If I understand correctly the ECU uses the information from the MAF to add the proper amount of fuel for the incomming air. This means it will automaticaly compansate for any Mods that allow more airflow by adding more fuel.

However at WOT the ECU ignores the MAF and adds a preset amount of fuel. This means guys with boost must add additional fuel managment (FMU) to get the proper fuel to air ratio at WOT.

Why is it done this way? Couldn't the ECU use information from the MAF at WOT? Are all cars like this?

 

The ecu does not ignore the maf at all. Us boosted guys add an additional fmu so that the fuel pressure raises for each lb of boost the maf reads

 

If the ECU is already compansating for the extra air won't increasing the fuel pressure add to much fuel and make it to rich?

 

No cause the FMU is boost relative. That means for each pound of boost that is added to the system, the FMU raises the fuel pressure X amount of PSI.

Dixit

 

At WOT, the ecu goes into a set a/f & timing programs to protect the engine. It's usually a little bit richer to keep the temperatures down and to protect against ping/knock.

 

The ECU ignores the O2 sensors at WOT, not the MAF.

Ignoring the O2 sensors is Open Loop mode, while using the O2 sensors' output is Closed Loop mode. WOT causes Open Loop, regardless of engine temperature/conditions.

 

so during open loop/cold engine/WOT the ecu reads from every sensor except the O2

 

This is a good question. apparently it can not correctly calculate the flow under pressure or something.

 

What Jeff and Spirilis said is correct. The ecu always uses maf information, but reverts to premapped fuel and timing maps when at WOT.

The A32 maf can accurately measure flow up to about 350 bhp, but the stock injectors (4th gen, that is) are only good to about 250 bhp, which is why boosted Maximas use a boost referenced fmu to increase fuel supply. Or they use a JWT programmed ecu with larger injectors.

 

It can. BUT what the ecu controls is the injector pulsewidth. It will continue to keep making the injector pulsewidth longer and longer until it goes static(dangerous/open all the time). You have to realize that the amount of fuel provided is a calculation of injector size, pulsewidth and fuel pressure. Since we can't easily change size and pulsewidth easy, we up the fuel pressue

 

Question for my infinite curiosity. So when the MAF sensor is telling the ECU that it has >250HP worth of air coming in, the ECU says "OK, let's compute the injector pulse width... oh ****, it's off the scale, so we're just going to max out the injector".
At that point, if there is more than 250HP worth of air coming in, the ECU doesn't actually have direct control over the fuel injection anymore, since it defaults to maximum pulse width all the time. So at that point, the FMU is in direct control of fuel metering, and it meters by raising the fuel pressure.

Is this how it works? Or does the ECU detect the higher fuel pressure and go "Oh, OK, we don't have to max out the injectors... the fuel pressure is now high enough to let us operate at less than maximum pulse width". In other words... does the ECU take fuel pressure into account (using some kind of fuel pressure sensor feeding into the ECU), or does it not?

BTW, the "250HP" number I pulled out of my *** was from Stephen Max's post above

 

I "think" the ecu is just reading a voltage off the maf. If the voltage is xx, then that tells the ecu to search for that voltage on it's fuel table and then fire the injector for xx time(at a certain fuel pressure) There are of course many other things affecting the pulsewidth also. Also you have the 02 sensor telling the ecu the car is running rich/lean and that also tells the ecu to fire the injector longer/shorter.

I think the ecu will keep upping the pulsewidth until it get static(open all the time) from the 02 sensor. Because I would have to assume at some point, the 02 sensor will read lean all the time.

Of course at WOT, the ecu goes to preset conditions. If you are on boost, the normally rich condition becomes a lean and dangerous condition

There is no fuel pressure sensor. I believe the ecu will fire the injector assuming set fuel pressures under certain conditions. wot and non-wot

 

From what others have indicated yes. The maf tells the ecu how much mass air is coming in and the ecu defaults into preset a/f (ie..ignores the 02 sensor)

There is NO fuel pressure readings. The ecu assumes a predetermined fuel pressure. ie.. xx psi as not wot and xx psi at wot.

