just wonderin wots best under engine brake.

is it ok to run mega lean?

 

generally, there is no fuel going in on overrun, apart from around the 4000rpm mark for piston cooling, then nothing again untill 1200 for idle - or thereabouts

 

Really does vary a lot from car to car, many manufacturers boost cut totally for the sake of economy, others piss fuel in at a huge rate for cooling.

 

but is it safe to run no fuel on over run, on a big bhp cossie?

 

bttt

 

yes - no fuel=no combustion=no extra temperature

 

mine has fueling on overun purely because if we cut it , then activate the als i get no effect till i hit 2000rpm and below !

 

cheers simon.

so its all boll-cks about puttin fuel in on ver run to keep cool.

 

No - Thats just a slow (old technology) ecu over fuelling because
as you shut the throttle the L6/L8 software takes time to recognise
this and still fuels for boost that has disapeared.

(This is pressure measurement integration/averaging for you tech heads.)

takes about 2-3 seconds on some weber ecu types/software versions

OR otherwise its just plain and simple bad mapping !!!!

 

Simon, I thought that the objective was to remove heat from the high loads prior to overrun rather than to manage cylinder temps during over run

Or am I totally wrong?

Could be an interesting post this

Neil.

 

My car was mapped by Karl and it has over-run fuel after coming off of boost to cool the pistons!

And I'm VERY glad it does! For piece of mind if nothing else!

 

Always makes me laugh this - whats cooler, rich combustion or no combustion....

 

Neil,

IMO, this is an explanation given by tuners to shut people up to explain crap mapping
and/or a complete missunderstanding of how an ecu works.
This situation typically affects modified weber ecu's that run over 25 psi boost
and usually ALL cars that run over 2 bar boost on a 3 bar map sensor.
The ecu has no choice but to "guess" fuel at this point.

No one likes thier car smoking and excess unburned fuel causes bore wash and fouls the plugs.

For example, if the car is flat out what is cooling it ??? - The cooling system of course !!!!!!

If the "overrun over fuelling myth" was true this means you are only supposed to run the car flat out for a few seconds then let it cool down !!!
Clearly this scenario is not desiried/true !

When flat out or mid range boost/high revs, an ecu needs to average the map readings to stabilise fuel delivery properly.
This averaging has the effect of have a rubber band on the input.
I.E, if you stop one end, the other end still moves until its out of energy.
This typically is more noticableon older OEM ecu's as they use older slower microprocessor and analog inputs.


However, cars running ALS do this deliberately but that really is a completely seperate issue.

Hope thats clear.

Any tuners care to add a comment to this ???

 

Cheers Simon!

So basically its caused by the time taken for the high MAP values to filter out of the time averaging period of the fuel calc's

Neil.

 

Surely combustion still takes place even on a very lean overrun Rick!

So what's cooler - rich combustion or lean combustion?

 

Having said all that, there is no doubt unburned fuel will cool a cylinder quicker than NO fuel but...

If the pistons are that hot when do you know they need cooling ???
When it melts of course.

IMO it is a myth this is any good as the only way it could be effective is you knew the car was running too lean for its application
and knew exactly when to back off !!!


IF a car is mapped correctly, cylinder temperatures will NEVER be an issue !!!

 

This place needs more tech posts like this

Neil.

 

Rich is cooler than lean....

No fuel is no combustion but theres no cooling from the incoming fuel either.



Alex

 

Neil S,

I agree and prefer such discussions ..... but most people are more interested in dirt dishing/shit stirring etc...

 

Yeah I know, I was just baiting Rick!

Simon - get posting then I love reading this kind of stuff

Neil.

 

J871yhk,

No combustion is cooler that some wether its rich or lean !!!

Why waste fuel on cooling pistons that should not be too hot in the first place.




Trouble is, people will always want the most power possible pushing everthing to the limit.

What would you prefer a safe cool engine mapped correctly or an engine that might blow up after 30 seconds boost if you dont back off !!!!

Its like locking the door after the horse has bolted

 

but its fun!

alex

 

thats sorts my issue out.

as on over run, my mapper has stopped all fuel, but i then heard (from dingy) he had a engine melt on over run due to no fuel bein put in. looks like it most of melted just b4 over run.

 

Can someone please explain then why on rolling roads, once a run has been made the operator starts giving the car revs as it slows down?

I always thought it was to dump fuel in?

 

learnt a lot from this one great post and info Simon I was always under the impression you should fuel on over-run but maybe I won't bother now!

mark J, I dont know but I assume on a rolling road where there is little resistance and less cooling the engine has the potential to get a lot hotter, hence the additional cooling needed?

 

also i guess on rr they blip the throttle, to keep the oil pressure up, to keep turbo and bore cooler.

 

absolutely correct

 

i do agree with simon, but something to think about

F1 cars overfueled on overrun

 

Interesting.

BUT then again I like flames on over-run so kinda xcheering for the "fuel is good" side on this one, lol

I dont know either way

 

what revs do they run and how quick does the system need to respond to give no fuel?

are we talking absolute ideals here or not?

you are trying to get the most out of your set up and pushing the limits of boost/detonation temps etc....

is it SAFER to fuel on overrun in terms of running lean etc?

or is it down to the driver aswell....

say you are doing top speed attempt - you reach your limit for distance... you brake and back all the way off the throttle what happens?

 

Flames rule, but so does having an engine that lasts longer than a bright idea in the house of commons.

 

Im confusesd too...

ZERO fuel on the overrun? Wouldnt that just be a non-running engine? No fuel means no exhaust that means no noise soon as you let off the throttle wouldnt it?

Never heard a car like that

 

I dont dispute fuel can be used to cool a cylinder BUT there is STILL combustion creating heat !!!!
(Assuming the spark is still active of course)

No combustion = NO HEAT !

As I said before, why waste fuel cooling a cylinder that shouldnt need cooling if the engine is mapped correctly.

So, lets suppose this "cooling" is being done deliberately....
You are driving up the road flat out...when do you know the cylinders "need" cooling ?

5 seconds, 10 seconds, 1 minute, 10 minutes ????

The answer = when it melts a piston because it hasnt been mapped properly or caused by an external fault.

Surprised the expert tuners are not adding their 2 pence worth .....

 

Steve, all modern cars cut the fuel on overun to save fuel !!!

Noise is still created as air is still being moved under pressure of the cylinder compression

 

im not sure what revs they was doing back then im sure the system was more than fast enough too notice a massive change in throttle angle

petrol do absorb heat alot faster than air does,,,but i agree with what simon is saying in that hopefully your cooling systems (both the water and oil) are up too the job of cooling your engine without the need to run fuel which i is only really getting on overrun (not much use when flat out for cooling then )

 

Yea i understand that some noise would still be there, but disdnt think itd be anything like the normal noise in the slightest

But fair enough

 

think i was a little slow there simon

 

what about a slight amount of water on the overrun?

know some WRCs run direct port water inj, making that possible

 

Its been a mandatory EU requirement to do this on all new cars since 1992 !!

So the esc cos (unless modified and decel fuel cut function turned off) does this !

 

RS1600i's had it too i think!