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.
