# Aw11 mr2



## Akya2120 (Jan 12, 2020)

Hello folks,

Excuse me for being new and potentially putting this in the wrong place. I'm intending for this to go from groundwork to moving vehicle in about one to three years.

I've been mulling it over for a while and I am going to start a project car. I would like to convert an AW11 MR2 to an electric vehicle. Originally I was going to do diesel, but it felt like a half measure. I want to have about 80 miles of range minimum, I want to make about 180-250hp, I plan on freeway cruising around 75 mph, I don't care about peripherals just yet (heating, ac, power steering).

My original thought was to use a zero bike motor and mate it to the stock MR2 transmission. Would make for a generally easy swap, but it doesn't quite get me to my power goals. Also, afaik electric motors tend to destroy manual transmissions when shifted (more on this later).

Then I considered using two of these motors and just mating them to the axles since the bikes are direct drive and good to 124 mph as is (I realize that's partially because of the reduction from front to rear sprocket). The hard part about this is how to control the motors when cornering (same power to both wheels will mimic a locked or welded differential). Simply using the electric motor on the transmission utilizes the stock differential and I wouldn't have to worry about that.

I also considered using a Tesla motor, but from what I understand this gets expensive quick, and I think there are more simple and cost effective alternatives that suit my goals.

Usually manual transmissions don't like the shock load from electric motors, which is what causes them to fail. Has anyone developed a shift controller that rpm matches the motor to the ground speed for a given gear? Wouldn't that remove the shock load and allow for clutch less shifting? I would imagine that would be fairly doable via a microcontroller and switched gates for the shift knob. Depending on which shift gate is triggered it would calculate and adjust the motor rpm until the gear is engaged (heck you could use the clutch start safety switch to engage the rpm matching mode). Would this still cause a bunch of shock to the transmission? I would imagine it could be fine tuned until it doesn't but then why has no one done it?

I am basically a novice when it comes to this kind of swap. I do have a lot of fabricating, design, and automotive experience. It also helps that I work in a shop and have a couple of machinist friends who I can trade work with. If anyone has recommendations and or advice I would love to hear it. Thanks for reading!


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## Immo1282 (Jul 12, 2019)

Shifting is pretty pointless in an EV. If the motor can deliver all the torque from zero RPM, and stilll adequate torque at high RPM - you've no need to shift while driving at all. Might be advantageous to put it in second instead of third if you're doing an off-the-line drag race, but "fixed" reductions can work just fine with most EVs. My project is going to be left in third gear almost all the time when it's built (I'm doing motor reverse too, so I won't even need to shift when it reverses...)

As for gearbox wear - if 99% of the time you'd have shifted with an ICE, you don't during normal driving - I'd have thought that wear (including clutch wear) would happen slower overall, despite higher instantaneous loads. Avoid over-engineering things if possible, as a conversion is going to be a pretty large project anyway!

Clutch vs clutchless argument seems a little silly to me really - the weight savings of ditching the clutch are going to affect range minimally, and having a mechanical disconnect seems like a good idea for safety from my perspective. Also, if you mount the motor directly in place of the engine, you will be able to skip the re-engineering a shaft that bypasses the clutch. If you've not a well equipped machine shop, this could add a fair deal of cost.

I'm definitely going to follow your project though, and wish you all the best! We're converting similarly laid out cars (mid-engine 2 seaters) after all


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## brian_ (Feb 7, 2017)

Akya2120 said:


> My original thought was to use a zero bike motor and mate it to the stock MR2 transmission. Would make for a generally easy swap, but it doesn't quite get me to my power goals. Also, afaik electric motors tend to destroy manual transmissions when shifted (more on this later).
> 
> Then I considered using two of these motors and just mating them to the axles since the bikes are direct drive and good to 124 mph as is (I realize that's partially because of the reduction from front to rear sprocket).


Don't underestimate the importance of that gear reduction, which is substantial. If you go with a single-ratio drive, it still needs to be a significant reduction. Production EVs typically run a 6:1 to 10:1 ratio.

