MegaSquirt Tech

How does a MegaSquirt compare to having my car ‘Chipped’?

It is possible to get many of the same performance benefits of the MegaSquirt by having your car chipped IF the tuner chipping your car has your car onsite and custom tunes the map for your specific configuration on a steady state/load bearing dyno.  If you are not the hands on type, or you don’t plan to EVER modify your car/engine again, then this is not a bad option.  If you do ever mod your car again it will likely need to be re-chipped, on a dyno by a qualified tuner, to get the maximum performance out of it.  If you simply send your ECU off to a chipping company who is supposed to load a map on your car, or buy a canned tune online to load on the chip yourself you’re likely not going to get the results you’re after.  In either case that tune/chip was NOT designed specifically for  your car on a dyno by a qualified tuner so you are simply not going to get the maximum performance possible out of the engine.  They simply can’t do it– they have to make a lot of assumptions with no scientific testing on YOUR car. 

However, if you really want to take control of things, if you do have plans to further modify your car and you don’t want to have to pay someone to rechip it every time you mod it, and if you are open to learning something in the process then by all means a standalone EMS is the way to go.  And there is no more powerful system per dollar spent than the MegaSquirt, in fact even with the affordable price out of the picture the MegaSquirt line of Engine Management Systems rival the featureset of many aftermarket ‘high dollar’ systems.  Particularly the MS3-Pro Stand Alone EMS.  We’re talking 100% of the functionality of many high-dollar systems, without the high-dollar price tag.

Don’t I need sequential injection to make big power? Does the MegaSquirt offer this?

Sequential injection attempts to only spray fuel while the intake valve is open for the cylinder that is about to fire, which in most cases is only possible at low engine speeds between idle and low speed cruising.  Benefits can be seen in emissions, drivability, and fuel economy at these low engine speeds.

Batch injection (which is what the MegaSquirt-I and and most MegaSquirt-II versions do) does not attempt to spray only when the valve is open.  It’s a fact that at higher engine speeds you won’t have enough time to spray all fuel while the valve is open, making this somewhat pointless, particularly for a performance oriented application.  At these higher engine speeds 99% of sequential systems (exceptions being race only systems with super massive injectors) cannot spray all of the fuel needed while the valve is open anyways, and sprays it both while the valve is opened and closed, very much like a batch system such as the MegaSquirt I and II.  We’ve dyno tested several cars with using batch fire and sequential injection on the same motor. Peak power was the same without using individual cylinder tuning. However, the sequential fire setup did run a bit more smoothly at idle and at light throttle angles.

The MegaSquirt III system with the MS3X Expansion board or MS3-Pro will do full 8 cylinder sequential fuel and ignition.  If you’re looking for the absolute best possible emissions and fuel economy, as well as the ability to fine tune each cylinder individually, then the MS3 system is the way to go.

There is generally no significant performance difference with a sequential system over a batch system unless you use individual cylinder tuning. There can be minor emissions and fuel economy benefits at very low speeds though. If your tuning budget allows for individual cylinder tuning, it may be possible to pick up 3-4% more power depending on how much cylinder to cylinder variation your engine has.

Is the MegaSquirt Engine Management System just for making more power? Or can I get better gas mileage out of it too? What about emissions?

MegaSquirt Engine Management Systems are most often installed in the pursuit of power gains, but often economy gains are just as possible.  In fact, due to the nature of programmable fuel injection usually you can achieve some of both: power when you are on the throttle and economy when you are lightly cruising around town.  It’s really all in the tune.  The factory will have tuned your gasoline engine to run around at 14.7:1 air/fuel ratio (AFR).  This is the ‘chemically correct’ or ‘stoicheometric’ air/fuel ratio, which basically means this is the air/fuel mixture at which the most complete burn of the fuel will occur.  It is also the proper mixture for the catalytic converter to best do it’s job.  In other words, your car comes tuned from the factory for reduced emissions.  If your goal is power, you’ll be tuning a somewhat ‘richer’ mixture than this; for a naturally aspirated car this is usually in the 12.6:1-13:1 range under throttle.  For economy you’ll tune somewhat leaner than this.  As lean as 16:1 possibly if your engine runs well there, or maybe even a small amount leaner when lightly cruising on the highway.  That can lead to quite an improvement in highway mileage over the factory 14.7:1 tune.  Not all engines like to cruise this lean so your ‘mileage’ may vary.  Our MR2 seems to love 16:1 on the highway– We were able to get 34.5 MPG on an 1100 mile trip at an average speed of 70 MPH- a decent increase over stock for sure!  And one of the great things about EFI is you don’t have to sacrifice WOT power to do this– the MR2 that’s getting better mileage than stock on the highway also makes more power than stock when you step on it– it’s an entirely separate part of the EFI map!

