A poser for the turbo experts...

[EvilScotsman]

Dan, I wasnt refering to you there mate.

ITRHondaboy, ok, no offence taken then. Im not trolling.

BenTaylor....recirculating air that is trapped is not the same as a leak. You dont need maths or physics to explain airflow or fuel mass? What would you use then??

And Ben & the others who confirm the Nissan ECU cuts fuel on the overrun, then should agree it shouldnt massively overfuel on throttle closure, except just for a tiny fraction of a second until it drops to cut-off or the throttle is opened again.

This is really getting boring now cos no one except us wants to know why it does this!!!!!!

[dan_t]

How does this work with regards not running a DV at all then? I've said above that I'm happy with the reasoning behind an atmosphere vent.

Ben I know you know your stuff with this

[itrhondaboy]

Its odd then that itrhondaboy has a car that overfuels at a time when the injectors are not firing? Maybe they are stuck open?

Just because you don't want to understand the working of something doesn't mean you should put other people off trying to find out for themselves.

Sure I'm being anal about the overrun situation but it just goes to show the amount of crap that can get spread around the internet.

I've never once said they don't overfuel. I think we have reached a semi sensible answer with this one now though. Although the overfueling on lift off really is for such a short duration that I really wouldn't worry about it.

A little fuel goes a long way with regards pops/bangs etc. I think its generally only made worse by the fact that the standard map is rich at the top end anyway.

by overrun i ment lift off, i do understand why it does it and have posted twice on why it does it.
I have also said it doesnt have to as it can be mapped out or a map sensor set up can be installed.

Wheres martin battye when you need him for a technical explaination!

[clive]

I'm sure the injectors are cut on overrun/engine breaking?

The nissan ECU cuts fuel on overrun.

but only after dumping in a load of fuel.

ill take a video of my afr and show you

[EvilScotsman]

ITR I know you meant lift off. Think about it....if the Nissan cuts fuel on the overrun / liftoff (as it should, everyone else does sInce 1985) then why does it run rich on throttle closure, unless the fuel cut is lazy, and what you are actually seeing is the result of a sluggish fuel cut to idle on CT.

If it is of such short duration, how can it be harmful.

Cold engines run much more fuel than 10:1 on cold start enrichment. More like 8:1. Doesnt break them.

[Actual_Ben_Taylor]

Dan, I wasnt refering to you there mate.

ITRHondaboy, ok, no offence taken then. Im not trolling.

BenTaylor....recirculating air that is trapped is not the same as a leak.

Yes, because it isn't trapped, it's recirculated back through the compressor of the turbo turbo (which will have slowed down, but not stopped) and into the engine.

It becomes a lot more noticable when you start running a bigger turbo with a lot more boost. You notice it more when you're coming off the throttle slowly (throttle plate and dump valve are open at the same time), if you're going through the gears really quickly its not so much of an issue.

[EvilScotsman]

I am NOT disputing the Nissan engine runs rich for a fraction of a second between gears, but assert that it is insignificant and meaningleass unless it runs rich constantly under normal operating conditions!

And its NOT going into the engine unless you open the throttle again!!!! If the throttle is closed the recirc air just spins round and round thru the turbine blades until the throttle is opened again.

[itrhondaboy]

i didnt think it was immediately harmful just not advisable to run it poping and flaming on every gear change for the entire life of the car. And the police dont take to kindly to having their front bumpers fried!

[Actual_Ben_Taylor]

I am NOT disputing the Nissan engine runs rich for a fraction of a second between gears, but assert that it is insignificant and meaningleass unless it runs rich constantly under normal operating conditions!

Its fairly insignificant between fast gear changes.

And its NOT going into the engine unless you open the throttle again!!!! If the throttle is closed the recirc air just spins round and round thru the turbine blades until the throttle is opened again.

Which is why its a big problem when you're decelerating on part throttle.

[EvilScotsman]

http://en.wikipedia.org/wiki/Stoichiometric

[clive]

ITR I know you meant lift off. Think about it....if the Nissan cuts fuel on the overrun / liftoff (as it should, everyone else does sInce 1985) then why does it run rich on throttle closure, unless the fuel cut is lazy, and what you are actually seeing is the result of a sluggish fuel cut to idle on CT.

