[radiantthought]

Found the source I was looking for.

The main technical reason for not using an off-the-shelf engine is that the Elio vehicle is much lighter than a standard vehicle. Meaning, the off-the-shelf OEM engine was designed to optimally operate at a different speed and load point than if it were placed into a lighter vehicle. Also, when a large carmaker designs a new engine, it is targeting multiple platforms. That often makes the engine good for all, but best for none. By having IAV design a power plant specifically for our vehicle, we can achieve our goal of 84 mpg. This is going to be a very slick engine.