Acceleration fueling

[mongoose]

Since changing my duff fuel pressure regulator,i have been tweeking my fuel map.This is because of two changes with the new one.I've gone from 2.5 bar to 3.0 bar,and i'm not running a vacuum pipe on the new one.With my pressure now fixed permanently,i have noticed that my acceleration fueling is leaner than ideal,i feel.With foot hard on the floor,the car accelerates and the AFR is always around stoich,to me this should be richer?If the ideal power AFR is say 12.8:1,then surely this is what i should be seeing on flat out acceleration?Is it that simple?Do i just need to increase acceleration fueling only,so that it reads 12.8?

[Mikey G]

Are you checking your EGT's?
As you say Stoich is probably a little lean, but to go to 12.8 maybe overkill if all you end up doing is cooling the gas temps down.

Edited by Mikey G on Wednesday 12th July 22:55

[trackcar]

there's a lot of parameters to consider in the accel fuelling Mike .. for example there's the fuel added whilst the throttle is opened,what dave walker calls the clamp (ie how long that overfuelling lasts for) the decay (ie how quickly that overfuelling blends back into the base map) and that's before you consider the rate of acceleration of the engine itself ..

[stevesingo]

Why are you running without the vacuum connected?

The reason this is connected is that the fuel pressure is constant in relation to manifold pressure/vacuum.

Can you give more details of your enngine i.e. N/A Turbo and engine managment.

Steve

[stevieturbo]

Why would you not run a vac pipe to the reg ??

If the fuelling under acceleration is wrong, then fix it !!! Simple as that. circa 12.8-13.0:1 for WOT on a n/a car would be fine.
Running leaner would give minimal if any gains, and start to border on risk of meltdown if held for any length of time.

Accelerator pump fuelling is a different matter altogether, and lasts for barely longer then the length of time it takes to move your foot to cause it to happen.

[mongoose]

stevesingo said:

Why are you running without the vacuum connected?

The reason this is connected is that the fuel pressure is constant in relation to manifold pressure/vacuum.

Can you give more details of your enngine i.e. N/A Turbo and engine managment.

Steve

Thanks for all the reply chaps.I cant see EGT by the way.
Hi Steve,the regulator isn't vacuum connected as it's a 3 bar regulator.With the injectors i have(stock Lucas)You can't run more than 3.2bar befor they wont open properly,so if i connect the vacuum pipe then the pressure behind the disc in the injector will be too great at WOT wont it?
Anyway 3 bar seems to provide more than enough fuel judging by the injector pulsewidth figures i'm getting.
The engine is a mildly modded RV8 5 litre in a griff.
Since i wrote this post i've been looking through my favourite read(How to power tune Rover V8),and it says(J Eales)"Induction systems are set to give the following,full throttle 0.88 lambda/13.0 afr.Acceleration phase rising to 0.85 lambda/12.5 afr."
Sorry to sound dense here,but am i to assume that at any time the car is inceasing in revs then i need to be seeing somewhere from stoich to 0.85,and that when i'm WOT it should always be 0.88?

Edited by mongoose on Thursday 13th July 10:26

[GreenV8S]

mongoose said:

With the injectors i have(stock Lucas)You can't run more than 3.2bar befor they wont open properly,so if i connect the vacuum pipe then the pressure behind the disc in the injector will be too great at WOT wont it?
Anyway 3 bar seems to provide more than enough fuel judging by the injector pulsewidth figures i'm getting.

That logic sounds backwards to me.

The vac reference line drops the fuel pressure under partial throttle. If you disconnect the reference line and run constant absolute pressure, then under partial throttle the pressure difference across the injectors will be greater (since they have partial vacuum at the outlet, rather than normal atmospheric pressure.

It doesn't make any difference under full throttle so not relevent to the duty cycle.

Who advised you to disconnect the vac line? Did they also tell you to scale the fuel map to account for the extra differential pressure under part throttle?

[trackcar]

here's a better idea .. get a very wide range regulator, put the same fuel number everywhere in the map (would make the ecu build very easy) and control the fuelling just on the vacuum pulled (ie engine load) .. no need for a laptop ever again.

[mongoose]

GreenV8S said:

mongoose said:

With the injectors i have(stock Lucas)You can't run more than 3.2bar befor they wont open properly,so if i connect the vacuum pipe then the pressure behind the disc in the injector will be too great at WOT wont it?
Anyway 3 bar seems to provide more than enough fuel judging by the injector pulsewidth figures i'm getting.

That logic sounds backwards to me.

The vac reference line drops the fuel pressure under partial throttle. If you disconnect the reference line and run constant absolute pressure, then under partial throttle the pressure difference across the injectors will be greater (since they have partial vacuum at the outlet, rather than normal atmospheric pressure.

It doesn't make any difference under full throttle so not relevent to the duty cycle.

