EFI Performance Fuel Systems "101"

This is a summarized but complete overview of the proper way to plumb a Mercury Racing EFI outboard powered boat.
Production EFI’s and ProMax series motors can benefit from these guidelines however they will not need the same size fittings, hoses or fuel pumps. These designs use vacuum pumps to feed a self-contained system therefore the plumbing is not as demanding. The majority of Performance Outboards are crankcase injected Mercurys. Therefore, they are the target of this topic.

We must start off by noting that this discussion applies to race boats, as certain practices do not always agree with USCG/marine survey/ insurance industry regulations. For example, the anti-siphon valve that no doubt came with your new boat, does not go well with that new drag motor you are about to install. The ultimate fuel system does not use stand pipes or shut-off valves, but rather fittings in the bottom of the tank. The good news is that outboard regulations are less addressed than their inboard counterparts. Engines, exhaust manifolds, belts, pulleys, alternators all in the enclosed bilge demand more attention than an outboard or two hanging off the transom. The most common rigs do utilize stand pipes in their tanks, which can easily be designed to work efficiently.

Perhaps, after some people finish reading this article, they may say “I don’t have it like that” or “never had a problem so why change it?” That may be so, but the beauty here is that the correct method is very inexpensive and the owner can easily do it themselves. When you do have an engine failure or are not running as quick or as fast as your buddy with a similar rig, it is nice to be able to rule out improper fuel delivery as the culprit.

We will not drag out fluid dynamics calculations that take into account internal hose friction, temperatures, or viscosity. That would be boring and unnecessary. This article comes from 28 years in the racing business both with cars and boats. Hardly a week goes by that we are not on the telephone explaining these principles that we are about to cover.

OK, now for the fun stuff.

One single sentence sums up this entire article.

“ Do not starve the suction side of the High Pressure pump”

Starvation is commonly achieved in many ways:

Small Hose
( we have seen people stretch -6 hose over a -8 barb)

Small Fittings
( Easily, the most common condition is the correct hose size coupled to a fitting with too small an inside dimension)

Collapsed and Kinked Hose
(We serviced a 30’ offshore cat with unmounted sponson bladders that worked the hoses to the point of internal breakdown)

Stand Pipes - Too Short, Cracked or Even Too Long

(plastic ones jammed to the bottom of the tank always raise an eyebrow)

Horribly Restrictive Screens at the Stand Pipe Entry
(a sometime necessary evil on boats with deteriorating built-in fiberglass tanks).

Most Often the Biggest Problem Starts at the Very Beginning
(The “Hard” 90 degree fitting at the top of the tank’s stand pipe with the infamous anti-siphon check valve barb may be lurking in your boat at this very moment)


Figure 1 shows one simple way of making a stand pipe for your existing tank. We use ½” o.d. soft copper tubing (the kind that is sold in coils). Tanks usually have a 3/8” pipe female threaded bung or bushing on the top, so simply machine a brass 3/8” pipe male to -8 adapter so that the straightened ½” o.d. copper tube slides inside. Solder them together with silver solder (lead solder works fine as well) and you are now well on your way to success.

Figure 2 are examples of what you do not want. These are however widely used and tolerable in another system’s design that use transfer pumps to fill a “catch” tank. We will discuss this later in the article.

Figure 3 Top off the new -8 adapter with a TUBE 90 degree elbow. Do not use any more 90 degree fittings en route to the high pressure pump inlet (suction side of system).

Figure 4
shows some samples of fittings that we had chromed. Fittings are available in a variety of metals and platings. Tube 90 degree elbows made in brass unfortunately, do not seem to be available.
When you use brass fittings in salt water applications (which is correct) spray them with a light coat of clear “krylon” to stall the inevitable green discoloration. Pick and choose the fitting alloy that suits you. In most of the rigs that we are concerned with in this discussion, electrolysis from dissimilar metals will not cause a problem. It certainly would on larger center console outboard boats left in marinas overnight. Shore power hook ups and many electrical devices on board would make bonding and zincs mandatory. A side note worth mentioning is to avoid the temptation to use stainless braided hose assemblies especially in salt water applications. They may be necessary in your blown 46’ Skater, street rod, or race car, but stay away from then in your outboard hot rod. The following are two examples of why you should not use them.

