Build Your Own
Water Injection System

Introduction

Why would you want to build your own water injections system? The answer is simple – to make more power with an internal combustion engine. The application is especially effective with forced induction systems such are turbochargers and superchargers, but benefits can be found in naturally aspirated engines as well when running high compression ratios. I’m not going to go into any detail about the specific reasons that water injection is so effective. If you want more information on the why behind the what, check out some of these resources on the web. Though this particular system is designed and fitted on a 1991 Mitsubishi Eclipse, one can adapt these same basic parts and principals to most any vehicle.

Please note: Injecting water into an engine’s combustion chambers can result in severe engine damage if not done properly. Please use this web page as a guide only. I do not assume any responsibility for damage that may occur in any car other than my own.

First Things First

Before you get started, you should formulate a game plan. How much time do you have to spend on this project and just when will you be able to do it? I found it most helpful to draw together a list of all the parts I felt I’d need, all the way down to items like mounting hardware, electrical connectors, etc. Avoiding repeated trips to the hardware/auto-parts store could save you hours in some cases. Most of the main components in the system were mail ordered, so I started collecting the parts early until I had everything in hand, ready to go. That’s when I started my two-phase attack.

The first phase revolved around building out all of the subsystems, and then laying everything out to make sure nothing was missed. I then temporarily assembled the system to make sure everything was working as planned. It’s important to make sure things work correctly before the water injection is actually installed. Troubleshooting is much more difficult once it’s on the car. It’s also is another safety check to make sure you won’t be damaging your engine unintentionally. Phase one should take two to three hours of total working time if you stay focused. There will be some dead time in the middle when glue and paint are drying. It took me a bit longer as I was photographing everything along the way.

The second phase was comprised of actually installing the system in the car. It took me about four hours to do so, mostly because I was taking pictures and also making decision about placement of components. I test fitted several items in multiple locations until I found what worked best. As you’ll notice in the photos, space is not very abundant under the hood, so I had to be creative.

Check out the water injection plans to get a better idea of what this is all about.

Parts

Assembly

Setting the Pressure Switch

Assemble the pressure switch by attaching the 1/8 in. NPT female connector and  the 1/8 NPT hose nipple. Make sure to wrap the threads in Teflon tape first to ensure a tight seal.

I started by setting my pressure switch to turn on at the desired boost level. My baseline setting is at 15 PSI, but can easily be changed after installation. The easy way to supply a pressure source is to use a vacuum/pressure pump. The MityVac pump I bought had a difficult time making 15 PSI, but I figured a way to get it to cooperate. Pressure is built when you let off the handle so if you push the plunger in by hand faster than it would have normally gone in, pressure builds faster and higher. I spliced in vacuum tee and hooked a tire pressure gauge to it. The MityVac only had readings for vacuum, not pressure. I might suggest looking for another brand of pump, as this one is kind of disappointing. On the bright side, the price was much less than some of the other brands.

I used a 6-volt battery to provide a power source. The test light will illuminate when the pressure switch reaches it’s set pressure point. Adjust using a screwdriver (on top of switch) – clockwise will set the pressure point higher. Later when you mount your pressure switch, you’ll want to be sure you can easily reach the set-screw in case you want to change the pressure point.

Building the Wiring harness

Save the packaging from the relay. It will come in handy when figuring out which pin outs go where. Pin 30/51 goes to your main power source. I hooked my system to the positive side of the battery since it’s so close. Pin 87 provides power to the pump and solenoid valve. Pin 85 goes to one terminal on the pressure switch – it doesn't matter which one. Pin 86 goes to a power source that will be used to arm the system. Since I removed my fog lights, the fog light switch is now unused. By tapping into the fog light fuse just adjacent to the relay’s mounting location, I can arm the system or turn it off from the dash, and I didn’t have to add any additional switches to the cockpit. You can also choose a source that gets power only when the ignition is on. This will arm the system as soon as you start the car.

