So I've been reading a few threads on here about how certain cams with fast valve opening rates, or steep lobe angles can create lifter problems and make the motor sound like a sewing machine. My recent motor with a 290-94H seems to have this problem. Is there a generally accepted way to determine what is too steep of a ramp angle for a specific build? It's at least dependent on the duration specs, both at .050" lift and whatever other spec is available for lower lift numbers, usually advertised duration or .020" or something. I use Solidworks at work and have tried sketching various lobes using published duration and lift specs to visualize the effect on the lobe of different durations for a given lift. However, this is a bit tricky without knowing the cam base circle diameter which is usually not published. If I assume the base circle diameter doesn't change, then here is my understanding. Somebody tell me if I'm wrong: 1. For a given valve lift and advertised duration, adding duration at .050" will create a faster opening valve with a broader lobe that has more area under the curve. 2. For a given valve lift and .050" duration, adding more advertised duration will create a slower opening valve with a flatter lobe angle. 3. For a given advertised duration and .050" duration, adding more lift doesn't change the angle, it just creates a sharper peak on the lobe.
I think the fast ramp cams were blamed for what they called lifter crash, where the engine would falter at higher RPM. When Jim Weise built my first engine, he tested 3 cams in my engine. One was a fast ramp Lunati. I’ll see if I can find the thread of my first engine build.
You would have to have access to the engineering data. Lobe profile values for opening and closing rates (or rates of lift in in/deg) will tell you. Hydraulic "intensity" - the difference in duration (adv to 0.050", or 0.050" to .200"), i.e. the "slope", can give an indication but not always. If the opening rate is too high, inertia forces overcome the check ball or check disc and spring, and won't seat, causing the lifter crash mentioned earlier. High closing rates cause the valve to bounce on the seat. Then there's the high deceleration over the nose, with inertia overcoming the spring force and causing the lifter to loft. Our small cam tunnel (which limits lobe physical size) is the cause of this
