After listening to all these "ECU" threads I thought I might try to add some insight. I have a fairly good understanding of the operation of the system (Spent alot of time in the IC engine lab back in college). I think the big question running around is does the ECU or does it not "detune" mods. In a nutshell, yes and no. Here is a semi-technical explanation of this :

First, the ECU knows nothing about horsepower. Like Chebesto says, it is a feedback and control system that reads inputs from a variety of sensors (MAP, Knock, Mass Airflow, IAT, O2 Sensor, Coolant Temp .. etc) then uses this data to change the operating parameters of the engine (Timing, A/F ratio etc ..) The Z's ECU basically runs in two modes, Open Loop and Closed Loop (like all modern FI cars). The majority of the time it runs in closed loop, reading the O2 sensor and adjusting the A/F ratio for best emisions (14.7/1 Air to Fuel). Closed loop is used for low/moderate load, light throttle (ie. normal driving). During startup, cold operation, and most important for us ... wide open throttle, the ECU runs in "open loop". This means, it is no longer reading O2 sensor voltage and reverts to a pre-programmed set of fuel and timing maps. Here is the kicker, a program in the ECU called "Fuel Trim" remembers the adjustments it has to make in closed loop operation to stay at 14.7/1 and after a period of time modifies the base map value. It does this so that the car can always maintain peak emmisions performance even as sensors and parts get old. After a period time, the long term fuel trim is used to correct the open loop maps used in wide open throttle. Back in the old days of OBDI, the maps where never modified and you could "bolt away" with the mods and the ECU would never adjust the open loop maps. Those days are gone.

So, what does all this mean ... well, say you bolt on a new cold air intake. Take a spin around the block and feel a boost in power due to more cold air. After 10 -12 drive cycles, the ECU tweaks the fuel trim to compensate. Some of the new found power is lost (Not all though as many mods improve the volumetric efficiency as well .. a whole other discussion). The good news is that an ECU reset will erase the long term fuel trim values and set them back to Zero. The bad news is that after a couple of weeks, you gotta do it again.

Have a great day !

 

Thanks for the info. Do you know of any text/artical that describes the operation of the ECU and other electronics? I have looked randomly in the automotive section of Borders and Barnes and Noble and haven't found much.

 

That was some kick-*** info......can i ask a question?......i know, that already was a question .......but seriously, can i keep these changes from happening with a piggyback ECM that will modify the MAF signal to the stock ECM?........something that tells the ECM that everything's A-O-K?.......i've heard a Nissan engineer said that this could be accomplished with a resistor on the right wire, but i don't wanna go low tech with it

 

Sorry, not off hand. Most of this stuff is in tech articles I've read through the SAE (Society of Automotive Engineers)

 

I would guess this is possible ? By no means am I an "Expert" but really, it would seem plausible. I agree, that this is how the piggy back units work. I had one of the Peter Farrell units in my 94 RX7 TT. Thats all it did ... +, - on the A/F and timing. After every mod (Down Pipe, Exhaust, ..etc) I would richen it up a bit to keep from detonating.

The resistor deal would be used on the signal for the O2 sender to modify the voltage signal. It sends the ECU a different voltage based on A/F ratio. I would imagine this being a crude approach though. My guess is that Chebesto's guys are not allowing the fuel trims to modify the base maps. I'm not a Computer Engineer (I'm a Mechanical) so I don't know the exact method used

 

This is where the APEXI SAFC would come into play or any fuel controller and you won't need to reset or alter because the fuel controller sends the last signal to the ECU with the new A/F correction. Than again their is always the standalone but I wont go into that.