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Discussion Starter · #1 ·
This has probably been discussed before but is there a simple relationship between vehicle weight and recommended motor power output ?

Is the original power output of the IC engine a good reference ?

My Subaru diesel is 148bhp and 258 lb-ft and weighs about 1.4 tons.
 

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I'm working on the same issue. I'm converting a 74 Beetle that has a factory weight of just under 1900lbs. I'm guessing I can go with a slightly under-powered motor, compared to the average conversion (just 57HP on the stock motor). This would be nice as it could allow for more range. But I'm still trying to figure out the right sized motor.
 

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Some sources, such as HPEVS, provide a general guideline for motor size based only on weight, but that's too crude to be an effective choice. Aerodynamic drag and other vehicle factors, plus performance expectations, must be considered for an accurate assessment of the power requirement.
 

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Some sources, such as HPEVS, provide a general guideline for motor size based only on weight, but that's too crude to be an effective choice. Aerodynamic drag and other vehicle factors, plus performance expectations, must be considered for an accurate assessment of the power requirement.
One immediate thought is power loss due to the drivetrain. A good point of reference is to look up the difference between the HP of the OEM motor compared to the actual horsepower at the wheels.

Generally, if you can't find that info:
-AWD loses more power than 2WD
-Configurations like RR, FF, and MR will experience less power loss than longer drivetrains in FR vehicles. It's just a function of weight.
-I'm just guessing, but power loss of 10-25% between the crank and the wheels is probable.

EDIT come to think of it, the easiest way to figure this out would be a simple equation that allowed you to input the old HP/Torque of the engine, at the crank, to determine what would be comparable.
 

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I'm guessing I can go with a slightly under-powered motor, compared to the average conversion (just 57HP on the stock motor). This would be nice as it could allow for more range.
There is no meaningful connection between motor power and range, because smaller motors are not more efficient and the motor weight is such a small part of the vehicle weight.

It's using the power that eats up range, not just having it available.
 

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Discussion Starter · #6 ·
For reference the more common Subaru 2.5 petrol is about 163 bhp (121 kW) at 5600 rpm and torque of 166 lb.ft ( 226Nm). So more power than the diesel but less torque.

My thought was that a motor can deliver all its output from a zero rpm so it might not need to have the same output to deliver a similar level of performance.
 

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My thought was that a motor can deliver all its output from a zero rpm so it might not need to have the same output to deliver a similar level of performance.
But that logic doesn't work. With an engine the clutch can be slipped, allowing the engine to run fast enough to produce its full torque, until the car is going fast enough in first gear that the slipping is no longer required... and that's only a second or two in the most extreme situation. As a result, if shifting for best performance, the lack of torque at low speed makes no difference to the performance capability with an engine and the only advantages an EV has due to torque output down to zero speed are less need to shift gears and no need for a clutch.
 

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Agreed but we rarely have to rev to peak torque to get the car moving. Maybe during a full sprint start, generally I think 60-70% of full torque is used in an ICE.
I agree that drivers rarely use the full capability of their car other than on a racetrack, especially from a standstill. So your logic is that much of the capability of an ICE is not used, so an electric replacement can be weaker? That works for starting from a standstill, but the performance limitation of a typical EV is at much higher speeds, where both the engine and the electric motor are able run at their full rated power.
 

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I've only worked on my projects so my experience is limited, but what I think I know is that torque determines getting going and hp determines top speed... Brian can confirm / shed more light...

Like Capt_orygun, the first thing to figure out is what % of your hp actually makes it to the driven wheels. My project is a bit more of an antique than yours, but in my case it's between 60% and 40% depending on transmission and AC and temperature and <long list>. My particular car had all options and an automatic transmission, so I'm at 40-45%. In the EV version most or all of those losses go away (the accessories still drain your battery, but they don't load the motor) so that helps.

Between eliminating that loss and the fact that you rarely rev the car to maintain cruising speed, I would think that a somewhat smaller hp electric motor should do fine. Like Brian said, the size of the motor is pretty much a moot point when it comes to efficiency/range, but trying to get high-hp motors can get expensive quick. Staying in the Warp-9/-11 range really helps the cost of a project stay "normal".

The one place where you do want "the power" is in getting off the line. That's when you rev an ICE to get the torque built up to accelerate, because an ICE has 0 torque at 0 rpm. The story is that an electric motor has full torque at 0 and that that makes it warp-speed / better than ICE. Well, it depends.

Like Brian says, you slip the clutch to let the engine rev up to get to torque. In my project, the ICE probably generates 250 ft lbs at 60% rpm, that's respectable (but the car weighs 6,000+ lbs so it needs it, it's no rocket). The E-motor equivalent generates 450 ft lbs, and that sounds great.

The Big Deal, the central decision here is whether to keep the original transmission.

