MegaSquirt is an aftermarket electronic fuel injection (EFI) controller designed to be used with a wide range of internal combustion engines. It is an open project headed by Bruce Bowling and Al Grippo, engineers that work on the U.S. East Coast. The project's do-it-yourself approach makes it the least-expensive system for this purpose. Basic costs are below US$200 as of 2005, although this can vary widely depending on application.

MegaSquirt is a successor of sorts to Bowling and Grippo's earlier EFI332 design, which was more complex yet more powerful system (at least initially). The EFI332 project started around 1995, and culminated in the release of about 200 kits in 2000. The system used a 32-bit MC68332 microcontroller from Motorola, hence the name. A steep learning curve is believed to have prevented the system from gaining wider acceptance.

The two engineers decided to simplify the design and focus on managing the fuel injectors (the EFI332 could also control the spark plug ignition system if so desired). The version 1.0 MegaSquirt used an 8-bit Motorola MC68HC908 microcontroller, but a later MegaSquirt-II upgrade included a 16-bit MC9S12. It is likely that a future version will use a 32-bit processor.

The assembled controller takes input from a few different sensors in order to manage the fuel injectors, including a throttle position sensor (TPS), exhaust gas oxygen sensor (EGO or O2 sensor), MAP sensor, intake air temperature sensor (IAT), and a coolant temperature sensor (CLT). The latter two sensors themselves are usually the General Motors type, although you can recalibrate the controller to use other sensors.

There are several related projects, including:

MegaJoltLite – an ignition system controller for the Ford Enhanced Distributorless Ignition System (EDIS)
UltraMegaSquirt – an integrated fuel injection and ignition controller