tip) and the camshaft's center of rotation (axis). The point at where the actual centerline of the intake lobe occurs in relationship to TDC position of the piston is defined in degrees of crankshaft rotation after top dead center (ATDC).
The intake lobe centerline is calculated by dividing the intake duration by 2, then subtracting the intake valve opening. For example, using the Andrews 64G cam specs in Table 2, it has 272 degrees intake duration and the intake valve opens at 30 degrees BTDC and closes at 62 degrees ABDC. This would result in an intake lobe centerline of 106 degrees: 272/2 = 136, 136-30 = 106. Dividing the exhaust duration by 2, then subtracting the exhaust valve closing calculates the exhaust lobe centerline. Again using the Andrews 64G cam specs, the equation would look as follows: 276/2 = 138, 138-30 = 108. The values of the cam's intake and exhaust lobe centerlines are used in the calculation of another specification called Lobe Separation Angle.
Lobe Separation Angle (LSA)
A major specification many racers note when selecting a camshaft is lobe separation angle (LSA). Lobe separation angle (also known as lobe displacement angle) is the distance measured in camshaft degrees between the centerline of the intake lobe and the centerline of the exhaust lobe for the same cylinder. Lobe separation angle is related to lobe centerline and both may be the same value, but they are not the same because they refer to different reference points.
As an example, LSA is one of the few occasions where cam specifications are specified in cam degrees instead of crankshaft degrees. Cam degrees are different from crankshaft degrees since the cam turns at half the speed of the crank. This results in twice as many crankshaft degrees for a given number of cam degrees. Additionally, unlike lobe centerline, LSA is ground into a cam and cannot be changed without regrinding the cam.
The importance of Lobe separation angle is that it has a direct relationship on overlap. Essentially, valve duration and cam lobe separation angle (LSA) determine the overlap period. When lift is added to the duration and LSA factors, the "overlap triangle" is defined. The overlap triangle represents a more accurate way of viewing overlap than only considering the amount of overlap duration. For a given LSA, the greater the duration and lift, the greater the overlap triangle will be. In addition, the wider the LSA (more degrees), the less overlap there will be. In contrast, the narrower the LSA is the greater the amount of overlap. However, keep in mind that two camshafts having the same duration and lift figures can be ground with different lobe separation angles, which results in different amounts of overlap.
Although there are exceptions for any rule, a tighter LSA usually improves midrange torque and usually results in a faster revving engine. A tight LSA also tends to produce a narrower powerband since the torque and horsepower peaks usually occur closer together. An engine with a narrow powerband usually requires higher ratio gears for optimum acceleration. In contrast, a wider LSA produces a broader powerband and more peak power. A wide LSA also improves idle quality and fuel economy while reducing exhaust emissions.
Overlap: How much is needed?
Overlap can have both positive and negative effects on engine performance. The ideal amount of overlap is related to engine displacement, rpm, and intake/exhaust flow characteristics during the overlap period. Exhaust system tuning also plays an important part because it has a major influence on airflow during overlap. Generally, the greater the engine displacement, the more overlap required for a given rpm band. Additionally, the higher the engine's intended rpm band, the more overlap required for optimized performance. Low-lift valve flow is another factor influencing overlap because the valves are only slightly off their seats during the overlap period. More overlap is usually beneficial when the cylinder head has poor low-lift flow, the combustion chamber has poor cross-flow during the overlap period or the valve opening acceleration rate is slow. An engine with a small carb or large lazy-flowing intake ports generally responds well to more overlap. These conditions generally work best with a tighter LSA.
Long overlap typically is what gives an engine the cool-sounding lopey idle that many riders desire. If matched properly to the engine combination, overlap can improve cylinder scavenging and power. Yet excessive overlap can over-scavenge the cylinder, reducing power and fuel economy. At low engine speeds, a large amount of overlap can allow reversion of exhaust gases into the combustion chamber and intake manifold, causing rough idling, poor throttle response, and increased exhaust emissions. Increased overlap also reduces vacuum in the intake tract at low-engine speeds, which can result in poor fuel atomization and throttle response. The benefits of overlap are maximized when combined with a tuned exhaust system. A long overlap period and poorly tuned exhaust system can produce significant dips throughout an engine's power curve.
