I am just curious! Sometime i read on this forum that some tuners try to set up a car with the perfict air fuel ratio. How is that done when i thought that the fuel pump is set at a specific value and then some people just add a 5 bar fuel regulator. Is their a acual way to dial in how mutch fuel a car need at a given time? Do they make an ajustable fuel pump or regulator? Or is it done by a programmer? I'm lost can someone clearify this for me?

 

Short answer.

The pump supplies a circulating flow of fuel through the fuel rail(s). This is more or less constant and the ecu has no control of flow on the 996tt.

The fpr restricts the amount of fuel that is returned to the fuel tank through the return line. If you swap a 4 bar fpr out to a 5 bar, you are increasing fuel pressure in the fuel rail due to less fuel being returned to the fuel tank.

So the fuel pump and fpr dont really affect actual AF too much, especially on modern cars that are able to correct and adapt fuel out the mixture

Actual AF is programmed by your ecu tuner. Your tuner specifies many many different values in many different areas of the ecu that are interpolated into an injector pulse width. This pulse width is more or less an "on time" for the fuel injector. The longer this on time, the more fuel is released into the cylinder.

Like I said, this is the short answer.

 

more detailed;
you have a "me7" ECU system on your car, inside is a flash (29f800) which stores the related maps and values. The fuel map is changed and the result is the AF ratio which is than measured by the wideband lambdasensors and in a closed loop mode the ECU can control the AF ratio.
Your ECU tries to get as long as possible a AF ratio of Lambda 1 only under full load it tries to get the AF which is stored in a so called "Bauteileschutz" map. This is necessary so your egnine doesnt get´s to hot, it richens the AF ratio.
It´s not a big deal in dialing all this values into the ECU if you have the right equipment.

regards

 

Stock the ecu tried to stay as close to lambda 1 for as long as possible. I would hope all tuners are telling the ecu to do other wise once power is increased.

 

Thanks Tony, this is very educational, great stuff.

I do have a question. This sounds like the 5 bar FPR is somewhat a patch/alternative indirectly increasing fuel by uping fuel pressure. At what point (hp or turbo psi) it will run out of fuel with a 5 bar FPR?

Thanks again.

 

Again, this is one I could write a book on....

You are correct the 5 bar is ONLY a patch or "band aid" . In most cases where a 5 bar is used, larger injectors would really be the better solution.

In general, you are "out" of fuel with the 5 bar at 575-580 crank hps. At this power level your injectors will be on 100% of the time or running at a 100% duty cycle.

Even at 100% duty you can retain safe a/f ratios over 600 crank hp's, but you have NO control over fuel, since the injector is on 100% of the time....

 

Wow, learn something new everyday. Thank you for another TT lesson.

 

Sorry I am not Tony but if you are running stock injectors ( I believe they are 44lbs on the 996TT?) and a 3bar FPR then you will be maxxing out at around 530 fwhp. Once you move to a 5 bar FPR the maximum output of those same injectors become 680 fwhp.

 

This graph is theoretical requested injector duty cycle of a k24 car with a stock fpr and stock injectors. This is not my tune, its from a car I logged for one of my dynoday customers in Dec 07.

This car was making less then 600 crank hps and from a good tuning standpoint was out of fuel at or around 4000 rpms.

From my testing, stock fpr and injectors are maxed well before 530 wheel horsepower.

 

If the car is out of fuel at 4000 RPM, what is the behavior of the car under these conditions? Will it sustain power and not increase or start to die off?

 

Yes, "from a good tuning stand point" the car is out of fuel. In some cases the car will be able to retain safe a/f and run at full power even while running 100% duty cycle on the fuel injector.

General fuel of thumb is a fuel injector should never be run at more then 80-85% of its capacity.

On most chipped 996tt's with stock fpr's and most if not all 600 level cars that are running stock fpr or a 5 bar, there is a certain point in the rpm range where the fuel injector just turns "ON". There is no ecu fuel control at this point, the injector just spraying 100% of the time.....This is not good tuning, however is how things are done in this market.

From a safety prospective alone, its better to have over head in your fuel system. If you are running at 100% capacity the ecu has no ability to enrich if it sees something it doesn’t like.

