Let's take generic cam specs such as: .500" / .500" lift and 228 / 236 @ .050" on a 112 degree spread fort both a flat tappet and a roller cam. To the layman's eyes you'd think both cams will perform the exact same, but nothing could be further from the truth for the simple reason that roller cams have MUCH faster ramps than flat tappet cams do.
I'm going to use generic numbers here just for illustration so it keeps things simple, so don't take these as actual numbers. Because the ramp is so steep and quick on roller cams, the valves literally slam open and slam closed. Look at the image to the right and see the side-by-side difference between what a roller cam lobe looks like (on the left) compared to a flat tappet cam lobe (on the right). This is why roller cams require much stiffer dual or triple springs. You can click on the image to enlarge it if you want to see it better.
So with the valve on the roller cam closed and just getting ready to open, it only requires about, say, 15 degrees of movement to get that valve from fully closed to fully open. Just look at how steep those ramps are on that roller cam on the left, where the flat tappet cam's lobes on the right aren't nearly as steep, so at that same 15 degrees of cam rotation, the flat tappet's valve isn't even going to be halfway open. It'll be more like 1/4th open. (using generic numbers here to simplify things for you)
Well, using those generic cam specs we have of .500" lift for both cams, the roller cam's valve is at full .500" lift while the flat tappet is only at about .150" or so. Which one is going to be flowing more fuel and air, or exhaust?
Now, that roller cam's valve is AT and STAYS at it's full lift, while that slower flat tappet valve is still trying to reach it's full lift, and as soon as it does, it starts to descend. So while that flat tappet valve is now descending, that roller valve is still wide open at full lift because it doesn't need to start closing yet because of how steep the ramp is. Just look at the huge surface area on the top of the lobe of that roller cam! When the flat tappet valve is at about at half lift on its way down, (and again only flowing less than HALF of what that roller cam's valve is still flowing), the roller cam valve begins to close. It will literally catch and pass that flat tappet cam's valve because the closing ramp is so steep and because it has much stiffer springs to slam that valve closed so quickly.
Overall, both cams opened the valves to .500", but the roller cam got it at its full lift much quicker, and held it there much longer than the flat tappet cam could, which means more flow to create more power. Both cams also held the valves open for the same amount of time, overall, but again, the roller cam held its valves open wider within that same period of duration time. Again, valve TIMING is every bit as important as all of the other "generic" specs you see on basic cam specs. This why a good, knowledgeable engine "builder" can pick-out a cam that'll run stronger than your typical guy, and why some guys have engines that out run other guy's engines.
The one draw back is that roller cams cost quite a bit more than your typical flat tappet cam. We fit our roller engines with high-end after market cams from Crane, Comp, Crower, Lunati, etc., not the factory stock stuff crate motors get, and yes, there is a HUGE difference between the two. For one; the lifters we use for both solid roller and hydraulic roller cams are much lighter and way stronger than what a factory engine comes with. We use light weight "transfer bar" type lifters from Crane, Comp Cams, Lunati, etc., which are designed for more abuse and higher RPM use, unlike what stock roller lifters were designed for. The down side to these lifters is the cost. They can run anywhere from $450 to over $550 a set depending on the specifics. So compared to a typical $65 - $75 set fo flat tappet lifters, a set of roller lifters is going to set you back a bit more $, but in the end, for what you gain in performance & reliability, it is well worth it.
The cams we use are also made out of MUCH better, stronger and more durable materials and have profiles that are WAY better than any generic cam, but they can easily run anywhere from $300 to $400+ depending on the material, base circle size, and so on. Sure, you see generic factory roller cams with mediocre profiles going for about $189 in magazines, but do you honestly think those are great cams? Do you think the fast guys are running those cams? Do you think those heavy lifters will be trouble free under serious conditions or at high RPMs with stiff valve springs? No way! Ford's "303" series cams are about as generic and as mediocre as you can get. The lifters weigh a ton and just can't take much abuse. The same thing goes for the factory GM cams and lifters being that they both use heavy slugs for lifters with H-bar retainers and/or a big, floppy tin pan to hold everything in place.
We only use killer, name brand cams that are custom designed for your specific application to squeeze-out every last bit of power and performance, yet still maintain reliability and longevity. These components aren't generic and they aren't cheap. As with anything, you get what you pay for. If you want to make a good investment and get great bang for your buck with something that'll out run and out perform any generic engine out there, then you are going to need the right roller cam & lifters in it, which aren't cheap.
Now, are flat tappet cams out of the question as far as performance and reliability? No. If you simply can't afford a good roller cam, then a good flat tappet cam can still be a great performer. As with what we said about our roller cams, we do the same thing with the flat tappet cams we run in that ALL of our cams are specifically designed for your specific engine and driving needs, and ALL of them are high quality, name brand cams & lifters, such as Crane, Crower, Comp Cams, Sealed Power, etc..
The down side to running a flat tappet cam is that you will need to break your cam in upon initial start-up. This means if you have performance heads that have dual valve springs, the inner springs MUST be removed before the cam break-in process can begin, and then they have to get re-installed afterwards, which means the whole valve train needs to be taken apart and put back together. Again, this is only on performance engines that use dual valve springs, and most performance engines WILL have dual valve springs.
Now, some guys will say you can get away with running a specialized 1.2 ratio rocker, which takes the load off of the lifter and cam during break-in, but these rockers cost as much as any set of rollers, and on top of that, not all roller rockers have the same dimensions. In other words, it is HIGHLY likely that you'll also need to purchase another set of push rods to maintain the correct contact area between the rocker's roller tip and the tip of the valve. That's quite a bit of money to fork-out just to break-in a cam, and even doing that, you STILL had to remove all of the rockers, and most likely swap all of the push rods too. Well hell... if you're going to do that, you might as well just save all of that money and just do what you were supposed to do in the first place by simply removing the inner springs for the break-in period and re-installing them once you're done.
You must also run a zinc additive in your oil for flat tappets, and you have to be really careful of what kind of valve springs you run. Flat tappet cams are more sensitive to stiff springs than roller cams. So, some people may look at the cost savings of running a flat tappet cam and think it's the way to go, BUT, if you have to pay to have your inner springs removed, then break the cam in, then tear the heads apart to re-install the inner springs again, that is going to cost some $, which in the long run could have offset the initial cost of going with a nice roller cam. It's an individual choice, but it's a choice we can help you make when you're ready for a truly Bad Ass engine.