My 1st gear is/was 4:1, so that 250 ft lbs from the ICE turns into 1,000 ft lbs at the wheel. Using the e-motor and no more transmission would now put me waaayy behind at 1/3 or half of what the original performance was, and that's before just flooring the ICE. The original 0-60 time was 10.6 seconds, and this would be more like 25 sec.

If you're keeping the transmission, then a lower-range electric motor will likely do fine. If you're ditching the transmission, you're giving up on all of the torque-multiplication so you'll have to buy a motor that can deliver them by itself. That makes a really big difference. A 250 ft lbs e-motor is "cheap", a 1,000 ft lbs e-motor is big and expensive.

In my project, because of the horrible hp-efficiency of the automatic transmission and because my e-motor is way too high in rpm for the transmission to handle anyway, I did ditch the transmission. To still get as much torque as I can, I've installed a permanent 2:1 gearbox (by TorqueBox). It lets the e-motor run at 6,000+ rpm so it's at peak efficiency, and it multiplies the torque to 900 ft lbs which is close enough to the original performance (or even better than in higher gears) to be "equal".

Anyway, my 2 cents
 

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The Big Deal, the central decision here is whether to keep the original transmission.

My 1st gear is/was 4:1, so that 250 ft lbs from the ICE turns into 1,000 ft lbs at the wheel. Using the e-motor and no more transmission would now put me waaayy behind at 1/3 or half of what the original performance was, and that's before just flooring the ICE. The original 0-60 time was 10.6 seconds, and this would be more like 25 sec.

If you're keeping the transmission, then a lower-range electric motor will likely do fine. If you're ditching the transmission, you're giving up on all of the torque-multiplication so you'll have to buy a motor that can deliver them by itself. That makes a really big difference. A 250 ft lbs e-motor is "cheap", a 1,000 ft lbs e-motor is big and expensive.
So thinking about this, if you were to keep your original transmission, then would I be right in thinking that the easiest way to match an electric motor to a chassis would be to find one that produces similar torque at similar rpm?
 

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So thinking about this, if you were to keep your original transmission, then would I be right in thinking that the easiest way to match an electric motor to a chassis would be to find one that produces similar torque at similar rpm?
If you were going to shift at the same speeds as with the engine, that would make sense... but there's no need to shift that way.

With an engine, you shift to keep the engine speed as low as practical (to reduce fuel consumption, noise, and wear) while still delivering enough power. With an electric motor you should shift to keep the motor at the speed at which it most efficiently produces the desired power... which will generally be much faster than the engine, even if their top speeds are the same.

I think it would make more sense to choose a motor which:
  • can produce enough power to match the original (or desired) performance, within the maximum speed appropriate for the transmission input; and,
  • can produce enough torque at low speed that when multiplied by the first gear ratio it provides the desired low-speed performance
 

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So thinking about this, if you were to keep your original transmission, then would I be right in thinking that the easiest way to match an electric motor to a chassis would be to find one that produces similar torque at similar rpm?
My recommendation (and it's just a recommendation :)) would be to take the bhp of the ICE at highway-cruising rpm, add maybe 25%, and select some electric motors that have that kind of range as their sustained / long-duration max power capability. Then, on the power-graph of each motor, figure out at which rpm it runs maximum efficiency (just to make the most of the bhp you've got, because losses there mean lost range).

That should tell you the best rpm to run the motor for cruising. Next, calculate the speed of the car based on the combined reduction ratios (gear box, differential etc) in 4th gear (highest gear before over-drive) when the motor runs around max efficiency. If that speed is off from where you want to highway-cruise by more than say 20%, you're probably better off picking a different motor.

Different motors with similar horse-power/kWh have quite different power graphs, max rpm is (from what I've seen) anywhere from 6,000 to 12,000 rpm, yet the rpm you need for cruising is likely only 3,000-3,500. In a lot of cases higher rpm means higher battery-voltage etc, and for charging purposes keeping the voltage from going way up is usually simpler. So, all this is a balancing act between the space you have available, the bhp/kWh you need, pack-voltage and just plain price-tags.


Once you've picked a motor and the speeds line up, you can do the multiplication to figure out how torque compares across the lower gears to say 60%-70% of max ICE torque (because you're not flooring that one either). Chances are you'll be more than fine in 2nd gear to pull away, then shift to 4th gear for cruising. The torque of the motor is probably so high that in 1st gear you'd snap the drive axles in half if you floored it....

All that said, there is an entire population out there who have figured out how to over-drive used forklift motors with tons of amps to make it all work. You could consider that route (I didn't, I went the stick-with-what-is-officially-rated route), or at least there is one lesson to be learned from that way of converting: you can over-drive a motor with proper cooling. If the motor you picked turns out whimpy, upping the pack voltage i.e. the power you push into the motor can compensate to some extent....
 
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