Whether we are talking about axle ratios, compression, carburetor size, timing, or cubic inches, our hot-rod language sets us apart. And nothing else is discussed as emotionally as camshafts. Lift, duration, lobe center, overlap, and much more confer just how the camshaft will or will not alter an (often theoretical) engine’s performance personality. The first step in any process is understanding camshaft basics.
In scientific terms, it is all about the area under the curve. Just like that calculus class you wished you did not have to take (or avoided altogether), the camshaft in your engine is based on the same mathematical rules. To allow more airflow into your engine, a camshaft is designed with a profile (or curve) that provides a specific amount lift to the valve.
The better we understand how each of these terms affects an engine, the better armed we are to choose the best camshaft.
Camshaft Basics
The key parts of any camshaft are the lobes. As the camshaft spins, the lobes open and close the intake and exhaust valves in time with the motion of the piston. It turns out that there is a direct relationship between the shape of the cam lobes and the way the engine performs in different speed ranges.
To understand why this is the case, imagine that we are running an engine extremely slowly — at just 10 or 20 revolutions per minute (RPM) — so that it takes the piston a couple of seconds to complete a cycle. It would be impossible to run a normal engine this slowly, but let’s imagine that we could. At this slow speed, we would want cam lobes shaped so that:
- Just as the piston starts moving downward in the intake stroke (called top dead center, or TDC), the intake valve would open. The intake valve would close right as the piston bottoms out.
- The exhaust valve would open right as the piston bottoms out (called bottom dead center, or BDC) at the end of the combustion stroke and would close as the piston completes the exhaust stroke.
This setup would work well for the engine if it ran at this very slow speed. But what happens if you increase the RPM? Let us find out.
When you increase the RPM, the 10 to 20 RPM configuration for the camshaft does not work well. If the engine is running at 4,000 RPM, the valves are opening and closing 2,000 times every minute, or 33 times every second. At these speeds, the piston is moving very quickly, so the air/fuel mixture rushing into the cylinder is moving very quickly as well.
When the intake valve opens and the piston starts its intake stroke, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel is moving at a high speed. If we were to slam the intake valve shut, all that air/fuel would come to a stop and not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel continues to force air/fuel into the cylinder as the piston starts its compression stroke. So, the faster the engine goes, the faster the air/fuel moves, and the longer we want the intake valve to stay open. We also want the valve to open wider at higher speeds — this parameter, called valve lift, is governed by the cam lobe profile.
Any given camshaft will be perfect only at one engine speed. At every other engine speed, the engine will not perform to its full potential. A fixed camshaft is, therefore, always a compromise. Therefore, carmakers have developed schemes to vary the cam profile as the engine speed changes.
Camshaft Lobe Lift
One of the camshaft’s most fundamental tasks is to transform its rotating motion into linear motion or lift. This is accomplished on a typical Chevy V-8 with 16 lobes (an intake and an exhaust)–all of them ground to an eccentric shape called a lobe to allow each lifter to raise and fall above a base circle. Just how far (mathematically) each lobe raises the lifter is called the lobe lift. The lift that is typically referred to in a camshaft catalog is the valve, or gross lift, and is achieved by multiplying the lobe lift by the rocker arm ratio. For most small-blocks, the standard rocker ratio is 1.5:1, so, by multiplying a theoretical lobe lift of 0.370-inch x 1.5 rocker arm ratio, we achieve a valve lift of 0.555-inch lift (0.370 x 1.5 = 0.555).
By replacing the standard rocker arm ratios with a higher-ratio rocker arm, you can add valve lift and some duration. If we use the camshaft with the 0.370-inch lobe lift and match it to a rocker arm ratio of 1.7:1, we realize a gross lift of 0.629 inches. Though these two gross lift examples are close to the upper end of a very high-performance small-block Chevy with a flat-tappet camshaft, the mathematical theory is the same for all engines.
Camshaft Lobe Duration
Camshaft duration is the amount of crankshaft rotation that occurs as the cam lobe moves the lifter off the base circle; it is measured in crankshaft degrees to make it easier to degree (check) the cam to make sure it is positioned properly in the engine. Determining the specific point that the lobe begins to move, the lifter off the base circle (upwards) can be difficult–so most camshaft companies use a standardized checking point to reference lift. Consequently, the camshaft industry has established a common duration checking point of 0.050-inch lift.
