Chris Hemer
April 1, 2001

Step By Step

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Distributor Gear: Small-block Fords have a tendency to wear the distributor gear prematurely in high-performance applications. If this has been a problem for you, consider drilling a small hole (on the order of 0.040-0.060 inch) in the oil plug just behind the distributor gear.
Lobe Centerline: Lobe centerline is usually defined as the relationship of the center of the intake lobe to top dead center of the piston, but it can also be described as the maximum lift point of the intake or exhaust lobe.
Lobe Separation: The number of camshaft degrees between the center of the intake lobe and the center of the exhaust lobe.
Roller Lobes: Used for decades by racers, roller cam lobes allow a radical profile, yet are easier on the valvetrain than their flat-tappet counterparts. The factory began using hydraulic-roller camshafts in the ’80s because they increase valvetrain life and reduce friction.
Ramp: How soft or steep the “ramps” of a cam lobe are affects the speed at which the valve opens/closes. A steep ramp actuates the valves more quickly, but it is also harsher on the valvetrain.
Base Circle: The backside or “heel” of the cam lobe. When a lifter is at the bottom of its travel, it is riding on the base circle of the cam.
Nose: The shape of the nose dictates the length of time that the valve will remain at or near its maximum lift figure.
Journal: There are five of ’em. They are what ride on the cam bearings inside the block.
Which camshaft you choose has a lot to do with the cylinder heads on the engine. For example, inefficient (read: stock) cylinder heads will need a cam that has more overlap. Good cylinder heads won’t require as much.
Turbocharged engines usually require a cam with a delayed exhaust-opening event to trap exhaust pressure, which is used to spool the turbo. An early intake event isn’t required because of all the positive pressure in the intake tract—a side benefit of which is better driveability in a low-compression street turbo motor. Other characteristics of a turbo cam are smaller intake lobes and larger exhaust lobes (relative to a normally aspirated application) and wide lobe separation. A free-flowing exhaust port is also a must.
Installing a hydraulic-roller camshaft such as those used in most 5.0 applications requires a liberal coating of cam lube (supplied with the cam or cam kit) and a steady hand. Use of a camshaft installation tool (such as the one shown) or even a long bolt inserted into the front of the cam can help you ease it into place without banging it into the cam bearings.
138_02z Ford_mustang Engine_view
Supercharger/nitrous cams share many of the same characteristics with turbo cams, except for the delayed exhaust opening event. Since the exhaust pressure won’t be used to drive the turbo, it can be released earlier. As a general rule of thumb, most cams that work well in a normally aspirated application will work well in a supercharged application as long as they don’t have excessive overlap.
One of the simplest and best ways to check piston-to-valve clearance is with the clay method. Place a piece of clay on top of the piston in the area to be checked (i.e., beneath the exhaust valve) and oil it so it won’t stick to the valve. Bolt on the heads, assemble the valvetrain, and rotate the engine several times. Remove the head(s) and measure the thickness of the clay—safe numbers are 0.120 inch on the intake side, and 0.150 inch on the exhaust. Remember, rocker-arm ratio changes the clearance, and advancing the cam reduces the intake clearance and increases the exhaust clearance. Milling the heads reduces piston-to-valve clearance, and larger valves usually require bigger valve pockets in the pistons.

Back in the day, there were only a few different cam-shaft designs available to hot rodders, and they were commonly known as either 3/4-race or full-race cams. The potential user really had nothing more to go on than the word of his buddies or the cam grinder as to how the new 'stick would work in his engine. A horrible idle or the total loss of low-speed performance were often the unfortunate results.

Today, all that has changed. Cam manufacturers offer hundreds of profiles to choose from, and they go to great lengths to take the mystery out of their products. Some will even work with you to custom-grind a cam to your specific needs.

The following is a primer course in the world of camshafts. It's designed to decipher camshaft lingo and give you a better understanding of how bumpsticks work. When it comes time to make your choice, however, there's no substitute for calling a manufacturer's camshaft hotline and talking with a tech who will help you choose the right cam the first time.