How a Sequential Fuel Injection Works: Pros and Cons

January 27, 2012

A sequential fuel injection is a type of multi-port fuel injection system in which each injection valve will open just before the cylinder intake valve opens. So in essence, the individual injectors work by themselves because they are fired individually. Other multi-port systems will fire all the injectors in groups no matter if the cylinder is ready or not. While sequential variable fuel injection has many benefits, direct injection is quickly becoming the system of choice and is replacing the SEFI systems.


The main advantage of a sequential system is that the entire system has a fast response time when the driver makes a quick change. More or less fuel can be released when the next intake valve opens. For systems that do not use sequential fuel injection it will be necessary for the entire engine to complete a revolution before the fuel to air ratio can be altered. This type of injection also produces much lower emissions and provides even better fuel economy. Because the fuel is altered for each cylinder, you can potentially get the best performance out of the engine that is possible. The throttle response is immediate, which allows for very rapid changes in the fuel to air mixture. Because fuel is used more economically, the emissions released are lower. This is very important where vehicles are being produced to meet stricter emissions.


Because there is a single injector for each cylinder, the larger the engine the more expensive the system. A V6 engine will need 6 injectors and a V8 would need 8 injectors. Sequential systems are also more expensive because they require an additional wiring harness that is used to ensure that the injectors pulse sequentially. The electronic control is more complicated and requires additional equipment to run properly. Because the system is more complicated it is very difficult to install and most drivers will require professional installation. This in turn also increases the cost of the system.

When to Use SEFI

Fuel injection systems are very common today. However, a sequential system is not the most common. For those that are looking for very fast handling response and the ultimate in performance, a sequential system is the best option. Almost all cars are sold with fuel injection, but to increase performance there are aftermarket systems available. While carburetors really are best for adding more horsepower, the SEFI systems require much less maintenance, are very reliable, make driving easy, and reduce noise and emissions.

Aftermarket System

Many drivers look to upgrade to aftermarket sequential systems because the stock systems are quite limited and the tuning software used is very inexpensive. An aftermarket system can supply real time tuning, particularly if the manufacturer has yet to make a tuning software program for your engine (not all engines have their own programs). Expect to find a computer, tuning software, wiring harness, sensors and cable for computer interface in an aftermarket kit. If you are planning to replace the entire multi-port system, then expect to pay about $4,000 depending on the make and model of the vehicle.

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