Exhaust brakes have become common place in the stock diesel marketplace. For older trucks with non variable geometry turbos one needs to be installed in the exhaust pipe. Most people understand their need but not clearly. A diesel engine does not have a natural compression braking effect that a gasoline engine has. Meaning it has no hold back when you let your foot off the 'go' pedal. I must note that an exhaust brake found in the light duty truck market is not the same as an engine brake found on heavy trucks. The base idea is the same but the way they achieve the effect is very different. Don't install an exhaust brake in your pick up and expect it to sound like a Peterbilt coming down Donner Summit.
To create compression braking a restriction is placed in the exhaust pipe (or via the turbo in a VGT set up). This gate in the pipe creates back pressure so on the exhaust stroke of the motor. That back pressure pushes through the exhaust manifold and into the cylinder and creates resistance on the piston as it pushes up on the exhaust stroke slowing the engine's rotational force. That back pressure is also pushing against the valve train, your valve springs now have added resistance. Those springs are designed with a certain amount of force in mind. Without properly adjusting the brake pressure to below what that spring is rated to and or changing the valve spring to stronger spring, that pressure can cause damage by forcing open a valve at the wrong time and that valve would contact the piston. For maximum braking effect the PSI of back pressure needs to be near the maximum safe pressure for the engine, 40 PSI for Powerstrokes, 55 for Duramax and 60 for Cummins (on some Cummins applications valve spring replacement is mandatory). The way the system works in a VGT (variable geometry turbo) is a set of vanes inside the turbo adjust to create the back pressure. The idea remains the same but the process is a little fancier.
An exhaust brake is a great addition to any diesel truck and is an absolute must if you tow or haul heavy loads. It is also a fine example of old technology being polished and refined in the new truck market.
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