I have heard these terms tossed around quite a bit and I understand that fuel injection is needed in order to inject fuel in to the engine (from a fuel rail/injectors?).

  • What is the difference between multi-port and combined-port fuel injection?
  • Are there any other alternatives to these methods of fuel injection?
  • Why can't people just upgrade fuel pumps and increase fuel line (pipe?) pressures to force fuel to flow in to the engine when the intake valves are open instead?
  • Max, didn't you already ask the first question here?
    – Zaid
    Commented Feb 11, 2016 at 18:44
  • Is multi-port the same as combined port injection? Commented Feb 11, 2016 at 18:45
  • They're both "port" injection. If you asked multi-port vs combined-port it would make for a better question in my opinion
    – Zaid
    Commented Feb 11, 2016 at 18:47
  • I've never heard the term combined port fuel injection. Maybe you mean central fuel injection?
    – Ben
    Commented Feb 11, 2016 at 23:56

2 Answers 2


"Why can't people just upgrade fuel pumps and increase fuel line (pipe?) pressures to force fuel to flow in to the engine when the intake valves are open instead?"

This is the job of the fuel injector/ECU. It is timed specifically so the proper amount of fuel is injected at the correct time. The ECU is tuned for a specific fuel pressure and can typically compensate if it changes slightly.

If you are asking in terms of modifying for more power...
The ECU and typically compensate for small changes
For larger changes, you will want to retune your ECU. You will not need the same fuel increase throughout the RPM range, and will need to tune increases based on RPM.
Typically, you only need to raise the fuel pressure if you are raising the pressure in the intake by way of turbocharger or supercharger, otherwise, you get injectors that flow more fuel.

"Are there any other alternatives to these methods of fuel injection?"

Carburetor, Mechanical fuel injection (seen in the 50s and 60s), Throttle body injection, multi port fuel injection, direct injection.

A carburetor is mechanical and injects fuel based on air flowing through, plus precise tuning.

Not sure on mechanical fuel injection, heard of it, but never looked into it.

Throttle body injection uses a single fuel injector to inject fuel right before the throttle body.

Multi port injection is the most common since the early/mid 90s. Each cylinder gets its own fuel injector mounted in the intake manifold.

Direct injection is not very common on gasoline engines, but is growing in popularity. Gasoline is injected directly into the cylinder. Haven't looked much into this either. I believe it is both the most efficient and the most complex.

  • On direct injection there is a low pressure and high pressure pump. You'll typically see pressure on the high side in the thousands of PSI if I remember correctly. Common problems with direct injection have to do with carbon build up on the intake valves. More likely caused by the EGR system.
    – Ben
    Commented Feb 12, 2016 at 20:35
  • Mechanical fuel injection is just that, mechanical. It has a set amount of fuel which is injected at a given time. There is no way to adjust fuel flow WRT how much the engine really needs. It just shoots more often as the engine RPM increases. It is all based on a set of assumptions designed into the system during the engineering phase. Commented Feb 14, 2016 at 19:05
  • That explains RPM. Could be attached to a cam. How would it compensate for throttle?
    – rpmerf
    Commented Feb 16, 2016 at 1:07

In multi port fuel injection the injectors are located near the Intake valve.

In central port fuel injection there is a single injector with multiple valves that route to their respective cylinders. Think the 90's Chevy Blazer with the 4.3l Vortec.

There is also throttle body fuel injection in which a single injector is located at the throttle plate.

As to upgrading the fuel system while you can upgrade the pump you would also need to increase injector pulse width while watching injector duty cycle. Typically when you upgrade a pump you also replace injectors.

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