New Modular ECU for MX5 Miata NB8B and NB8C

Hi everyone, this video describes the new Modular ECU for the MX5 Miata NB8B and NB8C, which includes the naturally aspirated, variable valve timing engine and the factory turbo, or SE – or in the US it was called the MSM for Mazdaspeed Miata.

MX5 NB8C-SE

Some people may ask why we did a Modular ECU for these cars when we already have the Select ECU. Firstly, as you can imagine we’re phasing out the Select ECUs, as the Modular ECU is far superior in terms of processing power, the fuel model which leads to how quick is it to tune and how nice the car is to drive. You’ve probably seen the modelled injector section in the software by now and that makes a big difference to how nicely they tune up and the amount of work involved.

Secondly, the Select ECU was a little bit short on outputs to control the NB8B really nicely. For example both thermofans were driven at the same time so this presented a big electrical load to the engine when they were first turned on. We got around that by pulsing the fans gently but it still wasn’t ideal. It seems also that many people want to convert to direct fire; which probably isn’t necessary unless you’re making very high power; MX5 Mania run factory coils in their turbocharged endurance race cars at more than 200 kW at the wheels, but some people want to do it anyway so it would be nice to offer that ability.

Lastly, we wanted to give people the option for more inputs; all the things that we recommend people put on cars eg fuel pressure monitoring and so on.

So, here it is!

The ECU takes the form of the other plug and play Modular ECUs, and has all the inputs and outputs of wire-in ECUs like the M2000, for example 8 injector and 8 ignition outputs. There are 2 internal locations for additional small modules if you want to add individual wheel speed sensing or drive by wire. There are also some things peculiar to this vehicle though so I’ll go through those now.

Firstly, there are two different versions; there’s the factory turbo version, ie the SE or MSM – from 2003 to 2005, and there’s the naturally aspirated version with VVT from 2001 – 2005. There is of course the SP which was an Australian only one which was VVT turbo and for the sake of this discussion you can consider it a VVT version with a turbocharger slapped on.

The turbo variant varies from the VVT variant in that there’s no variable valve timing. Instead, the positive wire for the VVT oil control valve becomes the positive of the boost control valve, and the negative of the boost control valve goes to ground.

This is in contrast to the standard way of wiring solenoid valves with the positive side connected to a 12V supply and the negative switched to ground via the ECU. Separately from this, the wire which is the negative of the oil control valve on the VVT engine becomes an auxiliary air temp sensor on the SE; it has an air temperature sensor just before the throttle body as well as at the air intake.

In the Select ECUs, we had an internal jumper in the ECU to change the pin assignment of the negative VVT solenoid or air temp sensor pin, but on the Modular ECU it’s done internally using a relay, driven by ignition output 8. So this means that all you need to do is to load in the base map for whichever vehicle you have, and the ECU changes the pinout automatically. The software also automatically changes the pinout shown in the pinout page depending on the map loaded.

The two base maps also have the air temperature inputs swapped around, so that on the turbo map, the MAT signal comes from the air temp sensor at the throttle body, rather than at the air intake. Note also that these two air temp sensors have different calibrations, just to keep lifeĀ  interesting.

If you wanted to do boost control on an SP, or you’ve turbocharged the VVT engine and you want to use an output to control the boost, then the best way to do that is as follows:

1) Disconnect the positive feed from the oil control valve. This is a yellow wire.

2) Connect the positive for the oil control valve to an ignition switched 12V source, for example from the idle control valve orange wire, or the injectors.

3) Use this yellow wire coming from the ECU as the positive to your boost control valve

4) Connect the negative of the boost control valve to ground – essentially mimicing the connection on the factory turbo car

5) Change the output function of injector output 8 from being “none, inverted” to be wastegate control, PWM, 33 Hz as usual.

Note that although the output to the boost valve is driven high, this is done by a circuit within the ECU, so there is no need to invert or select the “drive high”. When the output is “on” as far as the ECU is concerned, it will be driven high instead of driven low.

The other function on these cars which was a bit unusual at the time was that the alternator has no regulator internally; the regulation is done by the ECU. So this means firstly the ECU has to be working correctly to make the alternator regulate correctly, but also it has to receive correct power to make it regulate to the correct voltage.

To avoid voltage drops on the ignition supply, the ECU uses the constant power input voltage pin as the reference for the voltage regulator. This is regulated to 14.1V by controlling the field current on the field current pin. Furthermore the field current is fed into an external 0-5V input on the ECU, so that the ECU can have a measure of how much current the alternator is producing, which in turns gives a representation of the amount of mechanical load the alternator is placing on the engine. This is fed into the idle control algorithm via the alternator idle-up function and it allows excellent idle control in the face of large electrical loads like headlights, blowers and thermofans. There’s a slight dip when the thermofan turns on because of the very high startup current and the delay of the idle speed control but overall it works very well and is a lot easier than setting up additional idle control percentages for every separate electrical load.

Separate idle-up efforts are required for the air conditioner and the power steering though, because those also present a mechanical load to the engine.

A couple of points to be careful of; firstly, for some reason the factory turbo injectors are the same as the naturally aspirated injectors. Normally when we make a base map, we do so not with a completely stock car, but with a car with light modifications that’s representative of the point where people start to put on an ECU. Our test car for the SE had the factory turbo, but an upgraded intercooler, exhaust and air filter, and with those changes, it was out of injector on actuator boost level. I know it’s very common to install Mazda RX8 series 1 secondary injectors (390cc/min), but these are only marginally bigger than the ones which are being replaced. On the test car we put on ID1000s.

The second thing I’d say to be careful of is that I’m not an engine builder, but I’m told that the factory rods are not very strong. It’s possible to make decent power with these engines but they start to fall apart at fairly low power levels; about 160 kW at the wheels according to MX5 Mania. I know others say that they can tune them 200 kW and they go strong, there’s no problem but these people (at least the ones I’ve heard) do not have the years of endurance racing experience that MX5 Mania has, and at least some of their customers end up at MX5 Mania for engine rebuild jobs. I’m sure everyone has their anecdotes; my last engine I tuned up to 2.5 times the factory power output and ran it for a long time with no trouble but that doesn’t mean that it’s safe to do. Maybe it would have failed the following day, or maybe someone else that did the same thing would have failed after a couple of years. Maybe it would have failed if I’d circuit raced it, rather than just some drag racing which is over pretty quickly. I don’t know, but what I will say is that I’m happy to defer to people that have a more statistically valid sample size than individual anecdotes.

And just on a personal note, when I was street testing the SE for drivability (it felt nice on the dyno but it’s always good to check things like transient throttle after gear changes, low RPM jerkiness etc on the street), I felt a bit sad to have handed in my scissors earlier this year.

Thank you.