Modified Mustangs & Fords
Engine Building Part 3 - Power Principles, Heads, Cam, and Intake
Choosing the right Induction components
Let's go over some camshaft terminology and discussion now:
Duration: Camshaft duration is the amount of time (in degrees of rotation) that the cam holds the valve open. You'll commonly see durations listed as "advertised" or "at 0.050-inch valve lift." Essentially, when you see a number in front of the "duration," it's telling you how long the valve is open if you start at that number, open the valve fully, then bring it back to the position you started with. For instance, a camshaft with a 230-degree duration at 0.050-inch means that if you started with the valve 0.050-inch off the seat, then it would take 230 degrees to fully open it, then close it back to where it's 0.050-inch off the seat. Advertised durations are rated at different values, depending on the type of camshaft and the manufacturer. We typically see them rated at 0.006-inch and 0.020-inch. If you look in a lobe catalog, you'll also see durations rated at 0.200-inch.
So what do these numbers mean to you? If you look at the 0.050-inch duration, the 0.200-inch duration, and the advertised duration, you can get a very good handle on what the camshaft is going to do. For example, the 0.050-inch duration will give you a very good idea on where the engine’s horsepower peak will be (this also depends on head and intake flow, so you have to be careful here). A 347 with the typical 185cc AFR head package will peak at around 5,500 rpm with a 218-224-degree duration at 0.050-inch hydraulic roller camshaft. If you add 12 more degrees at 0.050-inch (i.e. 236 degrees at 0.050-inch), the horsepower peak can be moved up to around 6,500. That horsepower peak will give you clues as to how the rest of the horsepower and torque curves will look. If you have a very high horsepower peak, then typically the torque curve will follow suit, and both curves will have a sharper shape. Keep in mind here that if you shift the curves to the right, then you take away power from the left side of the curve. This means that you have high rpm power, but off idle and mid range power can suffer. Remember our discussion involving our 5,000-pound Thunderbird with the 302?
Advertised durations will help you with calculations for dynamic compression ratios and it will also clue you in on how aggressive the lobe is. Very simplistically speaking, if there's a smaller difference between the 0.050-inch duration and the advertised duration, then the lobe is more aggressive. For instance, Comp Cams has three popular hydraulic roller camshaft families: Magnum, Xtreme Energy, and XFI. The lobe's aggression increases with each family change. Let's take a quick peak here:
Comp Cams Magnum 280H: 280-degree advertised duration, 224 degrees at 0.050-inch=56 degrees
Comp Cams Xtreme Energy 276XE: 276-degree advertised duration, 224 degrees at 0.050-inch=52 degrees
Comp Cams XFI 274XFI: 274-degree advertised duration, 224 degrees at 0.050-inch=51 degrees
You can see that each lobe design has its own intensity, which means that it has a certain degree of aggressiveness. Looking at this a little closer, we know that advertised duration is measured at a very small amount of valve lift. The 0.050-inch duration is measured at 0.050-inch of valve lift. If you have a smaller amount of degrees between these two, this means that the valve is being jerked open, held open longer, and then effectively slammed shut. So if we look at our three cams up above, the cam with the Magnum lobes should be easier on the valve train than the XFI lobes. Of course, as with anything else, there are always exceptions to the rules, but this is a very good way of measuring how hard your particular camshaft is going to be on the valves, springs, retainers, and more. If you have an aggressive cam with inadequate valvesprings, then you can introduce valve float and valve bounce into the equation, which is a recipe for premature wear or engine failure. This is especially the case with roller cams, as the lifters and valvetrain components are much heavier.