Don’t be afraid to waste some wire, you can always trim the excess during the installation. I used four two-foot pieces to be on the safe side. When hooking the wires to the relay use the female spade connectors. Remove the hard plastic insulators and throw them away. They are usually crap. You can also buy connectors without these plastic pieces, but I found the ones with the plastic were actually cheaper. I also found that by buying them in a multi-pack (hard, clear plastic case with dividers) they were also way cheaper per unit. Cut generous lengths of the shrink tubing and place them over the wires before you crimp. Make sure the crimps are mechanically sound, or the wires might fall out later if tugged or stressed. To be extra cautious, you can also solider them to the wires. Use a flame or some other heat source to shrink the tubing over the wire and connectors. I also used a larger diameter of shrink tubing to help bundle all four wires together.

At this point, labeling the wires isn’t a bad idea. Getting them mixed up during installation could lead to hours of frustration, or even engine damage. I put both the fuse holders in-line with the 30/51 wire that goes to the positive post of the battery as well as the 87 wire, which feeds power to the pump and solenoid. Don’t forget to insert the 20 AMP fuses. Also note how I used zip ties to make sure everything is secure around the fuses. You don’t want all of your wires just flopping around. Try to make this as much like a factory harness as you can.

Plumbing

Connect the 3/8 in. hose nipples to the pump. To save a little cash, I only used the high-pressure version on the pressure side of the pump. The high-pressure versions have steeper angles on the barbs and are approved for use with fuel injection systems. The pressure side of the system also gets high-pressure fuel injection line vs. the regular fuel line I used from the reservoir to the inlet of the pump. Make sure to wrap the threads in Teflon tape first to ensure a tight seal.

Next, set up the solenoid valve and spray nozzle assembly. The picture shows everything together hand-tight, and without Teflon tape. You may need one or two washers to keep the screen (built into the nozzle) from bottoming out inside the brass elbow. Once again, everything goes together with Teflon tape and gets tightened down with open end wrenches. I had to orient my solenoid valve upside-down to fit under the hood. Save the hose cutting and fitting over the barbed nipples until the installation. It will be easier to cut to the correct length and will keep you from cursing at the high-pressure nipples from not releasing the hose! I almost think these things would hold a decent amount of pressure without t-clamps, but lets not find out.

Reservoir Tank

Some people choose to use their windshield washer bottles, but I wanted to keep the run of hose from my reservoir to the pump as short as possible. I also like the idea of being able to wash my windshield with “the blue stuff” and save my water/alcohol mixture for the combustion chambers. My homemade reservoir tank also holds a half-gallon, which is nice because I don’t have to fill it up that often. The problem I had was finding a place under the hood to fit such a bottle and also be able to get to it easily when you need to fill it. The only place I could find was right behind the engine, and at that it was only going to allow me to have a very narrow, tubular shaped bottle. I couldn’t find one just the right size for sale, so I decided to make my own -- here’s how.

You’ll need a 15 in. long section of 3 in. PVC pipe. You’ll also need a cap for one end, and some sort of removable lid for the other. I used a drain access cap that is threaded and has a threaded plug you can screw in and out. The cap end will be where the outlet is installed for the hose that goes to the pump. The nipple needs to be installed before you glue the cap to the pipe, or you’ll never be able to reach in and secure the lamp nut. I drilled a hole that was ever so slightly smaller than the diameter of the threads on the elbow. I then managed to get the first few tapered threads to start screwing into the PVC. The threads actually acted like a self tapping screw and screwed right into the PVC making a really tight seal. Note, that I also used a rubber o-ring between the elbow and the cap for extra sealing power. The elbow was so tight that I really think adding the nut was not needed, but I had it so why not have extra insurance? I looked all over for a nut that would help secure things, and finally found one in the electrical department near the lamp supplies. I was only able to get about two threads to catch, but it should be enough to keep everything together.

It’s now time to glue the cap to one end of the pipe, and the drain access fitting to the other. Let the glue dry and paint the outside black so it matches under the hood better.

At this point, you’ll be ready to pre-assemble your system. I laid mine out on the garage floor to give myself some room to work. No need to get too crazy about hooking connectors tightly and hose tightly. You just want to see that the solenoid and pump activate when pressure is applied to the pressure switch, and that you get a fine mist coming out of the nozzle. If all is well, you’re ready to begin installation.

