How to MegaSquirt your Mazda FC RX-7

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Updated 7/21/2010 (added notes on relay board)

The mods for RX-7 are a bit more complicated than many, but we've been working with Ken Culver to put together a daughterboard that makes this installation a paint by numbers excercise. It does, however, have a bit more numbers than your typical MS installation. This uses the Zeal Engineering Daughterboard, which we've tested extensively on several RX-7 test mules. You can find more extensive notes on it at this link. Here, we'll restate the basics of assembling it, and go over how things specifically work on the RX-7.

Applications: This covers the FC (second generation) RX-7, as well as first generation RX-7s converted to FC ignitions, such as the one that appeared in the August 2009 issue of Grassroots Motorsports. We'll also touch briefly on the FD in this article, though current MegaSquirt firmware does not control the sequential twin turbo setup found on these cars.

Based on our testing, we recommend the MS2 V3.0 or V3.57, running MS2/Extra firmare 2.1.0 or higher, for best results on a rotary.

ECU Modifications

First, we'll cover how to build a Zeal Engineering Daughterboard. You'll need to assemble one of these whether you use a V3.0 or a V3.57 board.

Now, the instructions split off depending on what board you're using. The daughter board will go in the case lid.

If starting from a V3.57 board, we'll assume you are leaving the IAC jumpers in place and use the DB15 connector to add the extra inputs and outputs required to run an RX-7 ignition. These directions start with a standard, off the shelf V3.57 that has not been ordered with any options.

A note on temperature sensors

RX-7s have a somewhat unusual set of temperature sensors with a much higher resistance reading than most. One popular mod is to replace R7 (and R4, if using the stock IAT sensor) with 39K resistors. This gives somewhat more accuracy. Use the Tools -> Calibrate Thermistor Tables in the tuning software to adjust for the different range.

External Ignition Wiring

Here's how to wire the MS2 up to the stock ignition components. The NE signal corresponds to a crankshaft position sensor signal on a piston engine, while G is equivalent to a camshaft position sensor. These colors are taken at the eccentric shaft position sensor.

For those using a relay board (useful if the stock harness hasn't held up too well and you're rewiring the car from scratch, or if you've transplanted a FC ignition into a first generation, carbureted car), here's how this translates into a relay board setup. Note that V3.57 setups do not run very many of the ignition wires through the relay board; they mostly use the DB15 on the ECU instead.

FD coils are similar, but there's no trailing coil select output. Instead, you'll use the output connected to Trailing Coil IGT for the trailing front output, and Trailing Coil Select is replaced by the trailing rear output.

Settings and Configuration

MegaSquirt, for the most part, will treat a 13B engine as a four cylinder (since it has two spark events every rotation of the eccentric shaft) with a 2600 cc displacement (since it draws in 2600 cc of air every two rotations of the eccentric shaft). The most important difference is that the rotary engine has two spark plugs per rotor, and unlike a piston engine with dual plug heads, the plugs don't fire at the same time. MS2/Extra code allows you to tune the split angle between the spark plugs. Positive split means the leading plug fires before the trailing, while negative split means the trailing fires before the leading. Here are the key ignition settings to use:

• Spark mode: Toothed Wheel
• Trigger angle: 0
• Ignition input capture; Rising Edge
• Spark Output: Going Low (Normal)
• Number of coils: Wasted Spark
• Spark A Output Pin: D14
• Dwell type: Standard Dwell
• Maximum dwell duration: 3.0
• Trigger wheel arrangement: Dual wheel
• Trigger wheel teeth: 24
• Tooth #1 angle: 0
• Wheel speedL Cam wheel
• Second trigger active on: Rising Edge
• And every rotation of: Crank
• Enable rotary mode: Rotary on
• Output mode: FC mode
• RX8 mode: Disabled

A brief note about the different rotary modes. The FC ignition uses a single timing signal (IGT) for the trailing coil, and second signal, coil select, to identify which rotor to fire. The FD uses separate IGT signals for the front and rear rotors, while the RX8 mode is a coil on plug setup with four separate signals, one for each coil. Using RX8 mode also requires enabling FD mode. Here's how the spark outputs work for the three different modes.

Sometimes on a first generation RX-7, it's easier to control the tach from the ECU than the coils. Wire DB37 pin 3 (for a V3.0 board) or DB15 pin 5 (for a V3.57) to the wire going to the stock tachometer, and set the tach output to JS11. If using our wiring harness, you'll need to crimp a pin to a length of wire to use pin 3.

Idle Air Control

These engines use a pulse width modulated (PWM) idle air control motor. A V3.57 board can control this directly. With a V3.0 board, you can use the circuit on the daughter board for this.

Other Things the ECU Controls

The stock ECU controls a couple other devices in these cars besides the injectors, ignition, and IAC valve. Here's what to do about the ones you will need to control.

Fuel pump: This is a standard output on the Megasquirt. Wire the Megasquirt pin 37 to the fuel pump relay wire that went to the stock ECU.

Air conditioning: While the stock ECU controls the air conditioner, you don't need a computer to make this work. Wiring the A/C request wire running to the ECU to the A/C compressor relay should make this work, although we have not tested this one for ourselves.

Boost: You can use a boost control mod kit to control boost on the Turbo II motor.

Fuel pressure regulator: One of the relay control circuits on the daughter board can be used to activate the fuel pressure solenoid while cranking, which can help reduce vapor lock.

Automatic transmission: On S4 cars, the transmission controller does not talk to the ECU. On S5 models, the ECU communicates with the transmission controller, so if you have an S5 or later car with an automatic, you'll probably need to do a parallel installation. (See this link for more information on how to do this.)

Gauges: These are not controlled by the ECU, so if you're using the stock ignition you have nothing to worry about here. See notes in the ignition for using the MS to control the tach.

Oil metering pump: S5 RX-7s have a stepper motor driven pump to mix oil with the fuel. The MS firmware does not control this; we recommend removing this system and pre-mixing 2 stroke oil with the fuel in the gas tank. S4 engines used a mechanical system that does not connect to the ECU.

What to Order

If you're buying an assembled unit, it will need special mods to work with the ignition, as described above. We can install these for you for an extra charge; please contact us for details.

If you're building your unit from a kit, here is what we recommend.

MS230-K MegaSquirt II V3.0 Kit

ZealDB1-K Zeal Daughterboard Kit

JimStim-K JimStim kit

MSHarness12 12' wiring harness

TuneCable6 serial cable

USB-2920 if your computer doesn't have an adapter port

IATwPiggy Open element IAT sensor - for removing the stock air flow meter

We also recommend a wideband oxygen sensor system for tuning.

Other RX-7 MS articles throughout the Web

There are several other, even more detailed, articles on how to MegaSquirt the RX-7. One of the more in depth ones we've found is this one written by Aaron Cake: How to MegaSquirt your 2nd Gen RX-7 - highly recommended for further reading.

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