Water Injection Question

If so, everyone would be doing it.

The water injection itself will not change mpg. However, an increased compression ratio will increase efficiency and mpg, but at expense of higher octane demand. Water injection raises effective octane of fuel. But to take advantage of it, you'd need to increase CR. That is not a trivial job. Further, if you live in climate where it goes below freezing in winter, you must use a water/alcohol or water/glycol mixture, and the cost of the antifreeze burned may be more than the fuel you save.

As others have already posted, water injection has some advantages, but not normally fuel efficiency. It can improve octane response. It can help decarbonize an engine, possibly.

In the case of tarlike fuel oils, it might even improve efficiency. But not in your gasoline engine.

Half true. Just adding water by itself will not increase your MPG. What you might be able to do is increase the engine timing a couple of degrees. That will increase your MPG, but for many cars it will also increase the chance of pre-ignition( knock ). By injecting water, you cool the intake and lessen the chance of pre-ignition.

-------------- Alex

Not likely on a 98.

I have been trying to remember why the popularity of water mist injection back in the late

You may remember the pollution control devices of those times were terrible...the engines ran on after you turned them off, they pinged badly under stress, etc. And there was little you could do (legally) to make them run right. Water injection helped some of those beasts and it was legal.

(I completely desmogged mine, saved the pieces, and just reinstalled them before every inspection.)