Who advised you to disconnect the vac line? Did they also tell you to scale the fuel map to account for the extra differential pressure under part throttle?

Hi Peter,the way i was thinking bout it was like this...the disc in the injector cannot open after 3.2 bar,this is pressure behind the disc,ie still fuel rail pressure.The difference the vacuum pipe gives to the regulator is to compensate between manifold pressure and rail pressure,therefore applies only when the injector is open.My injector wont open if i go 0.2 bar higher,and by using the vacuum pipe i would surely go above this at wot,therefore it just wouldn't get to open properly in the first place at wot if i use the vacuum pipe.
Have i got all this completely wrong..............,as usual

[mongoose]

Just read your post properly now Peter.so if i use the vacuum line then i will always get less than 3.0 bar unless at wot,then i get 3.0 bar?,

[stevieturbo]

At WOT there is no vacuum, so you will see exactly the same FP that you are seeing now.
Except at low throttle openings, when there is vacuum present, your FP will drop to as low as 2.5 bar, depending how much vacuuum is present.

Ive never used the injectors you are....but are they really so bad, that a simple 0.2bar change will cause them not to work properly ?

But if you have an ecu, that allows you to control fuelling, why should you feel the need to run a higher fuel pressure ?? The ecu should give you the ability to control the fuelling without making significant changes to fuel pressure.
What ecu are you using ?

You are mistaking acceleration fuelling and WOT fuelling from whatever you are reading.

What does your car do at WOT ?? I know mine accelerates. If it sat still, I'd be very very annoyed.

If I was to hold a sustained WOT load with no accerlation, I would want a richer AFR, than if the engine was simply accelerating passing through load sites quickly.

Ive never read the book you refer to so cant comment on it directly...but from reding otehr books, some writers get stuck with old fashioned tuning methods on some engines.
Buy some modern books.

A good one is Julian Edgars "21st Century Performance" It covers a wide variety of topics, in easy to read and understand format.

[Marquis_Rex]

mongoose said:

stevesingo said:

Why are you running without the vacuum connected?

The reason this is connected is that the fuel pressure is constant in relation to manifold pressure/vacuum.

Can you give more details of your enngine i.e. N/A Turbo and engine managment.

Steve

Thanks for all the reply chaps.I cant see EGT by the way.
Hi Steve,the regulator isn't vacuum connected as it's a 3 bar regulator.With the injectors i have(stock Lucas)You can't run more than 3.2bar befor they wont open properly,so if i connect the vacuum pipe then the pressure behind the disc in the injector will be too great at WOT wont it?
Anyway 3 bar seems to provide more than enough fuel judging by the injector pulsewidth figures i'm getting.
The engine is a mildly modded RV8 5 litre in a griff.
Since i wrote this post i've been looking through my favourite read(How to power tune Rover V8),and it says(J Eales)"Induction systems are set to give the following,full throttle 0.88 lambda/13.0 afr.Acceleration phase rising to 0.85 lambda/12.5 afr."
Sorry to sound dense here,but am i to assume that at any time the car is inceasing in revs then i need to be seeing somewhere from stoich to 0.85,and that when i'm WOT it should always be 0.88?

Edited by mongoose on Thursday 13th July 10:26

The fueling for best torque is known as LBT. Or Leanest Mixture for best torque. Going richer than this will LOSE you power. Going leaner than this will lose you power too.

LBT mixture changes with engine speed: it's about 13:1 at 1000-1500 rpm, 12.9-12.8:1 from 1750-3250 and then progressively richens to about 12.5:1 at peak engine speed. There's no need for absolute accuracy anyway, as long as you're close to these figures.


During acceleration or "fast transients", you NEED the mixture for best torque- and this should therefore be figures in the ball park of what I just stated. However it's NOT that simple:
Not all of the fuel injected into the engine is immediately inducted into the engine.
When the engine is running steady state- or continually running with constant throttle position the cylinder air charge for each cylinder is roughly constant and the fuel transport mechanism from injector to cylinder are roughly in equilibrium. Here the mass of fuel inducted is equal to the mass of fuel injected.
However during transients or "fast tip-ins"-which is what we're concerned with -and constitutes ALOT of typical PHers driving-the inducted fuel is very different to what is injected. Not all of the fuel injected goes into the cylinder, some of it remains on the port walls to form a puddle. This puddle mass stays the same during steady state operation, but during transients, say, when the throttle is opened more, the pressure in the inlet manifold rises and so does the puddle mass (when the inlet manifold pressure goes down-the reverse occurs). This means some of the fueling is lost to the newer bigger puddle when you open the throttle more. THIS is what you have to compensate for-which is why you need extra fueling during fast "tip-in" transients.

So we NEED to achieve our 12.8:1 on transients and want our AFR meters to read this, but more fuel will be required to do so to avoid lean spots on acceleration.

