To make choosing a cam for your engine less of a guessing game and more of a decision based on knowledge and experience, the basics of camshaft technology have been explained thoroughly in the last few How It Works tech articles. If you are building your first engine or are trying a new engine-parts combination, you are gaining experience as you go, and you may need to rely on a cam company’s experience to select the proper camshaft. It is important to know what information the cam company will need so it can give you a good recommendation. You’ll probably also have questions to ask of the cam company tech people after they have recommended a cam. These tips will make the purchasing experience much easier and get you the right cam for your application the first time.
What Is Needed To Select A Cam?
To get the engine performance you want, you need to provide the proper information to a cam company so it can recommend a camshaft for your engine combination. The most common engine built by the first-time hot rodder is a small-block—whether it’s a Chrysler, Ford or GM engine. The engines are in the 300- to 400ci range and have similar traits. Here, we have used the 350ci Chevy, the most common of these engines, as our example. And to make this example a real-world version of a first-timer’s engine, we’ve used as many factory components as possible to keep the cost down (cast crank, stock rods, pistons, valvetrain) and added an aftermarket intake, carb and headers. Custom machine work should be kept to a minimum to keep down the cost and complexity. This engine will cost from $1000 to $3500 to build, depending on how much scrounging you do to get the parts together. It can make between 250 to 300 horsepower at around 5000 rpm and about 350 lbs-ft of torque at 3500 rpm. The following information in the chart to the right should be determined long before you call the cam company.
That list makes a rough outline for a common engine combination that will have enough power to be fun and will be durable and inexpensive to build. Using the above information as an example, we called a few of the top aftermarket camshaft suppliers to get their camshaft recommendations. Their suggestions are listed to the right in Table 1.
Almost all of the cams recommended will have a stock-type idle with vacuum at idle being near the stock 18 to 22 inches of mercury (in/Hg). Since many hot rodders want their hot rods to sound the part, an idle with more lope to it might be desired. To get a stock-type engine to do this, a cam with more valve overlap—which can be achieved through either more duration or tighter lobe-separation angle—will be needed. Idle quality will decrease with the increase in duration or decrease in lobe-separation angle because the valve overlap causes weird vacuum signals at the carburetor, making the fueling inconsistent at idle. So if you are running a stock automatic transmission and converter and vacuum-assisted equipment (power brakes, for example), a lopey idle might not be what you want.
The reason most want the lopey idle is because race cars usually have a rough idle—and we want our cars to look and sound like race cars. The reason race cars have that rough idle is because the engines used are designed for high-rpm operation. To get the air/fuel mixture in the engine and the exhaust out at high rpm, race cars need a camshaft with as much lobe lift and as many degrees of duration as possible and a narrow lobe-separation angle, which results in valve overlap. A cam like this has extremely poor idle quality—the engine won’t run below 2000 rpm—but idle quality doesn’t really matter on a race car.
For the low-buck street guy to get a lopey idle and see an increase in power without a drastic decrease in driveability, he needs a cam with a lobe-separation angle of 110 degrees—the same lobe lift as recommended earlier—and duration numbers of about 218 degrees at .050 inch lifter movement. This would be roughly equal to 270 degrees of “advertised” duration. The reason you want to stay away from an increase in lobe lift is because of the changes you’ll need to make to a stock engine if the cam has more than approximately .460 inch lift. Any more than that and the valvesprings could begin to suffer coil bind, the retainers may hit the valve-stem seal, or the valve may hit the top of the piston. If you are trying to build a low-dollar engine, making changes to the valvetrain will run up the cost of the engine considerably.
This brings up another critical point regarding camshaft selection. The cam is just one component in the very intricate valvetrain system. Making a drastic change in the camshaft will most likely require changes throughout the valvetrain. As an example, for the cams recommended in Table 1, many of the cam companies recommend new valvesprings, rocker arms and pushrods. (The cams usually come with new lifters, and some come with new timing-chain sets.) They don’t recommend these products just to make money off of them. The stock valvesprings often need to be shimmed to obtain the minimum acceptable seat pressure, whereas the new springs will need much less in the way of shims. When you shim a valvespring you take away compressed height, something a stock spring is very close to running out of when you use an aftermarket cam.
There are several problems with using stock rocker arms: They aren’t designed for increased-lift cams, they can be worn from usage, and manufacturing variations can result in rockers with different ratios. Most of the aftermarket cam companies have competitively priced rockers that are designed and manufactured for street/strip applications. With these rockers, different pushrods may be required to maintain the proper valvetrain geometry to essentially keep the rocker tip working directly over the center of the valve stem throughout its full range of motion. As you can see, one change can mean making modifications to the entire system to make the whole package work properly.
Building your first engine is very exciting. Often, it is done on a strict budget with many used parts, and the car that the engine is going into will be driven daily. If you keep in mind the usage the car will see, and also consider future improvements to the drivetrain (possibly going from the 3.50 gear you have to a 4.10, or installing a 3000-stall converter), you can design an engine package that can be easily upgraded in the future. Cam companies can help ensure that your money is well spent if you provide them with enough correct information to make those decisions. Get some of the cam companies’ catalogs and read, or perhaps study, the pages on cam basics. The cam companies want you to be an educated buyer so you are a happy customer, and they try to ensure this by putting as much information in the catalog as possible. The more you know, the better the decisions you make with your money will be, which will most likely get you the engine combination you want—a strong-running, durable small-block.
