If we were to ask for a show of hands, it’s a safe bet to assume many enthusiasts would reluctantly admit that camshaft theory is one of those topics that causes more than a little consternation. You know the basics of lift and duration and probably the difference between hydraulic and solid lifter cams, but as for aspects beyond those, such as centerline, lobe separation angle, overlap, and more there’s no harm in admitting if you struggle a bit.
Hey, we get it. Camshaft theory is a complex science with a lot of ins and a lot of outs, to paraphrase the great Jeffrey Lebowski. More than how much the camshaft lifts the valves off their seats and how long it holds them open, centerlines, lobe separation angles, overlap, and more contribute to how it affects engine performance.
We’re not discussing the finer points of lobe ramp angles with this story, but simply explaining some of the basic terms and general theories of camshaft design and operation. It’s a back-to-basics overview intended to lay a foundation for greater understanding. And while it should help you discuss the subject more authoritatively and decipher the specs camshaft manufacturers list in their catalog descriptions, it’s not a guide to selecting the perfect camshaft for your engine.
You need to talk to the experts for that. All the major camshaft manufacturers have tech lines and online options to help select the best camshaft for the job. Use them and you’ll have a better idea of what the hell they’re talking about!
1. What are the differences between flat tappet camshafts and roller camshafts?
Simply put, a flat tappet camshaft is used with flat-faced lifters (well, mostly flat), while roller cams are used with lifters with roller tips. The roller tips greatly reduce friction, offer greater performance and efficiency while also reducing maintenance because the valvetrain requires virtually no adjustments. Flat tappet camshafts come in two varieties: solid and hydraulic. The solid flat tappet cam is commonly called a solid lifter camshaft because it has a solid interface with the pushrod. A hydraulic flat tappet cam uses lifters with oil-pressurized pistons to maintain their connection with the pushrods. Flat tappet designs require periodic valve lash inspections or adjustments, but not nearly as often these days as in the old muscle car era. They also generate considerably more friction than a roller design and wear much faster. Generally, flat tappet camshafts open and close the valves slower than roller cams, which tend to open them and snap them closed quickly. For a street or street/strip engine, the roller design is the way to go.
2. I know the basics of lift and duration, but what’s important about their relationship?
Lift is the measurement of how far the valve lifts off its seat, while duration is the amount of time the valve remains open, expressed in degrees of crankshaft rotation. That means a camshaft with 245 degrees of duration holds the valve open as the crankshaft rotates 245 of 360 degrees. Both aspects of valve actuation work together to optimize airflow in and out of the cylinders, but the lift and duration rates have different effects on performance. Generally speaking, more duration will help build horsepower at higher rpm, typically at the expense of low-end torque. A little more lift and a little less duration can help achieve comparable power without the torque penalty as long as the cylinder heads flow well. More lift is useless otherwise and, in fact, can work against the engine combination, because more valvespring tension is typically required with a high-lift camshaft—and that generates more power-robbing friction.
3. Aren’t there several types of lift specifications? Which one should I care about?
When reading advertised cam specs, the basic “lift” specification refers to the maximum lift of the valve off its seat and that’s the important number. The nuance here is that this valve lift measurement is based on multiplying two other specs: the gross lift of the camshaft, which is the distance a cam lobe moves the tappet (lifter) and the rocker arm ratio. Let’s say the camshaft moves the lifter 0.275-inch and the rocker arm ratio is 1.5:1. Multiplying them brings a max valve lift of 0.4125 inch. And if you’ve heard that increasing the rocker arm ratio brings an effective valve lift increase, you’re correct. Moving up to a 1.6:1 ratio rocker and staying with the same camshaft would change the max valve lift to 0.440 inch.
4. What is the difference between advertised duration and duration at 0.050-inch?
Good question. It’s all about where the duration is measured, which must be at some point of lifter rise. The measurement is the angle in crankshaft degrees between the matching lift points on the opening and closing sides of the cam lobe. Let’s say the measurement is 0.005 inch. The duration measurement starts when the lifter rises 0.005 inch on the opening side of the cam lobe’s rotation and concludes when it gets back to 0.005 inch on the closing side. The angle between those points—265 degrees, for example—is the advertised duration. The problem is, not all manufacturers’ advertised duration numbers are measured from the same point. Some are at 0.004-inch lift to show a really big number, while others stick to the SAE spec of 0.006-inch. The cam manufacturers also rate their duration specs at 0.050-inch lift for standardized comparison purposes. The numbers are considerably “smaller” than the advertised duration, but the 0.050-inch spec allows the cams of different manufacturers to be easily crosschecked.
