KJ Jones Senior Technical Editor
July 1, 2008
Photos By: The Manufacturers, KJ Jones
Along with the myriad of variables to consider when selecting a new camshaft for your 'Stang's powerplant, it's important that you think rationally and realistically about your performance goals for the engine as a whole (greater torque, increased tip end, and so on) before ordering a custom-grind or shelf bumpstick.

Horse Sense: Honesty is the best policy when selecting a cam. Wishful thinking may lead you to believe a long-duration, high-rpm, solid-roller cam is the perfect stick for your daily driven Mustang. The frustration, however, that results from the inability to get anywhere near its 7,000-rpm sweet spot will prove that bigger isn't necessarily better when it comes to camshafts for street 'Stangs. A mild hydraulic -roller would have been the better choice.

Although we've done several projects that involve designing and building 5.0-based or 4.6-liter 'Stang engines, we've never taken the time to determine exactly how many various parts go into making a hot pushrod or modular bullet.

The broad-scale answer is a lot. Obviously, there are hundreds of pieces, big and small, that contribute to a Mustang engine's overall makeup, and some components are much more important than others. As such, they require a significant amount of thought before you purchase one or several to eventually install in your 'Stang's engine.

We'd dare to bet that engine builders and hard-core engine enthusiasts will agree that camshafts rank high on the list of critical internal parts for any engine-possibly in the number-one position. Why do cams carry more weight than cylinder heads, a crankshaft, or a block? Well, one of the main reasons is rooted in the layman's definition of an engine that we're all familiar with: An engine is, and always will be, an air pump. Camshafts have the important responsibility in this air-pump system, as they ensure the valves open and close at their proper times throughout the combustion process, thus allowing fast-moving air and fuel to enter, be compressed and burned, and exit the cylinders in an efficient manner as engine speed increases.

Although they're OEM pieces in the supercharged 5.4 engines of Ford GT super cars, these FRPP high-lift bumpsticks (PN M-6550-GT) are now being applied to '07-'08 Shelby GT 500s. Lift is increased by approximately 1.14 and 1.36 mm on the intake and exhaust sides respectively. With a good tune and free-flowing exhaust, Shelbys can pick up a bunch of power at the rear wheels with these cams.

Actually, cams can be thought of as the biggest multitaskers in an engine. They're responsible for controlling the opening and closing of the intake and exhaust valves, as well as how much and how long the valves are open, individually and simultaneously. The period where intake/exhaust valves are open concurrently is known as camshaft overlap. It's measured in crankshaft degrees and occurs only at the end of the exhaust stroke. Overlap can be a blessing and a curse. An increase in overlap can improve top-end power, but idle quality and torque down low will suffer.

When asking what's the best camshaft for a particular pushrod or modular engine, it's important to have a good understanding of duration, lobe separation, and lift, from a raw-dog technical standpoint. Duration and lobe-separation angle play big roles in establishing an engine's peak power and usable rpm range.

Duration is basically the amount of time that an intake or exhaust valve remains off its seat during a cam lobe's lifting cycle, measured in degrees of crankshaft rotation. As a rule of thumb, cams with lower/shorter duration (below 250 degrees at 0.050 inch) idle fairly smoothly (some lope, but not a lot) and make power in the low-rpm range. On the other hand, camshafts with higher/longer duration bring on the steam at high rpm and have the aggressive, lopey idle that many of us crave.

Lobe-separation angle determines an engine's torque range and the rpm at which an engine reaches peak torque. It's the distance between the intake centerline and the exhaust centerline when their values are added together and the sum is multiplied by 0.5. The intake centerline is the maximum lift point on a cam's intake lobe, measured in cam degrees after top dead center; the exhaust centerline is the maximum lift point on a cam's exhaust lobe, measured in cam degrees before top dead center.

There are two types of duration that are most commonly referred to when selecting cams: advertised (seat) duration and duration at 0.050 inch.

Advertised duration on a cam card refers to the total time (represented by degrees of crank rotation) that a valve is off of its seat-from the time it opens until it closes. It's recorded when the rocker arm is raised to a predetermined amount-usually 0.004 inch or 0.006 inch-that's set by the cam manufacturer.

Duration at 0.050 inch is the value everyone refers to when discussing cams because it's recognized as the industry standard. It's also measured in crankshaft degrees, from the point when a lifter is 0.050 inch off the cam's base circle on the opening side of the lobe until the time it falls 0.050 inch off the base circle on the closing side of the lobe.