Shaft Shrift

You’d think national security was at stake, considering the way engine builders, racers, and some enthusiasts keep their high-power engine’s camshaft specs a triple-sealed, vacuum-packed secret. When you get up into the Pro Stock drag racing and Winston Cup circle track strata, where a fraction of a percent in power advantage can put you in the money or on the trailer, cam specs are a very real issue of team security.

One cam manufacturer with involvement in racing told us that because Pro Stock engines have altered lifter bore angles and much less pushrod deflection, they can shorten up duration (from where they used to be five or six years ago) and still gain lift. He said that typically, Pro Stockers use up to 1 inch of valve lift, a little more than 300 degrees of duration at 0.050-inch lift, and 114 to 118 degrees of lobe separation, depending on the combination.

As a comparison, Trans-Am racing engines, which must make power over a much broader range of engine speeds, have roughly 0.420-inch lobe lift, not more than 0.700-inch valve lift, 255 to 265 degrees intake duration, and 262 to 275 degrees exhaust duration. Lobe separation angles are in the 104 to 108 range, helping to extend the powerband down for good torque to accelerate off corners. Yet, engines never dip below 4,000 in a race, and T-A teams are rev-limited to 8,800 rpm.

For the vast majority of weekend racers and hot rodders, however, selecting the right camshaft for any application should not be perceived as a mystical, otherworldly task and should not emulate the aforementioned racing examples. It’s all absolute, tangible numbers and science. Yet still some buyers go wrong.

Lesson number one: Duration is not like chocolate, exercise, and sex. You actually can have too much. And many often do. Regardless of a cam’s lift, duration dictates the attitude of the engine. Long-duration, high-overlap cam profiles create lumpy idles, little vacuum to operate the assist for brakes, and higher levels of noise than those with moderate duration. It’s the internal clock that controls at what point the air/fuel mixture prepared by the engine’s carburetion will be introduced into the cylinders, and secondarily how much of it will be introduced.

Camshaft selection should have the same thorough thought invested in it as suspension design and setup do. Without knowing each of the following parameters, no professional engineer or consultant will make a cam recommendation: the end use, limiting valvetrain factors (such as stability and lift limitations), engine alterations from stock, transmission type (including converter stall speed), compression, vehicle weight, gear ratio, and others. But perhaps most important is operating range—the engine speeds in which the engine must make power. Only then can the consultant get an accurate framework in which to work.

One thing a performance camshaft cannot do is increase unobstructed airflow of the cylinder heads, one of the most important elements for making power. Intake and exhaust port sizes and valve sizes dictate this. All the camshaft does is orchestrate the amount and duration of each shot of air/fuel mixture into each cylinder. This begs a little airflow theory.

As engine rpm rises, the speed of the pistons naturally rises, too. But the velocity of the intake charge being introduced into the cylinder usually remains the same. Once the charge is moving, it will generally keep flowing into the cylinder after the end of the intake stroke—when the piston starts rising from BDC on compression stroke—if the valve is kept open longer.

Therefore, in the best of all possible worlds, your cam timing would optimally be retarded as the engine revs higher and higher. This way, it would make up for the charge’s velocity, which never changes. But since piston speed rises, the charge moves slower and slower relative to engine speed. Since the intake charge velocity is a constant (assuming no movable intake passages or flaps exist), but engine speed certainly is not, we have long- duration valve timing for engines that rev high. Since you can’t exactly change the cam as you’re racing, you must set an operating range for your engine and pick the cam that best serves that range and the vehicle’s use.

Here’s a rough guide that addresses cam duration (all at 0.050-inch lift) and characteristics, adopted from a guide by Crane Cams:

Under 200 degrees hydraulic: Smooth idle, good fuel economy. Use with a stock or low-cfm carb and stock or small-diameter tube headers. Can recurve distributor or use aftermarket ignition. Retain stock rear gear ratio.

200 to 220 degrees hydraulic, under 230 degrees mechanical: Good idle, low-rpm torque. Good for towing, RV, and off-road use. Works well with low-cfm carb, torque-producing manifold, aftermarket ignition, and headers. Good in cars up to 4,000 pounds and moderate-sized engines. Stock or mild gear changes are OK if not too high.

215 to 230 degrees hydraulic, 230 to 240 mechanical: Fair idle, good low and midrange torque and horsepower. Good in cars 2,500 pounds and heavier with small engines, or in cars up to 4,000 pounds with medium-sized engines (larger-displacement small-blocks). Good with low-cfm carb and aftermarket manifold, headers, and recurved ignition. Near-stock converter stall speed recommended. Don’t use with more than 10.25:1 compression. Moderately lower gearing OK.

