HotRod (Larry) states the following:
As mentioned above the tuning for WI is not a single pass process but rather an iterative multi step process.
With that said, I would tune for best power without WI, and find your MBT timing under that condition. This should change very little as you add WI.
Find your best/latest safe turn on boost point.
I agree it's iterative as is most tuning. One question for me is: should the iteration be adjustment of the amount of water or the fuel. This brings me to the question of how much water to inject in general. There are all sorts of opinions in this area, most of which are stated with conviction.
How much water to inject
From what I've read less water is needed at low RPM under load, and the need for water increases with RPM. The same is true with boost, as boost increases (and consequently heat, abdiatic heat) so does the need for water. Makes sense.
Richard Lamb has an interesting graph of water requirement that can be seen in the Forced Induction thread: "Driving Aquamist 2c high speed valve with stand alone EMS."
Water is injected based on boost, RPM. I assume, as I think other's did in that thread that these parameters are fairly universal (hence the MF2). How the proportions on this graph are interpreted is important, but once a maximum rate is determined, ther remaining values are just relative to the maximum. The maximum injection rate is typically 10-25% of the fuel injection amount.
To begin tuning, I think a 15-20% maximum would be a good starting point. More can be injected if maximum brake torque (MBT) cannot be achieved with ignition timing.
Tuning for power with or without WI, will be targeted toward MBT. For a given engine, as Larry states, the MBT will be fairly consistent (cluster) in terms of ignition advance. An engine's MBT will differ according to its stroke length. MBT occurs near or at a point of greatest leverage on the crank when cylinder pressure does the most work. The speed with which the fuel burns will have impact on when ignition spark advance reaches maximum torque. I agree the MBT can and should be determined with WI off. With WI on, the ignition advance may be further advanced due a slower burning of the fuel.
Water Injection Point
The consensus would seem to be turn the water on as late as possible, which is sometime after boost onset, and prior to the onset of knock. This point will have to be determined empirically.
Knock onset, would be measured with ignition advance in the MBT range. In other words, pulling advance to suppress knock is counterproductive when tuning for power.
Air to Fuel Ratio
In Ed's paper (Charged Performance) he has a great graph that indicates that best fuel economy occurs an AFR or 14.5:1 and best power occurs at 12.5:1. Leaning or richening the mix above or below 12.5 leads to less power.
Please chime in on this. I've never seen someone lean out to 12.5 for maximum power. While maximum power per unit fuel may occur at 12.5:1, the maximum engine HP power may occur with a richer mixture of say 11.5 or 11:1.
O2 sensors
I have only heard that WI fouls up O2 sensor readings. By how much, I do not know. EGT has been held up as a means of tuning when O2 sensor readings are not reliably known. One way that I have heard of this working is to add WI, and let EGTs fall. Fuel is then subtracted, and the EGT's rise. Fuel is removed until EGTs reach their pre-WI level.
In reality, I imagine the O2 sensor readings are not very far off. Water is a byproduct of combustion, but not to the levels normally seen with WI. Partial pressure of water may or may not interfere with O2 detection.
Trimming WI
While starting with 15%-20% maximum maybe enough to get a good tune. Thinning the water out preserves your supply and extra water may lead to less power (bogging). I don't know how to determine when "enough" water has been injected, especially when I'm injecting post-IC and just prior to the throttle body--and no sensor, but power travels with the water past that point.
As far as I know there is a wide range of water that will produce good or best power, so trimming water back may not be a big concern.
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