Engines such as the Shovelhead, which have a high-dome piston and obstructed combustion chamber, typically have poor combustion chamber cross-flow and require a cam with a greater amount of overlap for optimized performance. If you were to compare the amount of overlap for Shovelhead, Evolution, and Twin Cam engines, you would notice that Shovelhead cams generally have the most overlap, while cams for the Twin Cam engine have the least. The reason for this is that differences in combustion chamber design along with intake and exhaust flow characteristics have a significant effect on the optimum amount of overlap. Greater overlap usually improves performance when the intake charge cross-flow is obstructed, or the intake and exhaust ports are extremely large for the engine combination and inlet flow is lazy and slow moving. Less overlap is needed when port velocity is high or the exhaust-to-intake flow ratio at low lift is high. Additionally, for maximum fuel economy, reduced emissions and best low-rpm torque, keep overlap to a minimum.
Another consideration is that the benefits of overlap are very rpm-specific. Consequently, it is easier to design a cam for a race engine that operates over a narrow 2,000-rpm band than it is for a street engine that operates throughout a 5,000-rpm or larger band. A long overlap is typically associated with long duration and is best suited for high-rpm power and tuned exhaust systems. In the case of a street engine, a long overlap accompanied by long duration and a late closing intake valve will kill low-speed performance. In general, when selecting a cam it is best to keep overlap to the minimum that will achieve your power goals, and avoid cams with excessive amounts of overlap.
Changing the timing of a cam's opening and closing events can significantly change the engine's power curve. Of the four opening and closing valve events, the intake valve closing (IVC) has the greatest effect on engine-operating characteristics. The IVC begins the point where all valves are closed and starts the change from the engine's intake cycle to the compression cycle. All engine power is made when the valves are closed and cylinder pressure can build on the piston. All other things being the same, the earlier IVC occurs, the greater the cranking compression and low-rpm torque. As rpm increases, intake charge inertia increases and generates a "ram effect." Closing the intake valve later takes advantage of the ram effect at high rpm and increases cylinder filling for greater power. The higher the engine's operating rpm, the later the intake valve should be closed to maximize cylinder filling at high speed. However, the tradeoff is that low rpm cylinder filling will be reduced, thereby reducing low speed performance. Note the IVC when comparing cams with similar specifications. All else being equal, the later the closing, the higher the rpm peak torque and horsepower will occur. Performance benefits are sometimes realized by designing a cam with a different amount of duration and valve timing for the front and rear cylinders.
A cam can be advanced or retarded by removing the cam drive gear and pressing it back on at a slightly different position. This procedure advances or retards all intake and exhaust events on the V-Twin engine. To eliminate the hassle of removing the cam drive gear, some manufactures offer a multi-index cam shaft with multiple keyways machined into the cam, which allows advancing or retarding the cam about 4-degrees. Although advancing a cam advances all valve-timing events, it most importantly closes the intake valve earlier, which increases low rpm cylinder pressure and usually improves low speed torque at the expense of high-rpm horsepower. As can be expected, retarding a cam closes the intake valve later and improves top-end horsepower at the expense of low-speed torque. It is best to make sure the carburetion, ignition timing, gearing, and exhaust systems are already tuned before experimenting with cam timing.
IVC and Compression
An engine's mechanical compression ratio has an important relationship to intake valve closing. Mechanical compression ratio is a mathematically calculated number based on cylinder and chamber volumes. However, IVC occurs when the piston is past BDC and moving up the cylinder on the power stroke. Therefore, cylinder volume is less at IVC, which means the engine's compression is less than the mechanical compression ratio. Calculating compression and taking into account the reduced cylinder volume at IVC is called the corrected compression ratio.
In general, an optimized Big Twin engine will run on 91-octane pump gas without encountering detonation with a 9:1 to 9.2:1 corrected compression ratio. In some cases, you can go slightly higher, maybe up to a 9.5:1 corrected compression ratio, but the combustion chamber design, engine assembly and tune-up must be spot-on. Moreover, keep in mind that an engine with a 10:1 mechanical compression, long-duration cam, and late IVC could potentially have a lower octane requirement than a 9.5:1 compression engine with a short-duration cam and early IVC.
