This is sort of a two part question. The first part about cam specs, the second about DCR. I could be completely wrong here, so I hope everyone is in a good mood.

I'd like to know why (despite .006 adv for hydraulic and .020 or .015 for solids) it seems that for a hydraulic cam to have the same .050 numbers as a solid, it needs to be huge? Take for example two cams, a hydraulic roller 295/305 240/250 .006, .050 and a SR 276/284 245/253 why does the hyd. cam have to be 295 adv to get *less* .050 duration than the 20-degree adv smaller solid? It dosen't seem possible that it all has to do with where the 'seat' or advertised or whatever is measured?
And second part, how does this play in with DCR? if a hydraulic cam needs 295 or whatever to get 240 @ .050, and a solid only needs 276 adv to get 245, won't a solid cam always DCR better given the .050 #s? I guess what I'm saying is does the DCR calc "artificially" deflate the DCR # when you plug stuff in for a hyd. cam?

I know I'm confusing, I'm confused, but, if I wasn't clear enough for an answer, I'll try to reword it..
Its just I'm comparing two solid rollers and one hyd. roller, a bigger solid, a smaller solid, and a hydraulic that is smaller at .050 than the small solid, but WAY larger at adv but has a lower DCR..

matt

Matt,

Imagine you're at the drag strip, and on the starting line, all revved up. Your opponent is 50' behind the line, and when he gets to the starting light, it trips your green.
You wouldn't wonder about how he got a much faster ET and MPH, you'd know it was because he was going a lot faster when you started.
That's the exact same thing with hydraulic and solid or roller cams. Stock hydraulics open the valve at about .0005"/°, and even my mildest solids are faster than .002"/°, 4 times as fast. Because the solids and rollers are opening so much faster than hydraulics, it takes them much less time to get to .050".
I will not and can not give you answerss of the DCR. Obviously different cams caan all have the same closing point, but have different volumes of air/fuel in the cylinder, and therefore different cylinder pressures. I feel the DCR is a good rule of thumb, but not the final answer. Pat Kelly has done a great job with it, and certainly given everybody something to think about.
I hope this answers your questions, if not, ask some more...

UDHarold

The DCR concept, as used in the calculator, doesn't address cylinder pressure at all. It is a simple CR calculation based on the intake closing point. More aggresive cams stuff more mixture into the cylinder, raising the cylinder pressure. A very aggresive cam could cause detonation at a slightly lower DCR than a mild cam. However, I don't know of any way to calculate this. Chamber/piston shape, plug heat range, engine temp, and VE can effect detonation, again, rhe DCR calculator doesn't account for any of this.

Also, a cause of some "fuzziness" in the DCR calc's are that it asks you to input seat duration, NOT, seat duration at 0.0xx" of lift, so there you have the vagaries of the seat duration measurement at various tappet lifts. EG, I beleive the old UD seat measurements are at .026 lift, so the cams SEEM smaller on the seat than some other manufacturers. Seat durations are not standarized as are .050", .100" and .200" durations.

So it's an approximation. But still a useful one.

Harold,

Why can a solid flat tappet move the lifter at
.002"/deg while the hyd flat tappet only .0005"/deg?

SS Dave,

Because you can't go as fast in an 1/8 mile track as you can in a 1/4 mile track.
Hydraulic cams(ALL of them!) start the valve off the seat in .004" of ramp lift. I can get a valve moving over .001"/° in those .004", GM does only .0005"/°, same lift. Regardless of where cam companies rate their 'seat' durations, the .004" figure holds, except maybe with things llike Rhoades lifters.
Solid lifter cams have ramp heights of anywhere from .008" to some with over .020". Obviously you can accelerate to a lot higher velocity with 2 to 5 times as long a distance.
There are a lot of other factors involved, and most of them involve calculus and differential equations, and years of work. A lifter of a certain diameter is limited to a max rate-of-lift, whether hydraulic or solid. Roller cams do not have velociity, or rate-of-lift limitations, but they have other, just as severe, limitations.
Most cam designers who are any good reaalize it's almost entirely about ramp design.........

UDHarold