OSSEV Mazda MX-5 EV Conversion

This is an overview of our Mazda MX-5 / Eunos Roadster EV conversion. Our 'MX-5e' is our initial test platform to learn on and is also for testing components we plan to use in the OSSEV^®. It will not make financial sense to a lot of people but, we are not just investing in a car. The estimated £24,000 costs include a full restoration, an 'as new' rebuild, the EV conversion, a full respray of the car and to register it as an EV with the DVLA. We aim to have our MX-5e on road in the first half of 2024.

Unlike the sympathetic restoration that we did on Ethan's Yoko, we are planning to drag this 30+ year-old car into the future. It has to be a car that our whole family wants to drive and can enjoy, so it is not going to be an extreme sports car like Rob's Fisher Fury R1. It will be a proper driver's car though 😎

It will retain all the best bits of the Mazda MX-5 / Eunos Roadster and so it will be a lot of fun and provide great driver involvement. We are taking steps to improve it in this respect, including making it faster and lighter.
Our Eunos Roadster (MX-5 NA) originally had a 1.6 litre DOHC inline four-cylinder engine, producing 86kW (115bhp) at 6,500 rpm and 136Nm of torque at 5,500 rpm. As an EV it will have 136kW (182bhp) and 350Nm of torque from zero rpm. We also have an upgrade path to 215bhp/400Nm.
It will not have the safety systems and driver interventions that are a feature of modern EVs.

In March 2023 we purchased a 1992 Eunos Roadster V-Spec (Japanese import mk1 Mazda MX-5 NA) for £1425. It was an automatic with black paintwork and a tan interior. We aim to learn a lot whilst converting it to EV and are using it as a test platform for some of the components we are developing and using in our OSSEV^®. Some of the many changes are documented as separate projects.

The goal is that we end up with a better sports car than the one we started with. It will be lighter, faster, more involving to drive and also be a much nicer place to be. It will retain the simplicity and lack of intervention that a driver's car requires. There will be no anti-lock brakes, traction control, lane departure warnings or many of the safety systems that nag you in modern cars. It will have many useful features that you can use if you want to though.

Rob doesn't name his cars but, the rest of the family does. Over dinner, we agreed on a name which is a little bit more catchy than 'MX-5e'. Kaminari is (re)born. Or Kami for short. It is Japanese for lightning (or thunder). This kanji will feature on the completed car.

Our Plans

Removal

There are a lot of things that we have removed to reduce the weight, improve the centre of gravity and to maintain a good weight distribution. This includes, the air conditioning, the air intake system, the radiator, fans, header tank and cooling pipework, the luggage carrier, the engine and gearbox, the wiring loom and ECU, the exhaust system and heat shields, the fuel tank and fuel pipework, the soft top and frame, the power steering, etc.

We are maintaining a weights spreadsheet where we are weighing everything removed from the car and added to it. Some things are estimated for now.

Bodywork

The underside of the car has been stripped and all the old underseal removed. The brake lines, fuel lines, petrol tank, etc. were also removed. The car was then rust treated and painted with Buzzweld glass reinforced primer and then painted with several coats of Buzzweld WAR underseal.
The car will be fully resprayed 'Aurora Blue' as featured on the McLaren Artura.
A black front lip spoiler has been installed from BOFI Racing.
We briefly considered widening the wheel arches to give our MX-5e a meaner stance but, we don't want to fit wider wheels and tyres so there isn't much point.
We have bought new door mirrors, which will be painted to match the bodywork.
We have new trim pieces for the exterior of the car including the door/glass trims and the windscreen plinth.

The existing front air intake will be mostly filled by the front number plate. We don't need as much cooling as the MX-5 with an ICE. We have now purchased a stainless steel Zunsport grille in black and the towing eyes have been removed.

Brakes

All of the old brake pipes and fittings have been replaced using Kunifer (copper/nickel) 3/16" pipe.
The brake calipers have been refurbished as new by Bigg Red and painted gold at the same time.
New discs, pads and stainless-steel braided brake lines have been fitted.
A iBooster has fitted to provide electric brake pedal assistance. It's not 100% necessary on a car this light but, we want anybody to be able to drive it with confidence.
The existing MX-5 pedal box has been modified, strengthened and refitted, with the original brake pedal which has been refurbished.
The front/rear brake bias will be controlled by the existing brake bias valve.
The MX-5e retains the manual handbrake and new handbrake cables have been fitted.
There is no ABS.

Batteries

The high-voltage battery will be located low in the engine bay and possibly where the petrol tank currently sits.
The 400V battery pack will be around 30kWh, giving a range of around 100 to 120 miles. Probably closer to 80 miles with spirited driving 😊 Whilst this isn't much compared to the ICE equivalent, it is perfectly adequate for our needs.
The MX-5e will have configurable and personalised regenerative braking, set via the Driver Control Unit (DCU).

We are leaving the battery purchase as late as possible. This is because the technology is moving so quickly. We may end up with a much lighter car or with a much greater range by the time we need to install the batteries.

Charging

The current plan is for a type 2 charging socket hidden under the existing fuel filler cap. This enables charging at 6.6kW and this is perfectly good enough for our needs.
We are confident that we can fit a full CCS charging port to allowing rapid charging (120kW) but, this comes with significantly higher costs.

Electrics

The existing wiring loom has been completely removed and is being replaced with a bespoke loom that is much lighter.
A much smaller and lighter 12V battery sits under the bonnet as this is closer to where most of the 12V components are, resulting in a much lighter wiring loom.
Many electrical components will support CAN Bus.

