Mike Jones (Camking):
If you could make everything out of unubtainium and run 1,000# of spring pressure, that would be true, but it's not in the real world.
The acceleration limits of a roller are go beyond the most aggressive flat tappet cams.
If you tried to run the max acceleration on a flat tappet cam as high as I use on my inverse radius rollers, you'd need more spring pressure then a flat tappet cam could stand.
Here's some Max accelerations I just looked up.
Flat Tappets
Comp Cams restrictor plate cup cam: .000294"
Jones Cams restrictor plate cup cam: .000372"
My most aggressive flat tappet cam: .000422"
Rollers
My average IR cam: .000393"
A Pro stock IR cam: .000438"
A asymmetrical IR cam: .000463"
Like Harold said, the Max velocity dwell is a must.
We started doing it a couple of years after Harold(around '83)
We called those flat tappets our E.M.V. series (Extended Maximum Velocity).
I've refined the way we do it over the years, but it's still doing the same thing.
As far as ramps go, spintron testing and my own way of thinking have moved me in a different direction them most cam designers.
The roller will show benefit on any engine where the max velocity of the flat tappet is below what the engine wants.
BTW, I don't design flat tappet cams for pushrod engines. All my F.T. designs are actually roller designs, designed for a 52"-58" radius roller follower.
Harold Brookshire (UDHarold):
The cam used in Alan Johnson's Nr 1 Qualifier at the O'Reilly Mid-South Nationals the other week had a peak acceleration rate under .000330"....
And this was on a 70mm core, .850" roller wheel diameter. This is the one that would hold 10,500 for over 90 secs, 1.090" valve lift...
And EVERY SBC flat tappet cam I design has a peak acceleration rate of .000383", and a peak velocity rate of .0070400". Under .106389" I use the same ramp on all of them, ie, the opening ramp is that high, and of course, closing ramps are different. Of course SBFs and Mopar solids have different peak velocity rates, also.
Cam design is an infinitely challenging world.
If you are looking at a roller cam and a flat tappet, and they have very similar durations at .020", .050", and .200", you are looking at either a very well designed flat tappet, or a very poorly designed roller.
Back in the early 1990s at UltraDyne, I designed a .904" Mopar/AMC flat tappet that was, and still is, 282 at .020", 255 at .050", 172 at .200", and .3879" lobe lift.
A popular roller cam from Memphis is 288 at.020", 252 at .050", 169 at .200", and .420" lobe lift.
My flat tapppet is limited to a little over .0075"/* max velocity, and all roller cams are unlimited on max velocity.
My shorter seat-timing and fatter area at .200" gives a higher port velocity through part of the lift cycle. AFAIK, these 2 cams have never been compared in the same engine, so I do not know the power difference.
However, my solid lifter has always been a strong running cam.
My current roller cam is 283 at .020, 255 at .050, 180 at .200, and .4164" lobe lift. I also have this cam with .4544" lobe lift, and with .4854" lobe lift. They are all identical from .050" lobe lift on down.
This is the nice thing about roller lifter cams----Lobe lift is rarely a problem, unless limited by the core's capability.
I even have an .842" solid lifter profile that is 288 at .020, 259 at .050, 171 at .200, and .3783" lobe lift. It stays over .017" away from the edge of an .842" tappet.
We've come a long way in the past 30 years......
During overlap, it is the AREA-UNDER-THE-CURVE(Combination of valve lift and duration, of both valves) that govern what happens. Look at this:
UD 288 Flat Tappet--288@.020, 255@.050, ,128"@TDC on 104 ICL.
UD 288 Roller--------288@.020, 255@.050, .125"@TDC on 104 ICL.
The valve lifts, .200s, etc, are different.
Peak Velocitiy, Flat Tappet---.00705"/*
Peak Velocity, Roller----------.00940"/*
Dyno test, same engine, different lifters, cams, and springs---Pro Motor Engineering. Roller made 30ftlbs more at 5200, 60 BHP more at 7200.
Both were excelllent examples of the art, and are still good today.
Remember, the engine sees the total valve lift curve as the cam, not what it is at .050". The part before TDC on the intake is seen as bad, the good stuff occurs AFTER TDC.
Two cams with the same intakes and exhausts, but different LSAs, will dyno with 2 different power curves, even when both cams are put in with the identical INT CL. The differing positions of the exhaust cam give different cylinder pressures before the intake valve opens BTDC, and this affects the reversion, and the recovery, and the intake port velocity, and the cylinder filling.
Sorry I've missed so much of this thread, and this will be a short tid-bit for thought, as I have to take my wife to work.
