@MrHewes/YouTubeMany car and truck enthusiasts understand the impressive capability of some diesel engines, as the amount of horsepower alone can sometimes be very surprising. A great example of this is the massive Mirrlees JS8 inline-eight diesel. One of these engines with 700 hp and a 102.8-liter displacement was actually sitting inactive for years, but thanks to the Mr. Hewes team on YouTube, this engine was recently brought back to life.The 73-year-old engine was in rough shape, but had reportedly only accumulated about 720 hours of operation. When the team began digging into the engine, they found a significant amount of water in the sump, while the valves, cam followers, and injector pumps were stuck. The team had to free those components while also manually priming the oil system. Even the engine's decompression system was stuck, so they had to work on that before attempting to air-start it. After freeing the mechanism and working through the cylinders, they finally managed to turn the engine over.But the diesel fuel system still needed some work, so the team had to bleed the individual injector pumps and tackle the components that were still sticking. Eventually, they got the engine to fire up and run on six cylinders, which was two less than the Mirrlees had. The problem was two stubborn fuel injectors, but once they were freed, all eight injector pumps began moving. The engine could finally run on all eight cylinders, reaching nearly 800 rpm.The history behind the massive Mirrlees diesel@TANISLA2/YouTubeThe 1953 Mirrlees JS8 engine that was fired up on YouTube has an interesting history. The problem, however, is that the team behind its resurrection doesn't have all the exact details. What's certain is that this somewhat obscure diesel engine was originally ordered by the Air Ministry and delivered in 1953. It appears to be an auxiliary power unit (APU) and may have been used as a backup generator at an RAF base in Scotland. Additionally, the engine has an air-start system instead of a conventional starter, making it even more unusual.The JS8 engine comes from Mirrlees' J series, which the company began developing in 1947. It was designed for use in a variety of demanding applications, including marine equipment, locomotives, oil fields, and power-generation systems. The eight-cylinder version had a 9.75-inch bore and a 10.5-inch stroke, resulting in a total displacement of 102.4 liters. The J series covered a wide range of configurations and power outputs, from 135 to 2,480 hp and operating speeds from 600 to 1,000 rpm.Mirrlees didn't stop with the J series, as the company eventually developed even larger engines for both power generation and marine use. For example, the K Major was offered with power outputs reaching 13,140 hp. However, the company's most powerful engine was the MB430, which could produce 1,000 hp per cylinder, making the biggest version capable of producing 16,000 hp.