Also, even two the of the Zero motors would be inadequate for a car like this.



Akya2120 said:


> Has anyone developed a shift controller that rpm matches the motor to the ground speed for a given gear? Wouldn't that remove the shock load and allow for clutch less shifting? I would imagine that would be fairly doable via a microcontroller and switched gates for the shift knob. Depending on which shift gate is triggered it would calculate and adjust the motor rpm until the gear is engaged (heck you could use the clutch start safety switch to engage the rpm matching mode). Would this still cause a bunch of shock to the transmission? I would imagine it could be fine tuned until it doesn't but then why has no one done it?


This would work well, and would eliminate the need for both the clutch and synchronizers. It's beyond what DIY builders generally attempt, and even beyond what many understand.

This would be easiest to implement with a sequential shifter (such as in a motorcycle or automotive race transmission) since the direction of the shift (up or down) and the target gear (always the next one, never skipping ratios) would always be immediately known.



Immo1282 said:


> Shifting is pretty pointless in an EV. If the motor can deliver all the torque from zero RPM, and stilll adequate torque at high RPM - you've no need to shift while driving at all. Might be advantageous to put it in second instead of third if you're doing an off-the-line drag race, but "fixed" reductions can work just fine with most EVs. My project is going to be left in third gear almost all the time when it's built (I'm doing motor reverse too, so I won't even need to shift when it reverses...)


There seems to be a tendency to think of "all the torque" (meaning the motor's peak torque output) as infinite torque - it's not. With a high-voltage system it is normal to be able to produce the maximum rated power over a broad speed range, so multiple gear ratios are not required. With low-voltage systems based on brushed DC motors of moderate size, it is normal to need to shift to keep the motor in its relatively narrow peak power band over the range of road speeds, although only a couple of ratios is usually enough.


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## Akya2120 (Jan 12, 2020)

So the shift controller will likely come later if I decide to go the manual route. The stock 4-AGE which came in the MR2 made 115-130 HP (depending on year) and 110 Ft•Lbs. So a single zero motor is about on par with the stock engine. There was also a supercharged model which made a little more power, but probably about the same torque number as the zero.

What are my other options for motors? Should I be trying to canabalise a production vehicle like the leaf or bolt? I'd rather go with an AC motor, but I'd like some input on which motor I should be looking at, and why. I did think having independent motors for the rear wheels would be cool, but would add more complexity since at that point I need to figure out how to bias them for cornering.


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## Duncan (Dec 8, 2008)

Hi
I suggest the motor from a production EV - Leaf, Tesla whatever

As far as two motors is concerned don't overthink it - a standard open diff as used by 99.9% of cars on the road simply uses the road to equalise the loads - two motors run separately would be BETTER than that - and that is before you get into torque vectoring and such

If I was doing your car I would be sticking a Tesla or Leaf transaxle where your diff is 

A BIGGER problem is where to put the batteries!


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## brian_ (Feb 7, 2017)

Akya2120 said:


> The stock 4-AGE which came in the MR2 made 115-130 HP (depending on year) and 110 Ft•Lbs. So a single zero motor is about on par with the stock engine.


Those numbers look reasonable for a 4A-GE, but unless there is some higher-powered version of the Zero than I found, the zero motor is not even close to that. The original (DS) and higher-power (SR) models of the Zero appear to have 44 and 67 horsepower... way short of any 4A engine. Torque appears to be 106 lb-ft for the SR, which looks close but is presumably attained at only low motor speed. _Two_ Zero motors might be suitable.



Akya2120 said:


> What are my other options for motors? Should I be trying to canabalise a production vehicle like the leaf or bolt? I'd rather go with an AC motor, but I'd like some input on which motor I should be looking at, and why.


If you are willing and able to deal with the issues with controlling the motor from a production EV, that's definitely the most effective reasonably-priced source of a suitable motor for a car. It will also come with a single-ratio transaxle, giving the option of using the complete drive unit (motor with transaxle); that would mean dealing with mounts and axle shafts, but not needing to adapt a motor to transaxle or the motor shaft to a transaxle input shaft.

