I've read it repeatedly over the years that the larger diameter Ford and Mopar lifter allow for a wilder cam than the Chevy lifters do, and that mushroom lifters allow for even more. What about roller lifters? I can't find it now, but I thought I remembered reading that some company was saying that it's larger diameter roller lifters allowed for a wilder cam design than smaller lifters. I just can't see how that's possible, but maybe I'm missing something.

Dan

 

Very interesting. A larger diameter would give a larger contact area before full lift on a flat tappet. A larger diameter roller would put the point of contact "closer" to the line of action of the roller lifter. The angle from the center of the roller to the point of contact will be smaller. This way the lifter doesn't push as much on the edge of its bore. Same concept as a longer con rod. Probably a durability issue.

 

Think of the difference between rolling over a curb on a 27" mountain bike, vs. a skate board wheel.

Bigger roller allows steeper ramp. A little difference goes a long way.

 

I don't think you missed anything. Sounds like BS to me.

The reason the larger diameter lifter allows greater performance with a flat tappet cam is that the "tip" of the lobe can be made to come into contact with the lifter at its outside diameter at a greater distance from the center of the cam, which allows for greater duration, more lobe lift, and less wear all at the same time. But, since a roller lifter only contacts the cam at one place rather than the contact point being all over the base of the lifter at different points in the cam's rotation, and that one place is on a radius of the cam rather than a tangent at the lobe's surface, it makes no difference how large the lifter bore is with a roller. However, the diameter of the roller itself will have a very slight effect on the duration, but probably not enough to even be measurable in most cases.

BTW, the setup for flat tappet Chevrolet motors is to enlarge the lifter bores and use AMC lifters... same height, larger diameter.

 

I assumed the "larger diameter roller lifter" referred to the diameter of the roller.

No matter which you're talking about here, the cam needs to be designed with the intended lifter in mind.

 

Damn! And all this time I thought the trick setup for a flat-tappet SBC was a retrofit roller.

(Just jackin' you a little, RB!)

Another trick with any lifter is to undercut and elongate an oil groove in the lifter bore to provide better flow to all the lifters in the bank.

And you're right. Unless the the lifter has a roller of something larger than 48" in diameter, the contact point is essentially just that - one narrow point instead of the full diameter of the roller. It just doesn't make a lot of difference, except that the larger roller would be heavier. Another thing we don't need in an already massive valve train.