[Trevor 57]

A "jake brake" is a term for a Jacobs Brake. It is a system that sits on top of the valves on top of the engine, it opens the exhaust valves on the compression stroke and stops fuel going in, so it effectively enables the piston to operate like a bike pump with your finger over the end. It is trying to force a lot of air through a small hole, the purpose of this is to slow the 'push up' of the piston, therein slowing the engine down through down pressure on the piston, conrod, crank, gearbox, wheels. (simply put). Hence the noise, if you picture the bike pump process, you can understand the noise from the truck engine, it is virtually the same noise, only bigger. LOL

There are a few variants of this type of technology, there is the "C-Brake" which is the Cummins Engine variant, there is a Mack version, which I can't remember the name of. The Europeans (Scania, Volvo) have their own version as well.

There is also an "Exhaust Brake", this is very common on Japanese and Euro trucks, it works by closing off a butterfly valve in the exhaust system, therein slowing down the evacution of exhaust gases, putting back pressure on the pistons, etc, etc, (see above). There is a perception that this type is not as effective as a "Jake Brake", in some cases it isn't, but it operates on a higher rev range is all, the blue area on a Volvo tacho is the 'exhaust brake' range, nornally this takes the engine past the engine governed revs by a couple of hundred revs, safe in the manufacturers eyes obviously.

Jake changing is by having the jake brake turned on, not use the clutch and changing gears, what this does is drop the engine revs VERY fast so that quick gear changes can be done, this practice has become redundant with the new 18 speed boxes in common use, these have close ratios. This practice can also cause some damage to engine and driveline if not done absolutely spot on.