Ford's 4-cylinder EcoBoost engine remains one of the most underrated engines out there. Ford knows it, and tuning house Mountune knows it. Mountune is a UK-based company that maintains a close relationship with Ford as a "tuning partner." We've featured them recently for the work they've been doing building kits to boost Focus RS engines up to and over 400 horsepower.

Mountune also has a US-based workshop, which has just released a video stretching the Focus RS's 2.3-liter 4-cylinder EcoBoost to a staggering 684 horsepower.

The video is short in length as well as details, but we see the peak horsepower comes at 6,900 rpm. That 684 horsepower is a staggering 334 ponies more than the stock output, and just as jaw-dropping is the torque that's developed. It made 566 lb-ft of torque at just over 5,500 rpm in the test, which should have Mountune deluged with emails from Focus RS owners wanting to know when the kit is out.

However, we don't know exactly what this project is for. It could be Mountune finding out what the 2.3 EcoBoost is capable of when strung out to breaking point by its engineers. Or, it could be destined for bigger things.

It's also possible that Mountune is working on a crate engine as Ford is planning on using mild-hybrid tech rather than traditional tuning on the next-generation Focus RS. The chances of the next Focus RS coming to the US is incredibly slim but what we do have here is the Ford Mustang Ecoboost, and Ford has recently introduced a High Performance Pack using a modified version of the European Focus RS's 2.3-liter EcoBoost engine. If Mountune is eyeing the Mustang for big horsepower from a small and lightweight engine, the world could get very interesting very soon.