Generally, added duration is helpful in high-rpm engines, but not typically in low-rpm engines. This is because the added time that the valves are open in high-rpm engines allows the airflow additional time to enter or exit the cylinder. But at lower rpms, the added duration opens the valves too long in relation to the piston’s position in the cylinder, and the pumping pressure is lost.
It is also important to discuss the opening and closing points of the valves. The intake opening (IO) typically occurs before top-dead center (BTDC) and intake closing (IC) occurs after bottom dead center (ABDC). Exhaust opening (EO) happens before bottom-dead center (BBDC) and exhaust closing (EC) after top-dead center (ATDC). These points can be checked by degreeing the cam in the engine by degreeing or simply referencing from the information on the cam card supplied with each new camshaft.
These values can therefore be verified when the cam is installed and degreed. If you are not sure of the duration of your camshaft but have determined the intake and exhaust closing points, you discover the duration by adding the opening and closing point values to 180 degrees. For example: if the intake valve open value is at 0.050-inch lift of 2 degrees and has an intake closing value of 40 degrees, we simply add both amounts to 180 (2 + 40 + 180 + 222) and we have 222 degrees of duration at 0.050-inch lift. This works just as well for the exhaust side.
Camshaft Lifters
Camshaft lifters, also called followers, are offered in hydraulic flat-tappet, solid flat-tappet, hydraulic roller, and solid roller designs. The hydraulic flat tappet is the most used largely because it offers the benefits of quiet operation and is available with most all camshaft profiles designed for either pure stock operation to very high-performance applications operating to about 6,500 rpm. Once a good hydraulic lifter has been adjusted properly, it remains adjusted because of a valve-controlled plunger inside the lifter that maintains a preload from engine oil pressure.
Mechanical (or solid) lifters are used for applications where very high rpms are expected–typically to 8,000 rpm or more. Although a solid lifter has an oil-fed orifice, it is still a mechanical lifter. To allow for expansion that occurs from varying engine temperatures, mechanical lifters maintain a certain valve lash clearance at the rocker arm and require periodic valve adjustments.
Roller lifters employ a small wheel at the bottom of the lifter that allows it to negotiate a very aggressive lobe design with more lift at a given duration and reduced friction. Roller cams are offered in both hydraulic and mechanical designs in a variety of performance profiles and are typically more expensive to install than the flat-tappet varieties.
Single and Dual-Pattern Camshafts
If you have ever looked at a cam card (furnished with each new camshaft), you have noticed that some cams have the same specs for both the intake and exhaust (single pattern), while others differ (dual pattern). A dual-pattern camshaft offers more exhaust duration to improve the flow of an otherwise marginal cylinder-head exhaust port.
Intake Centerline
The cam’s centerline specification is used to determine the position of the lobes on both the cam and as installed in the engine. Most cam companies include an intake centerline number, which is the position of the intake lobe centerline (in crankshaft degrees) relative to top dead center on piston number one.
This can be altered by degreeing the cam after it has been installed in the engine. Moving the camshaft from its installed position affects all four opening and closing points; the most important is intake closing. So, by advancing the cam, the intake valve closes sooner. While this improves low-speed torque, it compromises top-end power.
Lobe Separation
Lobe separation angle (LSA) refers to the distance or spacing between the intake and exhaust lobe centerlines. This value is expressed in camshaft degrees, not crankshaft degrees. This is an important dimension because it establishes the amount of overlap between the intake and exhaust. Lobe separation angle (unlike the intake centerline) can only be determined when the cam is ground.
Finally
Choosing the best camshaft depends on vast criteria. If you use your car for daily transportation, you will want to consider a camshaft with shorter duration and relative lift. Most of the camshaft companies offer copious assistance to match the best camshaft with the type of driving or racing you do. If you are looking for something wild, remember to match the rest of your engine and vehicle to your cam choice. Transmission type, vehicle weight, stall speed, compression, cylinder heads, and much more only begin the list. In this section, we have only touched on the camshaft basics. If you would like to learn more, visit the some of the cam manufacturer’s Web sites (listed in the source box). They hold a wealth of camshaft knowledge that is ever evolving, and the tech sections of these Web sites are packed with information. Understanding the camshaft basics may be difficult at first, but after reviewing the information, you will find that you have learned quite a lot. The knowledge that you gain will surely help you choose the best camshaft for your engine.
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