230 to 245 degrees hydraulic, 240 to 255 degrees mechanical and roller: Rough idle, good mid- to high-rpm torque and horsepower. Good in cars up to 4,000 pounds with large (huge small-blocks, normal big-blocks) engines. Must have higher-than-stock-cfm carb. Intake, headers, and ignition should be matched to rpm range. Can port heads. High-rpm stall-speed converter and up to 4.5:1 gearing preferred.

240 to 255 degrees hydraulic, 250 to 265 mechanical and roller: Rough idle, mid- to high-rpm torque and horsepower. Cars up to 4,000 pounds with medium to large engines (big small-blocks and all big-blocks). High-cfm carburetion (can use multiple carbs) on a single-plane or tunnel-ram–type manifold with large tube headers, low-restriction exhaust, and hot ignition. Ported heads work best. Very high stall-speed converters and roughly 4.5:1 gearing or higher needed.

Many cam companies give a minimum and maximum compression ratio acceptable for a given cam in a noncomputerized engine. The minimum reflects a concern for sufficient torque output, and the maximum figure is based on keeping cylinder pressures within a range that pump gas can support. Normally, somewhere in the range of 9.0:1 to 9.5:1 with iron heads is safe, while aluminum heads can usually allow 0.5:1 compression more.

Computerized engines present another problem. Acceptable cam timing changes from stock vary based on the ECU’s program and the type of injection used. Speed/density systems are intolerant of vacuum changes and are restrictive with respect to any moderate engine changes. Mass air systems, however, are much more adaptive and can operate within a wider window of vacuum, air volume, and speeds. Most throttle-body systems are grouped with speed/density systems because they’re not intelligent; they can’t adapt and compensate.

You can also pick a cam based on airflow, but not on total flow numbers. Along with compression and operating range (engine speed at cruise, often dictated by gearing and tire diameter), the difference in airflow from intake to exhaust ports offers a guide for duration figures. For example, if your exhaust ports flow 60 percent of your intakes’ cfm figures (which is pretty restrictive), then use a lot more duration on the exhaust ports, regardless of lift. This applies to many street-driven cars and suggests choosing a split-duration cam, of which there are many to choose. Similarly, most dyno tests done both by manufacturers and sanctioned by magazines use dyno headers, which give much better exhaust flow than the corresponding car’s exhaust system. A dual-pattern cam with more exhaust than intake duration can make up for some of this backpressure.

Duration is also a concern when dealing with car weight. A 350ci engine in a light car with minimal exhaust restriction can get away with less difference in duration from intake to exhaust timing, whereas a heavier car will enjoy more of a difference in duration. Here’s an example: If we have a 2,800-pound ’68 Nova with a 350 and a fairly standard driveline and exhaust, it could use 222 degrees duration on both the intake and exhaust for good all-around street performance. However, in a 4,100-pound ’67 Impala with an engine of the same size and general state of tune and the same engine operating speeds, a split-duration cam with 222 degrees intake duration and 236 degrees exhaust duration would perform better (perhaps much better) than the single-pattern cam used in the Nova. In general, the lighter the car, the more free-flowing the exhaust; therefore, the less duration difference it can get away with. Not total duration, mind you, just duration difference from intake to exhaust. With some cams for late-model cars, 10 to 15 degrees of duration difference is commonplace.

Keep in mind that lobe separation angle has a direct effect on cylinder sealing (the portion of time both valves are open versus closed). Narrower angles seal the cylinder longer than wider angles do, increasing low-end torque. If intake and exhaust valves stay open too long at intake and exhaust stroke transitions, the exhaust can blow out the intake valve, and the intake charge can be shoved out the exhaust port. This makes idle quality very poor; therefore, we usually see “street” cams with no less than 110 degrees of lobe separation.

Also, remember that small-block Chevys use rocker arms that multiply cam lift by 1.5 from the factory. Cam specs frequently include lobe lift specs (read right off the cam) and gross lift specs (read off the end of the rocker arm for actual total lift at the valve).

In general, though, small-block Chevys like a lot of valve lift, and high-ratio rockers may fill the need without changing cams. In addition, according to Crane Cams’ Mike Golding, any vacuum loss caused by using high-ratio rockers (they can shorten the vacuum signal) is gained back by using a true full roller rocker, not a roller-tip rocker. The reduction in the roller’s friction gives this difference.

