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Old 12-30-2010, 06:40 PM
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sbc roller cam

3 yrs. ago a friend of mine purchased a SBC hyd. roller cam from a local hotrodder. He installed it with aftermarket hyd. roller lifters and a cam button. Last week during disassembly he discovered that the camshaft was in fact a hyd. flat tappet cam not a hyd. roller cam. I thought that ALL flat tappet cams were ground with a pitch on the lobes. How could this camshaft have worked so well for 3 years?

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Old 12-30-2010, 10:44 PM
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If you're saying the motor had hydraulic roller lifters on a flat tappet cam design, I don't see why it wouldn't work, having a cam button to check cam movement. You just wouldn"t realize the full potential of the roller lifters' ability to deal with steep, aggressive roller cam lobe profiles available.
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Old 12-31-2010, 12:00 AM
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I gues I just thought that the hardened roller on the lifter would destroy the cam lobe. Mostly because the lobe is ground at a small pitch and the roller wouldn't sit perfectly flat on it.
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Old 12-31-2010, 01:22 AM
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With that small a cam it may take a long time for it to destroy the lobe or a roller. I bet you can definately see the contact tracking pattern between the lobes and the rollers.

A stiffer sprung solid roller would have tore itself up in a short time.

Man, the engine had to have been down on power a bunch, the flat tappet lobe profile is nowhere near correct for the line contact of a roller follower. Lift might be similar but duration will be way short.
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Old 12-31-2010, 01:52 AM
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The cam was supposed to have 300 adv. duration but it never really behaved like it had that much duration.
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Old 12-31-2010, 03:00 AM
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Quote:
Originally Posted by Vix76
The cam was supposed to have 300 adv. duration...
That was likely the saving grace, because like has been said- that set-up would have been radically short of duration.
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Old 12-31-2010, 03:23 AM
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I really don't understand. If the cam has 300* of duration what difference does it make what lifter is moving over it. How can the duration change with the lifter?
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Old 12-31-2010, 12:05 PM
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You'll have to look at the lobe shape differences between the two along with the lifter contact point differences to see that roller lifter on a flat tappet lobe will lose a bunch of duration. The cam lobes have a radically different shape. In reverse, the flat tappet lifter on the roller lobe won't even work, the edge of the lifter will dig into the side of the lobe.

Maybe take a look at Isky, Comp Cams, Crane, Elgin, Crower or some other manufacturers web site with a cut-away drawing of the contact point, you'll be able to easily see what the difference is.

It is hard to describe without pictures and 3 pages of text
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Old 12-31-2010, 01:22 PM
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Please correct me if I'm wrong, but wouldnt that mess up the cam. I was told that a roller cam was cut on a steel billet blank, and a flat tappet was ground on a cast blank. Something to do with pressure angles and such. Am I wrong?
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Old 12-31-2010, 01:50 PM
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Quote:
Originally Posted by CJ_1080
Please correct me if I'm wrong, but wouldnt that mess up the cam. I was told that a roller cam was cut on a steel billet blank, and a flat tappet was ground on a cast blank. Something to do with pressure angles and such. Am I wrong?
Your correct, it will if run long enough, or with a decent high spring pressure. The surface of the cast cam will "eggshell" crack under contact with the roller but won't fail immediately in a mild spring application. Given some time it will flake and chunk out.
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Old 12-31-2010, 03:12 PM
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Quote:
Originally Posted by ericnova72
Your correct, it will if run long enough, or with a decent high spring pressure. The surface of the cast cam will "eggshell" crack under contact with the roller but won't fail immediately in a mild spring application. Given some time it will flake and chunk out.

I believe Comp cams sell cast mechanical roller cams that tolerate spring pressures around 400lbs.
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Old 12-31-2010, 03:19 PM
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Quote:
Originally Posted by Vix76
3 yrs. ago a friend of mine purchased a SBC hyd. roller cam from a local hotrodder. He installed it with aftermarket hyd. roller lifters and a cam button. Last week during disassembly he discovered that the camshaft was in fact a hyd. flat tappet cam not a hyd. roller cam. I thought that ALL flat tappet cams were ground with a pitch on the lobes. How could this camshaft have worked so well for 3 years?
I have to admit that I'm stupefied, I've seen this done a couple times and the result was a lot of metal shavings and chips in the oil. This does answer why the engine didn't behave like it had a 300 degree cam. That timing was never achieved, not even close. The reason is a flat tappet cam is ground for the fact that the lifter diameter has to be swept by the lobe, this distance takes time and is a huge portion of the open duration at the valve. A roller lifter makes an instant contact with the cam, it has no sweep diameter, so the lobe has to be shaped to provide that amount of duration. This is why a flat tappet and a roller tappet cam producing the same timing on the valve will look so differently with the roller have a very robust looking lobe in comparison the the FT cam.

Typically, the small and highly loaded contact point of the roller lifter when combined with the kind of springs one would think that a 300 degree cam would need would absolutely crush and roll the relatively soft surface of a flat tappet cam. This is the same process which you'd use to shape a piece of sheet-metal on an English Wheel. The pressure on the roller wants to roll the lobe's surface into a radius around the diameter of the roller. When mixed like this, usually the roller just unpeals the lobe layer by layer. This usually just flakes off into the oil. I would have no explanation as to why this failure mode didn't happen other than your friend has a well surface treated shaft to add hardness to the lobes and/or the valve springs aren't what they should have been, therefore, the lack of spring pressure wasn't strong enough to load the hard roller to where it unpeeled the surface of the lobe, especailly given the duration over the top of the lobe was very short. As far as the offset grind on a flat tappet lobe goes, there isn't enough there to usually make a difference to anything, the offset alignment of lobe to lifter and the convex surface of the lifter is what imparts most of the rotation to the lifter, it doesn't so much spin in its bore as slowly orbit.

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Old 12-31-2010, 03:37 PM
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Quote:
Originally Posted by 406 bug
I believe Comp cams sell cast mechanical roller cams that tolerate spring pressures around 400lbs.
Yep, and there are posts around various boards on the web of guys having flaking problems with them, especially with BBC's and their 1.7 rocker ratio multiplying the pressure the spring puts on the lifter compared to lower rocker ratios. Some guys won't use the cast core roller cams for this reason, and have Comp grind the same profile on a billet blank instead.
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Old 12-31-2010, 04:27 PM
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Then, there's a reasonably good chance that the cam in question IS, in fact, a roller cam from the start.

Any chance you could publish the cam ID numbers and/or a photograph of said camshaft? Just looking at it will tell anyone in a second if it's a roller of flat tappet cam.

The FT cam has "pointy" lobes, compared to a roller's "egg shaped" or much more rounded lobes. A roller cam will be a shiner-looking metal than the dark gray FT cam.

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Old 01-01-2011, 02:11 PM
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BTW, ignore the lame text attached to the above image. It's an oversimplified- bordering on erroneous- description that doesn't tell half of the story of the "why's and wherefore's" of a roller vs. flat tappet cam.
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