Quote Originally Posted by Bob Parmenter View Post
I've got that on my '09 Avalanche. It was standard equip on the 5.3 LS engine. Basically the computer shuts off the spark and injectors to 4 of the cylinders (no 6 cylinder step). It's virtually seamless when it happens. If I choose the computer readout on the dash I can watch when it switches mode, but other than that there's no feel to it. On my rig you pretty much need to be on dead flat terrain, or downhill, for it to be active. Throttle position is probably part of the data input the computer considers, but there are likely more data points that contribute to the calculations. I wouldn't imagine that fouling the plugs would be a consideration as there's no fuel to be burned when the cylinder is shut off.
The more complex computer controls coupled with direct port injection being much more common today make this a lot more manageable animal than it was in the '80's and '90's. The mechanical approach before was much more prone to problems. One thing that was a surprise to me was that by keeping the valves closed the trapped gasses in the cylinder are alternately compressed and expanded with the cylinder action, but the forces balance out with no work being added to the cycle. That said, it seems to me that an aftermarket application, with all of the needed inputs and logic, would be difficult to install and optimize on an older, non-computer controlled driveline.