I bought this Spearco Water Injection kit in the mid-90s with plans of installing it on my RX-7. However plans fell through and I never got around to installing it, so it has sat in a box for about 15 years until I just discovered it while unpacking a couple weeks ago. The kit is missing a few odd screws and vacuum lines but the major components are there: .4 gallon resevoir tank, electric water pump motor, boost-activated adjustable valve.
What does water-injection do for your car? Well, for turbo/supercharged or high compression motors, it adds a "buffer" to the combustion event which can reduce or eliminate detonation and pre-ignition. It allows you to run full timing and boost without fear of breaking something.
This is the old style hand-welded aluminum version before they went to cheap plastic. Very durable and hard to find. New Spearco water injection kits are $400-500, but I'm looking for $160 in BTC for this one, shipped within the US.