5. What the heck is a “dual-pattern” camshaft?
It’s a camshaft that has different duration specifications for the intake and exhaust lobes, whereas a single-pattern camshaft has the same specs for both. The dual-pattern cam has more duration (and more lift) for the exhaust lobes to enhance airflow for cylinder heads with less-than-adequate exhaust ports. Better-flowing aftermarket heads these days have reduced the need for dual-pattern cams, but they boost performance on older, stock head designs.
6. What is overlap and how does it affect performance?
Overlap is the brief period where the intake and exhaust valves for a cylinder are open (off their seats) simultaneously, measured in degrees of crankshaft rotation. This is intentional because as the exhaust gases are pulled out of the combustion chamber a scavenging effect is created that helps pull the next mixture into the chamber—and the greater the duration generated by the camshaft, the greater the overlap. The intricacies of overlap theory are very complicated, but generally speaking, more overlap benefits performance at high rpm because it helps fill the cylinders. Overlap also accounts for that awesome low-rpm lope at idle—lots of overlap to build power at high rpm. The trade-off is reduced torque and low vacuum production, which isn’t a problem on the dragstrip, but affects driveability in street cars. Then again, it sure sounds good on cruise night.
7. What is the lobe separation angle and why is it important?
Lobe separation angle (LSA) is the measurement in camshaft degrees between the maximum lift points—known as centerlines—of the intake and exhaust lobes. Simply put, it affects the engine’s power curve, idle quality, vacuum production, and more through its effect on valve overlap. A tighter (narrower) LSA contributes to a narrow powerband that moves torque lower in the rpm range, while increasing overlap for high-rpm horsepower. A rougher idle and lower vacuum at idle are byproducts. On the other hand, a wider LSA broadens the powerband and moves torque higher in the rpm range. There’s less overlap, which enhances idle quality and vacuum. Supercharged engines generally benefit more from a wider LSA because they don’t require the scavenging effects that come with greater overlap.
8. What is the camshaft centerline and why does it matter?
It is the halfway point between the intake and exhaust centerlines. The intake centerline is the point of highest lift on the intake lobe, expressed in crankshaft degrees, after top dead center (ATDC). The exhaust centerline is the point of highest lift on the exhaust lobe before top dead center (BTDC). What’s important here is the intake centerline on the intake lobe for cylinder No. 1, which is used to determine accurate camshaft timing. The manufacturer lists the intake centerline on the supplied cam card, but that specification may not translate exactly when the camshaft is installed, even when the marks on the cam and timing gear are aligned—although it is generally very close. Degreeing the camshaft will determine the correct installed intake centerline for spot-on timing.
9. What is degreeing a camshaft and is it really necessary?
Degreeing ensures the most accurate valve actuation by correcting errors and accounting for tolerances involved with the machining process of the camshaft. In other words, it is a procedure that verifies the camshaft’s just-right position for spot-on timing. It involves checking the cam lobes at a specified lift in relation to the crankshaft’s position. The larger question is whether it’s necessary. Short answer: Not really. For most stock-type rebuilds and even moderate performance builds, degreeing doesn’t necessarily deliver an advantage. The accuracy of computer-based machining has made modern camshafts very accurate out of the box. Most camshaft manufacturers build a little advance—about 4 degrees—into the grind. Degreeing the camshaft can’t hurt and is recommended for max performance, but don’t worry about simply lining up the dots on the timing gears and calling it good. It will work.
10. What does installing a camshaft “straight up” mean?
It means the camshaft is installed during degreeing to the recommended intake centerline specification. The spec is indicated on the cam card included with a new camshaft. The intake centerline, you will recall from above, is the highest point of lift on the intake lobe. The camshaft’s installed position can be advanced or retarded relative to the straight up specification, altering camshaft timing and therefore valve actuation to shift the engine’s torque band. For example, a camshaft with a 108-degree intake centerline will be centered at 104 degrees when installed 4 degrees advanced. Conversely, the intake centerline will effectively be 112 degrees when retarded 4 degrees. An advanced cam opens the intake valve a little sooner and generally enhances low-rpm power and throttle response. A retarded cam delays the intake valve’s opening, pushing power production higher in the rpm band. As mentioned earlier, most aftermarket camshafts have a little advance built into them.
Photos: Barry Kluczyk