Marine Fuel Injector Types Explained — Top Feed, Side Feed, GDI & OptiMax

If you've pulled injectors off an outboard — or you're about to ship yours in for service — it helps to know what you're looking at. Marine outboards use several injector types, and they differ in how they're built, where they mount, and how fuel gets into the engine. Those differences are also why our service pricing varies by type.

You don't need to know your injector type before you send them to us — we identify them when we open your package. But if you want to understand what's under the cowling, here's what to look for.

Top Feed Injectors — The Standard

Top feed injectors are what the vast majority of outboard owners will encounter. These are port fuel injectors, meaning they spray fuel into the intake port upstream of the intake valve — not directly into the combustion chamber. Port fuel injection operates at relatively low pressure, typically around 45 PSI on marine applications.

How to spot them

Top feed injectors are tall, cylindrical, and mount vertically. You'll see them hanging from a metal fuel rail that runs across the top of the intake manifold, with each injector dropping down into its own port. Fuel enters through the top where the injector plugs into the rail. The electrical connector sticks out from the side of the body. At the bottom, you'll see a nozzle tip with o-rings above and below it that seal against the intake manifold.

When you pull them, they slide straight out of the rail and manifold. They have a narrow, cylindrical body — sometimes described as "pencil style" — and you'll find a small mesh filter basket inside the fuel inlet at the top.

Which engines use them

The entire Honda outboard lineup is top feed port fuel injection — every BF model from the BF40D through the BF250D. Honda calls their system PGM-FI (Programmed Fuel Injection), and all Honda marine injectors are Keihin manufactured. Yamaha's F-series multiport range from the F25 through the F350 uses them, with Mitsubishi and Denso/Nikki supplying the injectors depending on the generation. Suzuki runs top feed across everything from the DF25A through the DF300. Mercury's four-stroke EFI line uses them on models from 30HP through 300HP.

If your outboard is a four-stroke and it's not labeled "HPDI" somewhere on the cowling, the odds are very high that you have top feed injectors.

Service is straightforward. These mount on the ASNU bench with a standard fuel rail coupling, take standard o-rings and filter baskets, and a full set of four to six injectors can be cleaned and flow tested efficiently. That's why this is the lowest price point at $25 per injector.

Side Feed Injectors — The Legacy Format

Side feed injectors accomplish the same thing as top feed — spraying fuel into the intake port — but the plumbing is different. Instead of fuel entering from the top through a rail that sits above the injectors, side feed injectors sit inside the fuel rail itself. Fuel enters through slots on the side of the injector body, and the rail wraps around the injector rather than connecting at the top.

How to spot them

Side feed injectors are noticeably shorter and stubbier than top feed. The biggest visual giveaway is the fuel rail relationship — if the injector appears to be nested inside the rail rather than hanging below it, that's a side feed. You won't see a distinct top inlet where the injector plugs into the rail the way you would on a top feed. Instead, the body sits in a cradle or block with fuel surrounding it.

Where you'll see them

Side feed is largely an older design. In the marine world, some Mercury OptiMax fuel injectors use a Siemens side feed configuration, and you'll occasionally find them on legacy platforms. They're far more common in older automotive applications — 1990s-era Nissan, Subaru, GM TBI, and Mazda engines used side feed extensively. In today's outboard market, you're unlikely to encounter a standalone side feed unless you're working on an older motor.

Service requires a different adapter block on the ASNU bench and a different parts kit — that's why side feed is priced at $30 per injector rather than the $25 top feed rate.

GDI — Gasoline Direct Injection (Yamaha HPDI)

GDI is a fundamentally different injection approach. Instead of spraying fuel into the intake port (which is what both top feed and side feed do), a GDI injector sprays fuel directly into the combustion chamber under high pressure. In the marine world, the primary GDI platform is the Yamaha HPDI (High Pressure Direct Injection) — a 2-stroke system that Yamaha introduced in 1999 and produced through 2013.

Yamaha's HPDI system uses a belt-driven, high-pressure mechanical fuel pump to deliver fuel at approximately 700 PSI — dramatically higher than the roughly 45 PSI of a port fuel injector. That high-pressure delivery atomizes the fuel into extremely fine droplets (just a few tens of microns, per Yamaha's documentation) for a more complete, efficient burn. The fuel is injected from the top of the cylinder milliseconds before the piston reaches top dead center, with the exhaust ports already closed — which is a major reason HPDI engines are significantly more fuel efficient and cleaner-running than conventional 2-strokes.

How to spot them

HPDI injectors are visually distinct from port injectors. They're shorter, heavier, and built with thick metal bodies designed to handle the high fuel pressures. They mount into the cylinder head rather than the intake manifold, and they're fed by high-pressure fuel lines rather than a low-pressure fuel rail.

