Thread: cam

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can someone explain the difference between a tappet cam and roller cam. is it the lifters only or the cam too
thank you

The lifters have a roller end supported by needle bearings to virtually eliminate any point load friction. The cam lobe profile is significantly different, as the roller allows a different form of acceleration ramp, and a fatter contour at the peak to stay more open, longer. From Lunati -

Originally Posted by back in bus.

can someone explain the difference between a tappet cam and roller cam. is it the lifters only or the cam too
thank you

Your nomenclature is a little skewed. They're all tappet cams, it's just that some cams use flat tappets, either solid or hydraulic, and some cams use roller tappets, either solid or hydraulic. So, you have flat tappet hydraulic cams and flat tappet solid cams and roller tappet hydraulic cams and roller tappet solid cams. Hydraulic cams adjust the lash (the play between the rocker arm and the tip of the valve stem) themselves and never have to be adjusted after the initial adjustment when you are putting the motor together. Solid cams are not self-adjusting, you have to adjust them once or twice a year after the initial adjustment when you are putting the motor together. Typical lash adjustment on a solid cam would be in the neighborhood of 0.020" (twenty thousandths of an inch), with some tighter and some looser. Generally speaking, a hydraulic cam would be the choice for a street motor, where you wouldn't want to be adjusting lash a couple times a year. Fellows who would go racing will usually, but not always, choose a solid tappet cam, either flat tappet or roller tappet because higher revs are more easily achieved with a solid lifter cam. And by the way, (lifter) (tappet), same thing.

Roller tappet cams can have a much more aggressive lobe design than a flat tappet cam, with lobes that would allow the edge of the flat tappet to dig into the cam lobe. Rollers have no such problem and can even follow an inverse curve on the lobe without digging in and destroying the whole mess. Some of the latest flat tappet grinds from a few of the manufacturers have lobes ground on a very radical profile, with the edge of the lifters just a frog hair away from digging into the cam lobe. These grinds, called something like "extreme energy" and other names, do allow more area under curve and make a little more hp (I've seen a 10 to 12 hp gain on the DynoSim) at the expense of a noisy valvetrain and possibly shortened service life. If you needed another 10-12 hp to drive around a competitor on the track, fine, go for it. But in a street motor, it makes little or no sense. You can tell the profile of a cam lobe by subtracting the 0.050" duration from the advertised duration.....for instance, a 280 degree advertised duration with a 220 degree duration @0.050" tappet lift would result in a Hydraulic Intensity of 60. Harvey Crane coined the term Hydraulic Intensity and ground all his cams on a H.I. of 62 for years and years. Some of today's radical profiles will result in a HI down in the 40's. Personally, I feel that being a responsible engine builder would dictate a lobe profile H.I. of around 55, with a low limit for my own personal use of no less than 52. On the other end of the scale, there are some really lazy profiles available from grinders such as Melling that have an H.I. of 70-something and perhaps into the 80's. Although these grinds will most certainly work, there are more modern designs available to us today that will generate more area under curve for the same amount of duration.

And by the way, you guys with SBC street motors need to get over the idea that you can set the motor on kill with 1.6:1 rocker arms. If you're racing and need another 10 hp, fine, but a street motor should not be punished with the added stress that 1.6 rockers will bring to the table over 1.5 rockers. A little more duration will be far more effective than more lift and will not stress the valvetrain like increased ratio rockers will. Hell, you can even destroy an anvil if you beat on it long enough and hard enough, so give the valvetrain a break and use only 1.5 rockers on a street motor.

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