I have two V-Rods and perform all of my own maintenance, including adjusting valve clearnaces, so I have a high degree of familiarity with the V-Rod. I have that hand memory of the engine you only develop from working on them enough times (not that it isn't reliable, it is, but I put a lot of miles on the bike and that drives maintenance actions).
If you think about a valve train, the valve spring has to control everything in the valve train, the valve, pushrod if any, rocker arm and lifter. Like any piece of metal, all of those parts have a resonant frequency, sometimes many frequencies and the only thing preventing resonant vibrations is the valve spring. The more mass that valve spring has to control, the less able it is to control all that mass, which means the more mass you have the slower you have to spin the engine to allow that valve spring to keep up, otherwise you get valve float (valve spring compresses and cannot close the valve fast enough). That is when pistons hit valves and bad things happen to your engine.
In general, overhead cams mean fewer parts and less mass for the valve spring to control. If you operate all the valves with one cam through rocker arms, the weight and flexibility of the rocker arm will limit how fast you can spin the engine before you risk breaking things, but with rocker arms you can use lock nut adjusters to set valve clearances, and this can be done without much work or disassembly. If you operate the valves with a cam located directly over the valve stem through a bucket, you minimize the mass the valve spring has to control and thus can use more rpm. With buckes you have to have means to adjust valve clearances, and most engine manufacturers use sized shims placed either under the bucket on top of the valve stem, or in a pocket on top of the bucket. With the shim under the bucket there is no way the cam lobe can scrape the shim off the bucket and do damage at high rpm, so most really high rpm engines use direct acting cams with shims placed under the buckets. The drawback to putting the shims under the buckets is that you have to remove the cams to replace shims, and you have to make excruciatingly careful measurements of the valve clearances before you pull the cams, and then measure the shims carefully so you can calculate what size shim you need to get the correct clearance. I have this down to a 7-8 hour drill on the V-Rod now, which is darn good. But my BMW K100s use shim over bucket, and I can check clearances in maybe 45 minutes, even if I have to swap a couple of shims along the way.
Remember horsepower = (Torque X RPM)/5252. As long as torque does not fall off, raising rpm gives higher horsepower and a faster bike. The limit is how fast you can fill the cylinder and how fast the flame can travel across the bore during combustion. The larger the bore, the more time it takes the flame to travel across the bore, so there are practical limits to how fast you can spin a big bore engine before you run up against the physical limits of combustion. No worries, just add more cylinders with smaller bores. That is why GP bikes are multi-cylinder screamers.
I live in a parallel universe but have a vacation home in reality.
Last edited by Desert Tortoise; 02-25-2014 at 06:04 PM.