Pretty general question that has bothered me sometimes...but after parking why do people rev their engines before turning it off? Is there any positive affect of this to the engine or do people do this for some other reason? jus wonderin.

 

Back when carburetors ruled - sometime in the late Jurassic, high-performance engines running astronomical compression ratios (12:1, 14:1) would tend to run rich at idle. The practice was originally thought to clear the combustion chambers of excess fuel, clear the plugs, and in some cases, prevent "dieseling" after the ignition was switched off by minimizing the available fuel in the cylinders.

Of course it made no sense, especially on anything that was street legal - and even less sense on a modern fuel-injected, computer-controlled engine. On turbocharged engines it only spools up the turbo(s) to let them run down after the engine has shut off, eliminating the oil supply (without a post-oiler) - not a good idea for bearings of any kind.

There were a lot of excuses, but I suspect revving up the engine and killing the ignition was done primarily to draw attention. It was a good way to produce a few choice pops and backfires from an open exhaust. You could shut off the ignition, open the throttle and if you could manage a brief flick of the ignition switch as the engine was running down to ignite the raw fuel being pumped through the cylinders into the headers, you got the immediate and favorable attention of everyone at the drive-in.

Of course our engines today are smarter than we are, and will have none of that foolishness. Key off, fuel off - simple as that.

 

It was not only fuel-related or entertainment-related. Back when alternators and generators were not as efficient or powerful as they are today, many of them could not charge the battery or electrical system at idle, so the battery was being discharged while the engine idled, especially with a lot of electrical accessories running (lights, radio, wipers, etc...). Running the engine above idle for a minute or so before shutting it off helped send a brief charge to the battery and helped insure a successful start next time......that charge was sorely needed on cold mornings when the engine had been sitting all night. Carburators, especially chokes and fast idles, were cantankerous, too, when cold......the engine often stalled and needed restarts while warming up.