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The question of whether a multi-speed transmission, a single-ratio transmission (reduction gearbox) or nothing is needed depends entirely on the motor and its power supply... so what motor is planned?
 

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Saying that a Nissan Leaf motor is a 250+ kW class motor is like saying that the 22RE currently in the truck is a 230 horsepower engine, because that's what they can do when race-prepared (even without a turbocharger). In stock form, with a typical battery used in a way that it will survive, Leaf motors are good for what the produce in a Leaf: 80 kW to 160 kW depending on battery and inverter.

So, alabamatoy, do you want to use a salvaged EV motor in stock form, a salvaged EV motor with modified inverter, an aftermarket motor sold for EV conversions, an old motor salvaged from industrial equipment such as a forklift truck... ? There are many options and many consequences for the work you will need to do to use them.

The 22RE was about 115HP and about 140LbFt torque. I would like to wind up with something a bit more than that, maybe in the range of 180-200 HP.
That (140-150 kW) has become a relatively common range for production EVs, but not those with the smallest batteries. The Nissan Leaf (only in the variant with the 62 kWh battery), various Hyundai/Kia models, the Chevrolet Bolt...
In aftermarket motors, that sort of power is only the more expensive options, and they need about the same battery voltage as the production EVs (typically 360 volts). Providing that voltage isn't a problem; it's just

That's in the category of aftermarket motors, in this case sold with an inverter so the package can be relatively easily wired up and controlled. The "rated power" shown by EV West of 120 hp (90 kW) is double what the motor can handle continuously without overheating. It's nowhere near your power target, even for a momentary peak; the continuous rating is quite a bit lower than the original engine.. but that may be enough. A more extensive source of information on this motor is the distributor: NetGain - HyPer 9 systems The actual motor and inverter are made by Dana TM4 (which purchased the original manufacturer, SME), but Dana TM4 doesn't provide detailed technical information to individual consumers.
 

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I seriously doubt an NP231 could survive 12 grand at its input shaft without fragging like a grenade.
I agree - about half of that would be a realistic limit. I don't know if the gear would grenade, or the bearings would seize and drag the vehicle to a skidding halt, or the seals would puke out all of the oil and the gearbox would overheat and bind up or grenade. It might be interesting to test. :unsure:

A stock 1987 Toyota 4WD pickup has P225/75R15 tires (28.3" or 719 mm tall). If it is moving on a highway at 130 km/h (81 mph) that's about 992 RPM at the wheels. With a 4.88:1 final drive ratio, that's 4840 RPM at the transfer case output. In high range that's the same 4840 RPM at the input, and that's about what the transfer case is designed to handle continuously. The numbers for the Wrangler are different to suit the tires and ratios, but the situation is similar.

The main transfer case is fine with that, but if you use the reduction gearbox from another NP231 as a "doubler", with a 2.72:1 ratio, the doubler will have a 13,170 RPM input for a 4840 RPM output. This is where the doubler approach gets in trouble: it is designed for much lower speeds, as in a normal use the driver would shift to high range (of the doubler) or a higher gear (of the transmission) long before reaching that transmission output speed.

The doubler is still a viable choice, if you get one which is built for the speed, or you only use its low range at lower road speeds... corresponding to input speeds under perhaps 6,000 RPM. Some motors are only suitable up to about 6,000 RPM anyway, and others would be fine using only the lower part of their speed range, as long as you shift to the lower gear range to get motor speed (and torque to the wheels) up at low road speed.
 

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Some of the higher RPM brushless motors are perfectly fine at half RPM. You get half the horsepower which is still more than you'll get out of a forklift or Hyper9, though you'll likely still need a higher voltage pack.

The "low" voltage HV pack is a double edged sword. Modern eAccessories run at 400V...
It can be far better than that. Typical production EV motors are limited in power by controller programming to protect the battery or the inverter (e.g. to 80 kW for 24 or 30 kWh Leaf, 110 kW for 40 kWh Leaf), so they have the same available power from the end of the constant torque region (e.g. ~2700 RPM for a Leaf) to the point that voltage becomes limiting (e.g. 9800 RPM for a Leaf). Not being able to use the top part of that motor speed range doesn't mean any reduction in power at all; it's being forced down into the bottom of the speed range by the tall gearing chosen to avoid high speed that reduces power available (and reduces efficiency)... at low speed. The fix, of course, is two or more gearing ratios, and the ability to shift.

If you can't shift, but there is a compromise overall gearing ratio that provides enough torque at low road speed yet doesn't run the gearbox input speed too high at top road speed, then you still have a successful system.

Not using the top of the motor speed range means not needing as much battery voltage, but if the controller can handle it anyway (and especially if it is needed to make those high voltage accessory system work), staying with a high battery voltage keeps DC link current down for the same power.

Motors themselves don't have this sort of flat power limit, so they have the single peak power point as seen in performance curves such as those published for the HyPer 9. Even matched motor and controller systems sold separately from a vehicle typically don't have a power "plateau", but in some cases they do because of the inverter. In this case the peak power for a given supply voltage is not at the highest motor speed so the top of the speed range does not need to be used, but power falling off at lower speed still creates the problem of finding a single compromise gearing or using a multiple ratio transmission system.
 
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