Supra Forums banner

1 - 7 of 7 Posts

·
Registered
Joined
·
1,523 Posts
Discussion Starter #1
Im trying to understand how a stock ECU fuel map works. Does the ECU adjust fuel according to RPM and air flow that has been programmed into the map. What I mean is lets say stock car im just throwing numbers in the air...makes 14lbs boost by 3800RPM. So obviously by factory they would have tuned the MAP to start adding fuel there. Alright now lets say you do some mods that suddenly make full boost 14lbs at 1800RPM. The ECU never saw that much boost down low. Does it just read the amount of airflow and pull the information from the map at the point from where the same airflow used be approached at 3800RPM. Or will it just go lean as hell. Sorry if its confusing but Im trying to understand the ECU logic.
 

·
Shawn Davis
Joined
·
1,662 Posts
When not at WOT and below 6 psi the ECU always seems to try to reach 14.7 AFR. When WOT or above 6 psi (with stock turbos in sequential you pretty much have to go WOT to get above 6 psi) the ECU refers to the fuel map (load and rpm based I assume). So if you're able to make 14 psi at 1800 rpm it goes to the map and pulls the fuel number from that cell in the map. That's my understanding from seeing the stock AFR numbers while driving.
 

·
Registered
Joined
·
1,523 Posts
Discussion Starter #3
oh ok..so even though the car from factory was never tuned at 14psi at 1800RPM since physically the turbos never could produce that....it knows to pull fuel number from the cell registering that amount of airflow/volume. I just ask this because if it didnt have this logic..that would mean a fuel map would be enormous and you would not be able to tune it for every possible situation. I get it now.
 

·
Registered
Joined
·
544 Posts
The fuel map is not the most determinant factor for fueling. The MAF input is. MAF based ECUs calculate the theoretically required fuel based on the air flow read by the MAF sensor and the desired A/F.

The fuel maps represent only correction factors due to the MAF sensor being an imperfect determinant of air flow. On Toyotas, in addition to the MAF correction factor, the ECU might also incorporate the desired A/F in the fuel map (ie, it calculates how much fuel is necessary for stoich based on the MAF sensor, then the fuel map correction factor richens it up). I have no idea. I know Subaru turbos incorporate the desired A/F in the MAF function. ie, target A/F is stoich at low flows, then it slowly transitions to a richer A/F as air flow gets higher. I guess the Supra ECU does use the MAP sensor for this, but it's probably not the only way it does it since the car runs fine with the MAP sensor disconnected.

So, yeah, you are right, at 14psi at 1800rpm represents some air flow, and the ECU will be able to calculate how much fuel is needed even if it was never programmed to. But it is not just pulling a cell from some fuel map, but rather performing calculations to determine the fuel needed given the MAf sensor reading.

Shiva
 

·
Registered
Joined
·
1,523 Posts
Discussion Starter #6
thats some good stuff man...

oh yeah I have that book already and read the whole thing. That book doesnt really get into major detail on stuff like this. Its kinda general
 

·
Shawn Davis
Joined
·
1,662 Posts
Shiva, I think we're agreeing. The use of a map of cells and a transfer function to calculate fueling for airflow is exactly the same thing if your map has infinite cells. I have no idea how many cells the map has or if the function is linear (in which case a map would only need two cells to perfectly replicate the function) but the same thing happens either way. The computer decides how much fuel to add for a given amount of air flow (load)
 
1 - 7 of 7 Posts
Top