Cam Basics

Webster's Dictionary defines a camshaft simply as "a shaft bearing integral cams." A cam is defined as "a disk or cylinder having an irregular form such that its rotary motion gives to a part or parts in contact with it a specific rocking or reciprocating motion."

In an internal combustion engine, the camshaft actuates the intake and exhaust valves, which feed and evacuate the air/fuel mix into and from the combustion chamber. In the case of an overhead valve engine (OHV) like a small-block Chevy V-8, with the valves located at the top of the combustion chamber, which is part of the cylinder head, the camshaft causes the lifters to reciprocate up and down. The lifters activate the rocker arms, which causes the valves and stems to open and close in a reciprocating fashion.

For this reason, the cam has long been revered as the "brain" of the engine, opening and closing the valves at the exact precise instant, coordinating the timing of each event to the exact location of the crankshaft and piston. The cam also has the remarkable ability of repeating this process over and over, as many times as necessary, in order to keep the engine running at the peak of its capability.

Selecting a camshaft is one of the most important decisions affecting overall engine performance and durability. And, the cam is the one area where the builder can, in a sense, put his personal signature on the engine by giving it the idle and power band characteristics that he prefers; you might say putting some "soul" in the machine.

Unfortunately, one of the most common mistakes is to put in "too much cam," resulting in a rough-idle drag racer sound; cool but impractical on a daily driver. Plus, if the airflow path through the engine isn't suited to higher capacities, increasing the "lift" and "duration" will not produce more torque and power. In fact, it's more likely to reduce combustion efficiency, power output--and economy. (As a rule of thumb, limit maximum valve lift to 0.500-inch and valve-open duration to around 290 degrees.)

Many radical cams produce little in the way of bottom-end torque, which means low rearend gears are a must to keep the engine rpm up and in its power range, at the sacrifice of highway driving efficiency. Additionally, such cams can cause the engine to have low manifold vacuum at idle, which interferes with the operation of some power accessories, including brakes and steering.

We wanted to see exactly what installing the "proper profile" cam would produce in a mildly modified engine. So we decided to install a COMP Cams CL-Kit (which includes a cam, lifters, instructions and decals) in one of our Quaker State Dream Engine Giveaway GM Performance HO330 350 V-8 "crate motors." COMP Cams makes products that work with stock, near stock and street performance components, while maintaining acceptable fuel consumption and "streetability." So they're perfect for this engine.

To have the cam installed--and get real, quantifiable numbers--we took the engine to Westech Performance Group (in Mira Loma, California), where figures before and after were measured by running the engine in a controlled test cell on their enhanced SuperFlow 901 Engine Dyno with Windyn software.

Earlier we said "proper profile" cam swap. That's because we had the winner of the engine (Doug Burba of Choctaw, Oklahoma) talk to Westech's Technical Director Steve Brule about what his specific driving and performance requirements would be beforehand, so the new cam is exactly the profile Doug wanted/needed. They decided on installing an Xtreme Energy XE262H-14 hydraulic cam (P/N 12-262-4) and High Energy lifters (P/N 812-16). Note: You should always replace the lifters when installing a new cam.

Xtreme Energy cams are COMP's newest series of hydraulic and hydraulic roller cams. They are designed to take advantage of the latest improvements in valve train components and the newest developments in camshaft design. Their aggressive lobe design produces great throttle response and good top end horsepower (better than many other cams with the same duration @ 0.050-inch) while delivering increased engine vacuum. It's also just "lumpy" enough to produce a "noticeable idle," while retaining manifold vacuum, so everyone will know that it's not stock.

High Energy Lifters have a lightweight check valve disc that allows for quicker response. This results in increases in engine speed before valve float. This check valve also maintains added control at all engine speeds and loads. Increases in engine speed can be attributed to this quicker response check valve.

