[ijames]

No matter what brand of vehicle, I've always been told that you should allow at least 30 seconds of very light throttle or idle after making any boost, to allow the turbo to spin down and cool off. I've always tried to do that and so far have never lost a turbo seal, knock on wood (25 years of spirited driving and drag racing a Buick TR and 6 years so far with a turbo Duramax diesel pickup).

Also, a spark ignited gasoline engine has maximum pumping losses when the throttle is closed. It isn't that the engine compression is slowing the car down, it is the pistons being forced down with the intake valve open and the manifold at maximum vacuum, so the vacuum is pulling upwards on the pistons. Any air that is ingested will be compressed as the piston goes up on the compression stroke, but then on the expansion stroke the air will be re-expanded and minus friction losses the work down on compression is recovered, at least until the exhaust valve opens and you lose the rest of the stored energy. Diesels do not have throttle plates (in general, I know there are a few but ignore them so we can keep this simple :-)) which is why they are so efficient at idle and light throttle; they don't have the pumping losses of the spark ignited gas engines. That's also why when you lift off the accelerator on a diesel you don't get nearly as much engine braking. Diesels move the same amount of air at a given rpm no matter how much fuel is being injected, at least without a turbocharger. The power level is controlled by how much fuel is injected. That is one of the advantages of the direct injected gas engine, which is why manufacturers are working so hard to develop it; it lets them do away with the throttle and inject the fuel at the last possible moment to effectively raise the octane so they can raise the compression ratio to raised the efficiency. You could test this with a fuel injected engine and a kill switch, just do a short coast down test in gear with engine off and throttle closed, and then repeat with throttle wide open. Yes, you will be injecting fuel at wot but if you only do this once each way it shouldn't hurt any thing, and you should coast a good bit further with the pedal to the floor which will demonstrate how great the pumping losses are at idle. Just don't turn the kill switch back on with the pedal still on the floor :-).