[quote=spdrun]Depends on the engine. Some have throttle plates, some don’t.

Trucks have a way to alter valve timing(?) to provide a braking effect – that’s the loud burping noise you hear when a truck slows down on a steep hill.[/quote]Diesels generally don’t have throttle plates. The ‘burping’ you are referring to is from something called a Jake Brake or Jacobs Brake. It is and isn’t an altered valve timing. Often it is an additional valve that gets/engaged used when the brake is engaged (ie for Detroit Diesels). The fuel injectors on the diesel get ‘disabled’ at the same time. The brake releases the compressed air in the cylinder near the top of the stroke, generally right after TDC. It is after TDC to avoid crashing the valve into the top of the piston. Piston clearances are much tighter on the top than gas engines.

There are other types of exhaust restriction brakes for diesels that don’t have the Jacob’s Brake. You will sometimes find them on Diesel pickup trucks. NOTE: It is not recommended to use these on some Ford Navistar diesels because of insufficient exhaust valve spring pressure.