Ken, I'm very sorry to hear about your car troubles, bad news <img src="/forum/images/graemlins/frown.gif" alt="" />, I hope you get it sorted.
In answer to your questions I lifted this information from the Aquamist web-site FAQs when I first had mine fitted:
WI In Winter
It is true that water droplet size wil be bigger in winter than in Summer. Regardless of the temperature, air coming out of the turbo will still be hotter than ambient, water injection should still be used, but on higher boost. Don't forget you will have the following conditions:
1) Colder ambient air produces more power so the combustion temperature will be overall higher. Detonation is still a threat.
2) Colder temperatures also means drier air, so switching off the water injection may product greater chance of detonation due to less water content in the induced air.
3) Water injection might not be as useful on induction cooling but the main cooling will be shifted towards in-cylinder cooling.
So ERL Aquamist recommend it for summer or winter use, however they are trying to sell more systems <img src="/forum/images/graemlins/grin.gif" alt="" />
The jet size would depend upon the system you are running; I have the System 2d and I use a single 1.0 mm jet, this is due to the accurate metered water injection offered by the 2d system, synchronised and controlled by the fuel injection pulses (and duty cycle).
Aquamist provide a method for calculating jet size dependent upon the amount of fuel injected.
The following are the calculations taken from Aquamist data and I adapted them for the 20VT:
For Water Injection
Aquamist recommend between 10% and 25% of fuel flow rate.
@80% injector duty cycle = 1,341.2 cc/min fuel is injected
10% = 134.12 cc/min to 25% = 335.30 cc/min
@95% = 1,592.68 cc/min
10% = 159.27 cc/min to 25% = 398.17 cc/min
@100% = cc/min
10% = 167.65 cc/min to 25% = 419.125 cc/min
0.5mm water jet gives 180cc/m @5bar
0.6mm water jet gives 225cc/m @5bar
0.7mm water jet gives 260cc/m @5bar
So from the above a suitable jet can be selected depending upon the injector duty cycle.