flat tappet cam vs. roller cam, early ramp rate question? .

[dizuster]

We all know that with standard lifters (not schubeks) that roller cams can and do have more peak acceleration then a flat tappet. But I also know that there is an acceleration of the acceleration (jerk, or 4th derivitive for you math guys! lol)

Here are my assumptions, correct me if I'm wrong. I am under the impression that due to the contact geometry, a solid flat tappet actually has faster early acceleration then a roller cam. Later in the duration, the roller cam will actually catch up, and pass the flat tappet in acceleration. That being said, I am assuming at 5~10° of duration, the flat tappet will actually have more valve lift then the roller cam will.

I'm thinking on a small duration cam, the roller may be of no value over the flat tappet. If the roller never gets a chance to get up to speed, the flat tappet will actually have more area under the curve then a roller cam will.

My question is, how long does it take for the roller cam to catch up in acceleration. Say, if the .050" duration is under 240° or something, will the flat tappet actually make more power at the same duration?

If this is true, it also has an effect on overlap. So a flat tappet cam wouldn't want near the overlap of a roller cam, because in those early duration stages, even though you may have the same degree's of overlap, the roller has way less lift (airflow) then the flat tappet does.

Also this would lead me to believe that on my particluar motor, which is going to be EFI, could actually get away with a little tighter lobe sep with a roller, then I could with a flat tappet.

Or is it still in the lash ramp during overlap anyway, so it doesn't matter?
Depends on duration I guess. . .?

I will make a seperate post for cam recommendations for my stuff.

Thanks for your help guys!
Scott

[rmcomprandy]

A flat tappet does have faster early acceleration... explaining it is beyond me...
Roller cams are maximum ACCELERATION limited...
Flat Tappet cams are maximum VELOCITY limited...

[beth]

Some of the best minds in camshaft development frequent here and I am sure one will have your answer. My thoughts (and they should only be taken as that) are that given the same lobe design your explanation is right. A flat tappet on a given ramp will lift higher sooner than a roller. The difference is in practice, roller ramps can be steeper and make this difference up. The ramps can be steeper because of the round shape of the roller (a flat tappet would ride on it's edge) and the higher spring pressures allowed with rollers.


beth

[plovett]

Well it depends on the exact lobe profiles. Some flat tappets are more aggressive than some rollers throughout the entire lift range. But what you are saying is true in general.

I looked back at an artilce, The Truth About Street Cams, written by David Vizard from the september 2005 Popular Hotrodding. In it he addresses your very question. He measured various cams at different lifts and found, in general, at advertised seat durations 270 degrees and less, the flat tappet will have more area under the curve and more power. Above 270 degrees seat duration the roller has enough time to catch up and pass the flat tappet in terms of lift and duration, and starts making more power.

This is copied from the article. I hope it comes out alright

Cam type/dur. @ seat/dur.@ 0.050"/dur.@ 0.0200"/ 1.5 lift /1.6 lift /lift 1.7
Short roller----249----------198-------------104-----------398------424---451
Short flat------250----------206-------------117-----------432------461----490
Med. roller----270----------218-------------139------------495-----528----561
Med. flat-------270----------226-------------139-----------482-----514----546
Long roller----300----------248-------------171------------563-----600----638
long flat-------298----------254-------------167------------549-----576----612

I'll see if I can find a link to the article.

paulie

[bigjoe1]

All I( know is, a good roller grind will beat out a flat tappet every time.The only time it will be close is when the lift is the same. You can always run much higher lift with a roller of the same duration. I have seen this same results many times. JOE SHERMAN RACING

[Cobra]

Flat tappet cams will eventually join the dinosaurs!

[plovett]

Flat tappet cams will eventually join the dinosaurs!

You're probably right, but flat tappets are still superior for the budget minded enthusiast needing good rpm capablility and good reliability. Solid rollers still don't have long term street durability and hydraulic rollers still often have problems with high rpm. And both cost more than flat tappets. Solid flat tappets hit kind of a sweet spot blending cost, performance, and reliability. Improvement to rollers may change that in the future

JMO,

paulie

[putztastics]

It seems to me the only way a roller would ever be slower per degree of cam rotation is if the lobe was not designed to take advantage of the roller, ie like running a roller on a flat tappet lobe design. If the lobe is designed with a roller lifter in mind the roller lifter/cam will always result in more total air flow when compared to a flat tappet.

Like others have said the roller by design allows a shorter duration per degree of lift everywhere on the lobe, lash ramp, opening ramp, closing ramp - everywhere.

