Water Injection Pre-Turbo (Pumpless/Passive) – DIY Guide

December 5, 2025 Off By RICHARD

This is a brief guide detailing how you can setup pre-turbo water injection on your turbo engine for very little monetary outlay. More info about pre-turbo water injection can be found here.

The concept of this water injection system is that it is reliable and inherently safe. We will be using the boost pressure from the turbo to power the system so water can only be injected when the engine is under boost (with some precautions detailed below).

This system is not designed to make more power, it is designed to try and increase engine reliability for the same power by making the engine more knock resistant and allowing the turbo to work less for the same boost. Check link above for more info on pre-turbo water injection.

DISCLAIMER: Use this guide at your own risk.

Tricky Parts – DIY Water Injection Pre-Turbo

The two most difficult parts of this process will be tapping into the 25mm hose that feeds your BOV/DV. We need to be able to attach an 8mm pneumatic hose to this hose. And mounting the misting nozzle in the intake pipework in a way that is after the MAF and air filter, and in a way that does not leak air (air leakage shouldn’t be an issue if you are MAFless).

To setup the nozzle you will also need a high pressure air supply to test the water flow of the system at different pressures. Knowing water flow against boost is essential. Process is described in more detail below.

Parts Required

T-off adapters. 25mm OD main diameter with 8mm outside diameter T, for use with pneumatic push fitting. This is the most difficult part to find, I 3D printed mine. Fits between BOV and BOV feed hose.

8mm OD PU pipe for use with pneumatic fittings. 3 meters, from your local garden centre.

8mm silicone hose, 1 meter is plenty

4mm silicone hose, 50cm.

8mm push t-fitting x1

0.5/1.5/3 litre fizzy drinks bottle (empty!)

High pressure tap for bottle

8mm adapter for high pressure tap x2 (assemble the pieces)

Misting nozzle (I used external mixing flat)

Adapter to hold misting nozzle in turbo inlet pipe and which is air tight. Actually this is probably the most difficult part to find, again, I 3D printed a nozzle mount.

1/8 to 8mm push fitting adapters, to connect misting nozzle to your 8mm PU pipe.

Manual boost controller. This won’t have push connectors, slide PU pipe into silicone, attach silicone to controller. Secure with clips.

Water filter, like this but 8mm, this is 6mm.

Plumbing – In order

Tap into the 25mm hose going to your diverter valve with the T-Off adapter mentioned above. This is the pressurised air source.

Add a push fitting and then a length of PU hose.

To manual boost controller. Adjust this to it opens at 5PSI plus. Without this your water will get used up quickly.

Push fitting t-piece

One side of t-piece goes to misting nozzle air side.

Other side goes to high pressure tap on bottle, side entry. Use silicone hose, 4mm from high pressure fitting centre to bottom of bottle, water exits here.

Centre of high pressure tap goes to water filter and then to water side of misting nozzle.

And that is the plumbing

When your engine is under boost and above the boost threshold (you set with the boost controller), an atomised water mist will be injected pre-turbo.

Plumbing Path

BOV/divert valve inlet hose > boost controller > t-piece

T-piece route 1 > misting nozzle air side.

T-piece route 2 > fizzy drink bottle inlet (side) > bottle out > filter > misting nozzle “water” side.

Mount misting nozzle in intake.

IMPORTANT Limitations & Considerations BEFORE Trying.

There are some limitations to this approach

Compressor Blade Damage

The elephant in the room. There are posts on the internet that say pre-turbo water injection will destroy the compressor blades. I have also see videos of pre turbo water injection were the water is not atomised, except by the in-rushing air and compressor blades themselves.

I have also seen posts on the Aquamist forum (appears down now) from people saying pre turbo was fine.

In my experience of this misting nozzle, at the volumes we are injecting, it is almost impossible to see water droplets coming from the nozzle, it’s a fog, you only notice the water when it collects in the measuring container. And because the source of the water pressure and the air pressure is the same, I feel confident this fog is not doing damage to the compressor blades but I have not checked yet.