Flat tappets are limited by velocity.
.842"=.00705"/*
.875"=.00733"/*
.904"=.00758"/*
This keeps a small distance between the edge of the lifter and the contact patch on the lifter. The smaller the distance, the smaller the area of contact, and the much higher the spring load in lvs/sq. in---Wipeout!
Rollers are controlled by max acceleration rate and BASE CIRCLE RADIUS--bigger bearings allow higher usuable max acceleration rates before too much negative radius of curvature. If the radius of curvature is too small, the cam cannot be physically made. I used to joke about designing cams with the radius of curvature so severe that the design was a teardrop of metal suspended above the base circle---the radius of curvatures had cut through to the other side......
Now on flat tappets. A Chrysler profile must be made as a roller to run in a Chevrolet, or does it?
Around 1990 Crane had, and still does, a Chrysler-only flalt tappet cam, the F-258/3735. It was 294 at .020, 258 at .050, 166 at .200, and .3735" lobe lift.
In April 1980 I designed the .842"-tappet design, my F4(4th flat tappet designed at UltraDyne). The F4 was 292 at .020, 259 at .050, 170 at .200, and .3677" lobe lift.
In 1995 I designed the .904"-tappet design, the NF63.
The NF63 was 294 at .020, 267 at .050, 185 at .200, and .4058" lobe lift.
Neither of my 2 flat tappets got any closer than .017" to the edge of the tappet, and the F4, the 292, has gone over 100,000 miles in a number of engines.
Think on that, I'll try to answer more later this afternoon.
The way I look at it, overlap is harmful to engines, not beneficial. And I call those derivatives 'snap, crackle, and pop', although Harvey Crane coined those terms. I like them a lot better than 'quirk'!!!
This information on overlap is irregardless of type of tappet. When cam lift curves are designed, the type of lifter lies in the head of the designer, and his choice of ramps.
Hydraulic-type lifters, both flat and roller, take all the compression out of the valve train by .004" lobe lift, according to the SAE. The valve is about .001" off the seat at this lobe lift. You can only accelerate the valve so fast in that short of a lift(Think 1/8 mile vs 1/4 mile.....) and hydraulic rollers, because of manufacturing difficulties, have trouble using very high acceleration values that low. When I designed the VooDoo hydraulic rollers, I, as always, used the same ramps regardless of cam lift. I had to redesign the ramps for the higher-lift cams because the base circle got too small. The ramp that worked perfectly for .510" valve lift caused trouble with .550" valve lift---in manufacturing.
I view the intake overlap as bad, and have designed my cams accordingly for 30 years. The sooner you open the intake valve, the more reversion you expose your engine to, and the more you delay filling the cylinder. I like to delay the intake valve's opening, then open it fast, both velocity and acceleration. Flat tappets, either solid or hydraulic, are limited by a maximum velocity , roller cams are not. Hydraulic cams generally run higher acceleration rates than solids do, this has a lot to do with maximum RPM expected. Roller cams, are as always limited by max acceleration rates, although I am improving my math abilities to raise the max rates usable.
Reversion hinders intake flow. Before TDC, when the intake valve opens, the cylinder has exhaust gas in it being pushed out of the cylinder. This exhaust gas has both volume and pressure, and when the intake valve starts off the seat, this exhaust gas enters around the intake valve toblock the intake port. Later intake valve openings reduce this reversion, allow earlier intake port recovery, and allow higher intake port velocities---IE, better cylinder filling.
At least it's worked this way for me for 30 years.......
Everyone is correct. Because of pushrod angularity, the valve only gets the Cosine of the lobe lift, which is varying as the lobe turns, but should always be above .975, or thereabouts....... Only OHC engines with direct-acting cam lobes deliver true lobe lift.
Pushrod deflection is also into play, and everyone should use the stiffest pushrod they can. On the cam side of the rocker arm, stiffest is most important. On the valve side of the rocker arm, lightest pays off a lot. This is why we have the return of the BeeHive spring---It lightens the valve side of the equation, a lot.
Angularity is a fact of life in engines, and the higher the lobe lift, the more angularity. Remember, the valve movement in the 75* ATDC is really more important than a few more thousands of valve lift. The conditions for good intake port flow are set up in those first 75*, not around max valve lift.
The higher the lobe lift, the larger variation in cosine, therefore net valve lift. Also offset pushrod seats/rocker arms, etc, alter the cosine angle, normally figured as a angle off of vertical. Now it is a compound angle.
