The Difference Between MFI & MPI

by Richard Rowe
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Cars and acronyms may go together like peas and carrots, but the resulting alphabet soup could drive Buddha to fits. So you're excused for feeling a bit like a monkey at the prom, and wanting to ram a sock down your own throat in frustration when trying to tell the difference between terms like MPI and MFI. Especially these terms, which, like so many others, since they can often refer to the same kind of system using slightly different nomenclature.

TBI vs Multipoint Injection

The old throttle body injection system, or TBI, is something of an evolution of the carburetor, a stop-gap engineered to help manufacturers transition from carburetors to the electronic fuel-injection systems used today. The TBI unit bolts in place of the carburetor and sprays fuel under pressure into the engine. A multipoint system uses one injector for each one of the engine's cylinders. These individual injectors spray fuel directly onto the back of the intake valve, ensuring perfect fuel distribution to each cylinder.


In the common parlance, MPI and MFI are actually two different ways of referring to the same system. MPI breaks down as "multipoint injection," and MFI stands for "multipoint fuel injection." You could also abbreviate the latter as MPFI, MI or Fi. BMW originally dropped all but the last letter when referring to the same system and still uses the old "-i" suffix in spite of the fact that the company no longer manufactures a car without fuel injection.

MFI -- Definition 2

The acronym MFI can also refer to mechanical fuel injection, a system in use since the earliest days of internal combustion. The primary difference between carburetion and fuel injection is that the former uses engine vacuum to suck fuel into the cylinders, while the latter sprays fuel into the engine under high pressure. A mechanical system is just that: it uses no electronic controls whatsoever, relying either on a fuel pump -- as many diesels do -- or a fuel distribution system to time fuel injection to crankshaft position.


Electronic fuel Injection differs from mechanical fuel injection in that it uses some sort of electronic controls to time the fuel injection cycle. The most basic type of EFI system consists of an oxygen sensor and a small control box that controls an auxiliary air valve. The fuel injection system is essentially mechanical, and the engine controls the air-to-fuel ratio by making minute adjustments the the amount of air going in. The modern EFI system uses a legion of sensors and electronic injectors to control how much fuel goes in and when.

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