The Leaf is the most common affordable EV, and it has a good well-proven motor. Some conversions have worked out ways to successfully control the Leaf motor outside of Leaf.

The GM EVs (Chevrolet Spark EV and Chevrolet Bolt) have the advantage of being configured with the motor and transmission on the axle line, making the whole package more compact and leaving more space between the firewall (behind the driver) and motor for everything you need to fit in (such as a battery pack). Of the GMs, the Bolt has a more powerful and smaller motor; it will be more common, but is newer so the salvage supply of Spark EV motors may be greater for now. Here's a successful project using the Bolt drive unit (and battery - but you won't be able to use a Bolt battery in an MR2):
_Westfalia T3 with Chevy Bolt drivetrain_


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## Akya2120 (Jan 12, 2020)

Well, I was interested in using two motors for exactly that reason. I don't like open differential vehicles. Two motors allows me to emulate a performance differential, but then I only have a set gear reduction which may not necessarily be a bad thing.

As for the batteries, the MR2 fuel tank was in what would normally be the transmission tunnel. So a few could probably fit in there. Maybe not 80 miles worth, but at least a couple. The engine bay is rather large compared to an ev transaxle so probably a couple could go in there. It also has a frunk and a trunk. I'd like to keep enough storage capacity to pick up groceries, I'd also like to keep the vehicle as close to a 50/50 balance as possible (I have plate scales to help achieve that).

The stock engine alone weighed about 350lbs. The transmission is probably another 100, the fuel tank was a little over 10 gallons, so about 80lbs. How much does the Tesla transaxle weigh? How much does 80 miles worth of batteries weigh?

Does the Tesla transaxle have torque vectoring, or some form of limited slip differential?


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## Akya2120 (Jan 12, 2020)

brian_ said:


> Those numbers look reasonable for a 4A-GE, but unless there is some higher-powered version of the Zero than I found, the zero motor is not even close to that. The original (DS) and higher-power (SR) models of the Zero appear to have 44 and 67 horsepower... way short of any 4A engine. Torque appears to be 106 lb-ft for the SR, which looks close but is presumably attained at only low motor speed. _Two_ Zero motors might be suitable.


The SR/F is the model with a 140 Ft•Lb and 110 HP motor it also has a 14.4 kWh battery pack. It seems like I would want about a 30 kWh battery pack to meet my range goal, but that's estimated from current ev ranges and battery packs. The other thing I like about using two motors is I have no transmission or differential to service. The zero motors are passively air cooled, which means I'd have to run a duct or a fan. Either of those things are not a problem. The hard part is going to be (once again) power biasing for corners. Or I could just run it like a welded diff and it would just be real drifty all the time haha.


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## brian_ (Feb 7, 2017)

Akya2120 said:


> Well, I was interested in using two motors for exactly that reason. I don't like open differential vehicles. Two motors allows me to emulate a performance differential, but then I only have a set gear reduction which may not necessarily be a bad thing.


It's certainly an effective way to go, but not trivial to implement.



Akya2120 said:


> Does the Tesla transaxle have torque vectoring, or some form of limited slip differential?


There are a few different Tesla transaxles (Model S and X are different from Model 3, front and rear have detail differences, and small-motor and large-motor Model S/X rear units are different), but none of them have any form of limited-slip differential and there is no way for any of them to apply different torque to left and right wheels... other than individual wheel brake application as in every other current production car.

It is possible to fit a different differential in a Model S transaxle, and there are even commercially offered Quaife units to fit. I haven't heard of anyone going to the trouble of doing this for any other EV transaxle, just because the Teslas have been around for quite a while (so they're commonly used) and are especially popular in high-powered conversions, but almost any differential can be swapped out for a Quaife or similar unit.



