A race camshaft can offer engine performance gains for the speed conscious. The outcome depends on many factors, since the camshaft is just one part of an internal combustion engine. It is very easy to select a camshaft that will diminish engine performance, so let's cover the basics.
What is a camshaft?
A cam shaft is basically a lumpy stick. At least one performance manufacturer brands their cams as "Bump Sticks." Regardless, a camshaft runs the length of your cylinder head(s) and has cams--offset, egg shaped lobes--spaced across its length. The number of cams is dependent on the number of cylinder valves in your engine, usually a multiple of four.
The camshaft turns in time with the crankshaft and actuates the valves into opened and closed positions. In each cylinder there is at least one intake valve and one exhaust valve. Cars may have two, three or four total valves per cylinder with most older cars having two and luxury or high performance cars having four per cylinder. Econo-boxes normally have four to squeeze the most out of smaller engines. The general idea is that the intake valve(s) open and let the fuel/air mixture enter the cylinder as the piston travels down during the intake stroke. All the valves are closed during the compression stroke to build the effective compression. The fuel/air mixture is ignited, forcing the psiton down to generate rotating force, which powers your car. As the piston travels back upwards, the exhaust valve(s) open to expel the exhaust. Suck, squeeze, bang, blow. This happens thousands of times a minute in your car.
There are many internal combustion engine valve actuation methods, but the common ones on the street are pushrod and overhead cam engines. The pushrod, or overhead valve engines rely on a timing chain connecting the crankshaft and timing gear, which is affixed to the camshaft, to actuate the valves. The camshaft sits in the middle of the engine, with lifters and pushrods sitting on top of the cams. As the cams rotate the pushrods push the rocker arms up which open the valves for a specific duration. As the pushrods move down, powerful valve springs under the rocker arms close the valves. This setup is effectively limited to two valves per cylinder and was used on V-8 engines. Note that all the muscle cars of the 1960s and 1970s ran on this configuration.
The overhead cam does not use pushrods but instead rests directly on the rocker arms or lifters. They are normally driven by belts, as opposed to chains. Single overhear cams use a, uh, single overhead cam to activate all the valves in each cylinder head. A double overhead cam uses a separate cam to activate both the intake valves and exhaust valves. Four cylinder cars and newer V8s use this setup. Overhead cams usually allow for higher engine RPMs due to the decreased rotating mass inside the engine.
So what's a "racing camshaft?"
Glad you ask. Your engine is nothing more than an air pump. The more air it can draw in and burn, the more power it can produce. Since your camshaft controls the air flow in and out, you stand to gain a lot of power by picking out a good cam.
You should therefore ask youself, "Self, what's a good cam?" As you browse through a list of cams, you will notice the advertised lift and duration of a cam. The lift is effectively the maximum height that the rotation of the cam will lift the valves. Therefore, a cam advertised at .498"/.512" will lift, or open, your engine's intake valves 498 thousandths of an inch and lift your exhaust valves 512 thousandths of an inch. This number can be increased with higher ratio rocker arms, but those pose their own set of hazards, like striking a valve on the top of a piston. The second number, duration, is the length of time that the valves will remain open with the particular cam pattern. So a cam advertised with 220°/230° @ .050" will keep the intake valve open at 50 thousandths of an inch or more for 220 degrees of camshaft rotation and keep the exhaust valve open at 50 thousandths of an inch or more for 220 degrees of camshaft rotation. Sometimes the duration is given without a rating at the normal .050" which just means the duration that the valves are open at all. Remember that this thing spins in a circle, so any given numbers are out of 360 degrees of rotation.
You'll notice that this will force a certain amount of overlap when both the intake and exhaust valve(s) are open simultaneously. This happens when the exhaust valve is letting exhaust out and the intake valve is letting fuel/air in. Sucking and blowing at the same time, if you will.
You still haven't answered the question!
Ok, fine. A racing camshaft will offer bigger numbers than your stock camshaft. The problem with getting wowed by numbers it the potential to lose power or damage your engine. In the worst case, if you have too radical a lift, you'll touch the valve to top of the piston. This ruins both the valve and your day and requires the engine to be taken apart in order to fix it.
So how do you lose power if you're letting more air into your cylinders? Remember that bit about your camshaft only being a single piece in a complex air pump? Well, your cylinder heads have a lot to do with that. Each cylinder head casting has its own air flow characteristics. Believe me when I saw that both cylinder heads and camshafts are their own science. They are most certainly their own industries as well. The important thing to consider is that each cylinder head pattern has it's own flow characteristics. Some of them will flow very high numbers, measured in cubic feet per minute, at very high RPMs. Others may flow well at medium RPMs and mediocre at high RPMs. If you are unsure, consult a speed shop that frequently deals with your type of engine or call the manufacturer of the parts you're looking at. Similarly, each camshaft has it's own pattern that has the best numbers at certain RPM ranges.
The most important thing to remember is that your engine components must work together for the maximum benefit. Both cylinder heads and camshafts have certain power bands that gain, maximize, and lose horsepower and torque during specific RPM ranges. If your camshaft has great low end torque under 3000 RPM and good mid range horsepower from 3000 to 4500 RPM, while your cylinder heads gradually climb to peak at 6000 RPM, you won't have near your potential for maximum power. Those ever important flow numbers will be off. Your camshaft is flowing the most air in the middle of your RPM range, then dying off, while your heads are most potent near the redline. This is why gearheads in training need to consider their long term project goals before rushing off to buy parts. A racing cam is only effective when it's swapped into an engine with a matching powerband.
There aren't really any drawbacks other than price. If you try to install a camshaft or two in your engine and screw it up, you can be in very deep trouble. But that's hardly the camshaft's fault! Otherwise, it's pretty much clear sailing. There are too many engine control systems on the road to make generalized statements about performance, driveability and fuel economy when changing your cam. The aerodynamics of air flow inside the the cylinder heads is a very complicated subject. Furthermore, the exact same cam will produce different results when used with a flat tappet setup versus a hydraulic roller setup. Every little bit makes a difference, for better or for worse. Something too radical will give you problems in your daily driver. However, moderate configurations in your pride and joy will help you get around town quicker. Gas mileage is hardly affected, except for the part where you start flooring it every chance you get.
Oh, and for those of you who want that seductively beautiful, head turning sound of a big lopey cam, get a cam with big duration. Lift helps, but duration is what makes it heard. Best when used on a non roller, flat tappet setup.