In summary, there is a potential risk and I don’t think there is a hard and fast answer, but I would say anything less than fog ie if you can see water droplets, then I think there is a greater chance of damage than injecting a fog. Although a fog may also erode blades over time. Test the system, see what the spray/fog is like, use your own judgement according to your situation and your budget.

If you don’t want the risk, don’t do pre-turbo water injection.

Adjusting Boost Threshold

It will probably take some off-the-car trial and error to have the boost controller opening at the pressure you want. It is important to have the threshold set to I would say, plus 5PSI so water is not used up too quickly. Set it higher if you wish.

High RPM

This approach does not work according to RPM, only boost. At 10PSI boost from example, the same amount of water will be injected at 3000RPM as at 6000RPM. VERY IMPORTANT TO TAKE THIS INTO CONSIDERATION.

Bottle Location/Type

Container needs to be able to work at the boost level your are running. In this case I have found that up to 21PSI, coke bottles are fine.

We need to be able to monitor bottle level easily and it needs to be easy to refill the bottle. This can make finding a location challenging. I suppose trunk mounting is ideal but more water line needed and more associated installation complications. May also introduce delay.. Personally I have bottle mounted in engine bay.

Syphoning of Water

I guess there is also a slight possibility of water being syphoned from bottle.

Bottle ****must**** be mounted lower than misting nozzle, I cannot stress that enough.

I have it setup like this and no issue but it may not work for you.

Nozzle Adjustment

Again, some off car testing will be required. Nozzle is adjustable with regards to water output vs pressure. Test nozzle at different pressures and adjust nozzle until you get the flow you want at the pressure you want.

Pressure Relief

It is essential that pressure is bled off through air side of nozzle as well as water side. We do not want the water bottle depressurizing ie putting water into intake manifold out of boost. By connecting air side of nozzle to boost T, pressure should be relieved through water and air lines equally and as quickly as possible. Correct routing of lines is essential.

Tuning

Because water flow is not connected to RPM, under no circumstances should the engine be remapped for use with water, using this method. This setup is solely to improve engine reliability by reducing intake temps, reducing load on turbo and reducing the chance of knock. Engine must run correctly before installing and no changes should be made to take into consideration this water injection system.

Adjusting Nozzle and “Boost” Controller – Our Experience

It did take a good amount of setting up to arrive at our current settings. Boost controller that controls pressure into the water injection system had to be adjusted to open at 5 PSI. We used an air compressor and boost gauge to get it set up.

Also the misting nozzle adjustment took a lot of trial and error. Ultimately we had to setup the system off the car and use an air compressor to power system at different pressures. We measured water flow out of nozzle at different pressures to find the max amount of opening we could have at low RPM. For example at 3000RPM engine makes 10PSI. We worked out how much air the engine was consuming at this condition, from there worked out the fuel flow, and then made water nozzle output around 20% of the fuel flow amount. To repeat, the fuel/water percentage will increase in favour of fuel as RPM increases because water output is fixed according to boost only.

Set max fluid flow to be compatible at the lowest air flow when boost controller is open.

For example, if you set controller so it opens at 10PSI, find the lowest RPM your engine can generate 10PSI. Then calculate the airflow at this RPM, then calculate the fuel flow at this RPM.

I’d say replacing 20% of fuel with water is a good place to start. 20% of the fuel flow equals the amount of water we want to inject. Adjust nozzle to flow that amount of water at 10PSI. And again, remember its a 80:20 mix at most, as RPM increases water volume will stay fixed as fuel increase. Perhaps at 5000RPM the mix will be 90% fuel and 10% water. I know I’m labouring the point here but it’s important.

Our Results

This setup has been used on a VAG 1.8T big turbo engine (300BHP approx). Without this water setup, the engine could at times, under high load, pull 9.5 degrees of timing. With the water injection, the max ignition retard the ECU commanded fell to a max of 4.5 degrees. We use a mixture of distilled water and windscreen washer fluid in a ratio of around 10:1. With spirited driving, 500ml of water will last around 20 minutes.