If tuning for emissions is your goal, it actually may be possible to do a better job than the factory did, though your task will be tougher as that’s at the top of the factory’s priority list.  Areas you may be able to improve:  Often factory cars idle richer than 14.7:1, and under power are again richer than 14.7:1, though they likely need to be at wide-open throttle.  With proper tuning, there is likely room for improvement in emissions and fuel economy.

Note– All parts are sold for OFF ROAD RACE-ONLY ground-vehicle use only. Aftermarket EFI/EMS systems are not for sale or use on pollution controlled vehicles. Alteration of emission related components constitutes tampering under the US EPA guidelines and can lead to substantial fines and penalties. Your state/district may also have specific rules restricting your tampering with your vehicle’s emissions system. In short, as stated before, our official policy has to be RACE or OFFROAD USE-ONLY in ground based vehicles ONLY.

The MegaSquirt 1 and 2 ECUs seem to have only four injector output pins (or terminals on the relay board) but they are labeled as pairs so this is even more confusing, do I have two or four injector outputs? And how does either two or four outputs allow me to run 6, 8, or 12 injectors?

With the MS1 and MS2 you have two ‘banks’ or groups of injectors.  You can use one or both of them. With 1-6 cylinders one is fine, though you can use both and generally would do so unless you were using the second bank for a different purpose, such as staged injection. With more than six you should share the load across both banks. 

Each bank has two terminals that you bring together to a nice fat (i.e. 14ga) wire that goes out to all of the injectors on that bank. For an 8cyl example you might choose to run two banks of 4 injectors each. You’ll run 12v to one side of all eight injectors that are hot in the ‘crank’ and ‘run’ positions. Similarly, you’ll run the 14ga wire out from one pair of terminals (one injector bank output) nearby the injectors where you’ll split that 14ga off into 4 smaller gauge (20ga likely) short wire runs to the individual injectors. Do the same thing for the other bank.

For sensor & wiring diagrams, please visit

Note that when running a MegaSquirt-III, you can still run batch injection if you choose, using the same wiring diagram as above.  However you can also purchase your MegaSquirt-III with the MS3X Expander Board, and then have up to 8 channels of sequential fuel and ignition outputs available.  (Not to mention all of the other features this enables, lots of I/O).

Check out the MS3X Wiring Diagram here.

Is there a central place where I can download maps that others have shared?

Maps can sometimes be found on the or forums that are similar enough to be used as a base map for your vehicle but it’s generally not encouraged to just grab someone’s map and run with it, as there are a few things to consider first. You want to ensure their setup is as similar to yours as possible, including the engine, ignition system, MegaSquirt version and firmware version they’re running.  If they were running a different MS version, or firmware version than you then you can still copy the information from their map manually, but don’t import it into your ECU as it’s very possible some of the data won’t import properly into a different firmware version.  This could help you get started in the right direction towards building your map.

Another important question to consider, especially if the ECU setup you’re using required mods to control your vehicle’s ignition system, is are the mods you’re using identical to the mods used by the person that created the map?  Are they using the same ignition components (ignitors, coils, etc)?  If anything is different here their ignition settings could be different from what you need, and could damage your coil or ignitor.  Things to watch out for are the SPARK OUTPUT setting (inverted or non) and the DWELL settings. 

The most important thing to remember is that no matter how good so-and-so told you their map would work on your car, you are still ultimately responsible for making sure your ECU is properly setup for your car, and that your car is properly tuned.  That means looking over the map’s settings BEFORE you start the car.  Do the ignition settings look right and does the ignition table look logical?  Is there 60 degrees of timing at WOT?  If so, that’s probably not right….  When you start the car you should still treat it like you need to tune it yourself from scratch, however you hopefully have a good head start.  Tune the idle first, then slow speed VE and ignition, cruise VE and ignition, and then work on up to higher loads and finally WOT VE and ignition.  Spend some significant time on   Another great resource is Matt Cramer & Jerry Hoffmann’s Performance Fuel Injection Systems.

Idle Air Control valves versus Fast Idle Valves— What does my car have and how can I tell?

Here’s a way to figure it out–

On a cold morning, when you start your car up on it’s stock ECU, it idles higher than normal right?  That’s because this valve is partially open letting more air in to idle the car up so that it warms up faster.  Now… if you sit there until it warms up, does the idle drop back down to normal all of a sudden?  If so it’s a FIDLE valve (generally two wires, ground and 12v+).

Alternately, does it slowly idle back down to normal a little at a time, gradually reducing the idle speed as the engine warms up?  If so it’s either a PWM or stepper motor valve.

To determine if it’s a stepper or PWM valve, find the valve on your engine, it will be attached to the intake manifold, either directly or via a hose.  How many wires does it have?  Generally a PWM valve will have 2 wires, sometimes 3, and a stepper motor IAC will have 4 wires, sometimes 6.  The MS2/MS3 stepper circuit was designed with GM/Jeep valves in mind, however it works great on many different stepper IAC systems, including most all USDM, Euro, and Japanese vehicles that use steppers, among others.

