How to Improve Water Pressure on Low-Water-Pickup Lower Units & Nose Cones
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Low-water-pickup gearcases are designed to let a high-performance outboard run higher while still supplying cooling water to the powerhead.
They are common on Mercury Racing, Sport Master, CLE-style, F1, SST-120, S2000, tunnel-boat, drag-racing, lake-racing, and high-performance bass-boat applications, as well as aftermarket nose cones from companies such as Bob’s Machine Shop and Buckshot Racing #77.
The goal is not simply to get more water into the lower unit. The objective is to deliver enough clean, pressurized water to the powerhead with the least possible drag and restriction.
Too little inlet area can starve the cooling system, but too many or overly large pickup holes can increase drag, disturb water flow around the nose cone and, in some cases, reduce effective pressure.
Mercury addressed this subject directly in Service Bulletin 99-7, “Improving Water Flow for F1, SST 120 & S2000 Racing Outboards.” The bulletin was written for VI SSM racing gearcases through serial number R96L069 and focused on three areas: water-pump capacity, water-inlet geometry, and actual water-pressure monitoring.
The bulletin is specific to that racing gearcase, so its exact machining dimensions should not be applied blindly to every Sport Master, CLE or aftermarket nose cone. The engineering principles, however, are very useful for almost any low-water-pickup system.
Start by Measuring Water Pressure
Before changing engine height, drilling pickup holes or modifying a nose cone, install a reliable water-pressure gauge and establish a baseline.
Mercury’s bulletin specifies taking water pressure from the 1/8-inch NPT brass pipe-plug location on the exhaust cover above the ignition coils. Mercury also noted that engine height can make significant changes in water supply, which is why pressure should be monitored while making setup adjustments.
Another common high-performance setup is to take water pressure from near the top of the powerhead. Many racers and engine builders prefer this location because it provides a reading after the cooling water has traveled farther through the system.
Buckshot Racing #77 offers a top-of-the-block water-pressure fitting for compatible Mercury V6 applications, allowing a mechanical or electronic gauge to monitor pressure at that location.
Whichever pickup point is used, use the same location consistently. Record water pressure at idle, during acceleration, at normal cruise, at wide-open throttle, and at different jack-plate heights. A telltale stream alone does not confirm that the powerhead has adequate pressure and volume at high speed.
Engine Height Can Change Everything
One of the biggest advantages of a low-water-pickup lower unit is the ability to raise the engine and reduce submerged gearcase drag. The limitation is that the pickup eventually begins operating in disturbed or aerated water.
As engine height increases, the water in front of the gearcase can be affected by hull turbulence, propeller aeration, trim angle and the running attitude of the boat. Mercury specifically warned that engine height can significantly change the water supply to the powerhead.
If an engine has good pressure with the gearcase lower and loses pressure as the jack plate is raised, first lower the engine and confirm whether the pressure returns. That tells you far more than immediately drilling additional pickup holes.
The best setup is the highest engine position that still maintains stable cooling pressure throughout acceleration, trim changes and sustained high-speed operation.
Water-Inlet Depth and Shape Matter
Mercury’s 1999 bulletin makes an important point that is sometimes overlooked: a pickup can look open from the outside and still be restrictive internally.
Mercury found that some older VI SSM racing gearcases had water inlets that were not machined deeply enough, which could limit water flow to the powerhead.
For that specific gearcase, Mercury specified an inlet depth of 0.230–0.250 inches, or 5.8–6.35 mm, while maintaining a 2-degree inlet angle. The bulletin even shows a fabricated metal depth gauge for checking the opening.
Those numbers are specific to the VI SSM and should not be treated as a universal nose-cone specification. The larger lesson is to inspect the full water path.
On a Buckshot Racing nose cone, Bob’s Machine Shop nose cone, or another aftermarket low-water-pickup system, look for epoxy, filler, paint, casting flash, impact damage, poorly blended transitions, or an inlet that is visibly shallow or uneven.
A clean-looking external opening does not guarantee that the internal passage has enough effective area to supply the pump.
More Holes and Bigger Holes Are Not Always Better
One of the most common mistakes in high-performance lower-unit setup is assuming that low water pressure automatically means the nose cone needs larger or additional pickup holes.
Every additional opening creates more exposed area on the gearcase and can increase hydrodynamic drag. Larger openings can also disturb the water flowing around the nose cone.
In a race or high-speed application, the correct pickup system is not necessarily the one with the most holes. It is the one that delivers the required pressure with the least unnecessary disturbance.
Multiple pickup holes can also interact with one another. Some aftermarket nose cones use several holes on the bottom and sides of the cone. If the lower holes are receiving stronger ram pressure while an upper or side hole is exposed to lower-pressure or aerated water, some pressure can potentially be lost through the less-favorably positioned opening rather than all of the water being directed efficiently toward the pump.
That does not mean a five-hole or multi-hole design is automatically wrong. It means that hole size, hole count, location, and internal plumbing must work together.
The practical goal is to use enough inlet area to maintain the required water pressure without creating more drag or unnecessary leakage paths than the system needs.
