How To: Choose A Camshaft
Most Mustang owners want crisp low to mid-range torque, good throttle response, some semblance of fuel economy, and a Mustang that sounds cool. Of course, some want that “rumpity-rump” idle and the roar of a hopped up engine during those exhilarating blasts down Main Street.
The right camshaft can help with both.
Before you start camshaft shopping, what do you want your Mustang to do? You can opt for a lumpy camshaft and that bad boy persona, but how is it going to feel driving to work in heavy traffic or headed to a car show 100 miles away? No matter what you’re thinking at your computer or easy chair, a lumpy race cam is unacceptable for daily driving. If you want aggressive traffic light-to-traffic light performance, you don’t need a hot cam to get the job done. You want a cam that’s going to deliver snappy low to mid-range torque without the theatrics. With abundant torque, you have the traffic light advantage.
We spoke to Chase Knight, who has been with Crane Cams over 40 years and knows a lot about camshaft and valve train selection. Chase has watched camshaft technology evolve over the years and tells us there has never been a better to time to shop for a camshaft. Even if you have marginal or no knowledge of camshaft function, you can go to Crane’s website or chat with the company’s technical staff to select the right camshaft.
What Camshafts Do
Camshafts and valves have a complex job. Valves have to operate in perfect unison with piston travel. If only it was as simple as “intake-compression-ignition-power stroke-exhaust,” but it isn’t. Because fuel and air ignite more slowly than actual valve and piston timing events, there’s more to this business than meets the eye. Valve timing, coupled with ignition timing, contributes to how engines function and perform.
Chase tells us that it’s easy to choose a cam if all you want to do is impress your buddies with a lumpy idle. All you need is long exhaust duration (valve open time) and tight lobe centers with five-degrees of advance. Valve overlap (both valves off their seats between exhaust and intake strokes) is what gives you a lumpy idle, along with poor intake manifold vacuum. Thing is, you have to live with that aggressive attitude when your buddies have gone home. Chase is suggesting you carefully evaluate what you want before committing to a camshaft profile you may not be happy with later on. This means putting a lot of thought into cam and valve train selection.
Cam selection normally begins with what you want your engine to do followed by what type of camshaft you want. Your first bullet point is flat tappet versus roller cam followed by mild versus aggressive. Although flat tappet camshafts have been around for as long as there have been automobiles, a flat tappet cam these days doesn’t make much sense considering the great technology available. Flat tappet camshafts were original equipment in Mustangs prior to the ’85 model year when Ford began use of roller tappets in the 5.0L High Output. A flat tappet cam consists of conventional cam lobes and flat-faced lifters that ride lobes in offset fashion to actuate pushrods, rocker arms, and intake and exhaust valves. Lifters ride the cam lobe offset in order to spin in their bores to achieve consistent wear.
The cost advantage of a flat tappet cam speaks for itself. They’re inexpensive. You save a bunch of money you can invest elsewhere in your engine. However, saving money with a flat tappet camshaft is but a short-term gain because you’re going to spend more money long term in wear and tear and miss the advantage of the performance you can get from a roller cam.
To help you make the right decision, we’re going to look first at all of the high-friction components most Mustangs came with from the factory, then look at ways to not only choose a great cam, but improve efficiency at the same time.
While we’re looking at how to choose a camshaft, let’s look at some terms you need to understand:
Lobe Separation (lobe centers) is the distance in camshaft degrees that intake and exhaust lobe centerlines (full open) are apart. In other words, from the time the exhaust valve is fully open until the time the intake valve is fully open. The span between both is called Lobe Separation. There’s also valve overlap, where intake and exhaust valves are off their seats for a brief moment between exhaust and intake strokes.
Duration is the period of time in crankshaft degrees that an intake or exhaust valve is open. “Duration @ .050-inch” is the point where the lifter rises .050-inch from the base circle as the cam turns. Lower duration cams produce the power in lower rpm ranges. Longer duration cams operate at higher rpm because they flow more air, but you will lose low-end torque. Chase adds that for each ten-degree change in duration at .050-inch of lift, the power band moves up or down in the rpm range by approximately 500 rpm.
Advertised Duration is the promoted duration number, but not always accurate. There are two key components for measuring duration—the number of degrees of crankshaft rotation and at what point of lifter rise the measurements are taken. Advertised durations are not taken at any consistent point of lifter rise, so the numbers you see in catalogs can vary greatly. For this reason, advertised duration figures are not good for comparing cams. Duration values expressed at .050-inch lifter rise state the exact point the measurement is taken. This is what you want to look for in cam specs, Chase tells us.