I’m considering an EFI conversion – converting my car from carbs to EFI – what do I need to consider?

Your fuel system will need to be converted to a high pressure system suitable for fuel injection (around 43.5 psi for MPI, less for TBI if you go that route).  If you can get a TBI unit (with TPS) that bolts to your manifold, that may be the simplest route (though their likely will be more power and better economy/emissions in a good MPI setup), otherwise you’ll need to buy or fab up an intake manifold and throttle body with TPS, fuel rails, etc.    We don’t carry intake manifolds at this time, though we do carry many of the fuel system components you will need, and of course the engine management systems and accessories.

FOR FUEL AND IGNITION CONTROL– We’d suggest running a MegaSquirt-II, or MegaSquirt-III, using the PCB v3 or v3.57.  You’ll need a compatible Crank Angle Sensor to determine engine position and RPM from as well as some way to fire the ignition — with the MS2 this is generally a distributor based solution or the Ford EDIS or GM DIS distributorless solutions which could be adapted to your engine (most adaptations use EDIS which also uses it’s own 36-1 Crank Angle wheel which can be mounted on the crank pulley).  You can read more about ignition options in the MS/Extra documentation at

FOR FUEL ONLY– You could run any of the MegaSquirt ECUs, the MS1, MS2, or the MS3.  We’d recommend PCB v3.0 or PCB v3.57.  Pickup up your ignition signal from the negative terminal of the coil, run your IAT, CLT, TPS, and O2 sensor inputs to the MegaSquirt, calibrate the MS to the sensors if they are not GM sensors and start tuning.

If you haven’t yet we’d again suggest taking a look through the MS/Extra version of the manuals at as it will go into detail about most aspects of the conversion/installation/configuration/tuning and help to make things clearer.

Here’s a list of what most people would commonly pick up:  We’ll list what we see as ‘required’ and then some ‘recommmended’ and ‘optional’ components.  Then we’ll provide a link that goes into some detail on why we made these recommendations and that will also help you to decide on the optional components.



(Note you can substitute DIY kits for the below to save some cash and learn more about your EFI system, and electronics in general, in the process.)


along with:


‘Highly Recommended’

Innovate LC-2 Digital Wideband Controller with Sensor

12′ MegaSquirt Wiring Harness (MS1 / MS2 Ready)  and possibly the MS3X Harness as well if you’re running the MS3X board.


MegaSquirt Relay Board – Assembled Unit

MegaSquirt-I Relay Cable (if you’re using the relay board and cable, you don’t need the standard MSHarness12, see note in product description)

GM Open Element IAT Sensor with Pigtail

GM Closed Element CLT / IAT Sensor with Pigtail

USB to Serial Adapter


Here’s the link mentioned above with more info on each —

What do I need?

Using the Relay Board with the PCBv3, what do terminals S1-S5 do?

You can use the schematics for the PCBv3 ECU and/or PCBv3.57 and the Relay board to trace it back. Here are the Relay Board schematics and here are the PCBv3 schematics and PCB v3.57 schematics.

Here’s the quick answer though mapping these terminals back to the corresponding pin on the PCBv3 ECU’s DB37:

S1 = 25
S2 = 27
S3 = 29
S4 = 31
S5 = 36

With the MS-I these were unused by default, available as spare inputs/outputs. With the MS-II and MS-III (legacy installation) S1-S4 are used for the stepper motor IAC, and S5 is the ignition output.

For more information on the Relay Board check out this page:

Note you can do the same cross reference for the PCB2.2 ECU and the Relay Board, just use the PCB2.2 Schematics.  Though the pinout is of course the same…

Can I use the stock IAT and CLT sensors with the MegaSquirt?

Yes you can in almost every case– the exception is that you sometimes remove your stock IAT sensor when you remove your MAF/AFM as it’s often built into stock MAF or AFM on many cars.  If this is the case, you’ll need to add another IAT sensor to the intake tube, usually just before the throttle body and after the intercooler, if you have one.  The idea is to measure the air temp as it enters the engine.  After anything heats it up (like a turbo/supercharger) and AFTER anything cools it down (intercooler, meth injection, etc).  You want the actual air temp of the air as it enters the engine.

Here’s an article that shows how to measure the resistance curve of your stock (or any really) sensors so that you can input these values into your MegaSquirt EMS via the tuning software for MS2/MS3 ECUs, or via EasyTherm for the MS-I, to calibrate your ECU for your sensors.

Tech Article:  Measuring and Calibrating your Coolant (CLT) and Intake Air Temp (IAT) sensors

Help! I broke the pins on my MS-II Daughterboard!

The pins are Digikey Part# ED7764-ND.

How do I power cycle or reboot a MegaSquirt?

Power cycling and rebooting simply mean turning the MegaSquirt off and back on again. Some settings only take effect when the MegaSquirt powers up. The latest versions of MS2/Extra and MS3 firmware will display a message in TunerStudio when you change a setting that requires power cycling to take effect.