Years ago, I read about a NJ firm, that was going to mass-produce car engine blocks out of a high-tmperature plastic compound. The idea was to save a lot of weight, and also, the blosks would nedd to machhining-they could be injected molded. Plus, they would be extremely cheap to produce-about 20% of the cost of an aluminum block.
Well, needless to say, it didn’t pan out. I think the company name was “polymotor”-anyone know more?
Apparently they didn’t work out.
Unfortunately, that Wikipedia article says nothing as to why plastic engines were considered to be a failure. 
The R & D money was required someplace else. Seems like there was potential, but there is a long ‘return on investment’ and with all the pressures in the industry, they were spending millions (I would guess) to drop 150 lbs from the vehicle?