Akya2120 said:


> As for the batteries, the MR2 fuel tank was in what would normally be the transmission tunnel. So a few could probably fit in there. Maybe not 80 miles worth, but at least a couple. The engine bay is rather large compared to an ev transaxle so probably a couple could go in there. It also has a frunk and a trunk. I'd like to keep enough storage capacity to pick up groceries, I'd also like to keep the vehicle as close to a 50/50 balance as possible (I have plate scales to help achieve that).


The tunnel space is great - well-located for mass distribution - but modules are rigid things and few if any are sized to fit in that tunnel. Of course the Chevrolet Volt (and Volvo hybrid) modules are designed to fit in a tunnel, but they go in cars with a tunnel designed to accommodate them. Do you have the dimensions (particularly width and height) of the tunnel space?

You might be surprised how large a typical EV drive unit is. Open the hood of a Leaf: the drive unit is as tall and wide (front to back) as an engine with transaxle; it just isn't as long (across the car). The inverter and charger are a substantial part of the height, but you need to those things somewhere. This is why I mentioned the Chevrolet bits, which sit further back due to the concentric design.

The rear trunk is bad for mass distribution (you don't want it at the ends of the car), but the front trunk seems like a good place for a small pack.

An MR2 isn't a 50:50 balanced car; it's rare for a rear-mid-engined vehicle to not be rear-biased, and as a rear-wheel-drive car designed to be rear-heavy, that's okay. It's slightly rear-heavy when empty (about 45:55, depending on roof style and engine, for the W10 generation), driver/passenger load is also rear-biased, and of course cargo load capacity is much greater in the rear than the front. Some change is manageable, but even without driving one I'm sure that I wouldn't want a front-heavy MR2, and even 50:50 wouldn't be an improvement over the stock distribution.


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## MalcolmG (May 9, 2017)

I usually just lurk here at the moment, but since your project car is one I'm particularly fond of and familiar with I thought I would chime in...

Regarding your thoughts around one motor per wheel - I think the effects of this won't be as severe as you're thinking. Obviously it depends on your controller arrangement as to what the behaviour is, but it shouldn't effect handling when coasting or totally off the power as there is nothing forcing the motors to turn at the same speed (unlike a locked diff where both wheels are always at the same speed). Probably the main time it would affect the behaviour significantly would be powering out of corners, but I think you may be able to overcome any unwanted behaviour there with changes to suspension setup and wheel alignment.

A slightly out of the box idea would be to consider mounting the motor in the fuel tank tunnel, with a driveshaft running through to a standard RWD differential (from an IRS rear end, not live axle). This would allow you to get a decent gear reduction (I guess if you search hard enough you may find a suitable diff with a ratio of 5:1? I think I remember hearing there were some quite short diff ratios available for the Toyota GT86), and it would also leave a lot of the engine bay free for batteries. That would keep the bulk of the new weight in about the same location as the original layout, and you could move the motor fore or aft as needed to help achieve the target weight distribution. Obviously this option depends on the motor being physically able to fit into the tunnel, and I haven’t seen many of the popular EV motors in the flesh to have an idea of the physical size of them.


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## brian_ (Feb 7, 2017)

MalcolmG said:


> A slightly out of the box idea would be to consider mounting the motor in the fuel tank tunnel, with a driveshaft running through to a standard RWD differential (from an IRS rear end, not live axle).
> ... it would also leave a lot of the engine bay free for batteries. That would keep the bulk of the new weight in about the same location as the original layout, and you could move the motor fore or aft as needed to help achieve the target weight distribution. Obviously this option depends on the motor being physically able to fit into the tunnel, and I haven’t seen many of the popular EV motors in the flesh to have an idea of the physical size of them.


I'm pretty sure that someone has built that configuration. One limitation is that the tunnel is not huge - it will limit motor diameter; production EV motors are big. And the space between the axle line and firewall will be substantially compromised by the shaft and the nose of the final drive. 

Here's an example in EV Album, from a decade ago:
1987 Toyota MR2 AW11 “Black Magic”
This one apparently used a Mazda MX5 (Miata) diff, which is the same Hitachi as used in many Nissan and Subaru models.

