Elmer Racing is a company in Kirkkonummi, Finland that produces custom parts for racing. One of their most famous products is the billet 4.0 L inline-four called Thor. The 106 kg motor is capable of producing 1500 horsepower in circuit racing spec and 3000 horsepower in drag racing spec. Oskar from Elmer Racing released several videos explaining how they developed and built the monster engine.
Update 7-1-20: Added new videos
Source: Elmer Racing