Figure 19 demonstrates electrolysis formed under the hose end at the stainless steel braid. This hose assembly was removed from the bilge of an outboard offshore race boat that was craned out every afternoon. It never sat overnight in a marina. Stainless steel and aluminum together may be the the best recipe for electrolysis. Stainless braided hoses are risky in another respect. In a thrash a steel fuel hose assembly accidentally draped across a battery will no doubt spoil the weekend. Some shops use plastic coated braided -4 trim hoses for this reason. (they do turn ugly amber color however).

Figure 4 Also shows a few stainless steel band clamps which are a tasteful choice over ugly, hose wrecking worm clamps. These clamps are available in all necessary sizes.

Figure 5
shows the sometime optional water/filter base. We say optional because drag race and even river race boats (single engine) consume most of their small tank’s volume on a daily outing. If you choose not to use the water separator, be sure to fill the tank using a “race car” funnel with an internal bronze screen which removes water before it contaminates the tank. Add a commercial coffee filter into the funnel to trap debris. This filter/funnel set up will be easier to use if you install an “aero” style cap onto the top of the tank. We prefer this method in lieu of a water separator filter in the small EFI powered hot rod boat.
Larger hulls with big or multiple tanks, require water separator filters in their system. Gasoline sitting in hulls for a time will collect moisture and fueling in marinas may pose a problem.
Years ago, Mercury Racing generated a service bulletin for the ProMax series motors prohibiting the use of hull mounted water separators. Mercury must have had warrantee issues due to inadequate fuel system rigging, citing that their powerhead mounted water separator was adequate; which it was. The problem arises in the real world. The following is one example. You are returning from a weekend of fishing and diving in your triple ProMax powered center console from Bimini (Bahamas) to Miami. Halfway across in five to seven foot gulf stream seas, your motors feel the ill effects of the bad load of fuel that you recently bought. The only fix is to constantly dump the water separators that are not safely mounted inside the bilge. Changing the motor mounted filter on the center engine is an annoying process inside the shop, never mind hanging over the transom while waves crash over you! The point of this story is that by properly plumbing any boat’s fuel system, you can enjoy unexpected benefits that may arise. When installed properly, all ProMaxes can safely run hull mounted water separators without a problem.

Back to Figure 5 We use a large style 3/8” pipe water separator base. The large 3/8” pipe openings (as opposed to Merc’s ¼” pipe one) allow us to easily ream the inside passages to .400” to improve flow. Blending the inside corners adds a nice detail as well. If you choose to install a water separator filter in your rig, this is the one to use.

Figure 6 is a sample of -8 Push-loc hose with a stainless steel Adel or cushion clamp. The -8 hose is used for the inlet side of the two most common Bosch fuel pumps. Be sure to double clamp this connection.


Figure 7 Shows a Bosch High Pressure fuel pump. These pumps were original equipment on Mercury Racing’s 260 and 280 H.P. motors. They can be installed vertically and horizontally. It is important to mount these pumps low in the hull. ( bottom of fuel tank is optimum) These pumps have extremely tight internal clearances that require clean gasoline. A vast majority of these pump’s failures are caused by dirty fuel.

Figure 10 From this outlet to the fuel rail use -6 (3/8”) hose. Alcohol fueled motors need larger, but that is a whole other story.



Figures 8 & 9 Inline filters protect the fuel injectors. They are 8 micron rated and available with both fittings and barbs. You need a filter with no less than 5/16” inside openings.


Figure 11 Fittings with fuel pressure (1/8” pipe) ports are available for use with  both fittings and barbs. We also have pressure port fittings that thread directly into the filter outlet. These are pictured elsewhere on our website.


Figure 12 shows a few samples of -6 elbows that attach to the 2.5 fuel rail inlet. The tube 90 degree hose ends are a nice touch, but honestly, their importance is hard to determine at this connection. Remember that the welded male flare that comes on the Mercury 2.5 fuel rail is called -6 J.I.C..  Dash 6 fittings (and -12) come in two sizes; J.I.C. and S.A.E. You need J.I.C. for this.