The last point is that the engine won't "burn out" or misfire if you run at stoich at WOT. I've seen this stated here and in another post by someone who runs a rolling road. All that will happen is that the car won't produce as much torque. Simple.

[GreenV8S]

Marquis_Rex said:

The last point is that the engine won't "burn out" or misfire if you run at stoich at WOT. I've seen this stated here and in another post by someone who runs a rolling road. All that will happen is that the car won't produce as much torque. Simple.

I thought that richening the mixture was often used to cool the combustion chamber, reducing the danger of detonation. In that case, removing the 'extra' fuel might lead you back into detonation?

[GreenV8S]

mongoose said:

Just read your post properly now Peter.so if i use the vacuum line then i will always get less than 3.0 bar unless at wot,then i get 3.0 bar?

If you have the vacuum reference line disconnected, the fuel rail runs at a constant absolute pressure e.g. 3bar above atmospheric. Since the manifold pressure goes down below atmospheric when you close the throttle, the pressure drop across the injector varies from 3bar at full throttle, to 3.5 - 4 bar when the throttle is closed. This pressure drop determines how much fuel you get, and also how hard it is for the injector to open.

If you run with the vacuum reference line connected, you would normally set the fuel pressure with no vacuum so that the pressure you're setting is what you'll get at full throttle. When you connect the vac line, this will pull the fuel rail pressure down as the manifold pressure goes down. Assuming you have a convenional 1:1 fuel regulator, this means the pressure drop across the injector will be constant (e.g. 3 bar) under all conditions.

Either way might work fine, but if you change from one to the other you'll have to scale your fuel map to account for the extra pressure drop across the injectors under part throttle when the vac line is disconnected.

I think that running with the vac line disconnected may exagerate problems in metering the fuel accurately at idle, since you'll need to reduce the pulse width to compensate for the extra pressure across the injector.

Unless you know what you're doing and are doing it for some specific reason I haven't thought of, I would say that disconnecting the vac line was probably the wrong answer.

The only reason I'd consider doing that, is to enable me to use fixed fuel pressure alarms.

[Marquis_Rex]

GreenV8S said:

Marquis_Rex said:

The last point is that the engine won't "burn out" or misfire if you run at stoich at WOT. I've seen this stated here and in another post by someone who runs a rolling road. All that will happen is that the car won't produce as much torque. Simple.

I thought that richening the mixture was often used to cool the combustion chamber, reducing the danger of detonation. In that case, removing the 'extra' fuel might lead you back into detonation?

It's true that additional fueling will tend to alter the knock limit, but it isn't typical to use fueling to cool the combustion chamber- better and a much quicker response to use ignition.

Additional fueling is often used to cool the catalysts or sometimes component protection also, such as exhaust manifolding, but sometimes this is one in excess and you'll lose power because you're far away from LBT.

Detonation is also different to burning a hole in the piston or misfiring though. It's unlikely to happen using stoich fueling at WOT though.
If the engines CR is specified too high, or the back pressure is very high or alot of boost is being used, there will be a point in the rev range at WOT (or close to) where catalysts can melt if you don't overfuel.

[eliot]

Have good read of this section: www.megamanual.com/v22manual/minj.htm#supply

In summary, connect that vacuum line.

[mongoose]

Thanks for all the replies guys,there some great learning there for me.I have been misled(unintentionally)by things that the guy who sold me my regulator had told me.I was told that without the vacuum the regulator would just run a constant 3.0 bar,and that the vacuum was used to increase pressure at WOT.These words were not given to me in the same sentance,and i,as usual,just put 2+2 together.
Stevieturbo:-the injectors dont work properly over 3.2 bar,as the disc struggles to open,hence my concerns in my initial post.The ecu i run is an Omex 710,and it is easy to adjust for different pressure,injector size etc.
It has become obvious to me now,that the reason i'm struggling to get the map back to how it used to run the car,is because it was obviously mapped when my original regulator was using this vacuum principle,whereas im just compensating the ecu data for a slighly higher fuel pressure,and things still were not coming out correctly,the AFR was not altering consistently throught the range of load sites.
Thanks for all your help once again everybody

[rev-erend]

Can I ask a question - what is WOT ?

[stevieturbo]

Wide open throttle

[vixpy1]

GreenV8S said:

Marquis_Rex said:

The last point is that the engine won't "burn out" or misfire if you run at stoich at WOT. I've seen this stated here and in another post by someone who runs a rolling road. All that will happen is that the car won't produce as much torque. Simple.

I thought that richening the mixture was often used to cool the combustion chamber, reducing the danger of detonation. In that case, removing the 'extra' fuel might lead you back into detonation?

I would have thought running an engine such as the Rover V8 at 14.7:1 on WOT would be det city..