Maintaning a constant and know fuel pressure is important. Not the readings as the ecu assumes a preset fuel pressure reading. (important for the next sentence). On boosted cars, if you don't run some type of fuel management, then yes, the injectors will just keep opening until they are open all the time(bad) But if you run a boost referenced fuel pressure regulator and up the pressure at a known amount per psi, you can roughly bring back the a/f ratios to a safe level. Remember the ecu is ignoring the pressure and will fire per a lower fuel psi level. (ie..supplying more fuel per puslewidth). The o2 sensor can refine things a bit too but you have to be in the ballpark with the FPR.


The purpose is to keep the a/f ratios within a safe level for safety and power. Add more air, you gotta add more fuel. You want the injectors also firing within a safe injector rate. If you ask the injectors to run over about 80% duty cycle, their ability to meter fuel accurately goes down drasticly.

 

Sorry I deleted my post when I realized I asked the exact same question as spirilis

 

So if you're running say 280 HP the injector is wide open under certain heavy acceleration conditions including WOT at high RPM. This is because the incoming air is off the charts according to the ECU.

 

It could be. But I would think the maf and injectors are matched. It's more because the 02 sensor is reading LEAN and is trying desperately to richen things up.

 

Thought O2 signals were ignored at WOT? Meaning there is no way for the engine to know it's running lean, or rich for that matter. They must follow the map then. I'm just confusing myself now.


I hope you know I'm not trying to be difficult, just talking tech.

 

At WOT, the ecu is just reading the maf values yes. But if the maf is seeing more air than the maximum amount that usually gets though there(ie.. na vs boost), it will run extremely lean. The maf assignes values for different amounts of air coming in. ie.. 1-5 volts for min-max. If the air coming is more than the 5 volts max, then the ecu cann't tell the injectors to fire anymore.

 

I realize that, but if the MAF can read up to 350hp and the injectors are only good to 250. Ratios will be correct up to 250hp worth of air, after that is when lean conditions begin because the injectors are wide open and can't deliver anymore fuel with the factory pressure.

To me it just doesn't seem right that the injectors are wide open above 250hp. It has to be though if MAF voltages are the only way it calculates pulse width at WOT.

 

Well a 259cc injector at 43psi flows enough for 230hp @ 90% duty cycle for a FI motor and 260hp @ 90% for a NA motor. So when the ecu is in open loop it will see the maf voltage pass higher than the (known)/rated flow of the injectors so thus it makes the injectors run at higher than 80% DC. I believe one could see this with consult by looking at the injector ON time in msec.

Now according to the formula's on RC's site the injectors need around 70psi to flow enough fuel to max out the stock maf's voltage assuming its ran on the lean side. I have a techtom mdm-100 so I can view maf volt, ignition timing, injector time in msec and other things. One thing I noticed was in open loop o2 sensor went to around 0.88-91v while injector on time was 14.7msec and then it dropped down to 13.9msec near redline, meanwhile the maf volt was going from 3.1v to 4.64v at 7500rpm. In the end the ecu reads MAF volt vs RPM to find the right pre-programmed cell.

 

Nice. Now, for us s at fuel management, what options are available for controlling the fuel system in a N/A motor?

Idea behind my question- I have a 5th gen N/A motor, almost completely stock. Is there some kind of device I can plugin which causes the ECU to aim for a slightly-richer-than-normal mixture under closed loop, without ripping out the existing ECU, and still being able to connect to the stock ECU via OBD-II and view sensor data, emissions status, etc.?
If such a beast exists, what's it called?
(if there's a site that explains all this please share the URL!)

Thanks

 

hmm, nevermind... sounds like an S-AFC is what I'm thinking of. And further searching reveals that S-AFCs require a lot of tuning, certainly not worth the effort on an N/A engine...

 

So what's the verdict, yes or no, those running over 250hp have their injectors wide open (100 duty cycle) at WOT at high RPM. (stock injectors of course)

 

Not really a whole lot. You can dial in whatever afr your looking for in as few as 2-3 runs. If you're trying to smooth out every little peak and valley, though, it may take more. But like you say, it's not really worth the $300 for the safc and the $75 or more for the dyno runs if you're NA.