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How to MegaSquirt your Chevy LT1, LT4 or L99 with Optispark IgnitionGetting MegaSquirt-II to work with the OptisparkUpdated 2/6/2009Thanks to "Mobythevan" for his notes on the settings! Application(s): All Chevrolet LT1, LT4, and L99 engines with the Optispark distributor. This includes Camaro, Trans Am, Corvette, and B-body (Caprice, Roadmaster, Fleetwood) applications. Level of control: Full Standalone EMS-- Fuel and Ignition Control
When GM went looking for a new ignition design for the LT1, they went with a somewhat unlikely source - Mitsubishi. They bought an optical distributor system that was already in use in many Mitsubishi, Chrysler, Subaru, and Nissan applications. The stock ECU uses the 360 slot ring to determine crank angle and the inner ring of eight slots to identify the cylinders. MegaSquirt does not need or use the information provided by the second ring of 360 slits or the length of the inner slits, so you just need to obtain a signal from the trailing edge of the ring of 8 slits. This writeup covers using a BIP373 Ignition Driver for controlling the coil. If you would prefer not to use the Optispark, check out our page on crank triggered ignitions. This can be used for either a distributorless ignition or using the Opti strictly as a distributor. ECU ModificationsWith this engine, we recommend running the MegaSquirt-II. It's stepper IAC motor control can drive the stock idle air control valve, which is one feature MegaSquirt-I does not have. For those looking to build this on a budget and who want to figure out another route to control your idle speed, we also have a write-up covering MegaSquirt-I.
WiringThe output is quite straightforward - you will just need to connect the negative terminal of the coil to pin 36. The positive terminal of the coil you can just leave wired to it's stock location. For the ignition input side (tach signal into the ECU), wire the low resolution input signal from the Optispark pin A (this is usually either black with a red stripe, or red with a black stripe) to the MegaSquirt pin 24. Optispark pin out
Idle Air ControlLT1s used a stepper idle air control motor. If you have a MegaSquirt-II, you can drive it from the MegaSquirt pins 25, 27, 29, and 31. GM used several different idle air control valves on the LT1 series; they don't all have the same connector design. This picture from the MegaManual shows which wire goes where on GM idle air control motors.
Other Things the ECU ControlsThe 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. Cooling fans: These cars generally had two cooling fan control outputs. Use one MK-RelayCtrl mod kit for each of them if you want to control the fans. If you have an MS2, you can use the on/off IAC output on pin 30 for one of the fan controls, and use just one relay control mod kit for the other fan output. We recommend using PA0 - Knock Enable for the output, wiring the input of the transistor to pin JS11, and using SPR3 to bring the output out to pin 3 of the DB37 cable. See here for a picture of an installed relay control mod kit. Vehicle speed output: Some Corvettes use this for features like active suspension control, and it is linked to the cruise control. Right now the easiest way to keep this active, if you want to use these features, would be to leave the stock ECU in place, just disconnected from what you have the Megasquirt controlling. 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. Automatic transmission: The Megasquirt itself does not control the 4L60E, but the GPIO board (currently in beta testing) will have this capability. In the meantime, if you have an automatic you will need to either control it with the stock GM ECU or a stand alone transmission controller. If you're doing a parallel installation to keep the stock ECU in control of the transmission, see this article for tips on parallel installs. MegaTune Configuration for MS-IISet ignition capture to "Falling Edge," and coil charging to Standard Coil Charging. Dwell will depend on exactly which coil you are using. Cranking trigger time must be set to Calculated. If using MS2/Extra, set the ignition type to Basic Trigger. Trigger angle is in the 90 degree range; this may vary depending on production tolerances, so confirm this with a timing light. Important when using the BIP373: On the 'Ignition Settings' menu make sure to set 'Spark Output' to 'Going High (Inverted)' and set 'Coil Charging Scheme' to 'Standard Coil Charge'. If you use someone else's .msq file we do recommend you check the firmware version the .msq file was built on as you cannot always use a file built on one firmware version with an ECU running a different version-- it's best to use the .msq files with the same version of the firmware that the file was built on. Alternately you can manually copy the settings over from the borrowed file.
MegaTune Configuration for MSnS-EOn the 'Spark' menu choose 'Spark Settings' and set 'Spark Output Inverted' to YES. It is critical to get the Spark Output Inverted setting correct, as setting this wrong can result in damage to the BIP373 or coil. For Trigger Angle, start with 68 degrees with a Trigger Angle Addition setting of 22.5 degrees. This may vary a little depending on production tolerances, so check this with a timing light. Dwell settings will depend on the coil used. A typical rule of thumb is to back off the dwell until you can just barely detect a misfire under lean cruising conditions, then add 0.2 ms of dwell time. If you use someone else's .msq file we do recommend you check the firmware version the .msq file was built on as you cannot always use a file built on one firmware version with an ECU running a different version-- it's best to use the .msq files with the same version of the firmware that the file was built on. Alternately you can manually copy the settings over from the borrowed file.
Suggested partsAs stated above, we recommend a MegaSquirt-II V3.0 or V3.57 to have full control over the engine. You will be able to use your existing sensors except for the MAP sensor which is built into the MegaSquirt. ECU options:
Mod kits discussed in this article:
Other recommended parts
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