When to activate Water injection?

[Philip]

Hi all,

Ive just about finished installing a Paxton supercharger onto my rover powered cobra replica. Im going to be boosting it by around 6 psi and want to use water injection purely as a precaution. I dont think i'll need it but better safe than sorry. My question is that im going to reprogram my ecu to control the switch on point of the water. As the ecu uses manifold air temp, revs, manifold pressure etc, i can use these to form a switch on point. So what sort of temp and boost level should i be looking for the water to come on at? My comp ratio is 9.75, its a 3.5L V8 if thats any help,
Phil

[Forum Admin]

I can not provide you with an answer on the specific temperature but will point to some issues to consider as you implement.

Where is the sensor taking the temperature from? This may seem obvious but to be helpful it needs to be after compression. It probably is, but just want to be clear for others who come accross this since some sensors are taking pre-compressed temps.

If you base it on temperatures before the jet - you may not know if you are using too little or too much water for your intent.

For purely an intercooler function rather than utilizing all the benefits of water injection, you don't want to inject too much water because if you are leaving your fuel settings rich so that the engine would run safely even without water injection you would likely lose power with water and rich fuel settings.

I think one application that would be very interesting if your ECU can handle this would be to have the ECU not only control the on/off function of the water injection but also the flow of water. This would require the jet to be before the temperature sensor - the higher the temperature the more water is injected - the lower the temperature the less water is injected. Will your ECU provide a pulse width signal from a map based on intake temperature as the load?

[Philip]

Hi,
thanks for that,
yes the sensor is after both where the water is going in and after the blower. (In the plenum) I havent got the room for an intercooler so i was hoping to use water for this purpose. I can reprogram the ecu to do what ever i want, im currently making it turn water on at 4psi boost as long as revs are over 4200 or manifold air temp is above 50degrees C. I havent got it running yet so these are all guessed figures. I could do as you say and start it at 50% duty cycle and work up to 100% as temp increases. Wouldnt be too difficult, just means a bit of programing and a driver pcb making up. DO these figures look ok to you or are they too low? Ive read that water shouldnt be turned on below 6psi, but i might not reach that level of boost,
Phil

[Forum Admin]

For this intercooling purpose if you have a valve in the WI system that can be driven as you describe based on temperature then you will be sure just to inject enough water to maintain the desired intake temperature. That would give you good reference for intake temperatures when programming your fuel without worrying about too much water. The temperature you are seeking is up to you really and may be a matter or trial and error. Also the exact IDCs and flow rates will take some trial and error. Without mapped water injection you may be injecting going well above and well below your target temperature rapidly and the on/off cycling of the WI may in that case cause inconsistent engine performance if it is more water than needed. I hope I have explained this in a way that you understand what I mean.

There is not solid rule on what boost level to inject on. The 6 psi reference may be for turbos and that level would be for a couple reasons. Turbo engines frequently have lower compression ratios and 6 psi would be a good measure to prevent running the WI during relatively low loads when it may not be necessary. Some GTP users for instance do not run much more than 6 psi of boost and are using fixed pressure switches that engage WI at 3 psi.

[Philip]

Thanks, I'll have to play with it when i get it running and see whats best for my application,
Phil

[M3Turboca]

I would say around 3 to 4 psi if you get no detonation and your air/fuel is fine then 3 to 4 psi is a good number.

What is your redline on your motor and at what rpm do you build max torque. Where do you build 3 to 4 psi at what rpm.

To trigger it by duty cycle will is nice but not necessary for the setup you have.

What water injection setup are you using if aquamist what size nozzle ?

[Philip]

Hi,
thanks for that,
my engine produces max torque at 4250 rpm and im limiting max to 5700rpm. Its a centrifugal blower so max psi will be at max rpm. Im going to use a 0.4mm aquamist nozzle but im using an adjustable pressure water pump ( 40-120 psi) and will adjust to get around 10% of the fuel usage at max rpm. My guess is the engine will use 16 gph at max rpm so i will adjust pressure for 1.6 gph from the nozzle. Does all this sound OK?
I haven't got the engine running with the blower yet, but as water screws up the readings from my Wideband Lambda sensor iv got to turn the correction off in the ecu when the water is turned on. So i might as well get the ecu to turn the water on. If Im doing that i might as well have a good control for the switch on i.e. Manifold air temp, Revs, Boost.
Phil
www.replica-cobra.co.uk

[Brad]

Philip
As your engine is using a Paxton supercharger boost is about the only trigger you need for activation. Boost will come on consistant with rpm. If you are only going to use 6psi turn water flow on at 2psi increase water flow propotional to fuel.
On a 3.5 liter engine with 6psi boost the normal jet size is 0.7 to 0.8 when increaseing boost to 10-11psi you may have to use two 0.8's and enough flow to keep up. You should alway make sure your system can put too much in. If it can not make the car stumble there is not enough water volume for proper tuning. All pump lose flow with time so make sure it is over built.
One of the great advantages of water injection is you can get very aggresive with the timing at 6psi. You should not have to retard but one or two degrees from stock if things are tuned correctly
One method of tuning is to not use water. get your fuel map and timing correct for the setup, then add water and increase timing.
Increasing timing will show a slight increase in O2 reading.

[Philip]

Thanks for that,
Ive redesigned the switching points to come on with boost (3psi) and Manifold air temp, probably dont need temp bit but its done now. Ive also added another switch point for around 5 psi. My idea is to start the pump with around 10-15% water to fuel at 3psi by switching the pump on with a lower voltage then increase it at 5 psi to get 10-15% water to fuel at that pressure,
Phil

[Brad]

Philip that is a good way to ccontrol flow. I have made many stepped over the years. They left me with uneven performance. I now use the Auamist high speed valve tied into fuel injection duty cycle. It gives a smooth progression in flow based on engine load. Fuel flow is based on engine load at every rpm. This has proven to be the easiest and smooth operation. We started tieing the high speed valve into injector duty cycle about 1999 have has been involed in over a 100 installs and upgrades.
The biggest thing is to use the correct amount for your engine, do not get too hung up on the math. Work with engine performance.