Though considered by many an old-fashioned approach to increasing engine longevity and octane tolerance, water injection is currently enjoying a resurgence in popularity due to the engine requirements of today's massively boosted engines. Water injection enjoys a number of advantages over typical gas enrichment or inter-cooler systems, including lower cost, higher latent heat absorption rate (inter-cooling effect) than gasoline and much lower nitric oxide emissions than any other system.
Though used on some high-compression, naturally aspirated engines, the water-injection system's forte is making power in boosted engines. Unlike an intercooler, which cools the compressed air before it gets to the engine, water stays in its liquid form until the moment of detonation in the cylinder. Though water by itself does not make power, its ability to prevent detonation in high-pressure applications means that the driver can increase the engine's timing and boost pressure, which does make a difference.
Diesel engines can also benefit from water injection systems, as they cool the engine's exhaust temperature greatly, allowing a greater amount of boost and increasing turbo longevity.
A small reservoir is used to hold the fluid, usually a mix of 50/50 water and alcohol with a bit of water-soluble oil. Far better than water alone (which obviously does not burn), the alcohol in this mixture is a fuel in and of itself and does not significantly affect the water's cooling effect. By introducing a small amount of this high-octane fuel to the engine's additional boost, the power gains become practically limitless with no alteration whatsoever of the stock computer programming or fuel system.
The water soluble oil is used only to prevent corrosion in the water injection system.
A big part of the water system's appeal is its cost. Complete kits can be purchased for under $300 (less than the cost of a computer re-tune), but the components are easy enough to gather for those taking a custom approach.
A small fluid reservoir (similar to a windshield wiper reservoir) is used to hold the fluid. A high-volume 150psi pump moves the juice. You can use a dedicated water pump like the SHURFlo from Northern Tool, but a new fuel pump designed for a fuel injected car is a good option, provided you always use the water-soluble oil. You will need to include a Hobbs pressure switch in the line to ensure that the system always has pressure.
The pump goes to a purpose built water solenoid, which turns the system on and off quickly. These solenoids are available from McMaster and other suppliers.
The solenoid feeds a sprayer, which could be as simple as a metal nozzle in the intake opening, to a nitrous spray-bar, to a purpose built unit like those made by McMaster. The only important factors in the nozzle itself is that it uses small enough openings to atomize the water spray, and that the flow is adjustable for tuning purposes. Some top speed racers have been known to use 1/4-inch metal brake line for the sprayer, adjusting the flow by crimping the engine end of the water tube with a pair of vice-grip pliers. This isn't recommended of course, but it has been done.
The only tricky part of installing a water injection system is the nozzle placement, which is a hotly debated issue and depends on the application. Some old-school land speed racers insist that injecting water directly into the turbo inlet atomizes the mixture better, but this can result in damage to the turbine blades.
The general consensus is that, much like nitrous, the best approach is to use a multi-port fogger system. This method guarantees even distribution of the coolant, but is complicated and can get quite expensive. However, if you're a dedicated racer with an $80,000 engine, you may want to seriously consider it.
The most common arrangement, though, is to either inject the water into both sides of the carburetor secondaries from the top or use a beneath-the-carb plate system. Nitrous and water injection are so similar that many people simply use ready-made nitrous plate sprayers for their water injection system. Using a nitrous plate offers the additional benefit of built-in adjust-ability via the replacement jets included with the plate.
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