One important detail: Yamaha specifically designed the HPDI injector to open internally — the nozzle does not protrude into the combustion chamber the way some competitors' direct injectors do. This was a deliberate engineering choice to reduce heat exposure and carbon buildup on the injector tip. That said, the high-pressure environment and direct combustion proximity still create heavier deposits over time compared to a standard port injector.

Which engines use them

The Yamaha HPDI was available on the 150, 175, 200, 225, and 250HP V6 2-stroke models from 2000 to 2013. If your cowling says "HPDI," that's what you have.

The HPDI injectors use a Mitsubishi platform with a specific top o-ring and split washer that aren't interchangeable with any other type. Service requires specialized couplings and a more involved cleaning process to address the deposits that accumulate under high-pressure direct injection conditions. GDI service is $30 per injector.

Mercury OptiMax — The Dual Injector System

This is the one that trips people up, and for good reason. The Mercury OptiMax is a 2-stroke direct fuel injection system, but it works completely differently from Yamaha's HPDI. Instead of high-pressure fuel injection, the OptiMax uses a low-pressure, air-assisted system developed by Orbital Engine Corporation of Australia. It uses two different injectors working together on every cylinder — a fuel injector and an air injector — to get fuel into the combustion chamber.

How the system works

The fuel injector meters the correct amount of gasoline and sprays it into a pressurized chamber on the air/fuel rail. The air injector — which Mercury calls a "direct injector" — then fires a blast of compressed air (roughly 80–90 PSI from an onboard belt-driven air compressor) into that same chamber, atomizing the fuel into an extremely fine mist and injecting the air-fuel mixture directly into the combustion chamber.

There's a critical pressure relationship in the system. Fuel rail pressure runs about 88–92 PSI, while air rail pressure runs about 78–82 PSI. The fuel pressure has to stay about 10 PSI higher than the air pressure so fuel can flow into the air chamber and mix properly. If that pressure difference drifts — from a failed regulator, a worn tracker valve diaphragm, or an air compressor issue — you'll get poor atomization, rough idle, or the engine may not run at all.

How to spot them

The OptiMax rail assembly is distinctive. The fuel injectors sit on one side of the rail (typically above), and the air injectors — the "direct injectors" — sit on the other side, closer to the cylinder head. You'll have two injectors per cylinder — so a V6 OptiMax has 12 injectors total: 6 fuel and 6 air. The air injectors have a cone-shaped spray tip that sits in the combustion chamber, and you'll often see carbon buildup on that tip because they're directly exposed to combustion.

If the cowling says "OptiMax" or "DFI" (direct fuel injection), you have this system.

Which engines use them

The OptiMax line covers Mercury 75HP through 300HP 2-stroke outboards, produced from the late 1990s through 2018. Models include the OptiMax 75, 90, 115, 135, 150, 175, 200, SST200, 200XS, 225, 250 Pro XS, and 300XS. These engines are still extremely common on the water — they're reliable and fuel efficient when maintained, but the injection system is sensitive to fuel quality and ethanol damage. Ethanol draws water, and the combination of water and ethanol creates residue buildup inside both the air and fuel injectors and causes corrosion on the aluminum fuel rails.

When you send in a set of OptiMax injectors, include both the fuel injectors and the air injectors. They work as a matched system and should be tested together. A V6 has 12 injectors total — 6 fuel and 6 air.

Service on the OptiMax is the most involved of any marine injector we handle. Two injectors per cylinder, specialized Teflon seal kits, dedicated adapter couplings, and the unique deposit patterns from the 2-stroke DFI system all add time and parts to the job. OptiMax air injectors are priced at $40 per injector.

Quick Reference

Type	What to Look For	Common Engines	Service
Top Feed	Tall cylindrical body, hangs from fuel rail above intake manifold, rubber o-rings top and bottom	All Honda BF series, Yamaha F25–F350, Suzuki DF25A–DF300, Mercury four-stroke EFI	$25/ea
Side Feed	Short stubby body, sits inside the fuel rail, fuel enters through side slots	Some older Mercury/OptiMax fuel injectors, legacy platforms	$30/ea
GDI (HPDI)	Short heavy metal body, mounts into cylinder head, high-pressure fuel lines, cowling says "HPDI"	Yamaha HPDI 150–250 (2000–2013)	$30/ea
OptiMax Air	Dual rail (fuel + air), two injectors per cylinder, cone-shaped air injector tip, cowling says "OptiMax" or "DFI"	Mercury OptiMax 75–300 (1998–2018)	$40/ea

The Fastest Way to Know What You Have

Check the cowling. If it says "OptiMax" or "DFI," you have the dual fuel-and-air injector system. If it says "HPDI," you have Yamaha's high-pressure direct injection. If it doesn't say any of those things — and it's a four-stroke outboard from any of the five major manufacturers — you almost certainly have top feed port injectors.

When you fill out our shipping form, just tell us the engine make, model, and year. We handle the rest. Your diagnostic report will document exactly what we serviced, including injector type, flow rates, spray patterns, and variance across the set.