Hymee

iTrader: (1)

I just made a couple of posts where I mentioned "Closed Loop" and "Open Loop". I thought it might be a worthwhile thread if I started an explanation of what this all means, and how it works in conjuntion with piggy-back ECU's.

Closed Loop
This condition is normally encountered at part throttle openings, such as encountered in "normal" driving. A simple explanation of what is happening here is that the PCM (Powertrain Control Module) is continously monitoring the O2 sensors in the exhaust. This device tells the PCM if the engine is running Rich or Lean. The RX-8 also has what is called a wideband O2 sensor, and this is used to tell how rich or lean the engine is running - hence the PCM gets a very accurate reading.

Locked away inside the PCM is some reference tables where, for a particlar load/RPM/thottle condition, the target AFR is found. The PCM continously monitors the AFR and adjusts the fuel delivery constantly to try to obtain that AFR. Constant feedback/adjustment, hence the term "closed loop"

There are a few other things that come into play here, known as STFT and LTFT - and this is all related to "learning". I'll leave a discussion about that untill another time.

Open Loop
This condition is normally reserved for WOT. The feedback loop from the O2 sensor is no longer operational. Hence the term Open Loop. In this case, the computer determines the amount of fuel delivery (and spark timing) from some calibration tables that are again locked away inside the PCM.

One of the main sensors the PCM uses to determine the fuel requirements is the MAF (Mass Air Flow). The way piggy back ECU work is that they alter the signal the PCM is getting from the MAF in such a way that the PCM grabs a fuel "doseage" value from a diferent part of the fuel table. If you alter the MAF signal up or down by the right amount, you can pinpoint more or less fuel - hence adjust the mixture richer or leaner. Depending on the fuel maps in the PCM, this altered MAF signal probably needs different adjustmets at all RPM points.


So where does this leave us?

What is all means is that a piggy back fuel controller shouldn't make any difference to normal, part throttle (Closed Loop) driving. It should not make any difference to economy either, or to emmissions.

Why is that? Because in Closed Loop, the computer is continually adjusting the fuel delivery to obtain a target AFR. The target AFR is locked away in the PCM. To my knowlede the fuel controllers don't fool the PCM with a butchered O2 reading. Over time, the different amounts of fuel required at different load points will be learned by the PCM (this is where those STFT and LTFT come in), and negate the changes to the MAF signal.

That is also one reason not to play with the MAF too much - in closed loop there will be no differences, and if you fiddle with screens etc you will bugger up the accuracy - they rely on a nice laminar flow of air.

But with a fuel controller that has been calibrated properly to fool the computer to think a different amount of air is incoming at WOT, then since the computer is in closed loop, you can alter the performance of the engine. A "Piggy Back ECU" is really fooling the computer. Done properly by an experienced tuner, one can achieve measured gains - if the state of tune is conservative in the first place.

The other answer is to get inside the computer and find all the look-up tables, understand their structure, and change them. That is a lot harder to do.

I hope this has been an interesting lesson in de-mystifying the working of your cars "computer" to some extent. I love understanding what is going on inside there.


Cheers,
Hymee.

PS - I should put this in the Tech Garage...

https://www.rx8club.com/showthread.p...threadid=18833
Post all replies there, and I'll close this one