This may be a stupid question, but my reasoning for asking is...Both are a hardend core, both use very close to the same spring pressures, and there is no reason a Hyd. Roller lifter could not follow the cam profiles. Again, do not think I am stupid, it is just one of those what if questions.

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Black 89 IROC...Mods: 355, 10.5:1 comp, Big Solid Crower Cam, TFS heads, Victor Jr, Holley, and a 5 spd...

 

Roller cam lobes are ground perpendicular to the roller but non-roller lobes are ground at a slight angle so the lifter will spin. The roller lifter therefore wont make complete contact with the lobe. I believe this is why roller cams need the thrust bearings and the flat tappets dont. I am sure RB, BOR and Vader among others could elaborate on this.

 

Merf is right. On a flat tappet cam, the lobe surface is not parallel to the axis of the cam, but rather the side of the lobe toward the front of the engine is slightly taller. The lifters are not flat either, instead they are slightly convex. The combination of these two things makes the point of contact between the lifter and the cam to be near the outer edge of the lifter, and near the front edge of the lobe, which both forces the lifter to rotate on the cam and forces the cam rearward in the block.

The surface of the roller on a roller lifter is exactly parallel to the cam axis. Since the roller would only contact at the very edge of the lobe in the situation you describe, I'm sure it wouldn't last long.

Also, even though the numbers may look similar, a flat cam and a roller cam have nowhere near the same profile. For instance, I used to have a Comp 282S and now have a Comp XR282HR; very similar numbers, you'd think the lobes would look similar... wrong. The S has typical egg-shaped lobes that come almot to a point, where the HR has square-looking lobes where the valve stays at or very near peak lift for more than 60° of its rotation, and therefore much steeper ramps. That's what makes roller cams so much better (except of course for factory ones): the steepness of the ramp on a flat tappet is limited by the geometry of the contact point, where there is no such limit on rollers.

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"So many Mustangs, so little time..."

 

Well, at least to a practical point. There must be SOME ramp, or the lifter bores would wear out in pretty short order. But for all practical purposes, the peak velocity of a roller lifter is "limitless" compared to a flat tappet.

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Later,
Vader
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"I'm gonna talk about some freaky sheet now..."
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