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How to MegaSquirt your Mazda FC RX-7

If you haven't already, please read the 'disclaimer' at the top of the parent page here.

Updated 10/7/2010 (Added MS3X)

The mods for using MS2 on an 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 exercise. 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.

The other option is to use an MS3 with MS3X. Rotary support has been a part of the MS3 for a long time; the first car to run production MS3 hardware was an RX-7. The MS3X already has the second VR conditioner and spark outputs already there.

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 either the MS2 V3.0 or V3.57, running MS2/Extra firmware 2.1.0 or higher, or an MS3 with MS3X.

ECU Modifications - MS2

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.

Daughterboard Assembly

  1. Install all resistors in the indicated locations and solder into place.
  2. We have found that the second signal conditioner works better with the ground jumper installed in the two holes next to U2. Install a length of snipped resistor lead to bridge these holes, and solder into place.
  3. Install diodes D2 through D5, with the banded end towards the lower end of the card.
  4. Install C2, C3, C4, and C6.
  5. Install U1 and U2.
  6. Install C7.
  7. Install the Q1 through Q4 transistors, with the flat side on the transistor pointing to the flat side of the silk screen.
  8. Install Q5, again taking care to match its outline to the silk screen.
  9. Install C1 and C5. These capacitors are polarized; the long lead goes in the + hole and the white striped side goes in the other hole.
  10. Note that the VR connections (which you will hook up in the next steps) have the positive terminal next to the VR1 or VR2 marking, and the negative terminal furthest away from that marking.

At this point, you'll have a complete daughter board, and we'll assume you are starting from an unmodified MegaSquirt. Here's how to set up the MegaSquirt for this.

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

Using the MS-II PV3 ECU

with MS2/Extra 2.0.1 or later code

  • Omit the stepper IAC jumpers in step 22 - you'll need the IAC terminals for your ignition output. If you're going with an assembled unit, remove these jumpers
  • Build the ECU up to trigger from the VR Input. Jumper TachSelect to VRIN and TSEL to VROUT.
  • We will need three ignition outputs, each with a 5 volt pull up.
    • Run a 1K resistor from the 5 volt source in the proto area or the right side of R24 to the top (negative) lead of D14, and run a jumper wire from the negative lead of D14 to IGN. This is spark output A.
    • Run a 1K resistor from the 5 volt source in the proto area or the right side of R28 to the top (negative) lead of D16, and run a jumper wire from the negative lead of D16 to IAC2B. This is spark output B.
    • Run a 1K resistor from the 5 volt source in the proto area or the right side of R25 to the top (negative) lead of D15, and run a jumper wire from the negative lead of D15 to IAC2A. This is spark output C.
  • When building the IAC circuit, omit R19, Q4, Q20, R39, and D8. You'll use the driver on the daughter card instead.
  • Now it's time to connect the daughterboard to the main board. Use these connections:
    Daughterboard connection Main board connection
    12V S12
    5V S5
    GND Proto GND
    GND SG
    VR2 + IAC1A
    VR2 - IAC1B
    VR2 Out JS10
    IDLIN Bottom end of R19
    IDLOUT Banded end of D8
    IN1* JS11
    OUT1* SPR1

    * The tach signal connections at IN1 and OUT1 are optional - they make wiring things up easier on a first generation RX-7 running a FC ignition, but the FC does not need an ECU tach output, so you can ignore this step on an FC.

That's it!

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.

Using the MS-II V3.57 ECU

with MS2/Extra 2.0.1 or later code

  • Set up the ECU to trigger from the VR Input. Place JP1 in the 1-2 position and J1 in the 3-4 position.
  • Remove the jumper going from JS10 to the center hole of Q16, if fitted.
  • We will need three ignition outputs, each with a 5 volt pull up.  These directions use the built in pull ups on the LED circuits.
    • Run a jumper from PAD1 to PAD16.
    • Run a jumper from PAD3 to PAD14.
    • Run a jumper from PAD2 to PAD12.
  • Now it's time to connect the daughterboard to the main board. Use these connections:
    Daughterboard connection Main board connection
    12V S12
    5V S5
    GND Proto GND
    GND SG
    VR2 + PAD15
    VR2 - PAD13
    VR2 Out JS10
    IN1* JS11
    OUT1* PAD11

    * The tach signal connections at IN1 and OUT1 are optional - they make wiring things up easier on a first generation RX-7 running a FC ignition, but the FC does not need an ECU tach output, so you can ignore this step on an FC.

