I don't have an MR2... but am thinking of taking the plunge for a weekend strip/track car. Am running an Aquamist 2d system on my WRX. Not cheap, but comes with many safety features. Other kits range from the SMC alky kits to more DYI kits such as the packages offered by coolingmist.com. There are directions in the modification section of projectwrx.com for a DYI approach. Basically these kits consiste of a high pressure (100+ psi) pump, a check valve or solenoid, a pressure switch, an injector, hoses and a water supply tank.
I am running with a blend of water/methanol. Generally these systems -- when tuned for it -- can add 20+ awhp. Not sure what the rwhp add would be. The trick is the ECU (or standalone, etc.) must be tuned for it. There are several different tuning philosophies for tuning with water/water-alky/pure alky, which basically fall into these two camps:
a) lots of boost, lean out the mixture, don't advance timing much
b) advance timing, lean out mixture
More people seem to use a) approach than b) at least in the subie community but I'm no expert. If you just slap on a WI/alky kit without tuning, you'll actually lose noticeable amounts of power b/c you will be running way too rich. With the injection of water and/or alcohol, you will cool down your cylinders and should be able to lean out or AFRs. I'm running the WRX between 11.5 and 12.0 AFR, and some of my colleagues say I could run it as lean as 12.5 with the WI. My EGTs peak out a bit about 1400 F (measured at the hotest of the 4 cylinders at the exhaust manifold), whereas before water injection I could see 1525 or higher. I'm probably leaving some power on the table, and this summer may try leaning it out more.
I'd recommend some sort of safety mechanism (ranging from low water supply warning switch/light, to pump activation lights, pump failure light, to flow meters, to boost cutoff switches triggered automatically, etc. depending on time / $$$) because if you loose your pump at WOT or your water supply is interrupted... you'll suddenly be running VERY lean and... in the words of engine builder Q. Crawford, WI is great for horsepower and new engine sales.
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