If it is of such short duration, how can it be harmful.

Cold engines run much more fuel than 10:1 on cold start enrichment. More like 8:1. Doesnt break them.

no one has said its harmful for the engine... but is for gaskets/weak exhausts cats etc

Yes, because it isn't trapped, it's recirculated back through the compressor of the turbo turbo (which will have slowed down, but not stopped) and into the engine.

It becomes a lot more noticable when you start running a bigger turbo with a lot more boost. You notice it more when you're coming off the throttle slowly (throttle plate and dump valve are open at the same time), if you're going through the gears really quickly its not so much of an issue.

yes you do notice it more when coming off the slowly.

when i go through the gears quicky it dont do it at all... i think thats because the engine burns the fuel before chucking it out the exhaust (im no scientist or mechanic)

[sideways14a]

Over fueling (in this case) aint a problem for then engine, but it is a problem for the cats and the rozzers.
Run a VTA for a while with its tendency to send flames down the pipes and you will have a roasted kitty (although the time scale aint to clear).

[redlinezak]

Just registering here in the hope that the discussion and name calling stops and we can get back to an interesting discussion. I think the OP was not trying to argue for arguments sake more just looking for a technical reasoning behind the result. I personally think that someone with some fairly obvious technical knowledge should be welcomed on here as you can never have enough help and advice.
I know pretty much diddly squat compared to lots of people who have posted on this thrread but from my experience of running a VTA i noticed that it was on part throttle that the hesitation or misfiring became apparent not on totally closed throttle. I also didnt seem to get as much popping as otheres have. I now run a MAP based system so i dont have any issues with it.

Lets get this back on track guys and not take somones inquisitive nature as offensive.

Welcome to the club mate stick around.

[nitrodave]

an inquisitive nature is one thing, but getting angry and insulting becasue nobody was able to talk physics and numbers is another all together. WTF did he expect at midnight on sunday, a collective of physics geeks waiting and ready for a reasoned debate?

whatsmore, several times was an answer given but the evilscotsman got angry, quickly, for no justifyable reason. If he never comes back on this site it is no loss whatsoever

[redlinezak]

Gotta say i disagree with you dave but either way i still think it would be best to forget the arguing and get to the original discussion as i think it could be interesting even if nothing gets finally resolved. Technical discussions although after way over my head are interesting and can only serve to increase understanding of various parts and workings of the cars.
Maybe midnight was not the best time to post initially with us bunch of alchy's lol.

[voodoo_melon]

This is really getting boring now cos no one except us wants to know why it does this!!!!!!

I'm baffled as to how this has gone on for three pages

As soon as the throttle shuts and the air starts recircing, there's no more air entering the afm, so the ecu will cut the fuel to match the amout of air going in. When the throttle is opened again, the air starts flowing so the ecu increases the fuel supply to match.

[BBB]

I'm baffled as to how this has gone on for three pages

As soon as the throttle shuts and the air starts recircing, there's no more air entering the afm, so the ecu will cut the fuel to match the amout of air going in. When the throttle is opened again, the air starts flowing so the ecu increases the fuel supply to match.

[kilt wearing voice on]
But you're not listening or using physics and you're all barstewards and myth propagators who know nothing about the 200SX
[/kilt wearing voice]

get with the program voodoo. It can't possibly be that simple

[dan_t]

I'm baffled as to how this has gone on for three pages

As soon as the throttle shuts and the air starts recircing, there's no more air entering the afm, so the ecu will cut the fuel to match the amout of air going in. When the throttle is opened again, the air starts flowing so the ecu increases the fuel supply to match.

OK I agree with this... Throttle closes so the injectors are cut. Now if the injectors are cut, who cares where the air goes? Its never going to run rich if the injectors are cut as there will be no combustion at all.

[dan_t]

This thread is bloody stupid.

[stu_]

I always thought that when you closed the throttle, another valve let air in to burn off the fuel that was about to be injected. However if you vent that to atmos, there's less air than has been metered.

...although I can't find a reference