I was surprised to find about fifteen MR2 conversions - of all generations but mostly W10 - in the EV Album. Generally they adapt a DC motor to the stock manual transaxle and stack some battery above it, with LiFePO4 cells (since none are recent), but there are some AC motors and even some lead-acid batteries. None of the others use the motor-in-tunnel solution.



MalcolmG said:


> This would allow you to get a decent gear reduction (I guess if you search hard enough you may find a suitable diff with a ratio of 5:1? I think I remember hearing there were some quite short diff ratios available for the Toyota GT86)...


The GT86 (and Subaru BRZ, and Scion FR-S) use the same final drives as some other rear-drive Toyota models (despite being built by Subaru and based on the Impreza); apparently 4.10:1 and 4.30:1 have been stock but 4.56:1, 4.67:1, and 4.88:1 are all available. Apparently the big and strong Toyota diff is not the one in the GT96, but the 7.5" F-series, and there are some more extreme ratios of that, but those ratios will be in obscure models and likely difficult to find... but still a possibility.

I don't know which diffs would be easiest to adapt to MR2 axle shafts.


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## Akya2120 (Jan 12, 2020)

I like the idea, it gives me an idea that I think is even more useful. I'd try and mount the motor up front, then just use the trans tunnel for the driveshaft. I'm sure I can find an IRS diff second hand or in a junk yard. I'm not extremely familiar with all the details of modern motors but I understand their operating principles.

Can anyone tell me, what is an efficient motor around 250-300hp/~180-230kW output? Likely most modern EVs are going to be my choice. Is Tesla the best choice for weight, performance, and efficiency? Or something else?

How does one go about controlling these motors? I'm guessing it's mostly just hardware that gets bought and calibrated. Does it make more sense to buy a totaled Tesla/Chevy/Nissan/BMW to get the batteries and motor, or hunt for them somewhere else?

Using the engine bay for batteries does seem to make sense since likely, unless I chose to have very short range, there will be more weight in batteries than motor. Do the batteries and motor get liquid cooled?

Sorry for all the questions, I'm familiar with this stuff but not on the level of the minutiae that allows it all to function cohesively. If someone has a good article or three to read, that would be awesome. Thanks for all your help so far folks!


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## brian_ (Feb 7, 2017)

Akya2120 said:


> I like the idea, it gives me an idea that I think is even more useful. I'd try and mount the motor up front, then just use the trans tunnel for the driveshaft.


That means not being able to use any of the space in the front or in the generously-sized tunnel for battery, and still having a final drive unit (differential) in the original engine space, keeping the battery pack from being able to use all of the space to the floor level. This seems like a strange way to use a vehicle designed for a mid-mounted engine.



Akya2120 said:


> Using the engine bay for batteries does seem to make sense since likely, unless I chose to have very short range, there will be more weight in batteries than motor.


I agree with this part.


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## Tremelune (Dec 8, 2009)

Even the "easy" route is not easy...I would recommend taking that approach first: Adapt a motor to the manual transmission. Put it in a high gear for highway, low gear for around town. Shifting will be exceedingly rare, and the clutch/transmission should suffer very little from shifts. I do wonder what motor torque the transmission can handle, though...

The Leaf motor puts out 210ftlb of torque, spins to 10k rpm, and can be bought for about $1000 with inverter (plus another $500 for something like a Thunderstruck controller). I think you'd struggle to beat this specs for the money.

Battery weight and size depends on what car you get them from. 80mi would be about a full Leaf pack from a 5-year-old car. You're looking at about 400lb, $3k, and a 1'x2'x3' brick of 48 textbook-sized modules (that are easily separated). Tesla batteries would be smaller and lighter, but cost more, require liquid cooling, and might be in a more cumbersome form factor per module.

You'll also need space for the charger, BMS, controllers, contactors, cooling, maybe a DC-DC converter, fuses, relays...It's hard to slice a highway-capable conversion for much less than $10k in parts. You could do $5k by buying a leaf and using _everything_, but that everything includes the display, shifter, and possibly the brake/throttle pedal assembly...


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