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Bump and Grind

Camshaft Variables Explained on the Dynamometer

Scott Crouse Feb 5, 2004
148_0405_lobe_01_z 2/8

Airflow in and out is what building horsepower is all about. Every month we attempt to deliver a killer airflow engine combination and uncover the mystery of a particular performance part. This month we take a hard look at camshaft design.

Rather than building a specific engine combination and handing the answers out, we'll go over camshaft principles and offer the information necessary to choose a cam for a winning combination. While diving right into the opening and closing points in relation to lobe separation angles and intake centerlines would produce the answer most seasoned veterans are looking for, we also understand that it's equally important to review basic terminology and principles before such technical answers can be explained.

The camshaft's function is to time each valve opening and closing point in relation to the piston and the combustion process to every degree of the engine's 360-degree rotation. In order to achieve accurate timing, the camshaft's timing gear has to operate at exactly half the speed of the rotating bottom end.

During the camshaft's installation, it is extremely important to make sure the crankshaft and camshaft's timing gears are properly aligned by placing the dimple on the crankshaft timing gear in the 12 o'clock position while the dimple on the camshaft timing gear is positioned at 6 o'clock. Of course, this assumes the camshaft is being installed without any advance or retard. Before we go any further, it's important to define some common terms associated with camshaft measurement and construction.

Valve Lift

Valve lift is probably the most mentioned term when it comes to bench racing and bragging about whose camshaft is bigger. In cam speak, there is only lobe lift, not valve lift. The cam lobe raises the lifter, which in turn raises the pushrod and actuates the rocker. From there, the rocker multiplies the lobe lift by the rocker ratio to determine the actual amount that the valve is pushed off its seat. Basically, as the camshaft rotates in the engine, the lobes lift the valvetrain and actuate the valves accordingly. One way to effectively increase the amount of valve lift without actually changing the camshaft is to utilize a higher-ratio rocker arm. For example, a lobe lift of 0.310 inch multiplied by the typical small-block Chevy 1.5:1 rocker ratio would provide a valve lift of 0.465 inch. On the other hand, simply changing to a 1.6:1 ratio would deliver an additional 0.031 inch lift to move the valve a total of 0.496 inch from its seat. Of course, with this additional lift comes additional duration as well, which brings us to our next subject.

Valve Duration

The next most popular camshaft measurement is the duration of time a camshaft lobe keeps the valve off its seat. The purpose of a valve is to regulate the flow of air. In order to measure a camshaft's duration, you must use a degree wheel in conjunction with a predetermined lift point. The most common duration references are advertised duration measured at 0.050-inch lift. While the advertised numbers are supposed to be measured at 0.020-inch lift, some cam grinders measure farther down the lobe to make their camshaft look more radical than it really is. Because of this confusion, most engine builders use the 0.050-inch duration figure, as will we throughout the rest of this story.

Duration is probably the biggest factor in determining an engine's overall character. When the length of the duration is increased, the engine's maximum top-end potential will increase as well. This is due to the intake and exhaust valves being held open longer to move more air and fuel through the cylinder. While this long-duration technique is great for making upper-rpm power, there is also a negative effect that must be considered. When the valves stay open longer, it requires them to leave the seat sooner and close back down on it later, which causes an overlap condition. This allows the combusted cylinder pressures inside the cylinders to fall at low rpm, which in turn creates a loss of low-speed torque. >> As for upper-rpm cylinder-pressure bleeding, there isn't enough time for a considerable amount of pressure to disperse before the next engine cycle can begin.

Sooner or later, the question of ultimate valve timing manipulation will enter the picture and issues of piston-to-valve clearance will become a problem. While most enthusiasts believe that total valve lift is primarily responsible for piston-to-valve clearance, it actually plays only a small part. Pistons contact the valves due to extensive amounts of duration. Camshafts that typically measure less than 0.550-inch lift and 220 degrees of duration (at 0.050-inch lift) are usually safe with flat-top pistons, but even then they should be checked for acceptable clearance.

Overlap

While valve duration is the amount of time a valve remains off its seat, overlap is a measurement of time that the intake and exhaust valves in the same cylinder are both open simultaneously. This overlapping of the valves is required to take advantage of an engine's maximum potential but comes with a few disadvantages. The first drawback: When a cylinder fires during its combustion stoke, both valves are closed in order to contain compression. However, to maximize exhaust potential, an engine builder may use a camshaft that opens the exhaust valve before the piston reaches bottom dead center (BDC) and closes slightly after the exhaust stroke passes TDC during the early induction cycle. The benefit here is a well-scavenged exhaust stroke but it comes at the expense of some bled-off cylinder pressure and a slightly contaminated intake charge. Along with a disturbed intake charge, the engine's vacuum signal is weakened because the piston's vacuum is not only drawing its intake charge from the induction system, it's also pulling dirty air from the open exhaust valve. This weakened air signal raises emissions and lowers idle engine vacuum, which plays a major role in the operation of engine-driven accessories. Along with the previously mentioned problems, excessive overlap also creates a weak torque curve, as the induction system is not functioning at its maximum potential when the cylinder is not being filled properly. In this case, low- and mid-speed torque are traded for peaky upper-end horsepower.