Fuel Rail

Figure 18 We should mention a few things regarding the return from the regulator. Ideally, it needs to be back into the main tank far away from the pick-up. The Pro Max (“Laser”) style system came to be because the Coast Guard cringes at the thought of a fuel line returning to the boat. Finding an empty port in the main tank is the first choice. If there is no fitting available, fabricate an aluminum (Stainless, brass, bronze) splice to install into the fuel fill hose with an angled fitting or barb to accept the return line. Once in the main tank, the return fuel can cool and dissipate any air bubbles that may have formed. If a line directly to the main tank is impractical, it is okay to plumb the return fuel into an empty “in” port on your water separator housing. This method has inadvertently saved many people from problems. In the late eighties, we would weld a fitting to the side of the Kinsler filter on the suction side of the Weldon pump. This was a quick cure to remove the bog from hard turning tunnel boats. Theoretically not the best method but it worked. If you use a catch tank system, then use the port you hopefully welded on top.

This photo shows pumps that are more necessary for the competing outboard drag racer. Large volume pumps seem to be necessary to maintain stable pressure on the race track. These large pumps satisfy the needs of large 4 stroke engines and yet we still need them on our little two strokes. Here we can skip the theories and go with what works. Typically a small fuel cell is used with fittings located in a small sump at the rear bottom. A single -10 fitting must be installed, however most cells come with two -8 fittings. Run the two -8 lines to a “Y” block with a -10 outlet and you will be just fine.

Figure 14
Aftermarket inline filters are available from a few sources. Be sure they have -10 connections. Figure 13 this is the Weldon pump which Mercury has supplied on and off since the 2.4 Bridgeport era. These are noisy, expensive, battery killers, but otherwise fabulous pumps. When purchased from Mercury Racing, they include the Kinsler filter so Figure 14 will not be used.

Figure 15
is a -10 hose sample with a stainless steel Adel clamp. Drag racers can also use the two -8 fittings in their fuel cells to feed two Bosch pumps in parallel into a “Y” block having a -6 outlet. We had our customers use this method back in the early-mid nineties. Some still use it today. If your stand pipe style tank has a bung for a return line, you can install a second stand pipe in that return line port. (you must find a new place for the return hose) Bend this new pick up towards the front of the tank to help solve slosh problems that can arise from larger un-baffled tanks. Join the two stand pipes to a “Y” block and then complete the system.

Figure 16
is our Bosch Magnum pump. It uses a -10 inlet with a built-in 20 micron screen. This pump fits the traditional mount, but we always end up fabricating custom ones. Soon we will have a “standard” custom mount for sale.

Figure 10
once again is a sample of -6 outlet hose.




Figure 8,9,11 are filters and fittings.




The final topic that we have not yet covered are boats that require
“Catch Tank Systems". This is where a low pressure transfer pump fills a small vented tank (vent line feeds back to the main fuel tank) from which the high pressure pump can draw the fuel. Big hulls, high speeds, tight turns, and high seas all require this simple but necessary system. Hard turning smaller tunnel boats can use this system as well, but a set-up with just one pump often is preferred. These small single tank boats easily can use custom tanks that feature baffles, sumps and one way trap doors to accomplish anti-sloshing.
A common inexpensive transfer pump is the “Red” Holley. Do not waste your money on the “Blue” model as it is the same pump with a different bypass spring. The “Blue” pump comes with a regulator needed for carburetors. You will not use it so why pay for it? On vessels running in salt water it is necessary to paint these pumps and mounts because they tend to get ugly fast.
The pick-up side of a catch tank system is more forgiving than the other type because the low pressure pump does the work from the main tank. "Hard" 90 degree fittings are prevalent in larger boat’s stand pipes due to clearance issues (decks and bulkheads) and boat builder unawareness.
The fuel is then pumped to the catch tank where hopefully the air bubbles will be dissipated. The catch tank supplies the high pressure pump with a consistent and unrestricted fuel supply and sends it on to the fuel injectors.

In closing, we cannot stress the importance of free flow to the inlet side of the EFI pumps in all applications. Tees and 90 degree fittings are commonly used by people because they lay out nicely on a bulkhead and because they are space savers. “Hard “90 degree fittings are a commonplace at the inlet side of water separators to avoid “unsightly” loops in the hose. Here is an old rule of thumb to keep in mind. A “hard” 90 degree fitting on the suction side is like adding 15’ of length to your hose. Give your current lay out a closer look; you may be shocked to see how restrictive it is.

Hopefully, the outboard hot rodder can benefit from these simple guidelines. The properly rigged boat will give you safety, reliability, and top performance.

See you at the races,
Joe and Marty Signorelli