How To: Choose A Camshaft
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.
Summit Racing Equipment