The first time I learned you can increase, sometimes significantly, the performance of a car by upgrading the oil cooler blew my mind a little bit, but of course in hindsight it makes perfect sense.

WW2 planes engines

I'm aware of some auto engine conversions - the 13Bs out of the RX-7s and related could do it, the older Subaru engines (EA81) would do it, but they're just a small aircraft engine in a car, design-wise...

Looking over the Viking engines, I don't think it's fair to call those "automotive engines," because my read of the description of what they do means they're pulling it apart and replacing an awful lot of pieces. They claim they're "based on" the latest Honda engines, and once you're starting to change the cylinder bore offset, custom hone the cylinders, replace your pistons and conrods, etc, it's no longer a car engine.

If changing those minor attributes makes an engine "not a car engine" then the majority of race cars also don't have car engines.

On their About page, they themselves say:

> Companies like Honda and Mitsubishi have spent millions of dollars refining and testing. We don’t claim that we can outsmart either company when it comes to the design of any internal running component of these engines. We don't change vital engine parameters around and claim we have different models. The compression, etc. is all stock. Every part is OEM Honda / Mitsubishi, nothing is new aftermarket from unknown origins.

> We start with the most up to date year engines with super low mileage, derived from fender benders across the US. When customers want the information about where there engine core came from and how many miles it had - we have all that documentation readily available to show.

Literally, every one of these engines came out of a car. I think that plainly qualifies it as a car engine.

those mods sound very similar to typical mods found in drag race cars.

100% duty cycle doesn't really exists as stand alone concept. It is always in reference to the application. A typical car engine is at 100% duty cycle at around 30 horsepower, but the engine can produce 200+ horsepower for short bursts (enough to do the 0-60 test that everyone loves to quote). With more cooling you can run more power from the same engine, but the additional stress will mean the engine doesn't last as long before it breaks.

Marine applications generally means salt water, so they will run at higher power is the salt is what gets the engine eventually not the stress of high power output. Airplane engines are expected to be reliable and so they run at relatively lower power output. There are many small niches in between that have their own idea of what 100% duty cycle is.

I agree, it can mean different things. To be more specific, the video I linked says Kia tests at full load, WOT, and maximum engine speed for 300 hours continuously. Then they run it 10-20% faster than the specified maximum engine speed for another 10-20 hours.

This exceeds most definitions of "100% duty cycle".