Lobe Separation Angle (LSA)

The next concern should be lobe separation angle. We know that the intake valve opens to create power and the exhaust valve follows to keep that power building. Lobe separation angle (LSA) basically describes the amount of lifter movement time between the intake at its maximum lift point and the exhaust at its maximum lift point. Of course, this is relative to the camshaft duration, which determines how long it takes the lifters to travel to and from their maximum lift points.

Refer to the diagram of two camshaft lobes. Points A and B are the beginning and end points of measurement for the camshaft's LSA, C and D refer to the intake's 0.050-inch duration measurements, and E and F indicate exhaust duration.

Notice that points E, G, and D form a triangular area, which is called the camshaft area of overlap. Once all of these points are understood, it's easier to see how a wider (higher numerically) LSA would create a smaller area of camshaft overlap. Now assume we tighten (numerically lower) the LSA. Points A and B would move closer together and cause overlap points E, G, and D to form a larger triangular overlap area. This means that the LSA can be used to alter the overlap of the intake and exhaust valves, and a wider LSA will offer the best idle vacuum and emissions, and allow the camshaft to carry out its power curve for a longer period of time.

This doesn't necessarily mean the engine will make more power the higher it is spun, but rather that the engine will not nose over quite as quickly as a more peaky engine would. As for tightening the LSA, valve overlap will become more evident and provide a number of benefits. Improved peak power potential is one and a stronger vacuum signal at maximum airflow efficiency is another. The strong maximum efficiency signal is also a way of saying that responsive power (peak torque) at or near the same maximum airflow efficiency will improve. As far as making the right choice, a street-driven vehicle should look for a wider LSA, while a race car can benefit from a tighter one. Either way, once the principles of camshaft design are understood, they can be used in conjunction with one another to develop the ultimate grind.

Intake/Exhaust Centerline

Anyone other than a camshaft engineer or competitive engine builder rarely discusses the subject of intake and exhaust centerlines. The reason for this is because they are both ground into the camshaft as initial timing points and allow an engine builder to phase the camshaft in with the rotating assembly. Earlier, we discussed how a camshaft must spin at half the speed of a four-stroke bottom end in order for it to operate the valves and deliver the spark at the correct time. The engine builder only needs to know the specifications in order to degree the camshaft in with the bottom end and check its accuracy to make sure the cam was ground correctly. Most builders only check the intake centerline because the exhaust centerline is phased off the intake, which means that if one is correct then the other is too. Unlike LSA, these centerline specifications are predetermined and ground into the camshaft at the time of manufacture. This means they cannot be altered unless the camshaft is completely reground.

Proving the Point

Once the idea of independent variable camshaft testing entered our minds, we had no other choice but to turn our dreams into reality. Our 383ci Smeding Performance motor set the stage for a high-performance foundation and would allow us to run various cam tests (this engine will ultimately be part of our Project POS Camaro). The Smeding engine was originally equipped with a hydraulic flat-tappet camshaft; we swapped over to a hydraulic-roller setup for lifter-reuse reasons. Smeding offers its crate engine combinations with the hydraulic-roller option, but since we didn't originally order it that way we had to change ours around. This required new lifters, lifter tie bars, longer pushrods, a cam retainer plate, and the roller camshaft itself. The Smeding small-block crate engine features a GM-style camshaft core using a recessed nose that allows the cam retainer plate to hold the camshaft in place. Read ahead to find the method we used, which called for a standard-style cam core outfitted with a cam button placed inside the timing-chain center hole. This cam button takes up slack >> between the camshaft nose and the timing-chain cover, making sure the camshaft will not walk out of the cylinder block. Either way, all small-block Chevy roller camshafts require a retainer to hold the cam in place.

When making the testbed camshaft selections, we looked for grinds that would show considerable power changes between duration and LSA. We called the Lunati tech line for some expert camshaft advice and a helpful representative recommended we come up with our own custom grind. The actual figures of these camshafts were not chosen to boost the power of the 383 but picked to show what happens when specific camshaft variables are altered.

