the fiesta was already an efficient platform to build on. as i recall those things got like 35mpg around town, and that is running a compression ratio of around 9:1. this guy has pumped his compression ratio up to 12:1, and then credits the water injection for improving fuel economy. well i can tell you it isnt the water injection, but the higher compression ratio is where the added fuel efficiency if coming from. the water injection allows him to run the higher compression ratio.

he also stated in the article that water injection was used to get more thrust from WW ll engines, that is not quite true. again the water injection allowed the use of higher boost pressures because it prevented detonation, thus allowing more boost so the engine could make more power.

this is a systems approach here, and everything needs to work together.

by the way, even modern jet engines use water injection, but turbines can use it for more thrust since they operate on volume. the harrier uses water injection to cool the engine in the hover mode to prevent overheating, and even then you get 90 seconds.