Outboard Fuel Injection Vs Carburetor Vs Direct Injection!


With an ever fluctuating price on fuel, you might be wondering to yourself what type of outboard fuel systems are available?

There are three different types of fuel systems found on an outboard. Fuel Injection, Carburetion, and Direct Injection. Carburetion is the oldest, Fuel Injection is the most common, and Direct Injection can be considered the most precise.

Here is the breakdown of each one of these different fuel delivery systems, how they work, and which is the most fuel efficient!

How An Outboard Carburetor Works

Carburetors have been around for many, many, many years. They are a great and simple component that uses physics to deliver fuel to the engine.

In short, carburetors have what are called jets. These are basically little tubes that direct fuel and air throughout the carb.

There are separate jets and passages for different speeds as well. Normally, a very small one for idle speeds.

And a much larger one for wide open throttle. That can deliver a much larger volume of fuel to the engine when it needs it.

The carburetor uses what is called a venturi effect. Which is a physical event that happens to fluid and air that passes through a smaller or constricted hole.

One of the biggest down falls to a carburetor, is that there is virtually no way of regulating the amount of fuel the carb delivers.

Making it one of the least fuel efficient ways to power an engine.

An advantage to a carburetor though, is that they are extremely simple.

They are easy to clean, they don’t have a lot of moving parts, they are usually easily accessible, and outside of getting dirty, they are extremely durable.

Here are a couple of videos from our Youtube Channel showing how to clean a carburetor and some other things that can affect the carb.

How An Outboard Fuel Injection or EFI Works

When we move away from the carburetor. This is when it will get a little more complicated. And that will go for both of the next topics.

Fuel injection is a term that is also called EFI for short. Which just means Electronic Fuel Injection.

This type of system is most commonly found on four stroke outboard engines. And it has been around for over 20 years.

There are a few carbureted four stroke engines available. But the vast majority of them will have fuel injection.

Fuel injection is where the engine has a fuel delivery system built onto the engine. That will take fuel from the fuel tank.

Then it will pressurize that fuel in either a VST or an FSM. (Here is an article that explains in detail the break down of these two components!)

Once the fuel has been pressurized to usually around 42-50 PSI. Which is done by a high pressure fuel pump.

(And you can learn about testing this pump and what to do if it is failing here.)

It will then be contained and routed to what is called a fuel rail. This fuel rail is where we have what are called injectors.

These injectors are little electronically controlled components. That open and close whenever they are told to by the engines computer.

They will open for longer durations of time to deliver more fuel. But this is a great delivery system.

The fuel injection system uses these injectors to spray fuel into the manifold, or air intake of the engine.

Then the engine will open up intake valves that allow this fuel and air mixture to be delivered into the engine’s cylinders and consumed!

Note that on this system the fuel is delivered into the air intake, not into the cylinder.

How An Outboard Direct Injection Works

Now when we move over to the direct injection side of things. This system works very similarly to the fuel injection system.

With only a slight change. Instead of being delivered into the intake, here the fuel is delivered through the injectors directly into the cylinders.

Most direct injection systems require more fuel pressure than most fuel injection systems produce.

Which is generally in the 45 PSI range. The three most common direct injection gasoline outboard engines are Yamaha’s HPDI or High Pressure Direct Injection.

Mercury’s Optimax, and now Yamaha’s 425 horsepower XTO engine. It’s important to note that neither the HPDI or the Optimax are in production anymore.

Currently the only direct injection gasoline outboard on the market is Yamaha’s 425! Just something to note.

But both the HPDI and the Optimax are still very present on the back out boats to this day. So you can definitely still see them in the field.

The HPDI and the 425 have fuel pressures ranging from 1200 PSI all the way up to 3200 PSI. Which makes the servicing and troubleshooting of these engines.

A completely different animal to tackle than your normal fuel injection troubleshooting.

But their biggest advantage is because the fuel is going directly into the cylinder. They are one of the most fuel efficient engines there are!

Here is a video on an Optimax for some reference, as well as a video about servicing an HPDI! Before we talk about the fuel economy!

Which Has The Best Fuel Efficiency AKA The Best Fuel Economy!

When it comes to the issue of fuel economy. By far, the carburetor will consume the most fuel out of any of these.

With that being said though, it also really depends on the size of the engine. Because most carbureted engines available today.

Are going to be under 25 horsepower. And the others that are available, are only available outside of the United States.

So, when we talk about fuel economy, these small engines are already super fuel efficient, so you really won’t notice the difference here.

When you get to the mid and larger horsepower range though. The fuel injected engines are going to be the next fuel efficient engine’s out there.

But these also cover about 80% of the outboard market, so the technology is there to make them the most fuel efficient.

Especially with things like Suzuki’s lean burn, and Mercury’s Advanced Range Optimization or ARO.

Where they will actually allow the computer to read certain parameters coming while the engine is running.

And the engine will change the timing to lean out the engine. Which basically cuts the fuel being delivered back, saving you money on fuel!

Now the direct injection system was the most fuel efficient, before the lean burn became a mainstream thing on outboards.

But the cavoite to that is that with the 425 being the only direct injection engine available.

You really aren’t going to get the best fuel efficiency when we are talking about a 1,000 lbs engine that produces a massive 425 horespower!

At wide open, any engine producing that amount of power is going to be drinking or well guzzling fuel! So hopefully that isn’t an issue!

Here is a video that we made that will help you out with a trick that can increase your fuel economy!

Check Us Out!

We hope that this has been insightful and allowed you to understand a little bit more about the different outboard fuel systems available!

Which ones get the best fuel economy and how do they all work!

If you have more questions or want to learn more about your boat and its different systems. You should consider joining our Boating Academy where we have created HUNDREDS of video courses teaching you basically everything about your boat!

There are also even more helpful videos on our Youtube channel where we create even more boating videos!

You can also save some money by using our coupons when you need to buy stuff from your boat by using any of these coupons here at Partsvu!

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Here are some other super helpful articles that you might find interesting!

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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