Some motor approximations:
For motors of the same physical dimensions:
If it spins to twice the RPM, it can make roughly twice the power.
If it has twice the copper fill, it can make roughly twice the power.
If it has twice the airflow/heatsink area, it can make roughly twice the power.
If it goes from 80% to 90% efficiency, it can make roughly twice the power.
This means, if you run a motor at 1/3rd the RPM, that is 80% efficient, you need to be 6 times physically larger to equal a motor at 90% efficiency with 3x the RPM. Fortunately, larger motors naturally have more area to transfer heat into the air passing over them, so in practice it might only need to be 4x larger.
Here is an example of how foolish it is to judge an electric motor by it's size, or voltage etc.
These electric motors weigh 1.4lbs, run on 24v, under 1" thick, and 6" in diameter that can do
7hp output continuously.
http://www.launchpnt.com/portfolio/aerospace/uav-electric-propulsion/
Likewise, there are motors running on 200VDC, weigh 190lbs, are the size of a large cooler, and make
4hp
http://www.ebay.com/itm/T-B-Woods-3HP-1750RPM-TEFC-180-200VDC-Motor-M300A-/270823166566?pt=LH_DefaultDomain_0&hash=item3f0e519266#ht_2052wt_994
Why can the giant motor only make 4hp at 200vdc while the motor that is lighter than just the end of the shaft alone of the big motor can make 7hp at 24vdc?
Because only a f*cking tool tries to judge a motors output by size, and then tries to throw voltage into it as well somehow? As if voltage is in anyway related to a motors output potential???
I've got a dyno, and a portable dyno. However, it's 100% useless to try to measure the power output of race bikes at the track. It's effortless to retard timing on the gas bikes to reduce power to any level you desire (from 20hp to 1hp even), and then slide it back advanced to go race. Likewise, have a 3 position control switch hidden on your ebike (and it's impossible to find well hidden switches, they can be magnet switches for wiring mounted in the frame etc), go roll on the dyno and lay 1hp, get off the dyno, take your tiny scrap of magnet away from some point on the bike, and have 20hp.
Bringing the dyno out could be fun for folks interested in seeing how much power they make, but it's useless for trying to patrol a class.
PaulD and I have talked for hours together about how to make fair classes for gas and electric bikes. It always comes back to Tiberius' original suggestion of weight maximums. Why weight? Because you can check it in 10 seconds as a bike is rolling up to go on. It balances modifications amazingly well. If somebody wants to run motorcycle tires for example, well, they were a 7.5lbs addition to my bike over bicycle tires, so I would have to run either less battery, a lighter controller, a lighter motor, etc to still make the weight. As a race gets longer, ebikes fall victim to battery weight in a drastic way, as there energy is single use disposable liquid, and ours is an elaborate reusable container for electrons.
We discussed having ebikes weigh-in with no battery on board, and gas bikes weigh-in with no fuel on board. It does allow for races to stay balanced at any practical distance.
Where it fails badly, is rider weight impact. This is why every karting class always weighs kart/rider together, and lead is bolted to the karts that fall under the minimum. For bicycles, this isn't really a safe practice, and would get pretty extreme with realistic 100lbs differences between riders body weights.
Likewise, heavier riders need heavier brakes, shred tires faster, and require more power to accelerate at a competitive rate. For example, PaulD's bike was amazing with his light body on it, but when I ran some laps on it, it was flexy, badly under powered, and the brake faded unsafely for me while only riding 7/10ths intensity.
This brings us to bike weight as a percentage of riders body weight. Why? Because it's measurable in seconds with a sub 1% margin of error, it lets big guys run safe bikes and little guys run safe bikes, without giving a big advantage to either, and it lets gas bikes run against electric bikes. To prevent anyone from being a huge 400lbs guy on a motorcycle, you have a fixed bike weight cap at something like 100lbs (or whatever line you want to say is when a bicycle stops being a bicycle).
Where does it fail? Non-race bikes, like big heavy cruisers with crap power, both electric and gas, end up not being fairly classed at all. However, there is an easy solution, if you brought a non-race bike and just came to have fun, then have a fun class for those folks to go ride.
Also, to touch on the topic of this thread, I think the concept of running ebikes by themselves, or 2 stroke, or 4 stroke or whatever by themselves is really really dumb. I would never even bother to attend an event that was just e-bicycles, it has no appeal to me at all. 100% of the appeal is in different types of bikes competing against each other. I don't actually think ANYONE likes grouping races by bike type rather than by folks that want to race against each other. It's like going to drag racing events, domestic only races and events suck, nobody cares, import only races and events suck, nobody cares, but DOMESTIC VS IMPORT events draw huge crowds, everybody is emotionally wrapped up in the races for one team or the other, and the races are 10000 times more exciting to watch.
IMHO, to do this right, you have 2 classes, possibly 3 classes (if you want 2 different weight groups). You have the fun-run folks who are just there to enjoy going around a track and didn't bring race bikes, and they don't weigh-in, they just go out as a group and ride. Then you have the class for people who come to race on things made for racing, and they have to weigh-in and meet the rider/bike weight spec. A light weight group for folks with perhaps under 0.4lbs of bike weight per 1lbs of rider, and a middle/heavy bike group for riders with under 0.55lbs of bike per 1lbs of rider (or whatever is determined to work out well).
Power can't be measured while on the track for gas bikes, and it's damn easy to fool with electrics. Weight can be measured with dirt cheap equipment to extremely reliable numbers, everyone can check at home with a bathroom scale to see where they are at so there are no surprises at the track. Set the values so it forces bicycle to be bicycles, and let the weight maximum be the limiter for the engine/motor/CVT/gearing/controller/wheels/tires etc.