Okay, this is for people familiar with carburetors, because I grew up in an age of fuel injection. I believe a few friends of mine picked up a habit not needed.
Before shutting off their vehicles, they will rev them up and just as it's coming back down shut the key off.
I'm not certain which is why I'm posting, but wasn't that more for carburated engines? Something about getting a little extra fuel in so it starts easier or something?
Seems it wouldn't do anything on a fuel injected engine.
It is of no benefit but it might put extra gas fumes into the manifold. They should be flooring the gas pedal (once before starting it) to close the choke (engine cold). I've tried the other method and saw only needless wear on the engine and wasted fuel.
If I remember it has something to do with when the engines were hand-crank started. If there was gas in there and it was compressed it would make it easier for the engine to start or something along those lines. Then along came electric starters and people still did that though they didn't need to and it's just kinda stayed.
Okay, just for the record. There are some engines where it is (or was) advisable to let the engine sit at idle for a few minutes before shutting them down. You did this to scavage the excess oil out of the engine (dry sump) so you had oil when you restarted the engine. But these were jet engines mounted in aircraft, not piston engines with a wet sump system. I think that everyone agrees that your friends have bought into an old myth that may have once had merit, but now serves no real purpose.
The reason for doing this is that if the timing is advanced a might too much, the car will diesel (or run on) after you turn the key off. It is caused by carbon deposits that get too hot and cause the car to keep sputtering for a few minutes after the key is turned off. The way to stop that is to give the car an extra shot of gas, which would either burn off the deposits, or flood them so that they cooled down enough that they would not ignite the gas. This was such a common problem with cars that had the rudimentary emission controls of the 70's and early 80's that manufacturers installed systems on the carbs to hold the throttle open when you turned off the engine. However, it wasn't much of a problem on the older cars.
Electronic fuel injection cured the problem, as the electronics would not allow fuel to flow.
Another reason that people revved their engines, usually people who had done some building on their engines, was that they liked the sound and wanted everyone else to hear it, just like they do today.
Actually, it was the other way around. Most carbs in those days had anti-dieseling solenoids. They would not "open" up the throttle, they would close it down completely when you shut off the ignition. When you started those cars, you would pump the pedal a time or two, this would set the choke plate, and allow the anti-dieseling solenoid to open up the throttle plate to the normal idle position. Once you cut the ignition, the solenoid would lose power and retract, completely closing the throttle plate.
It won't do anything for any engine. But a lot of people do a lot of strange things with their cars because somewhere along the way, they picked up some bizarre rituals/habits about how they drive/start/shutoff/change oil....etc....
Most of it is nothing more then myths and legends. But interestingly enough, even myths and legends probably had some truth to them at one time.
About 40 years ago, when I was young and stupid, we'd do that and turn the key back on just before the engine stopped turning. Nice loud backfire. Thought it sounded cool. Later on, if you had loud pipes you had to rev it before shutting it off, again for the cool factor. I'm past that stage now...mostly.;-0 BTW, doing that will kill a turbo. H
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