I might have the loudest jaking non straight piped kenworth on earth in my driveway, these guys are all correct, a jake is called a jack because most of your engine brakes are made by jacobs...I hardly have ever seen a jake on a cat, cummins, or detroit that wasnt made by jacobs...all three of our trucks have them, and all three are jacobs...I would say most of your bigger trucks, by the major manufactures are gonna have engine, not exhaust brakes....they are really a neat deal and I am a jakin' bandit....use it on the way up to make shifts quicker, and on the way down because if i was to ever touch the brakes I think my dad would castrate me