Installation

Mount the Pressure Switch, Relay, Pump and Reservoir

I wanted a location what was easy to get to and also that made the wiring as simple as possible. Since my pump location choice was not flexible, I tried to keep the other components nearby as well. I found a great ground location, which was also in use by the ABS system. You’ll find it much easier to hook your ground wires to this location before mounting the pump. Check out the “after” top view photo. Here’s a front shot of the mounted pump.

Mount the pump by looping a large hose clamp behind the ABS bracket and then tightening down. The relay gets mounted to the ABS electrical box, using a small hole and self-tapping screw. The pressure switch just zip ties in place. I secured it to a wire bundle. It’s easier to attach the vacuum hose to the pressure switch assembly before it goes in. Snake the hose under the ABS and over towards the compressor bypass valve. That’s where you’ll tap in for a boost source using a vacuum tee and hose clamps.

Now you’ll want to attach the pump inlet hose to the pump as well as to the outlet from the reservoir. Then slide the reservoir into place. I was originally planning on securing it with a large hose clamp, but it didn’t need it. It fits very tightly behind the rear motor mount and stops near the steering joint. You may have to move the heater hoses around slightly to get the reservoir to slide in. Use the regular hose clamps to secure the outlet hose. It will not be under pressure.

Next attached a length of high-pressure hose to the outlet of the pump, and secure it with a t-clamp. Cut the hose to length and run it to where the nozzle assembly will be located. Now you’ll be ready to install the nozzle and solenoid valve.

Install the Nozzle Assembly

Start by choosing a location that will let you access the inside of the throttle body elbow with a combo wrench. You’ll need to do so when tightening down the nozzle. Next you’ll need to drill a hole for the nozzle. I was able to leave the TB elbow in place by stuffing a rag inside, and then vacuuming out the shavings with a Shop-Vac. In either case, you’ll need to unhook the upper intercooler pipe to gain access to the inside. I just propped mine up and out of the way. The hole should just be big enough for the nozzle threads to pass through it. You don’t want to create any leaks by making the hole too big. Test fit the nozzle -- be sure to use an O-ring to help it seal.

Next, attach the nozzle assembly to the nozzle and tighten with a box end wrench. Make sure this is nice and tight, or you’ll be sure to have a leak. Don’t forget to attach the other end of the high-pressure hose that runs from the pump. Here’s a side view.

Connect Wires

You’ll be glad that you spent the time to make a clean, well-labeled harness. Attach the power wires to your battery, solenoid, pump, etc. I found it easier to ground the solenoid to the fire wall, rather than running it back to the ground location used for the other components. Follow this diagram to help set up your wiring.

Test Operation Before Boosting

Once everything is set up, you’ll want to verify once again, that the entire assembly is working correctly before you start driving the car – especially with added boost pressure. Since everything should be buttoned up by now, which means reconnecting the IC pipe and such, you’ll not be able to actually see the nozzle spraying. Don't forget to fill the reservoir with a 50/50 water alcohol mix!

I let my car idle, and then used a test lead to bridge the two terminals on the pressure switch. This simulates a boost condition activating the system. You’ll be able to hear the pump and solenoid valve working. You’ll also notice the engine idle go down and a bogging condition will start. You’ll know it’s working at this point. All that’s left is to tune the setup to work with your specific application.

Tuning

Trial and Error - There's no Other Way

My car is running a 20G turbo, so I may need more flow from the water injection system than those moving less air. You'll need to experiment with nozzle sizes to find what works for your setup. After some playing around, I’ve found the 9.49 nozzle worked the best. The system kicks in at 15 PSI, and lets me run 21 PSI on pump gas with the stock intercooler. I’m running a 50/50 mix of filtered water and alcohol, but you may with to use more water to save a few bucks. Use too little alcohol and you risk freezing in the winter. Use too much and you'll be transporting an explosive cylinder that could pose a danger in the presence of heat or in an car wreck.

 I get a bit more knock than I’d like to see, but it’s livable. I also suspect with more careful tuning and messing with “when the system is activated”, along with how much fuel I add, it may reduce knock to nearly zero.  I’ll post what I find out. In any case, this system has made quite a difference in the amount of safe boost I can run on the street, and the price was fairly cheap.

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