Our first camshaft called for 0.326 inch of lobe lift (0.489 at the valve) and 215/224 degrees of intake/exhaust duration measured at 0.050-inch lift and founded on a 106-degree LSA. This camshaft was small for a 383 but would soon prove a valid point. Our next camshaft choice called for the exact same grind with the exception of the LSA. Here we added 8 degrees of LSA (quite a bit) for a total of 114 degrees. Obviously, we would be scrutinizing LSA power differences between the first two cams. The third camshaft added slightly taller lobes and drastically longer duration points, with 0.510/0.525 inch intake/exhaust lift and 232/242 degrees of duration at 0.050-inch lift.

After a few warm-up pulls on the dyno at Vrbancic Brothers Racing, we set the testing points between 2,500 and 6,000 rpm and let the motor rip. The low-end and midrange torque numbers were strong with the first cam, and the horsepower posted a steady 405hp peak at a rather low 5,000 rpm. We made several more pulls and found the camshaft would barely pull 6,000 rpm. During one pull, the engine just nosed over at 5,700 rpm and wouldn't rev any higher. Making 415 lb-ft of torque at 2,500 rpm was impressive, but the low 12 inches of manifold idle vacuum >> and its inability to carry the power curve much past 5,000 were not.

With the second cam, we changed nothing except for a wider LSA. If theory held true, we would see an increase in idle vacuum and perhaps even a little more low-speed torque with a slight loss of peak horsepower. Well, torque fell slightly across the curve and we lost some horsepower too. Engine idle vacuum came up to 14 inches and the cam pulled well beyond 6,200 rpm, but this wasn't worth the loss of power. Torque and horsepower had diminished because of the increase in camshaft overlap. Less overlap means the valves will close sooner and affect upper-end peak power. Torque usually increases, but the short duration of this camshaft didn't >> create enough overlap to bleed off a detrimental amount of cylinder pressure. Therefore, at low rpm, torque was already maximized, and when we further decreased overlap, we hindered the torque by closing the valves too early. A solid street/strip compromise would be a few more degrees of duration with an LSA of 110 or 112 degrees.

By now, our technical spirits were tickled, and we couldn't wait to dive into the next camshaft swap. The third camshaft sported slightly more lift (0.510/0.525 inch) with a lot more duration (232/242 degrees at 0.050) all ground on the wide 114 LSA. Ideally, a separate lift and duration test would have been nice, but every degree of 0.050-inch duration is typically worth much more power than 0.010 inch of lift.

This time we expected to see a shift in the power curve to produce improved peak power at the expense of some torque. As it turned out, we saw a huge loss of low-speed torque, a slight loss of midrange torque, and a moderate gain in horsepower almost 600-rpm higher in the power curve. Engine-idle vacuum also fell considerably, forcing us to raise the idle and take a steady reading of 10 inches at 300 rpm higher.

What happened? Increased duration kept the valves open longer to raise the rpm and peak power of the engine. The large loss of torque occurred due to the bleeding off of excessive cylinder pressure below 3,000 rpm. As rpm increased, the torque falloff wasn't as great and we posted a peak number that was 13 lb-ft down from the first test. Had we been chasing more top-end power, a narrower LSA would have boosted peak horsepower while allowing peak torque to flatten. In this case, valve overlap played a major role in the mannerisms of our Smeding 383 ci.

What It Means

According to our test, the first camshaft would have made a great street machine choice. There was plenty of vacuum to run accessories and enough torque to pin the passenger against the seat. As for the short rpm curve, this cam would work well in a towing rig. Ideally, we'd add a few more degrees of 0.050-inch duration combined with a slightly wider LSA (somewhere around 110 to 112 degrees).

The second camshaft featured a wide LSA that hurt the engine almost everywhere in the powerband due to a small duration effect. This cam was merely a test subject and would work better if altered. We'd recommend the third cam for a street/strip car with some gear and a loose torque converter. The torque below 3,000 rpm would hurt street performance while the extra 20 hp up top would add a few miles per hour. Once a small-block cam reaches the 240-degree, 0.050-inch duration figures, we'd recommend foregoing streetable torque and tightening the LSA up to something like 108 or 110 degrees to watch horsepower climb. In any case, it's important to remember that these cam events were chosen for testing purposes only. We did not include the grind numbers because the Lunati catalog has many others that are capable of producing better results. When the time comes to purchase a camshaft, use this story to grasp the principles of camshaft selection and contact your favorite cam company. The answers are out there.

Sources

Smeding Performance
Boerne, TX 78006
877-639-7637
www.smedingperformance.com
Vrbancic Bros Racing
Ontario, CA 91761
909-947-3575
www.customcarbs.com

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