Considering the IVC of most street cams, achieving a corrected compression around 9.2:1 generally requires a mechanical compression ratio in the range of 10:1 to 10.8:1. By comparison, a race engine with a 16:1 mechanical compression ratio will have about a 13:1 corrected compression ratio and will run without detonation if designed correctly and fed race gas.
Calculating corrected compression is helpful when installing a long duration cam with a late-closing intake valve because knowing corrected compression will allow the engine builder to optimize the mechanical compression ratio for the best low-rpm performance.
Camshaft Selection Factors
- Application and Riding Style
- Engine Displacement
- Mechanical Compression Ratio
- Important RPM Range
- Total Bike Weight
- Drivetrain Gearing
- Induction System Design
- Exhaust System Design
- Stroke Length
- Rod Length
- Rod/Stroke RatioTable 4
Although many factors must be considered for optimized cam selection, some of the most important are engine displacement, compression ratio, gasoline octane, gear ratios, total vehicle weight, and most importantly rpm range. Selecting too large or small of cam will maximize horsepower and torque at the wrong rpm. Maximizing horsepower and torque in the correct rpm range is more important than high peak-power numbers.
When choosing a cam, start by matching duration, lift, overlap, and the IVC timing to the engine's application and parts combination. However, keep in mind that cams with similar catalog specifications may perform differently due to slight differences in lobe profile and opening and closing ramps. The following are additional cam selection tips for normally aspirated engines. Power-adder applications such as nitrous oxide, turbocharging, and supercharging require additional considerations that should be discussed with your cam manufacturer.
Duration Considerations: An engine with large displacement will usually run well with more duration than a smaller engine. A heavy bike and/or one with low numerical gearing requires a cam that will provide strong low and midrange power to help the bike accelerate. In contrast, since a lighter bike requires less low and midrange power to accelerate, it can tolerate a longer duration cam and later closing intake valve while still having acceptable acceleration. Keep in mind that for a given engine displacement, increasing duration moves the power band to a higher rpm, improving top-end horsepower at the expense of low-end torque. And the same holds true for closing the intake valve later.
Compression Considerations: For pump-gas engines, which are octane limited, remember that the intake valve closing and mechanical compression ratio must be coordinated to eliminate detonation. The engine's mechanical compression ratio also has a relationship with the exhaust valve opening. Since a higher compression engine has increased cylinder pressure and faster combustion speed, it can benefit from an earlier opening exhaust valve. In the case of a lower compression engine, the exhaust valve opening should be delayed to extend the time available for cylinder pressure to react on the piston. Increasing the mechanical compression ratio when increasing duration or closing the intake valve later will minimize the loss of low-speed torque.
Airflow Considerations: The cylinder head's exhaust-to-intake (E/I) airflow ratio can provide clues to matching the cam to the head. For example, a head with a good intake port and excellent E/I ratio (high 70-percent range or greater) would probably do well with a single-pattern cam profile (assuming valve lift is optimized to port flow) unless the exhaust system is very restrictive. A single-pattern cam has the same amount of duration for the intake and exhaust. Many cams ground for Harley V-Twin engines are a dual-pattern design, having more exhaust duration than intake duration. Greater exhaust duration helps compensate for a weak exhaust port or restrictive exhaust system by providing additional time for scavenging the cylinder. If an engine has roughly a 70 percent or less E/I ratio, start with a dual-pattern cam having more exhaust than intake duration. Power-adder applications typically benefit from a dual-pattern cam because they generate a large exhaust volume. Engines with an E/I ratio between 70 and 80 percent should be tested with both single- and dual-pattern cams to determine best power.
Valve-lash Testing: If you are running a solid-lifter cam, experimenting with valve lash can give an indication of whether the installed cam is optimized to the engine combination. Increasing lash opens the valve later, closes the valve earlier, shortens duration and decreases net lift. If power increases with more lash, the cam may have too much duration for the combination. However, if power goes down, more duration may improve power. Valve-lash testing works on both the intake and exhaust sides. Since large lash variations are hard on the valvetrain, keep test time to a minimum. Experimenting with different rocker ratios is another method for determining whether an engine would have better performance with more or less cam. And unlike valve-lash testing, rocker ratio experimentation works with both solid and hydraulic lifters.
