How does a Jake brake work?
 

Does it work by relieving pressure off the engine or by restricting the exhaust?

the Jake on the 5.9 Cummins work by restricting the exh flow right after the turbo . there is a butterfly valve that turn and restrict the flow of exhaust .

An exhaust brake works by restricting the exhaust flow after the turbo. A true Jake brake modifies the timing on the exhaust valves so that, when braking is desired, the exhaust valves open right as the piston reaches the top of the compression stroke. The energy gathered in the compressed air is released, so the compression stroke actually provides braking power. The most times the engine is built with a Jake brake, an exhaust brake can be added on.
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infidel,

You are dead on except for the last comment. Most jake brakes are added. Infact, I believe that this was the case exclusively up untill the Signature 600 came out which incoperated the Jake brake into the actual casting of the Cylinder head juring production of the motor. If you need an example, the Cummins C-brake on the N14, I believe it is, or maybe the N11 would be a perfect one. If I remeber right the C-brake just simply bolted onto the top of the Cyclinder head, sandwitched between it and the valve covers.

On the ISX which is also the Signature 600 , the engine has two camshafts , one being the injector cam and the other being the valve camshaft. on the valve cam there is a engine brake lobe just for the brake lever between the intake and exhaust lobe on every cylinder, this is one very effective engine brake that can be activated only on one cylinder or as many as you want and is not a add on, bobtailing you rarely have to use the brakes except to make a full stop with the engine brakes on. On the N14 , the Jakes or C brakes goes inbetween the rocker housing and the valve cover, it is a add on . you can also get Jakes now on the L motor which is a ISC on steroids as an add on and is also a compression brake like the big boys , as I stated earlier the 5.9 ISB only has a exh brake available which work by restricting the exh flow.

It would be very unusual to see a real jake (of the valve floating variety) on an engine smaller than 10 liters displacement. Our exhaust brakes get refered to as jake brakes becuase they are offered for our engine by the same company that made the valve floating jake brake famous. (Jacobs Manufacturing)