The purpose of the camshaft is to synchronize the timing of the intake and exhaust valves, and the amount of opening and closing, to yield optimum combustion pressure in the cylinder. This results in the most pressure being applied to the piston/rod assembly to rotate the crankshaft to produce torque. This timing is a vital function in the engine because the camshaft controls how much to open the valves and at what rate, and when to open them and when to close them. It's a straightforward mechanical timing device essential to engine efficiency. Here is an overview to help you choose the right one for your racing.

Mistake No. 1
One of the most common errors you can make is to "overcam" an engine, which means to install a camshaft with too much lift and duration (explained later) for the type of racing for which the engine is used. The result is like putting too large a carburetor on an engine-the engine is inefficient (lazy) in just the rpm range where you need it to operate the best. This ideal rpm operating range is where the camshaft will produce the best power, and it is approximately 500 rpm wide.

You are trying to pick and install a camshaft that will put this 500 rpm window at the point on the track where it can best be used given your car's gearing and tire diameter. When you choose a camshaft, you can raise or lower this rpm range.

Of course, we can't cover every engine model or racing type. We're going to use the typical in-the-block camshaft setup of the overhead valve (OHV) V-8 engine using pushrods operating one intake and one exhaust valve as our working example to explain camshaft terms.

While we'll cover some of the basics of camshaft operation here, it is a very complex subject and beyond the scope of this article-and frankly, probably beyond what most of us want to know about camshafts. We want to get on track and race, not design camshafts. Consequently, the best advice we can give for choosing a camshaft is to become friendly with your performance aftermarket camshaft manufacturer's tech line. They have invested years of researching, developing, and testing camshafts and valvetrain components for every racing application. By answering their questions, you can get a camshaft very closely matched to your racing and engine type.

Four Stroke Engine Cycle
Before we examine the camshaft's function and its relevant terms, it's prudent to review in general terms the four strokes of a typical racing engine cycle so we can see how the camshaft is essential to their operation and achieving maximum power. Remember that each stroke lasts for one half of a crankshaft revolution, and because the camshaft turns at half the speed of the crankshaft, each stroke is only one-quarter turn of the camshaft. This will be an idealized and simplified discussion. Not all of the following mechanical action operates instantly, nor right on the nth degree of crankshaft rotation (there is a reason OHV and pushrod valve actuation is called "monkey motion" by its detractors), but it will be sufficient to get us started.

Power Stroke
Let's start with the piston at the top of the cylinder (Top Dead Center, or TDC) when both valves are closed and when the spark plug has just fired. This is the start of the power stroke-the air/fuel mixture has exploded and the expanding gases move the piston down. When the piston nears the cylinder's bottom (Bottom Dead Center, or BDC), the exhaust valve starts to open. Ideally, the mixture is fully burned and the remaining cylinder pressure starts the evacuation of the cylinder.

Exhaust Stroke
The piston moves past BDC and starts upward. This begins the exhaust stroke. The shrinking cylinder volume forces the spent gases out the exhaust port, and as the piston continues up to TDC, the exhaust valve fully opens, travels through its maximum lift (extension), and begins to close.

Intake Stroke



Compression Stroke


Camshaft Terms




Duration: The amount of time that a valve is open. It is specified in degrees of crankshaft rotation. Shorter-duration camshafts yield power in a lower rpm range. Longer-duration cams work better at higher rpm, at the expense of bottom-end power as duration time is increased. Camshaft duration is defined at a specified lift point, and confusingly, there are two types of duration usually specified: advertised duration, and duration at 0.050-inch tappet/lifter lift. Duration at 0.050 inch is the one to use when comparing camshafts.

Two values are critical in specifying duration: crankshaft degrees of rotation, and the point of the tappet/lifter rising at which the measurement is taken. Advertised durations are measured all over the map; that is, not at any regular tappet/lifter rise value. In our valve lift curve plots, the advertised duration is measured at 0.006 inch (the line just above the horizontal axis). It can vary from one cam manufacturer to another. Remember, too, that tappet/lifter lift is at 0.006 inch, and actual valve lift is this value multiplied by the rocker arm ratio. In this example, it is 0.006 inch x 1.5 = 0.009-inch lift at the valve.

The opening and closing points of each valve are circled in this plot (just above the horizontal axis), and counting the number of crankshaft degrees between them gives us an advertised duration of 270 degrees for both exhaust and intake lobes. See this value specified by the dimension arrows below the horizontal axis. When the duration is the same for both lobes, the cam is called a single pattern cam.

A dual pattern cam is one that has a different profile on the intake and exhaust lobes. The lift and duration on the intake may be smaller or larger than the lift/duration on the exhaust lobe.

Duration at 0.050-inch tappet/lifter lift in this example is specified farther up the valve lift plot line; and in this example it is 224 degrees duration. Keep in mind that actual valve lift at this tappet/lifter rise value is 0.050 x 1.5 = 0.075 inch. That is where the second set of circles is placed up the valve lift curves.

We can see that in the simplified marketing world of "mine is bigger than yours" that using advertised duration instead of duration at 0.050-inch tappet/lifter lift might be a perceived advantage. Always rely on duration at 0.050 inch to compare camshafts.

Lobe Separation Angle: The number of camshaft degrees separating the peak lift point of the intake and exhaust lobes. Lobe separation angle is fixed in the camshaft after its initial grind and can only be changed by regrinding the camshaft. In our valve plot, the lobe separation angle is 110 camshaft degrees-the number of degrees peak to peak.

Lobe separation determines where peak torque will occur in the engine's power rpm range. A lobe separation of 106 degrees is "tight" and causes maximum torque to start earlier in the engine's rpm range. It will increase quickly and then peak early. A lobe separation of 110-112 degrees is more "open," or broad, and spreads torque across the ideal operating rpm, and improves power at upper rpm.

Intake And Exhaust Centerline:
These centerlines are sometimes confused with lobe separation angle. We noted that lobe separation angle is fixed by the camshaft grind. Intake/exhaust centerline is referenced to piston TDC, thus it is adjustable by where the lobe's centerline is installed. This centerline is the position of the centerline-the peak lift point on the intake/exhaust lobe-in reference to piston TDC. It is the maximum lift point of the lobe related to TDC in crankshaft degrees. It can be changed-either advanced or retarded-and doing so is called degreeing a cam. Camshaft manufacturers supply a recommended intake centerline installation position.



Need To Know

* Track type/size
* Engine size and number of cylinders
* Bore and stroke
* Compression ratio
* Connecting rod type/length
* Rocker arm ratio
* Carburetor size
* Intake manifold type
* Intake/exhaust valve size
* Transmission type
* Torque converter stall speed (if automatic)
* Rear axle ratio
* Rear tire size
* Fuel type and octane rating
* Car weight* RPM powerband operating range of engine

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