Just to add a little more info...
If you run a heavy high power motor you will also need better brakes and more weight in the front to get the bike to handle decent. If you don't have a reasonable weight distribution, when you go to turn the front wheel will slide and you will end up having the earth to come up and meet you very quickly :lol: . I speak from experience :wink: .
So everything needs to be considered and balanced...a huge, heavy, high powered motor like a QS273 is great but remember you have to feed the motor so a larger battery, a larger controller, larger brakes, larger tires, stronger wheels, etc. may be required. I am not saying you can't or shouldn't do it...just that it isn't as simple as buying a bigger motor.
You need to start with a good frame and one option I discovered is the "Nyx" frame: http://www.nyxbikes.com/nyx-frames. Turns out, doctorbass who commented above was involved in the development of the frame so there is somebody with some actual high powered ebike experience behind it
. There are a lot of other good options out there as well.
I'd also recommend paying fairly close attention to the comments by MadRhino...he has built several high powered bikes and is another one with experience.
Both doctorbass and MadRhino as well as a bunch of others have been very helpful to me. BUT I came to the conclusion that "speed" was not my primary objective...especially since dv/dt can get pretty traumatic the faster you go. For those not aware, dv/dt is the change in velocity vs the change in time and it basically means the faster you go the worse you are going to get hurt when you crash AND everybody crashes at some point.
My preference is to build a bike that has high acceleration from 0 to about 28 or 29 MPH. Acceleration is what your body feels and gives you an adrenaline rush...you can't tell the difference between 10 mph and 800 mph, they both feel the same assuming you are in an enclosed space.
My approach is to use the motor that has the highest acceleration per amp. It is a MAC geared hub motor. If you run every motor in the menu on the Grin Tech Motor Simulator, the MAC has the highest acceleration per amp except the 750W Bafang geared hub motor but the Bafang hub motor actually has little bitty phase wires so you can't feed it much amperage in reality...therefore the MAC is the king as far as acceleration per amp and the MAC only weighs 4.3 Kg.
The MAC does have limitations! If you want to go over about 30 mph the MAC is a bad idea...because it is a geared hub motor it does not reject excess heat to the atmosphere very well so if you want to push more than about 20A continuously (which equates to roughly 30 mph on level ground) through it then you need to find a different type of motor like a direct drive hub motor i.e. the QS 205/273, a Cromotor or etc. You can run much higher amperage through the MAC for short periods of time...see below
.
If you want to feel a lot of acceleration (from 0-28/29 mph) and have a fairly light bike (my steel frame hardtail with a MAC and a 14s6p battery weighs 62 lbs) an 8T or 10T MAC may be the best motor for you
.
Regardless of what type of motor you think you want, I'd highly recommend you run it through the Grin Tech Motor Simulator and see what the temperature does and compare it to other motors for acceleration and speed.
I have been running my MAC with a 52v battery and the 12 FET controller set to 40A battery and 112A phase current for several years and have never had a problem with the gears or the clutch. I ran both a 10T MAC and a Cromotor through the Motor Simulator and the MAC peak acceleration is 8.02 mph/s and the Cromotor is 6.98 mph/s...once you exceed about 20 mph, the Cromotor has higher acceleration.
The MAC is just another option...just be sure your circumstances do not exceed the MAC's capabilities. I have mine set to start reducing the power if the core reaches 130C and to shut down if the temps get to 140C. I came up with those numbers after Justin told me he ran a MAC at 145C on the dyno without any damage.
Remember...if you want to go faster than about 30 mph then go with a direct drive hub motor and if you want to ride off road then I'd recommend a mid drive like a BBSHD but for "fun per dollar" or fun per pound", it is tough to beat a MAC
.