John
I believe the original JAKE brake was more than just an exhaust brake. The newer exhaust brakes are much quieter. They usually install between the maifold and the exhaust pipe. They restrict but do not completely close off the exhaust. As I recall, the duramax allows about a 65% restriction. The spec for the Cummins is probably very similar.
The tow / haul mode built into most of the new trucks with automatics depend on the natural restriction of the exhast for braking. This braking comes from the back pressure created by the piping, muffler and, if you have one, the catalitic convertor.
Banks and several other aftermarket companies make these devices and several have the installation manual on the web. The Banks appears to be one of the better devices but, as Illapah said, it is more than just bolting in the valve itself. The banks unit monitors some of the information coming from the engine computer to make better decisions about when to close.
Pricing appears to run anywhere from 700 to 1200 dependig on how many features the device has.
Dave
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