Of course, the best way to install a cam is to "degree" it into the engine so you know exactly "where" the cam is located. Advancing the cam will move all of the lobes to open and close sooner in the cycle. Generally, advancing a camshaft will increase low- and mid-range torque at the expense of some top-end power. (For more information on degreeing a cam see "Perfect Timing," LRM April 2004.)

The new cam yielded obvious torque and horsepower improvements in the low- to mid-range rpm range, which is where you do most of your driving. We'll be bringing you a detailed "how-to" on installing a cam and lifters in the coming months in our "383 V-8 Rebuild" series. For now, check the graph for detailed results of the COMP Cam install.

Conclusion Armed with these basics of camshaft operation, you should be able to see why selecting a camshaft is not as easy as you might have thought, and that this simple-appearing device is actually an incredibly complex set of mathematical models that have a headlock hold on the engine's power curve.

The good news is that most street performance camshafts are relatively affordable and easy to install in the engine. So if you make a mistake, it is easy to rectify. But regardless of whether you're a baller or cruiser, a hard-core engine master or a first-time builder, there's a ton of available information to learn so that you can be the cam guru on your block who everyone turns to for the correct information.

Camshaft Terminology

There are several abbreviations and terms that are used when discussing camshafts. The following abbreviations have to do with the location of the piston in the cycle.

TC - Top center (piston at the highest point)

BC - Bottom center (piston at lowest point)

BTC - Before top center (piston rising)

ATC - After top center (piston lowering)

BBC - Before bottom center (piston lowering)

ABC - After bottom center (piston rising)

Symmetrical - A cam that is symmetrical has both sides of the cam lobe exactly the same. In other words, the intake ramp of the cam lobe that accelerates the lifter to actuate the valve has the same shape as the portion of the ramp on the downside of the lobe that lowers the lifter. These designs are very easy on the valve train as it is a smooth transition from open to closed.

Asymmetrical - An asymmetrical cam has opening and closing ramps that are different. These profiles are usually found on high-performance cams and offer a high-velocity opening and a lower velocity closing ramp in order to snap the valve open quickly and then set it back down more gently.

Dual Pattern - A grind that is usually found in a high-performance cam. The intake lobe configuration is different from the exhaust lobe. Usually the exhaust lobe is ground with slightly more duration that the intake lobe.

Cam Walk - A phenomenon that occurs with roller cams due to slight inaccuracies in the lifter bore spacing. Most roller cams use a cam button to control the tendency of the cam to unscrew itself from the block. Bushing the lifter bores can control this problem but is very expensive. A cam button will work quite well in 99-percent of cases.

Valve Opening and Closing Angles - The angles (usually measured in crankshaft degrees) when the valves first leave and then return to their seats.

Duration - This is the number of degrees the valves are "off their seats" in the four-stroke cycle. Duration is usually expressed in crankshaft degrees. Duration may also refer to the number of crankshaft degrees that the lift is greater than a specified value; e.g., duration at 0.050-inch lift.

Lobe Lift (Gross Lift) - The difference between camshaft lobe height and lobe width. For an installed cam, lobe lift may be measured from the lifter.

Valve Lift (Actual Lift) - This equals lobe lift less the tappet gap measurement or it is the lift measured from the top of the valve.

Valve Clearance (Tappet Gap) or Valve Lash - The maximum space between the end of the valve stem and the lifter.

Overlap - This is the number of degrees of duration that the intake and exhaust valves are open at the same time. Overlap should not be present in extremely low rpm engines. In fact, the stock Ford cam has 13 degrees of negative overlap. Overlap is present in most high-rpm engines and most of the reground cams.

Full Lift or Lobe Centerline - The centered point of the highest lift of a cam lobe, expressed in crankshaft degrees.

Lobe Center Angle or Lobe Separation Angle - The angle formed between the full lift of the intake lobe and the full lift of the exhaust lobe for a cylinder, expressed in camshaft degrees.

Lobe Terminology - Some of the terminology, which describes a single lobe (as shown in Illustration A).