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Originally Posted by some dude on a caddy board with over 10K posts who sounds like he knows what he's talking about
There are other things done with the idle speed, fueling, spark advance, etc. in concert with the cylinder cut out but the main function that makes the limp home system work is the cylinder cut out. Remember, the valves are still operating so the cylinder still pumps air thru it...making it an internally air cooled engine when in limp home mode. This is often confused with the DOD (displacement on demand) system upcoming on GM engines and the older V-8-6-4 system on the 81 Cadillacs. They are not the same as the Northstar limp home mode. In DOD the valves are actually disabled so that they stay closed to disable the cylinder. Just turning off the fuel to disable a cylinder causes huge pumping losses that are immediately evident trying to drive the car. With only 4 cylinders running and 4 dead but still pumping the engine has the net output of about 1.5 cylinders as much of the power goes into the pumping loss of the dead 4 cylinders. In DOD, the gases trapped in the cylinder act as an air spring returning the power used to compress them to he piston on the down stroke.
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Found this on a caddy board. Sounds like a real bad idea.