Understanding Cam Specs: Part 2

In this video, we’ll explain what we like to call the “big three.” The three most important specifications to consider when selecting a camshaft for your engine are lift, duration, and intake closing time.

Lift

The first of the big three is lift. Lift is defined as how far the valve moves off of the valve seat, or in other words how far the valve opens. The distance that the cam moves the tappet and pushrod is known as cam lift or tappet lift. It’s the difference between the radius of the base circle and the distance from the centerline of the camshaft to the nose of the cam lobe.

The pushrod pushes one end of the rocker arm and forces it upward. The rocker arm is nothing more than a lever. The valve side that pushes down on the valve stem is longer than the pushrod side. The valve side of the rocker arm actually pushes the valve down farther than the cam pushes the pushrod end up. The ratio of the length of the valve side and the pushrod side of the rocker arm is called the rocker ratio. If you multiply the cam lift by the rocker ratio, you’ll get the theoretical valve lift.

Lift is important because opening the valve further allows air or exhaust to flow in or out more quickly. You get a better cylinder fill and more efficient elimination of exhaust gases.

If you plan on running stock valve springs in your engine, you need to select a camshaft that does not exceed the maximum lift of your springs. These are called bolt in cams. A true bolt in cam does not require any modifications or other components for installation. Stock tappets, pushrods, rocker arms, and valve springs can be reused. Cam timing is also appropriate for a stock engine.

Using stock valve springs is perfectly OK for most street applications. But, in addition to the limitations on lift, you can’t exceed the stock rev limit. If you plan on operating the engine and at higher rpm, you should install a set of performance valve springs.

When a higher lift cam is installed, the valve may no longer be the most restrictive part of the system. The air cleaner, carb, throttle body, intake port, exhaust port, and the exhaust system also play a part in restricting the airflow. A performance high flow air cleaner assembly, and a performance high flow exhaust system are necessary to help move air quickly and efficiently through the engine. Performance air cleaner and exhaust help the cam to reach its full potential. A larger carburetor or induction module can also be helpful or necessary.

Duration & Intake Closing Time

Two important valve timing events are the valve opening & the valve closing times.
Intake closing time and duration are two very closely related events, so it is important to look at them together. Intake closing time is related to duration because it actually plays a part in determining duration. Closing the intake valve later in the cycle increases duration.

A cam with a longer intake duration will have a later intake closing time. Duration is defined as the number of degrees of crankshaft rotation that a valve is open. It’s determined by the point when the intake valve opens and the point when it closes. The earlier it opens and the later closes, the greater the duration. The later it opens and the earlier it closes, the shorter the duration.

There are horsepower cams with lots of duration & late intake closing time, and there are torque cams with shorter duration and earlier intake closing time. Torque cams and horsepower cams are actually two ends of the camshaft spectrum and specialize in making either horsepower or torque, but not both.

A cam that falls in the middle of the spectrum, sometimes called a mid range cam, may suit you better. Duration and intake closing times are somewhat more than stock, but they’re not extreme. There is an increase in horsepower but low-end torque is generally increased as well. Mid range cams will not provide as much of an increase in torque or horsepower as the specialty cams, but they provide a general increase in performance across the rpm range.

After watching this video, you will have a good grasp on what to look for when choosing a cam. If you are looking for low-end torque or a dependable mid-range cam, you will see that bigger is not always better. If you want to go big, you will also need a big budget and then, hold on tight!

If you missed Understanding Cam Specs: Part 1, be sure to watch that video as well for a comprehensive understanding of cam specs.

Next, watch Understanding Cam Specs: Part 3 here.

Still hungry for more? Check out these other upgrade projects from Fix My Hog:

Cam Upgrade and Tensioner Upgrade
Cam Upgrade and Gear Driven Cam
M8 Cam Upgrade

Thank you to S&S Cycle for sharing this video series with Fix My Hog.