Choosing the Right Performance Camshaft - Camshaft Basics

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.