Reving the throttle right before shutting of the car??

Habit? Blip the throttle when "down-shifting" from "neutral" to "off".

 

Dubai had it right.

On heavily camed engines the valve overlap is such that at idle the engine "lopes" and does not effectively burn the fuel off the plugs. Bringing the RPM up to a point where the engine "comes on cam" and smooths out will then burn the fuel off the plugs when they are still hot allowing for an easier restart. On the obsurdly camed engines we have to shut off the fuel supply with the engine running, bring the RPM up to 2500 and wait for it to start missing then shut down the ignition. Start up is the reverse, get it cranking with no ignition and once the oil pressure is showing, throw on the ignition.
Another occation where it is of a little benifit is just before checking the oil on a dry sump car after a prolonged idle time. Bring the RPM up to 2K for a few moments will run the scavage pump faster and make sure the engine sump is emptied completely into the tank. That said, there is usually far more scavage pump volumn than pressure pump volumn so the effects are questionable.

Dave

 

Dave, Isnt' the Lobe on engine with hot cams from the carb losing signal and messing up the mixture? EFI engine, like the Indy car that started the thread shouldn't have a lopping idle or the loading up that goes with it....at least in theory.

In the motorcycle world, when the race bikes still had carbs, the racing carb often didn't even have idle circuits in them, so if you wanted it to stay running, you blipped the throttle....but I don't think that applies to car carbs.

 

+1 This is what I also heard years ago when we all had carbs and dinosaurs walked the earth

 

Quotation from a Len Deighton spy novel:

"he revved up his engine before shutting down, in the manner that racing drivers are reputed to do"

It was master spy Harry Palmer's disparaging view of his American boss.

 

LOL!

Must have had fresh plugs.......

 

No.

 

No water vapor in a hot Ferrari exhaust, unless you've blown the heads.......

That's only on cold start, from leaving it in New Orleans overnight....you should drive down for FFQC dinodan!

 

+1 You hit the nail on the head Dave....rev it up for burn off...flip the EMS switch off which stops the injectors(on FI engines) and exit the car for a 2gal swig of Gatorade..more if you're not wearing a cool suit in a closed cockpit endurance car.
All the race cars I've built had EMS disconnect switches..for start up....EMS off....crank till oil pressure was up.....flip the ole switch to on, fire it up, and hang on for the adrenaline rush..lol

I can remember back in the good ole days when we were running mechanical FI....even the ignition cut-offs sometimes didn't save you. The Porsche 917's in particular....if you waited to build full oil pressure before switch-on, the plugs had already fouled. If you timed it just right you had to keep the revs above 2000rpms or they would be soaking wet with fuel, and not a d@mn one of them would be firing. I can't tell you how many drivers I've cursed, while replacing 12 spark plugs on the starting grid...lol

 

From my experience with racing... when racing cars rev up and shut off the engines its to burn unwanted fuel/ oil from the plugs. make it easier to get the car running when starting. especially back in the carb days.

Today it may to ensure you have a clean stoichometric measure on the O2 sensors, and knock sensors. "clear out the carbon" prior to shutting it off.

 

Correct! That's the reason.

 

No our cars are fed by an electric fuel pump, turning it 'on' puts fuel into them...........

 

No fair; you cheated Big Tex. You didn't read the whole thread The comment about carb cars was in the context of old carb cars. I agree with the three gents who state that throttle reving prior to shut down originated to fill fuel bowls in preparation for the next start.

But the thread originated with a question about fuel injected race cars. The response related to being "on cam" prior to shutting down a heavy overlap motor is the most plausible.

 

If one was able to design a purpose built engine from scratch this might be true about the idle.

The last 348 race engine (EFI injected) we built used cams with a duration on the intake in excess of 340 degrees and intake valve opening before 70 BTDC. At idle there is near as much air being pushed out the intake as is going in. This engine doesnt come on cam until 5000 RPM and will not idle below 2200. Lopes enough at idle we considered putting limit cables on the engine mounts. If we shut it off with wet plugs there is no chance for a restart.

I will, to avoid confusion, change my above reply to..... This is why "I" bring the RPM up before shutting the race engines down. There is no reason to do it on a street car.

Dave

 

Bingo. Old Lotus' do that too until you change the jets.

Ken

 

My cars sit so long between drives you are only insuring a new supply of varnish in the fuel float bowls!!! LOL!

 

-If the fuel is transported by an electric pump then there's no need to fill the bowls because they'll always be filled. Revving the engine just uses MORE fuel, thus insuring they'll have LESS fuel in them, if bliping does anything.

-If the fuel is transported by a mechanical pump (like my air-cooled VW) then as long as the engine is running, fuel is being pumped into the bowls. At the end of a drive, those bowls are still filled with fuel. Revving the engine will, again, just use more fuel. The fuel pump will pump faster, of course, in keeping with the engine so it fills the bowl in time just as it would if the engine was idling.

So this theory is hog-wash.