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.
Valve Lift is how far the valve opens. However, lift or lobe lift is how much lift there is at the cam lobe. When you add rocker arm geometry, such as 1.60:1, you multiply lobe lift by that amount. In other words, .400-inch lobe lift becomes .640-inch valve lift with the rocker ratio multiplication. Valve lift and the speed it opens and closes determine when and how power is made. Valve Lift effects performance by allowing more or less air into the engine depending upon lift involved. How quickly the valve opens and closes also determines how much air is ingested in a given power cycle.
Intake and Exhaust Centerlines at either the intake or exhaust lobe are the theoretical maximum lift points of the lobes in relationship to Top Dead Center (TDC) in degrees of crankshaft rotation. They are shown at the bottom of the camshaft specification card as “Max Lift.” The centerline of the cam may be moved by advancing or retarding the camshaft in the engine. If you advance the cam, you improve low-end torque. If you retard the cam, you improve high-end performance and power. Always check valve to piston clearances in any case.
Advancing or Retarding Camshaft Position determines where peak power will happen. Advance cam timing and you improve low-end torque. Retard cam timing and you raise the power band. This is done two degrees at a time, paying close attention to valve-to-piston clearances. Begin your experimentation at zero, per the cam card, when you are degreeing. Advancing the cam two degrees moves the power band downward 200 rpm. Retarding the cam moves power upward by 200 rpm.
Compression Ratio affects cam selection by working hand in hand with the cam profile. Compression ratio is one of three key factors in determining cylinder pressure (working or dynamic compression). The other two are cam duration at .050-inch lifter rise and cam position (advanced or retarded off zero). These three factors interact with one another to determine the amount of cylinder pressure the engine will generate. This is usually expressed as the “cranking pressure” that can be measured with a gauge installed in the spark plug hole. Compression ratio should also match the recommended compression ratio for the camshaft you are selecting. What you don’t want is too much working compression. Too little compression or too much duration will cause cylinder pressure to drop. This will reduce power output. With too much compression, cylinder pressure (working compression) will be too high, even though cranking compression might not be. When working compression becomes too high, you risk detonation and possible engine damage.
Octane and Cylinder Pressure are important to consider as they relate to each other. In basic terms, Chase tells us, the more cylinder pressure, the more power the engine will produce. But watch fuel octane ratings. Today’s pump gas will not tolerate compression ratios above 10.5:1 and high cylinder pressure above approximately 165 psi cranking without risking detonation. Fuel octane booster or racing gasoline (100 or higher octane) will be necessary if too much cylinder pressure is generated.
A roller camshaft reduces internal friction significantly at the lobe and lifter. However, there are more benefits. Roller cam technology also allows a more aggressive cam without rough idle and drivability issues that plague radical flat tappet cams. By reducing internal friction, roller cams shed less metal particles into the oil. You don’t have to sweat breaking in a roller cam or using special additives either, though engine break-in procedures should always be observed.
A roller cam is only the beginning of what you can do to reduce internal friction and improve performance. Roller rockers and a dual-roller timing set contribute greatly to friction reduction and improvements in performance. And if you want to reduce friction even more, go with a Torrington thrust bearing on your cam sprocket. Though these expensive modifications seem like overkill, they make a huge difference in efficiency and performance when you consider how much power is lost to friction.
Set Up Time
Blueprinting is just a fancy word for detail. When you install a cam, be sure to degree that cam and be positive about valve timing events. First, establish true top dead center (TDC) using a degree wheel and dial indicator. Gently roll the crank back and forth until you are spot on TDC.
Once you have established true TDC, you can move to degreeing the cam, which is where you ascertain valve timing events as they relate to your cam card. Degree your cam and take meticulous notes before moving on. See how your notes compare with the cam’s specifications. You want to look at both duration and duration at .050-inch just for starters. And while you’re at it, check valve to piston clearances using putty.
And finally, when you’ve installed your cam and valve train, adjust the valves with a thickness gauge (mechanical lifters) or by the book with hydraulics. When we say “by the book,” we mean Father Ford’s instructions per the shop manual. When you adjust valve lash on a small-block Ford, you want .020- to .060-inch of lifter preload. Upstairs, this means having the cam lobe at the heel (lifter completely at rest) with the valve closed. Twirl the pushrod while slowly tightening the rocker arm adjustment nut. When the pushrod becomes impossible to turn with your fingers, tighten the adjustment ½-turn. Make the rounds of all 16 rockers—then do it again. Perform valve adjustment when the engine has been sitting and the lifters have bled down.
If you’re doing a mechanical tappet valve adjustment, make sure the cam lobe is at the heel with the valve closed. Valve lash specifications can be found in the shop manual.
In recent years, there has been concern about the removal of zinc from engine oil and its affects on flat tappet camshafts. Truth is, flat tappet cams need zinc as well as other important additives to prevent wear of both the cam lobe and lifter.
ASL CamGuard has its roots in aviation where you can’t just pull off to the roadside should the engine quit. CamGuard is a time proven successful blend of additives that fortify the engine oil to protect engine parts by providing a wear barrier. However, it also heads off corrosion in engines that sit for extended periods because it has staying power. It also stands up to high operating temperatures.
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