Changing Cam Timing: Retarding or advancing a cam 4-degrees will move the engine's power band either up (retarded) or down (advanced) approximately 300 to 400 rpm. Closing the intake valve later and/or opening the exhaust valve earlier tend to move the power band higher. Basically, retarding a cam increases top-end horsepower, while advancing a cam improves low-end torque. If power improves when the cam is retarded more than 4-degrees, the cam is too small and requires more duration. On the other hand, if power increases when the cam is advanced more than 4-degrees, the cam is too large and less duration would probably improve performance. All else being equal, a cam with a later closing intake valve requires an increased mechanical compression ratio to maintain a given low speed performance.
Cranking Compression: An octane-limited engine with excessive cranking compression is prone to detonation and can result in disappointing top-end power. Engine cranking compression can be an indicator of how optimized a cam is to an engine. Excessive cranking compression can indicate too small of cam-one with too little duration and an intake valve that closes too early. An engine that runs best with the ignition advance set much less than its normal timing range is another indicator of excessive cylinder pressure and possibly too small of cam. Retarding the cam is one way to reduce cranking compression. Installing a longer duration cam with a later closing intake valve is another.
A pump-gas engine with low cranking compression, roughly 165 psi or less will have poor bottom-end power, throttle response and fuel economy along with lazy acceleration. Note that cranking compression will vary, depending on whether the reading is taken at sea level or high in the mountains. Low cranking compression can be crutched by advancing the cam, which will increase low-speed cylinder pressure and power at the expense of top-end power. Correcting this problem may require a shorter cam with less duration and earlier closing intake valve along with a higher mechanical compression ratio.
Although dyno testing is a quick way for testing cams, remember that the best cam on the dyno may not be the fastest cam on the street or racetrack because gearing, traction, g-forces, weather conditions, and other factors have a major influence on performance. Clearly, there are many tradeoffs to make when juggling cam specifications. Simply put: Cam selection boils down to a fine balancing act.
Start out by matching duration, lift, overlap and IVC to the engine's application and parts combination. If you are building a serious performance engine and want to crank out the utmost in power, select a few cams that appear to match the engine's application, then dyno or trackside test each cam for best performance. Most engines perform best with a certain LSA. Try to identify that LSA. Then, experiment with various lifts and durations using the same LSA. In the end, it all boils down to: There is no such thing as a free lunch. With a bunch of money, gobs of time, and lots of dedication, your engine can have perfect timing and a winning personality.
|SAMPLE TWIN CAM CAMSHAFTS|
|Company||Grind||Valve Timing||Duration at .053"||Duration at .020"||Valve Lift||Overlap at .053"||Lobe Center||Lobe Separation Angle||TDC Lift At Valve|
**Table 2 ** Valve lift with 1.625:1 rocker arm ratio. *Gear Drive, reverse rear cam rotation.Note the TW37s and TW37b cams have the same amount of duration and lift but the opening and closing valve timings are different, by changing the timing of the opening and closing events, Lobe Center, Lobe Separation Angle, and the amount of overlap change. Also note that duration values change when measured at different tappet lifts, such as .053" and .020". Accurate comparisons can only be made with measurements taken at the same tappet lift.
|LOBE SEPARATION ANGLE CHARACTERISTICS|
|CONDITION||NARROWER SEPARATION ANGLE||WIDER SEPARATION ANGLE|
|Intake Event||Starts & Ends Earlier||Starts & Ends Later|
|Exhaust Event||Starts & Ends Later||Starts & Ends Earlier|
|Valves Closed Simultaneously||Increased Time||Decreased Time|
|Power Band||Narrower & Peaky||Wider & Flatter|
|Low Speed Cyl. Pressure||Higher||Lower|
|Low Speed Torque||Increased Potential||Decreased Potentiall|
|High Speed Cyl. Pressure||Lower||Higher|
|High Speed Horsepower||Decreased Potential||Increased Potential|
|Detonation||Higher Potential||Lower Potential|
Tests For Determining The Best Cam* Advance or Retard Cam* Experiment with Valve Lash* Change Rocker Arm Ratios* Check Cranking Compression
**Table 5 **This table lists tests that can be used to determine whether a given cam is optimized for an engine combination.
The Performance Pro Series
by Crystal Publications**