"Reverse radius" or "inverted flank" lobes offer the most lift per degree but it would be impossible to run a flat tappet on these lobes, the lifter edge would dig in.

[Ron Golden]

David Vizard's book on camshafts covers this question. He found hyd.cams smaller than 230* @0.050' actually opened the valve faster than the roller. However, like most of you pointed out, roller cams are the way to go.

Ron

[dizuster]

I have no doubt an any racing type application, a roller cam will make more power. But what I'm wondering, if it is true that flat tappets open earlier, then how does that effect the comparisons of flat tappet/roller durations/lobe sep. It would seem that if the flat tappet is open further sooner, and stays open longer, that to optimize a roller cam, it would need a few more degrees of duration at the opening and closing points to equal the flat tappet.

For example, lets say you have a motor that you have optimized with a flat tappet cam that is 260°@.050" By optimize I mean that the cam has a closing point, which runs the motor, at a RPM range the heads can support.

If it's true that the flat tappet has more early/late lift, the roller cam would need more duration to acomplish the same actual optimization of overlap/closing point.

The only thing I can think that we don't see is, As I understand it, at some point of RPM, the head always becomes the restriction, not the valve. When the mach index/FPS gets out of hand. Which means even if the roller cam has faster acceleration, which will essentially slow down the late FPS airspeed in the port because the pressure differential should be lower due to the added "area under the curve". Maybe since the airspeed is slower, the motor doesn't need the same late closing point as the flat tappet due to the slower airspeed?

But that doesn't solve the overlap issue. Maybe a roller cam needs slightly tighter lobe sep, to acomplish the same actual overlap flow as a flat tappet?

Sorry, hope I'm not rambling too much, I'm just trying to theorize and understand what effect roller cams have on overlap, and valve timing, all relative to flat tappets of course.

This is my first post, so hopefully I'm not embarrising myself!!!

Thanks fo the input guys.

[putztastics]

David Vizard's book on camshafts covers this question. He found hyd.cams smaller than 230* @0.050' actually opened the valve faster than the roller. However, like most of you pointed out, roller cams are the way to go.

What roller designs were those flat tappet cams compared to?

Commercially available cams? Custom cams?

Until we know we can't be sure a blanket judgement like "hyd.cams smaller than 230* @0.050' actually opened the valve faster than the roller" is true can we?

Compare generic flat tappet cam specs for different lifter diameters. What you will see very often is the same lobes (ramp rates) on Ford Chevy and Mopar cams.

Larger diameter lifters allow faster ramp rates but generic lobes do not always take this into considereation.

I wonder how many low duration roller designs take advantage of the roller's capability for faster ramp rates than flat tappet?

But what I'm wondering, if it is true that flat tappets open earlier, then how does that effect the comparisons of flat tappet/roller durations/lobe sep. It would seem that if the flat tappet is open further sooner, and stays open longer, that to optimize a roller cam, it would need a few more degrees of duration at the opening and closing points to equal the flat tappet.

In order for a roller to be slower than a flat tappet it would have to be running on a lobe with much less than optimum roller ramp rates - so I do not think it is true that flat tappets are "faster" or open "earlier". If it is true
(with some cams) it is not because of the roller lifter, instead it is because of slower than optimum ramp rates used with the roller.

>If you run a roller at max flat tappet ramp rates if will be "slower" than a flat tappet.

>If you run a roller at max roller ramp rates it will be faster than flat tappet.

A flat tappet will not run (for very long) at max roller ramp rates.

By design roller lifters can follow much faster ramps than flat tappets can doesn't matter what the duration range of the cam is. Faster ramp rates = more valve open time = more flow = more power.

[rmcomprandy]

More lift per degree everywhere...??? HARDLY...you can't grind a roller lobe profile hollow enough to match the initial flat tappet opening rate...the bigger the lifter...the faster the flat tappet can initially be...

UD HAROLD ... where are you...

[putztastics]

How many degrees of duration does the "initial flat tappet opening rate" cover?

It doesn't make sense to me that a roller lobe cannot be designed to open just as fast as (if not faster than) a flat tappet during this initial opening phase, just move the ramp opening point to the roller and it will won't it

[putztastics]

I do see what you are saying Randy ... be interesting to see what a cam guru says.

The only reason for flat tappet cams these days are rules and economy, and even the ecomomy of a flat tappet is questionable with engine oil changing like it does. One failure and you would have been better off with a roller.

[rmcomprandy]

To match a flat tappet you would need the roller on the end of the lifter to be about 45 to 60 inches in diameter and that of course...is impossible to achieve...