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Engine Rebuilding: The Camshaft

What to know about changing a camshaft

In spite of all the talk these days about engine management of the electronic kind, the camshaft is still calling most of the shots. Virtually every parameter such as peak torque, a usable power band, peak horsepower, manifold vacuum, fuel mileage and more are determined by the cam's timing specifications. No other single modification can produce as much horsepower per dollar. No other single modification can cost you as much power, efficiency and driveability if you select the wrong cam.

First Steps

So where do you start when selecting a camshaft for your resto or restomod project? If you're doing a restoration by the numbers, obviously you need a cam with factory specs. If the factory has discontinued grinding the cam that you require, chances are one of the larger cam companies are reproducing them or grinding one with similar specs. Most companies have comprehensive catalogs and excellent tech lines to guide you in the right direction. The information they provide includes the usable rpm ranges and recommendations for other changes that suit the camshaft such as lower gears, a high stall torque converter, exhaust headers or a larger carburetor.

If you are interested in the restomod approach for upgrading the power and efficiency of your vintage engine, then you can take advantage of more modern grinds that produce more torque and horsepower by opening and closing the valves faster within the same duration specification. Not all 280-degree camshafts perform the same. If you open the valve faster and close it at a faster rate, hence later, the camshaft thinks that it has more duration than it actually does. Adding more duration to a camshaft tends to reduce low-end torque and driveability. Opening and closing the valves at a higher rate does not and provides more effective duration.

Duration

So what's the practical limit for most restored engines without special modifications to accommodate the camshaft? Most engines respond well to cams with "advertised" durations of 260-270 degrees of duration. A more scientific method of measurement is duration measured at .050-inches of valve lift. Here you are talking 215-220 degrees of duration. Anything more and manifold vacuum drops to where the power brakes don't work too well and the engine is pretty sluggish off the line.

Duration isn't the only camshaft specification that's important to watch. Lobe separation angle, often referred to as the camshaft centerline, makes a significant difference in the way cams perform. Most resto cams are ground on 110-112-degree centerlines, a good compromise of power and fuel efficiency. Some drag race cams are ground on 104-106-degree centerlines to produce more mid-range torque, but they require many other mods to work successfully.

Another important camshaft spec is the amount of valve lift. More lift is good, but there are physical limits to take into account such as valve-to-piston clearance and valve spring retainer-to-valve guide clearance and valvespring coil bind issues. A good rule of thumb on modern V8s is .450-.475 inches of maximum valve lift. Consult with the cam manufacturer or physically measure these critical dimensions to be safe, especially if your engine has had special machine work such as decking the block or milling the heads. These measurements have a way of stacking up against you.

Once you have come up with the cam grind that is going to make your engine really sing, installing it is fairly simple. Some tips that make the job go more easily and ensure a successful break-in period are to thoroughly coat the cam bearing surfaces and lobes with a quality assembly lube. Screw a long bolt in the end of the cam to work as a handle to guide the camshaft through the cam bearings. Install a new timing chain and gear set carefully aligning the timing marks on the cam and crankshaft gear. Missing by just one tooth is not acceptable.

Heavily lube the valve lifters with assembly lube and adjust the valve lash before starting the engine. Make sure that the coolant level is sufficient and that you have an external shop fan to blow air through the radiator for break-in. Camshaft break-in is extremely critical to prevent your new camshaft from flattening one or more lobes. Have a timing light hooked up to the engine and the distributor loose before starting the engine so immediately when it starts you can set the timing and take it to a steady 2000 rpm in neutral and hold it there for 15 minutes. At a steady-state 2000 rpm, the valvetrain places the least possible load on the cam to lifter interface. This all may sound difficult—it's not! Just take your time and do it right. The results are worth the effort.

Tips

> Do your homework and select the camshaft that is right for your engine and drivetrain combination.

> Check the critical interference points caused by too much valve lift.

> Always install new valve lifters with a new cam. Old lifters have wear patterns in them that will quickly destroy a new cam.

> Follow the camshaft break-in procedure religiously. Failure to do so may cause the cam lobes to wear flat.