Camshafts- Three-Way Cam Lobe Shootout

Why Roller Cams Are Better
This is some serious stuff, so you'll need to get rid of your normal distractions for a few minutes. It is possible to accurately compare a hydraulic flat-tappet cam with a hydraulic roller or even with a mechanical roller cam, but there are some important stepping stones to getting there. To begin with, all cams are rated for duration, based on the lobe profile and expressed in crankshaft degrees. For example, lift is expressed on the cam card in terms of valve lift using the stock rocker ratio. But what we should really be looking at with any style camshaft is the duration of valve opening. According to Comp Cams, the best way to rate a hydraulic lifter cam at the valve is to assume 0.004 inch of lifter piston bleed-down before the lifter begins to move the valve through the rocker arm ratio. In our chart the duration number 283 degrees indicates that the XE flat-tappet cam measures cam lobe duration at 0.006 inch of lifter rise (advertised duration), while the second column indicates that after 0.004 inch of tappet bleed-down and the lobe multiplied by the 1.5:1 rocker ratio, the duration at the valve is actually 282 degrees at 0.006-inch tappet lift.

You can't tell much about... 

   

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You can't tell much about a cam by looking at it. Even the two roller cams look much the same, but they perform very differently and require specific spring-pressure approaches.

We can use that same 0.004-inch lifter deflection figure to rate hydraulic roller cams. Notice that despite the fact that the flat-tappet and the hydraulic roller cams are only 2 degrees apart at 0.050 inch of lobe lift (240 versus 242), the hydraulic roller offers 5 more degrees of duration at the valve at 0.200 inch of lobe lift and an impressive 16 more degrees of duration at the valve at 0.400-inch lobe lift (from 107 to 123 degrees). This indicates the higher lifter speed capability of the hydraulic roller design over the hydraulic flat tappet. So while at 0.050 these cams appear the same, this number by itself is deceiving. Looking at a basic lift curve, the hydraulic roller achieves a given lobe lift such as 0.200 inch much more quickly and therefore creates more area under the valve lift curve. This means more air and fuel will enter the cylinder to make more power. Now let's look at the mechanical roller lobe.

Mechanical lifter camshafts are more difficult to evaluate because you should not use advertised duration as an indicator for several reasons. First, because of 0.016 inch of lash (clearance between the rocker arm and the valve), a lobe duration number measured at 0.006-inch tappet lift is meaningless because at a rocker ratio of 1.5:1, that 0.006-inch lobe lift number is worth 0.009 inch of movement at the rocker tip, which is still short of taking up the 0.016 inch of clearance. Even at 0.050 inch of lobe lift (duration at 0.050), this calculated number of 253 degrees of duration does not take into account the lash. Going back to Godbold's rule of thumb, every 0.001 inch of tappet lift is worth roughly half a degree of duration. In order to account for our rated 0.016 inch of lash, we must remove 8 degrees from the 0.050-inch duration figures, which means the 248 degrees at 0.050 is really 240 degrees and therefore exactly the same lobe duration at 0.050-inch tappet lift as the flat-tappet hydraulic camshaft. But notice the tremendous velocity the mechanical roller cam can generate throughout the entire lift curve, offering up a serious 197 degrees at 0.200-inch lobe lift. Compared to the hydraulic roller and flat-tappet cams, you can see why the mechanical roller is superior. At the extreme, the mechanical roller offers a staggering 21 more degrees of duration at 0.400-inch lobe lift than the hydraulic flat-tappet cam, and 5 more degrees than even the hydraulic roller (123 versus 128). What this means is that the intake valve is held open at the same valve lift for a much longer period of time within the cycle from when the valve first opens until it closes. This is why the mechanical roller cam can make more power than the hydraulic flat-tappet. Because of additional duration and greater lift, the mechanical roller lifter is traveling faster than its more conservative hydraulic counterparts, which is why lighter components and stiffer valvesprings must be part of the overall package.