Quick rev before shutting off motor?
Jun 29, 2006, 01:59 AM
#16
I might also add when the ignition system is on, the fuel is being BURNED and at leaner AFRs when at low load. The problem is when you shut down, the fuel entering the engine at this point may not have been burned as the ignition system is turned off. So some of this fuel may hang out in the cylinder until the next time you start it.
Shutting down from idle may or may not cycle the engine enough to clear this, so shutting down from a little higher RPM can give the engine more momentum to cycle a few more times than possible from idle.
I don't know how much fuel remains in the cylinder after a shutdown, nor do I know how many cycles are needed to clear it. This is just what I think the reasoning is.
It's a choice; don't do it if you don't want to. BUT I don't think it's harmful to do in any way.
Shutting down from idle may or may not cycle the engine enough to clear this, so shutting down from a little higher RPM can give the engine more momentum to cycle a few more times than possible from idle.
I don't know how much fuel remains in the cylinder after a shutdown, nor do I know how many cycles are needed to clear it. This is just what I think the reasoning is.
It's a choice; don't do it if you don't want to. BUT I don't think it's harmful to do in any way.
Jun 29, 2006, 02:06 AM
#17
Now that I think about it, maybe it would be better to shut-off while the throttle is cracked open, as you rev up. When you shutdown and the engine cycles while turned off, hold it wide-open to cycle more air through it. Remember, the fuel system is off regardless the of throttle position. More air seems like it would be more effective. Maybe this is the way to do it.
Jun 29, 2006, 02:36 AM
#18
Originally Posted by Ike
This is something that could help prevent flooding, but in todays cars unless you drive a rotary it's useless. If there is another reason to do this I'm unaware of it...
So, I don't know if having a little excess fuel in the engine really that bad. I can see how it might be bad if you didn't change your oil frequently because it might saturate the oil. But, I've never heard of that being a huge problem in EFI engines. That said, if your goal is to have the least amount of residual fuel in the cylinders as possible, I think your best bet is to shut it down while it is hunting for stoichiometric in open-loop and not after you just gunned it. Any fuel left over from near-stoichimetric combustion should evaporate at those elevated temperatures anyway, wheras if you just gunned it and there was enough fuel to pool in the cylinders, it wouldn't necessarily all come out simply because the engine was still spinning, as it is probably a liquid at that point and not a gas or vapor. That's how I see it anyway...
Last edited by machron1; Jun 29, 2006 at 03:54 AM.
Jun 29, 2006, 03:34 AM
#19
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Originally Posted by Ike
This is something that could help prevent flooding, but in todays cars unless you drive a rotary it's useless. If there is another reason to do this I'm unaware of it...
Jun 29, 2006, 04:04 AM
#20
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Originally Posted by EVOgasmIX
Well, think about it. When you shut off, you shut off the ignition and fuel is going to be unburned in at least some of the cylinders. The engine comes to a stop from idle pretty quickly and may not have ejected all of the fuel out. Giving the engine a little more momentum before shut down gives it a few more cycles to pump out any remaining fuel. Even with a few hundred RPMs, the momentum keeps it going for a sec or two. From idle it seems over in a fraction of a second.
I'm sure it's not going to be that big of a deal, but I don't think it could hurt. Having just air and oil in the cylinder sounds better than having air, oil, fuel, water, etc.
I don't know all though. I figured I'd get an opinion here.
I'm sure it's not going to be that big of a deal, but I don't think it could hurt. Having just air and oil in the cylinder sounds better than having air, oil, fuel, water, etc.
I don't know all though. I figured I'd get an opinion here.
As mentioned earlier, THIS IS NOT A GOOD THING AND WAS DONE in the 50's - 70's with Carbs (left fuel in the system to facilitate startups). This had NOTHING TO DO WITH "CLEARING THE SYSTEM."
You should not do this with FI cars.
Maybe we could get some Mitsubishi engineers to come on occassionally and answer these sorts of questions, along with the more technical questions such as ECU programming
....
Jun 29, 2006, 04:42 AM
#21
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this is not a good practice unless you have carburetted engines. it makes no sense at all except waste gas, 
unless you want to hear your engine hum or your fart muffler. 
the owner's manual advised against doing it just before shutting down so as my lexus and honda owner's manual.
Jun 29, 2006, 05:17 AM
#22
Originally Posted by czevo
People used to do that with carburated motors but its pointless in our cars its
not great for the turbo hence people run turbo timers
not great for the turbo hence people run turbo timers
not a good idea imo. unless your looking to do a turbo upgrade.....then i say rev the livin' **** out of it.
Jun 29, 2006, 06:13 AM
#24
never heard of anyone with an evo doing it
Originally Posted by EVOgasmIX
This is not the case. When you shut the motor off, the fuel system shuts off with it. This happens at idle as well as at elevated RPMs. The difference is the engine cycles a few more times before it stops than it would at idle, pumping air through the engine while the fuel system is off.
Also when I shut down, I'm not holding the throttle open when I turn of the key; the throttle is closed when I do this. I only throttle to get the RPMs up. Once they are up it doesn't need to be open anymore and they start to fall, then I shutdown.
The extra fuel isn't why it takes longer to wind down; it's the rotational inertia that does this. It's not like the ignition system is on to ignite the supposed fuel anyways.
Also when I shut down, I'm not holding the throttle open when I turn of the key; the throttle is closed when I do this. I only throttle to get the RPMs up. Once they are up it doesn't need to be open anymore and they start to fall, then I shutdown.
The extra fuel isn't why it takes longer to wind down; it's the rotational inertia that does this. It's not like the ignition system is on to ignite the supposed fuel anyways.
Jun 29, 2006, 07:19 AM
#25
I don't do this. I have alot of friends with Evos and a few who have worked/built/raced turbo cars and they don't rev their cars before shutting it off. I'd rather follow their example than someone who doesn't even own a turbo car.
Last edited by rsboy; Jun 29, 2006 at 07:22 AM.
Jun 29, 2006, 07:36 AM
#28
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I don't think this is really needed either. Our systems are all electrical and computer controlled, so I'd think as soon as u shut that key off, the injectors, ignition everything will shut off. Plus, I don't think he really means "rev" it up then kill it. He's meaning just bump the revs up a little above idle and shut it off, and the extra inertia will get rid of the excess fuel. But like I said, I've never done it on any of my cars, and don't really see the need for it.
Jun 29, 2006, 07:50 AM
#29
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Originally Posted by EVOgasmIX
Now that I think about it, maybe it would be better to shut-off while the throttle is cracked open, as you rev up. When you shutdown and the engine cycles while turned off, hold it wide-open to cycle more air through it. Remember, the fuel system is off regardless the of throttle position. More air seems like it would be more effective. Maybe this is the way to do it.