CENTERLINE — A cam's centerline is the fixed point on its lobe that occurs exactly halfway between when the valve opens and when it closes. Locating a cam's centerline isn't as easy as it sounds, though, since peak lift doesn't last for just a single degree of crank rotation. All you've got to do is look at the shape of a cam lobe to see how difficult pinpointing its centerline can be. Cam degreeing is all about locating a cam's centerline and positioning it so that it occurs—in crankshaft degrees—exactly where the cam manufacturer says it should. If you've ever played with adjustable cam gears, then you've essentially done something similar, albeit somewhat arbitrary.
|This cam diagram reveals the cams' relationship to the four-stroke cycle. The big red curves represent each cam's profile. As their centerlines are adjusted by the degreeing process, their curves may shift either left or right across the diagram.|
LIFT — Cam lift is the maximum distance a valve opens away from its seat and is typically represented in inches or millimeters. Imagine a room full of people trying to exit through a single doorway. They can open the door wider (lift) or leave it open longer (duration). Either way, more people are gonna get through. To understand why you should care about lift, you've first got to understand the relationship the valves share with the cylinder head. Although valves are necessary for the engine's cyclical process, each valve's head (which is necessary for sealing against the valve seat) restricts airflow and must be moved as far away from its seat as practical to allow air to pass by. Increasing valve lift introduces more air and fuel into the combustion chamber and increases the rate at which exhaust gases may exit. Generally speaking, adding lift results in more power. However, the valves should also open and close as quickly as possible without causing valve float or damaged valve seats caused by excessive impact; the farther the valves open, the farther they've got to travel for each cycle. Cam degreeing doesn't affect how much lift will occur but it does determine when it happens.
DURATION — Increasing valve lift is an effective method for introducing more air into the cylinders, but holding the valves open longer (more degrees of crankshaft rotation) has a similar effect. Unlike lift, duration is measured in degrees of crankshaft rotation; it's the segment of the cycle that a particular valve is held open. You can't quantify duration in units of time, though, since at lower engine speeds the valves stay open longer than at higher engines speeds, despite equal durations. It makes sense then that adding duration holds the valves open longer—at any engine speed. Duration is fixed according to the cams but, like lift, cam degreeing determines when it begins and when it ends.
Lift and duration are independent values but do complement one another. Together they result in a cam's profile, otherwise referred to as its “rate of lift” or “ramp.” If a cam isn't degreed properly, its profile cannot fully be taken advantage of.
LOBE SEPARATION ANGLE — A cam's lobe separation angle is the angle measured in crankshaft degrees between its intake lobe centerline and its exhaust lobe centerline. Dual-overhead cam lobe separation angles can be manipulated with adjustable cam gears. This, combined with lift and duration values, will determine how much overlap there is. Cam degreeing not only locates the correct centerline but also allows the lobe separation angle to be realigned for optimum performance. Although trial and error on the dyno can lead to an ideal lobe separation angle, the risk of piston-to-valve contact is greater and the process can take significantly longer.
VALVE OVERLAP — The point at which both the intake and exhaust valves are opened simultaneously is known as overlap. Overlap generally occurs for a short period of time—when the intake valves are opening, the exhaust valves are closing, and that cylinder's piston is at TDC. Overlap might seem counterintuitive but the process helps draw exhaust gases out of the combustion chamber more efficiently. Also known as “scavenging,” the process can potentially increase horsepower by allowing for a more efficient cylinder fill by evacuating unusable gases more quickly. Too much overlap isn't always a good thing, though, and can draw the intake charge right into the exhaust, wasting both air and fuel.
Cam degreeing directly affects valve overlap and timing. Depending on which way the cam is adjusted during the degreeing process, overlap can increase or decrease as cam timing advances or retards. The important thing to remember is that, once degreed properly, valve overlap and timing are where the cam manufacturer says they should be. Small adjustments can then be made with the help of a dyno but, chances are, that might not be necessary.
|The anatomy of the camshaft. This diagram reveals some of the fundamentals that you should understand before attempting to degree cams.|
TOP DEAD CENTER — The piston reaches the top of its bore at TDC (top dead center). Although TDC is indeed a specific point, it's tricky to pinpoint since the piston can dwell at the top of its bore for several degrees. Dwell is simply a byproduct of the relationship between the crank and its rods. Other important reference points when degreeing cams include BTDC (before top dead center) and ATDC (after top dead center).
BOTTOM DEAD CENTER — The piston reaches the bottom of its bore at BDC (bottom dead center). Like TDC, BDC is also a specific point but can be just as difficult to locate because of piston dwell. Other important reference points when degreeing cams include BBDC (before bottom dead center) and ABDC (after bottom dead center).
Familiarize yourself with each these locations on the degree wheel before getting started.
Before moving on to the degreeing process, let's take a look at the intake cam we'll be using, which has a duration of 264 degrees. This particular cam begins to open its valves at 41 degrees BTDC, peaks at 98 degrees ATDC, and fully closes at 75 degrees ABDC. We won't need to concern ourselves with the cam's lift value in order to degree it.
WHAT YOU'LL NEED
- You can't degree cams without a degree wheel. A degree wheel will reveal all sorts of information relating to valve timing events. Larger wheels typically yield more accurate results.
- You'll also need a dial indicator with some sort of base that'll attach firmly to the cylinder head. If your dial indicator's got a magnetic base, bolt a scrap piece of steel to the cam caps and place it there.
- Some sort of pointer, like a coat hanger or a stick of welding rod, along with a piston stop will also be necessary.
- Your cam manufacturer should've provided you with a cam card. It's got all sorts of important information on it that you'll need to degree your cams.
- Gather whatever hand tools you'll need to access your valvetrain, adjust your valves, and rotate your crank.
|This simplified degree wheel doesn't have any more marks or information on it than necessary and is small enough to fit onto the edge of the crank with the engine still in the car. Numerically, this wheel is suitable for clockwise- or counter-clockwise-spinning engines.|