Take care of what you can; Make the pushrod very stiff and rigid to accurately transmit the lift curve from the cam lobe to the rocker. Quite often the variation is about within the range of valve lash changes, and no one complains about those. As Harvey Crane says, "WEW!!"---What Ever Works!! If you are supposed to have an .810" valve lift, and instead you have .800", or .795", it is OK.
The actual rocker ratio, the base circle diameter, and the spring loads, as well as the pushrod stiffness, are the culprits in lost valve lift. Going from a stock SBC core size to a 60mm, with everything else the same, makes the cam over 140% stiffer. Just changing the base circle size 10% makes the core 21% stiffer. These little facts are one of the main reasons for core diameter growth.....
These are all tools in a cam designer's tool box.
I have been using dwell at max velocity since 1980, with 90% of the UD flat tappets being that way, and at least 50% of the hydraulics.
The UD solids and hydraulics were done on a Texas Instruments pocket calculator. All of those UD printouts were 2 sheets of notebook paper.
I have never designed a roller using dwell at max velocity---It is not needed---and I know of no designers who do.
All the VooDoo hydraulics and the new solids have dwell at max velocity.
The main questions is always: How much?
This is the only way to achieve high lobe lift with a limited-diameter flat tappet, either hydraulic or solid, and keep a reasonable nose radius.
The only other way is to use higher and higher nose accelerations to keep the max velocity on the tappet, but this requires higher and higher open load on the valve springs, and the nose radius gets smaller and smaller, and the smaller nose radius is less able to withstand the open spring pressures.
Not all flat tappets, solid or hydraulic, have dwell. In fact, looking at all cam companies, probably more do not have dwell than do have dwell. Sometimes when I do hydraulics or solids that have a decent duration and not too high of a lobe lift, I don't bother with dwell either. Some of UD's most successful hydraulics, and some damn good solids, do not have dwell. Dwell is only used when it needs to be used, as it is more difficult than designing cams without dwell.
The 'No-Pulse' ramp is best explained by Harvey Crane, as he thinks it is his baby, and he charges for it, as that is how he makes his living now.
The reversals in positive acceleration are very common, on almost all factory designs, and used by most cam designers. I use them quite commonly on closing ramps. It is just a function of the exact velocity, acceleration, and jerk values you are using, and the duration of the ramp.
The charts on Harvey's website are tremendously informative, but they do not tell you everything, and all the charts are symmetrical. You need to study the charts, and do a lot of thinking.
I hope this has been of help in answering your questions.
ook at the middle chart you posted from Harvey Crane's website, the 'No-Pulse' ramp.
There is a line in red towards the bottom of the chart. This is the Jerk curve, and careful study of it will reveal some secrets.
It is not easy to design, but it works extremely good on the opening side of cam lift curves.
The 'Acceleration-Reversal' ramp is used quite commonly on the closing side, and helps ease the lifter down on the base circle.
At UltraDyne, my 288F was master F1, done in April 1980.
The following UD designs were all done using the computer printout of the 288F opening side:
F9 276 243 .3450"
F18 280 247 .3500"
F8 284 251 .3533"
F1 288 255 .3600"
F4 292 259 .3677"
F13 300 267 .3823"
F12 308 275 .3933"
The other, bigger, cams were either 4-equation cams, or else Dwell-at Max-Velocity designs where I traded dwell for a lower nose acceleration and a more rounded nose, for higher open pressures and RPM.
My 288/296H8 hydraulic, .485"/.507" valve lift with 1.5s, was my first hydraulic with dwell at max velocity.
These were my more successful cams for all of UltraDyne's life.
Jay Wiles (Warp Speed, dyno operator at Hendricks)
Efi and roller cams have had a minimul impact on power output!
We actually lost power going to EFI, but were able to get most of it back due to the ignition system, and being able to optomize the ignition map through the whole rpm range.
Roller cams essentially do very little, as we are velocity limited by current valve spring technology already.
Mike's response to the above:
That's really not relevant to this application. NASCAR had learned to get around the max velocity limits of the flat tappet lifters, by increasing the rocker ratios(up to 2.4:1), so they could get the valve velocity the engine wanted, without over-riding the tappet diameter. The roller profiles they're running in NASCAR now, have basically the same valve lift profile as the flat tappet cams they were running.
That's not going to be the case, with conventional rocker ratios.
If you're dealing with a rocker ratio in the 1.5 to 1.7 range, if you didn't pick up 20hp going to a roller with similar duration, it would have to be one real bad cam design.
As Jay points out, you can take care of what is happening at the valve, with a flat tappet, with very large rocker ratios, since what is happening at the lobe is limited. On the rollers, they have dialed out rocker ratio in turn for 'fatter' lobes.