Building a motor doesn't have to be an exercise in constraint. If you have the raw materials, the time and the equipment, you can construct an internal-combustion dream machine with any amount of power. One engine builder in Michigan shot for the moon for a client's one-off supercar, and the result is an output in excess of 3,000 horsepower.
Steve Morris Engines built the mother of all engines almost entirely in-house. It's a 16-cylinder engine with four turbochargers, and it already has a body waiting for it. The motor's supposed to be installed in the Devel Sixteen, a car first displayed at the 2013 Dubai Motor Show. It promised 5,000 horsepower, but that's the sort of claim that requires evidence to back it up.
That evidence is in the video below. After being built, Steve Morris Engines attached its creation to an engine dynamometer, which measures tractive effort and calculates the resulting horsepower and torque figures. With a "conservative" tune using pump gas, the engine puts out more than 3,000 horsepower, and the company believes that race gas can bump that figure up to 4,500 or so.
If you want a couple thousand horsepower, you can slap together a motor using plenty of off-the-shelf components. However, custom engine development like this is a ridiculously expensive enterprise that requires both a madman behind the CNC mill and a madman with the money to pay for it all. You've got to respect the fact that this even exists in the first place, especially as electrification and alternative fuels continue to gain traction with your average consumer.
Here's to the crazy ones.