 

Hi Tony

I mentioned flywheel horsepower in my post, not whp. I said stock injectors with stock 3 bar FPR will max out at 530 FWHP, and that is at 100% duty cycle. If you feel comfortable with 85%, then it is 450 flywheel HP maximum. So on a chassis dyno, you should see no more than 400 rwhp with stock injectors, and if you are, you know it is is due to dyno optimism (a good way to know how off is a dyno in fact )

That 85% might be somewhat low however, I know that up to 88-90% one is perfectly safe provided the tuner knows what he is doing. So a 5 bar FPR will take you to around 580 Flywheel HP on stock injectors, at 85% duty cycle, I would say 600-630 Flywheel HP is the safe limit (around 90% duty cycle).

Obviously the injector HP capacity is irrespective of the turbos used except when related to the amount of air the turbo consumes and hence amount of fuel it needs to maintain a certain AFR, but that is dependent on the tune, and specifically the BSFC. Two different turbos used at the same CFM levels will need the exact same amount of fuel.

If you know what BSFC your engine is running, the maximum HP of a certain injector can be very simply and accurately calculated.

 

Thanks to all the responses!!! You guys realy know your stuff. So correct me if im wrong a fuel regulator will fix or i should say change the amount of fuel going to the injecters only up to a certain amount and then after that a ECU proggramer is the one who can make drastic changes to the fuel.

 

Lennyspoolin

Each unit of air gives you a certain maximum amount of HP and each unit of fuel as well.

Very simplistically, there is only so much air an engine can absorb, you can do certain modifications that improve that maximum air intake. However a certain type of engine (M96) can only give you a certain amount of HP per unit of air, and you cannot improve that efficiency by much (~5-10% maximum AND provided you do extensive intake and exhaust modifications, head work, compression etc)

So now that your engine can "consume" more air via modifications, if you put the appropriate components to absorb and burn that additional air and give it enough fuel to match that amount of air, you will be producing more torque, and HP.

Increasing boost brings more air to your engine, therefore allows you to produce more HP provided you have enough fuel to maintain the right mix of AFR. However, it will not improve the amounf of HP per unit of air or fuel absorbed (efficiency), so for every unit of incremental boost you want to use to produce HP, you will need a similar relation of incremental fuel. Until you cannot use more boost because you have no more fuel. This is theoretical and excludes any turbo limitations etc.. for the purposes of this conversation

Your injectors flow 44lbs/air per minute if I am not mistalen, those will give you a limited amount of HP at a certain ratio of fuel to air. If the engine does not absorb enough air to burn correctly that fuel (correct mixture, AFR), you will not produce as much HP in relative terms per unit of air. So you need to optimize the AFR levels that will give you perfect combustion, and each tuner has certain preferences or beliefs, within a certain narrow margin.

If your injectors and FPR max out in their fuel delivery (100% duty cycle), the tuner cannot do anything to give you more HP, he will need to go leaner on his AFR (more air and less fuel), which will cause trouble and risk of detonation, disaster, if for example you have a sudden increase of boost or air density change etc... So to be safe you only use 85%-90% of the injector capacity, this way you keep a reserve.

If you have 44lbs injectors and a 5 bar FPR, and after around 630 flywheel HP (assuming a reasonable BFSC and 90% or so duty cycle), you will need to start looking for an injector upgrade that can flow more fuel to your engine, so that you can maintain a safe air to fuel ratio and ideal combustion. The tuner cannot do much to give you more HP other than narrowing the safety limit of fuel delivery of those injectors, maybe another 30-40hp at quite high risk.

This is a very simple calculation and tuners cannot go wrong on this, they will tell you what is the minimum injector size or FPR that you need, but if you know the basics, you can do some due diligence as well.

Hope it helps.

 

Well if you are getting that technical then do the math with a 3.8 bar FPR,that is what comes stock on our cars.

Even know some tuners say its safe to run 90%+ duty cycle, you start to sacrifice the spray pattern at anything over 85% it starts to look like a garden hose and does not atomizes as well.

Chris Green