Hydraulic cams—by far the most common—require no maintenance to speak of (aside from checking for proper preload) and operate quietly, though they begin to poop out at roughly 6,500 rpm in most V-8s.

Solid cams (with the requisite solid lifters) are able to cope better with higher rpm because they don’t depend on small oil-fed valve assemblies inside them. Where they are allowed by class rules, solid cams and lifters are always used in racing engines. Both solid and hydraulic cams are commonly referred to as “flat-faced,” as are the attendant lifters. In general, a flat-faced cam is good for an operating range about 3,500 rpm wide.

A variation on both is the roller cam, which uses lifters with roller bearings. No solid cam should be mated to a hydraulic lifter, and vice versa. Roller cams frequently outperform flat-faced cams in operating range, yielding a 4,000- to 4,500-rpm-wide operating band. Either of these two cam types will work on either side of these windows, but the best power will be within those points.

Related Stuff

After you’ve picked out your cam, you’re not quite done. You still have to make sure the valvesprings generate the correct pressure and an acceptable amount of valve travel, given the balance of your valvetrain. The simplest way to assure yourself of this is to order matching valvesprings and retainers from your cam manufacturer, but this isn’t always necessary. If you’re going to rev the engine far higher than intended by the factory, you’ll need different valvesprings. If you’ve increased lift appreciably, you must be sure the spring has sufficient clearance at full lift, too. Always match valvespring pressure to the cam company’s recommendations. Pressures that are too high will increase friction and therefore wear.

Degreeing The Cam

Degreeing the cam in your performance-oriented engine is crucial in order to maintain the cam’s phase with the crankshaft. The cam grind you choose is supposed to open and close the valves at a specific moment during the crank’s 720 degrees of rotation. Manufacturers work from the lobe centerline—the point where a valve is at maximum lift (not lobe separation). When the cam isn’t installed on the prescribed lobe center (a few degrees advanced or retarded), valve action will not be what the manufacturer intended. Since there are tolerances to deal with, timing-chain sprocket marks that line up perfectly may not reflect a cam installed on centerline.

To degree a camshaft, you can either measure lobe centerline or check valve opening and closing at 0.050-inch lift. The latter is often called the “timing card” method. It’s best to use the cam manufacturer’s recommended method, since the specs given are reached by that particular method. All the manufacturers have instructions on how to degree a camshaft, and you’ll also need to obtain a degreeing kit (also sold by the cam companies), which comes with complete directions.

Professional engine builders sometimes alter the cam’s factory-intended phase with the crank for power curve reasons. By advancing the camshaft as little as 3 to 4 degrees, a builder can boost low-rpm responsiveness. Alternately, he can retard the cam by a few degrees to increase high-rpm power. Typically, one shouldn’t have to do this, but if the powerband itself (a useful operating range of, say, 4,000 rpm from 2,500 to 6,500 rpm) is too low, with the car getting too much wheelspin low down, the cam can be retarded. This will bring that 4,000-rpm-wide band up a bit, softening low-end grunt while netting a bit more upper-range power. Degreeing the cam greatly is dangerous, though, because valves come closer to pistons—never a safe situation.

Common Problems

Often, poor valvespring pressure acts like float-bowl drainage due to poor fuel pressure. First and Second gear may be quick, but the engine softens in Third and Fourth because the float bowls have depleted their fuel, and fuel pressure isn’t filling them back up. It may seem like a fuel problem because you usually have enough power in the lower gears to spin the tires, therefore masking the problem early in the run. (If the problem is indeed fuel, check the fuel pressure at the higher rpm safely.)

Many late-model factory heads can be restrictive in spring pressure and travel. When checking clearance on a hydraulic-lifter engine, people frequently turn the engine over to measure it. While they’re doing this, the lifters are bleeding down, giving them an indicated 0.120-inch additional clearance that, when the oil pressure is up, won’t be there.

Another area that must be checked in late-model factory heads (when installing a higher-lift cam or higher-ratio rockers) is retainer-to-valve-seal clearance. This must be done with the valvespring off.

Crower, Isky, Crane, Lunati, Competition Cams, and all the other cam specialists agree on one thing, however. The number-one most common mistake people make is second-guessing the pros and using too much duration. Small-displacement street engines cannot get away with a lot of duration the way larger-displacement engines can. As individual cylinder displacement increases, the duration can increase modestly, yet the engine’s idle and powerband behavior will not change.

…Another benefit of many cubic inches.