# Planning Mazda MX5 mk2 conversion



## Rory166 (Jul 6, 2014)

Yesterday I collected a year 2000 1.8 MX5 UK spec. Now I wonder if I have done the right thing. It seems to weigh 1250 Kg as opposed to 1000 for the Mk1. Where has all the extra weight come from? The plans that I ordered turn out to be for the Mk1 only.

I have an AC-35 motor with matching Curtiss controller with display. I would ideally like to dispense with the gearbox entirely. I will need to check if the Motor can do the RPM. I am not sure if there is room to connect direct to the rear axle.

Rory


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## EVisa (Jun 30, 2014)

Hi Rory,

I'm new to this forum, so probably can't advise you much. I do assume that plenty of info about the mazda mx-5/miata is available on the internet, about added features/weight. Also check out the garage of this forum, plenty of mx-5's/miata's there. 
Personally, I'm not a great fan of direct drive, but as a newbie, others might correct. Do you have any pictures, expectations about top speed/ range, have a battery pack in mind?


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## skooler (Mar 26, 2011)

Ac35 might be slightly underpowered.... you'll definately need gearing. What are you range and performance.goals?


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## Rory166 (Jul 6, 2014)

I was hoping to keep everything as light as possible and using components that I have available. The hpevs AC-35 motor Curtiss 1238/7501 and 25 Cells Lifepo4 at 80 Ah. The cost so far is approaching 2,500 pounds. I realise the range will not be huge. I will need a contactor and all the mechanical adaption items yet.

The weight of the vehicle is still a mystery Wikepedia says 1065Kg but the plate in the engine compartment say 1250Kg and the registration document 1315 Kg. Wikepedia says the Mk1 is 940Kg. so the difference is 125Kg which would allow a lot of battery. It would be nice to have scales available. If only Elises were not so expensive.

Rory


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## skooler (Mar 26, 2011)

Ok makes sense. I can provide an ev200 contactor if that helps. Send me a pm and we'll discuss details.

I think an ac35 at ~84v is going to feel sluggish.

There are other small, lightweight options. I think the MG F / TF is quite light?


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## dougingraham (Jul 26, 2011)

Rory166 said:


> I have an AC-35 motor with matching Curtiss controller with display. I would ideally like to dispense with the gearbox entirely. I will need to check if the Motor can do the RPM. I am not sure if there is room to connect direct to the rear axle.


You are going to need the transmission. There simply isn't enough torque to do without it. You will also want to run the highest voltage the controller will allow. If the 1238 controller you will want to run 34 or even 36 cells. This will widen the torque band so you get maximum use out of your transmission. The only exception would be if you decided to make 45 mph your top speed and changed out the final drive ratio to allow for this. The motor can do up to 8000 rpm but at the low voltage you mention in your other posting there wouldn't be much power left when it gets there.

I think an AC-35 will work ok in this car but you will want to keep the transmission and clutch. If you don't it will look like a Miata but it won't act like one.


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## Rory166 (Jul 6, 2014)

Hi Doug

Seems like I need a different controller to get more power from the ac-35. The one I have 1238-7501 is specified for 82-96 volts.

My Leaf works well with no gearbox but that has 109 bhp.

Today I wasted some time trying to get the ICE to run. some blown fuses on the floor and more in the fusebox. I replaced these and the ignition light came on, it turns over but does not start. I suppose it could be immobilised and that is a whole nightmare. Pity as the engine would be more saleable as a runner.

Rory


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## dougingraham (Jul 26, 2011)

Rory166 said:


> Seems like I need a different controller to get more power from the ac-35. The one I have 1238-7501 is specified for 82-96 volts.


I think the 1238 can operate on up to 36 LiFe cells. That would be a nominal voltage of 115 volts and a peak voltage of 122 volts. 96 volts is the Lead Acid nominal voltage but hot off the charger it would have as high as 115.2 volts. I think the 7501 is the 550 amp version?

I believe you would see something like 90ft-lbs of torque up to a little over 3000 rpm with that controller. Peak HP would be at a little over 3k rpm and would be in the low 80 HP range. 



Rory166 said:


> My Leaf works well with no gearbox but that has 109 bhp.


The difference is the Leaf motor has a final drive ratio of 7.9377 while the MX-5 you have probably has a 3.909 (or 4.1) final drive ratio. The Leaf motor has 210 ft-lb of torque. The torque amplification of the AC-35 at 90 ft-lb and 3.909 final drive would give 352 ft-lb at the tires. The leaf in contrast would see 1667 ft-lb at the tires. In the Leaf this would be around 1/2 G acceleration up to the point where the torque starts to roll off. I was looking for a dyno plot to see at what speed this happens but didn't see one. I am guessing it is above 60mph. While the HPEVS motor can do 8k rpm the Curtis controller is after all a forklift controller. It is a testament to overengineering that it works as well in a car as it does.


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## Rory166 (Jul 6, 2014)

Doug

Thank you indeed for your expert advice. I had not considered that the controller input voltage rang of 72 to 96 volts could be nominal and not peak. If I can use more cells this will indeed help the performance, I must try to research the curtiss controller more for peak voltage rating. There are motor curves published for up to 144 volts but I assumed that was a different controller model than my 1238-7501. The number following the hyphen indicates a voltage rating 7 means 72-96 volts. As you say the rating is indeed 550 Amp which is the second digit after the hyphen.

The 96 volts is a system voltage ie 8 12v lead acid batteries in series. As you say the peak voltage when charging SLA batteries would be 120V. To me this represents 30 cells of 4v Max or 33 cells at 3.65v max. I suppose 36 cells could be possible using a contactor to keep the Pack disconnected from the controller during charging.

Thinking through what I have available to use I may start with 28 cells and see how it goes.

Rory


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## dougingraham (Jul 26, 2011)

Rory166 said:


> Thinking through what I have available to use I may start with 28 cells and see how it goes.


I remember reading that with 36 cells fresh off a charge sometimes the controller would signal an overvolts error. You could clear this just by turning on the headlights and draining off the surface charge with the DC-DC converter load. I think 34 cells pretty much always works.

There is a different model of 1238 that has higher current FETs and can do 650 amps. The other controller to look into is the 1239 which can do 144 volts but only at 500 amps. This turns out to be about the same power as the 650 amp 1238. But to get there you need to run more cells which is a good thing. I believe people use 46 cells on that one.

Best Wishes!


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