Overlap
What It Is: The amount of time, measured in crankshaft degrees, that both the intake valve and the exhaust valve are open. This situation happens at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift or duration or decreasing lobe separation angle increases overlap.Why It's Important: Some overlap is desirable, since the outgoing exhaust gases help pull in the fresh intake charge. Too much, however, leads to a contaminated charge that doesn't burn well. An increase in overlap generally improves top-end power at the expense of low-end grunt and reduces idle quality.
Single-Pattern Cam
What It Is: A cam with identical intake and exhaust lobe profiles, employing equal amounts of duration and lift. For example, a Crane Energizer 266 H10 (PN 10004) has 266 degrees advertised duration, 210 degrees duration at 0.050 inch, and 0.440 inch lift on both the intake and exhaust sides.Why It's Important: Some engines, especially those with heads that flow well on the exhaust side, may work best with a single-pattern cam.
Valve Lift
What It Is: Lobe lift multiplied by the rocker arm ratio. For example, if we have a lobe that measures 0.334 inch and are running a 1.5:1 rocker arm ratio, 0.334 x 1.5 = 0.501 inch valve lift. Running a 1.6:1 rocker arm creates 0.534 inch valve lift.Why It's Important: It's the distance the valve is lifted off its seat at the cam lobe's highest point and influences how much power an engine will make within its rpm range.
Valvetrain Checklist
If the most frequent mistake cam swappers make is choosing the wrong cam, the second is most certainly neglecting to upgrade the rest of the valvetrain to go with the new 'stick. "Many people may not fully appreciate how each part has to work together," says Knight. "And creating a good, stiff valvetrain is critical." Special attention should be paid to the valvesprings- just because what's already in your cylinder heads can handle an increase in lift doesn't mean they've got the necessary spring rate or pressure for the new cam profile. There's a host of things to check when stepping up your cam-run through this list and you'll have your bases pretty well covered.
Lifters
* Make sure you use the correct lifters for your application. "You can't run hydraulic lifters on a mechanical cam, either flat-tappet or roller," warns Knight. "The incompatible lobe designs can possibly lead to the lifters literally exploding."
* Never reuse flat-tappet lifters-either solid or hydraulic-on a new cam. These mate to the cam lobe during initial break-in and won't wear in properly on a new cam.
* Be sure to use a break-in oil and either oil additive or diesel oil when first running a hydraulic or solid flat-tappet cam. It may also be a good idea to remove the inner valvesprings for break-in if you're running high spring pressures.
* Roller lifters-either solid or hydraulic-can be reused if they're in good shape.
Pushrods
* Many guys try to save a buck when it comes to buying pushrods, but investing in good, stiff pushrods can pay dividends. "We took a 'nothing' motor, put in good pushrods, and all the little sags and dips came out of the powerband," says Knight. "We've seen increases of 7-9 hp with a pushrod upgrade."
* Hold off on buying pushrods when you buy your cam and lifters. Instead, use a pushrod length checker to determine the proper pushrod length, then order them. In short, the proper pushrod length is the one that yields correct valvetrain geometry.
* Check pushrod-to-head clearance whenever changing a cam or other valvetrain components.
Rocker Arms
* Traditional stamped-steel rocker arms are fine for stock applications, but any performance application should be using a good set of roller rockers-they're more rigid and cut down on power robbing friction.
 Crower has camshafts for just...  Crower has camshafts for just about any application, though we think it's fair to say much of its clientele leans toward the hot street and racing side of the game. The cam shown is a solid roller 280R piece intended for the former, along with some of Crower's Severe- Duty cutaway lifters and titanium retainers. The company makes aluminum stud-mount rockers, ofcourse, and its shaft-mount systems are available in aluminum or stainless steel, in a variety of offsets. |  "One of the absolute killers...  "One of the absolute killers to highend horsepower is lack of valve train stiffness, Comp's Billy Godbold tells us. "You lose stability, and even if you can control the valve with its spring, things start to bend." As a result, duration-and therefore high-rpm horsepower-is lost. When should you look into using a stud girdle? Godbold actually surprised us with his answer.... |  ..."If you're going racing...  ..."If you're going racing with studmounted rocker arms, you should use one, but if you have the right cam, you should be running fairly close to its limit speed; you should be on its dynamic threshold." In other words, when you've already upgraded everything else in your valvetrain and are running near its limit, it's time to consider a stud girdle. "If you're after that last 5-10 hp, you should look into it," says Godbold. "It lets you lock all the studs together, and if you have a stiffer upper valvetrain you have more duration." |