Check the Water Pump Before Reworking the Gearcase
Mercury’s first recommendation in Bulletin 99-7 was not to enlarge the water inlet. It was to upgrade the affected VI SSM gearcases to the radial-discharge water-pump system, Mercury P/N 849006A1, because Mercury identified improved water-flow characteristics with that pump.
That same troubleshooting order makes sense today. Before modifying a nose cone or lower unit, inspect the impeller, water-pump housing, wear surfaces, drive key, water-tube seals, guide tube, grommets, and internal passages.
A weak impeller, worn housing, leaking tube connection, or damaged seal can create the same symptoms as a restricted low-water pickup.
The cooling system should be viewed as one continuous path from the nose cone to the top of the powerhead. A restriction or leak anywhere in that path can reduce pressure.
Mercury’s “3/4-Inch” Big Water Tube
Mercury’s bulletin also required the affected VI SSM radial-discharge pump conversion to use what Mercury referred to as a 3/4-inch water tube. The bulletin notes that the exhaust plate had to be machined for the larger tube.
The terminology can be misleading because Mercury’s so-called 3/4-inch big tube is not an ordinary 3/4-inch copper plumbing tube that should be purchased at a hardware store. It is a specialized Mercury cooling-tube size and configuration used in higher-flow performance applications.
The tube must properly match the water-pump outlet, seals, exhaust plate and upper cooling passage. Simply installing a larger piece of generic copper tubing does not create a proper Mercury big-tube conversion.
Mercury-style big-tube cooling systems continue to be used on a number of high-performance configurations, and Buckshot Racing #77 also offers a titanium version of the big water tube for builders who want a corrosion-resistant, high-quality alternative.
What a Big-Tube Conversion Requires
A big-tube conversion must be treated as a complete cooling-system change rather than simply a tube replacement.
The top of the water-pump housing or pump outlet must be configured for the larger tube, and the exhaust plate must also be machined or changed to accept the larger tube. The seals and upper connection must also match the larger configuration.
If the water tube is enlarged but the pump outlet or upper passage remains restrictive, the larger tube cannot deliver the intended benefit.
The correct way to think about the system is as one continuous flow path: pickup, water pump, water tube, exhaust plate, and powerhead cooling passages. Every section needs to support the desired flow.
Small-Tube and Big-Tube Mercury Cooling Systems
Many classic Mercury 2.0L, 2.4L and 2.5L V6 engines use what racers commonly call the small tube or 1/2-inch water tube. Higher-flow Mercury Racing configurations use the larger tube commonly referred to as the big tube or 3/4-inch tube.
Neither nickname should be interpreted as an instruction to buy generic plumbing material. Both systems rely on Mercury-specific dimensions, seals and mating components.
Buckshot Racing #77 supplies both small-tube cooling components and specialized big-tube options, including the titanium version for high-performance applications.
Understanding Mercury’s 14 PSI Specification
For the F1, SST-120, and S2000 VI SSM application specifically covered by Service Bulletin 99-7, Mercury specified a minimum of 14 PSI for at least half of each race straightaway.
That is valuable engineering information, but it should not be treated as a universal minimum for every Mercury V6, Sport Master, CLE, Buckshot Racing nose cone, or Bob’s Machine Shop nose cone.
Different engines, cooling systems, pumps, and pressure-tap locations can produce different readings. A pressure reading taken near the top of the block can also differ from one taken closer to the exhaust cover.
Use the factory specification for the exact engine whenever possible and pay close attention to pressure stability. A system that repeatedly falls from healthy pressure to very low pressure at speed needs attention even if it briefly reaches a good peak number.
A Better Way to Diagnose Low Water Pressure
The best diagnostic method is to work from measurement toward modification.
Start by verifying the gauge and pressure source. Then lower the engine and establish a baseline. If pressure improves immediately, engine height or pickup exposure is likely contributing to the problem.
Next inspect the water pump, water tube, seals, and upper cooling path. Once those areas are known to be healthy, inspect the pickup geometry and internal water passage for restrictions.
Only after the system has been measured and the actual problem identified should the nose cone or pickup opening be modified.
This approach prevents a common mistake: permanently enlarging a gearcase when the real problem was a worn pump, leaking tube seal, or simply excessive engine height.
Buckshot Racing #77 Tech Perspective
Mercury’s Service Bulletin 99-7 provides a very good engineering framework for solving water-pressure problems: measure the pressure, make sure the pump can supply enough water, verify the inlet geometry, and understand what engine height is doing to the water supply.
For today’s Mercury Racing, Sport Master, CLE-style, Buckshot Racing and Bob’s Machine Shop low-water-pickup nose cones, we would add one more principle: do not create more inlet area than the engine actually needs.
More holes can mean more drag. Poorly positioned holes can also reduce effective ram pressure instead of improving it.
The best high-performance cooling system uses the minimum effective pickup area necessary to maintain stable water pressure, combined with a healthy water pump, the correct Mercury-style water tube, and an unrestricted path into the powerhead.
That is the balance racers and performance boaters should be looking for: adequate water pressure, reduced drag, consistent cooling, and better engine reliability.
Download the original service bulletin Free in PDF:
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