That's it!

JimStim testing on an MS2

Testing the Zeal inputs on a JimStim can be tricky (and they won't respond to a regular Stim at all). The LM1815 will not work with a square wave. You'll want to use a 12 volt pull up on both the primary and secondary tach signal. For the primary tach signal, you can set the jumper for VR emulation. There is no correspoinding jumper for the secondary tach signal. Instead, put a 0.1 uF capacitor inline between the "2nd Trigger" connection on the JimStim and the VR input on the MegaSquirt. This will give a signal that can trigger the LM1815.

ECU Modifications - MS3

This requires considerably fewer changes:

Using the MS-III PCB V3.0 or V3.57 ECU

And the MS3X expansion board

  • Set up the ECU up to trigger from the VR Input. Jumper TachSelect to VRIN and TSEL to VROUT for a V3.0. For a V3.57, set the JP1 jumper on the main board to the 1-2 position, and set J1 to the 3-4 position.
  • Make sure the JP7 jumper on the MS3X is off, as you don't need the pullup on the second trigger.
  • On a V3.0, the easiest way to get idle control is to use the PWM IAC driver on the MS3X board. If you wish to use the main board IAC driver, you'll need to install a TIP120 mod kit - see the MK-PWMIAC catalog page for details.

That's it!

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 MegaSquirt 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.

Function Stock wire color MS2 V3.0 connection MS2 V3.57 connection MS3 / MS3X connection
NE + Red Pin 24 DB37 Pin 24 Main board pin 24
NE - White GND (pin 2 on our harness) GND (pin 2 on our harness) GND (Main board pin 2 on our harness)
G + Green Pin 25 DB15 pin 3 MS3X pin 32 (Cam input)
G - White w/ black stripe Pin 27 DB15 pin 4 Main board GND
Leading coil 12V Tan 12 volt switched power 12 volt switched power 12 volt switched power
Leading coil IGT Red Pin 36 DB15 pin 10 MS3X pin 14 (Spark A)
Leading coil ground Coil body Engine GND Engine GND Engine GND
Trailing coil 12V Tan (2 wires) 12 volt switched power 12 volt switched power 12 volt switched power
Trailing coil IGT Red Pin 31 DB15 pin 11 MS3X pin 33 (Spark B)
Trailing coil select White Pin 29 DB15 pin 12 MS3X pin 15 (Spark C)
IGF Yellow Not used by MS Not used by MS Not used by MS
Trailing coil tach signal Black To tach To tach To tach

The black wire on the leading coil is not connected.

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. Same for MS3s - they run many of the signals through the upper DB37.

Function Stock wire color MS2 V3.0 connection MS2 V3.57 connection MS3 / MS3X connection
NE + Red Relay board tach terminal Relay board tach terminal Relay board tach terminal
NE - White sensor return terminal sensor return terminal sensor return terminal
G + Green S1 DB15 pin 3 MS3X pin 32 (Cam input)
G - White w/ black stripe S2 DB15 pin 4 sensor return terminal
Leading coil 12V Tan 12 volt switched power 12 volt switched power 12 volt switched power
Leading coil IGT Red S5 DB15 pin 10 MS3X pin 14 (Spark A)
Leading coil ground Black Engine GND Engine GND Engine GND
Trailing coil 12V Tan (2 wires) 12 volt switched power 12 volt switched power 12 volt switched power
Trailing coil IGT Red S4 DB15 pin 11 MS3X pin 33 (Spark B)
Trailing coil select White S3 DB15 pin 12 MS3X pin 15 (Spark C)
IGF Yellow Not used by MS Not used by MS Not used by MS
Trailing coil tach signal Black To tach To tach To tach

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) for MS2, Going High / Inverted with MS3X
  • Number of coils: Wasted Spark
  • Spark A Output Pin: D14 (MS2) or MS3X (MS3)
  • 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.

Spark output FC mode FD mode RX8 mode
A Leading Leading Front leading
B Trailing IGT Front trailing Front trailing
C Trailing coil select Rear trailing Rear trailing
D - - Rear leading

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. Note that the MS3X has a built in A/C control option.

Boost: You can use a boost control mod kit (or the MS3X) 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 an MS2 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.

Click Here if you have questions or would like to contact us!
 

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