Electronics

A CAN Bus will be fitted.
Wireless communications (4G SIM) will be fitted, to enable peer-to-peer control via the OSSEV^® app.
It will feature a Driver Control Unit (DCU), which will enable a personalised driving experience with driver profile and driving profile.

Entertainment

The entertainment system is covered as a separate project. We have installed a 10" touchscreen with Apple CarPlay and Android Auto support above the 'tombstone'.

HVAC

The chosen OSSEV^® solution for Heating, Ventilation and Air Conditioning (HVAC) will NOT be fitted to the MX-5e.
We are using a PTC heater and the existing heater matrix and fan controls. The heater matrix has been fully reconditioned.
The existing air conditioning system has been removed and a bypass pipe fitted.

Instrumentation & Controls

In both the MX-5e and OSSEV^®, we are retaining physical switches and buttons for all of the EV's main features. There are no touch screens used (entertainment system aside).

We are using the digital throttle pedal from an NC MX-5/Miata and an upgraded pedal box.
New instrument cluster with digital speedometer as will be used in the OSSEV^® and fitted within the existing binnacle.
We are developing an open-source, custom Driver Control Unit (DCU) to enable a fully personalised driving experience, with many advanced features and keyless security.
Many of the switches will be replaced or upgraded in terms of quality and feel.

Interior

The interior has been changed to reflect the planned interior of the OSSEV^®.
The tan dash and interior door panels have been changed to better quality materials, black or carbon-fibre effect in colour.
Many plastic elements will be hydro-dipped with a carbon fibre effect.
A wireless smartphone charger has been 3D printed and installed in the centre console.

Lighting

All of the lighting will be replaced with LED lighting and smoked lenses used (headlights aside).
We are developing our own bespoke LED side / indicator lights which will also work as daytime running lights.
The pop-up headlights will be retained for dipped beam and main beam functionality. We have added independent 'wink' capability to the pop-up headlights.
The rear light clusters will swapped for LED lighting units and this may mean remodelling the rear light cluster area.
We are upgrading the high-level brake light to LED.
We are developing our own bespoke fog light which will integrated into the bodywork.

Doors, Windows & Roof

We have a remote central locking module that locks/unlocks both doors.
The soft top and frame will be removed. We have a black hard-top that we can use if required.
A later project will be to develop a buttressed 'roll bar' with integrated rear screen and a two-piece removeable hard top that can be stowed inside the car. The plan is that this can be retro-fitted to any NA MX-5 or Eunos Roadster.

Seats

We have fitted a pair of second-hand MX-5 mk 2.5 black leather seats which are in excellent condition and have been cleaned and refurbished. These have heaters and will be included in our rewiring project.

Security

The existing after market immobiliser will be replaced by the OSSEV^® integrated security solution, the Intelligent Security & Personalisation (ISP) system.
We are also planning front and rear 'dashcam' cameras.
The MX-5e has remote central locking/unlocking.

Steering

We are using a black 350mm Momo steering wheel, with a black leather rim and a custom 3D printed horn button.
The existing power steering has been removed completely, to improve the steering feel and driver involvement.
There will be no electronic steering intervention on both the OSSEV^® and MX-5e, so no lane departure warnings, etc.

Suspension

We are fitting all new Energy Suspension bushes front and rear. These are not quite as compliant as the factory rubber bushes but, they are much easier to fit and will result in improved handling.

Transmission & Motor

After looking at the various motor options, our favourite for the OSSEV^® is still the Telsa SDU but, we were strongly advised by Fellten not to modify the rear sub-frame if we wanted to register our MX-5e as an EV. With this advice taken onboard, we decided to use the Nissan Leaf motor in the MX-5e and to use a reduction gearbox to drive the propshaft and an upgraded LSD differential.

Our MX-5e is using the EM57 motor and standard ZE1 inverter (3^rd generation Nissan Leaf), providing 110kW (149bhp) / 320Nm and spinning to 10,390rpm. Once we have it running we will look at fitting the Nissan Leaf Gen 2 'open inverter' controller to upgrade this to 136kW (182bhp) / 350 Nm. If we really feel the need for a bit more power, we can fit the Nissan Leaf e+ motor and inverter giving 160kW (215bhp) / 340Nm and spinning to 11,330rpm. This will increase the top speed from 109mph to 119mph and this is why we have fitted a 120mph speedometer.

We are using a fixed ratio reduction gearbox by Inductive Autoworks fixed to the output shaft of the Nissan Leaf motor, with a 1.8 : 1 reduction ratio. This is connected to the MX-5 LSD differential using a custom propshaft. The output flange matches the differential input flange with four M10 bolts on a 96mm PCD.

We don't need the MX-5e to go much faster than 100mph, so our differential ratio of about 3.636 : 1 is perfect and gives a top speed of 109mph (119mph with the Nissan Leaf e+ motor and inverter).

The 1.6 differential fitted to our car when we bought it used a 6" crown gear and this is not as strong as later versions, which used a 7" crown gear. This is much stronger and good for 400+bhp when properly upgraded. The LSD differential is documented in a lot more detail on our diff refurbishment & upgrade project page.

Wheels & Tyres

We have fitted these Rota Slipstream wheels in black. These are 15" × 7" with an ET40 offset. We wanted a little bit more width to help get the power down at the rear. Ideally, the front wheels would remain at 6" wide but, different sized wheels front and rear complicates things. These wheels weigh 5.9kg each, including the Tyre Pressure Monitoring System (TPMS).

We have fitted Yokohama Advan Neova AD08RS tyres in 205/50 R15 86V spec. Each tyre weighs 9.3kg. They provide excellent grip (in the dry) and very good feedback through the steering wheel. They are very progressive on the limit too.

Mazda MX-5 / Eunos Roadster EV Conversion

Kaminari