Does the advantage of a roller cam compared to a flat tappet increase, as duration increases?

[CamKing]

Thanks! Can I ask this, too? Does the difference in area between the two, the roller and the flat tappet, become more and more with added duration?

There are multiple variables, that effect the advantage of going to a roller.
Duration, lift, tappet diameter, base circle diameter, available spring rates, Engine RPM.

You can't just say, the advantage will be bigger, with more duration.
If you look at the flat tappet restrictor plate cams we ran in NASCAR a few years ago(mid 250's at .050"), because of the 2.37:1 rockers, limited RPM, ultra-light valvetrain, there would have been zero advantage going to a roller.

[CamKing]

How much does the overall quality of the roller lifter come into play when you grind a lobe with a lot of positive acceleration? Does the greater pressure angle have a dramatic effect on the life of the lifter and the lifter bores in the engine? Do you have any recommendations in that department? Do you recommend a coated lifter, a bushed lifter bore etc?... Is it one of those things where it gets to a certain point and then service life really drops off quickly and the technique is only suitable to short duration applications?

The biggest issue, is side loading. The farther up the wheel, the contact point is, the more the lobe is trying to push the lifter sideways, instead of up.
That's why big bearings, and big base circles are so much better. You get less side loading, with the same accel rate.
The more side loading you have, the shorter the lifter life. That's why I don't use a lobe designed for drag racing, in a circle track car.

[DaveMcLain]

Thanks! Can I ask this, too? Does the difference in area between the two, the roller and the flat tappet, become more and more with added duration?

There are multiple variables, that effect the advantage of going to a roller.
Duration, lift, tappet diameter, base circle diameter, available spring rates, Engine RPM.

You can't just say, the advantage will be bigger, with more duration.
If you look at the flat tappet restrictor plate cams we ran in NASCAR a few years ago(mid 250's at .050"), because of the 2.37:1 rockers, limited RPM, ultra-light valvetrain, there would have been zero advantage going to a roller.

I think I've got one of those cams here that came in a bunch of old junk. 245/248 with a .352 and .345 lobe ground on 111 the intake lobe uses .850 of the lifter but the exhaust uses .830. I always assumed it was for a restricted engine and I always wondered about how much rocker arm would have been used.

[Newold1]

Me thinks CamKing's excellent answers and explanations have put this post to bed! Thanks CamKing!

[plovett]

Thanks! Can I ask this, too? Does the difference in area between the two, the roller and the flat tappet, become more and more with added duration?

There are multiple variables, that effect the advantage of going to a roller.
Duration, lift, tappet diameter, base circle diameter, available spring rates, Engine RPM.

You can't just say, the advantage will be bigger, with more duration.
If you look at the flat tappet restrictor plate cams we ran in NASCAR a few years ago(mid 250's at .050"), because of the 2.37:1 rockers, limited RPM, ultra-light valvetrain, there would have been zero advantage going to a roller.

Thanks!

[Stan Weiss]

How can you tell that Vizard's rule of thumb does not apply?

It hasn't applied in 40 years.

With the availability of a 2.5" radius grinding wheel, I can design roller cams with an acceleration rate higher then what a cast iron flat tappet cam would allow, due to the amount of spring it would require to control it. If the pressure required to control the acceleration rate will cause the flat tappet material to fail, you can't really use that acceleration rate.

What if the flat tappet cam and lifter were made from better materials? The same thing happened with roller cams many years ago. Somewhere I still have a GK roller cam that was done on a cast core.

Stan

[CamKing]

What if the flat tappet cam and lifter were made from better materials?

That helps. It removes one of the issues. Like I said, the NASCAR restrictor plate flat tappet cams gave up nothing to a roller,
but they're powdered metal with DLC coated lifters. Their 2.37:1 rockers were the biggest help, because the high rocker ratio reduces the velocity and acceleration requirements at the lifter. You're talking, the best of the best, and it was no better, or no worse then running a roller.

[RDY4WAR]

Good information. I've wondered about if that 270 rule was still true since I read it. I learned something new today.