Thanks Mike..I took that off in fear of being labeled a double standard kinda dude...

 

The PCM learns two major items: fuel correction and spark timing. (It also learns other things like the crankshaft position sensor deviation, idle air flow, etc.)

1) Fuel is learned based on a Ltrim (long term fuel trim) and a Strim (short term fuel trim). These adjust for changes in fuel pressure, MAF calibration (i.e., new intake - K&N, volant), etc. based on input from the precat O2 sensor. The trims will be adjusted to keep the air-fuel ratio to 14.7:1 during part throttle. If you disconnect the battery for a while the PCM will re-learn these values. This is one reason why the aftermarket intake makers tell you to disconnect the battery - usually the PCM will learn faster from ground zero then from re-learning from previously learned values.

2) Spark timing - there are two timing tables: high octane and low octane. The PCM will adjust timing as you drive to be somewhere inbetween these two tables based on knock retark input from the knock sensor. The PCM will reset the timing (start again on the high octane table) if you add XX number of gallons of gas or if you disconnect the battery for a while. From what I've seen, the high octane table is based on 87 octane. This is why you'll see a minimal performance benefit from using higher octane (no way to increase spark advance beyond the high octane table) unless you have high air intake temperatures that you benefit from less knock retard lowering your spark advance.

 

So then it DOES re-learn based on air intake and fuel octane. If that is so, which one of you are right then?

 

I think Dennis and I said the same thing. The PCM learns or adjusts the engine operation based on sensors inputs. The PCM does not learn or adjust for the driving habbits of the individual - excluding abusive maneuvers.

 

We both are Mike...He is defining every car in manufacture as to how it manages engine emisssions, which it learns to adjust for on a day to day basis depending on the weather via sensor inputs..It DOES NOT learn your driving habits..i.e., are you a jack rabbit at take off, or a grandma?

LOL..before I posted this I notice Chevy said the same thing

 

I got it. It doesn't matter how you drive, what octane you use, etc, because every time you start the car, it will automatically adjust to what octane gas you have, what air flow you have, etc. If you add a cold air induction kit, it will automatically adjust for that, and in theory, if you turn the car off and add the stock system back in, it will automatically adjust for that again, without trying to remember anything.

I see the light!

So, it forgets the second you take the key out of the ignition, and is a newborn baby again later on when you start it.

 

The PCM has a continuous +12 volt line that lets the PCM retain its memory when the key is off. So you have to disconnect the battery to force the PCM's learned values to zero to force an all new relearn.

 

So, if I plan on using 91 to 93 octane gasoline from now on, and i have a tank of it, I disconnect the battery, and only use higher octane gas from now on, will that be to any advantage to me at all as far as performance or mileage, or is it just a waste of time to have th eengine re-learn using the higher octane.

 

Just a guess - performance will be slightly better (~ 3 - 5 hp) - may not notice a difference between 89, 91, and 93 octane. MPG may be slightly better perhaps 0.2 - 0.3. This is just a guess. For example, I've increased the timing by 2-3 degrees and don't see any knock with 87 octane - this is with true cold air entering the intake. This shows how much headroom there is in timing for 87 octane with cold air. Typically, with the higher octane gas, you can increase the timing to make more power. An old rule of thumb is that you can increase timing by 1 degree for every single digit increase in octane (i.e., 89 to 90 octane). You can increase timing with a colder engine block or colder intake air.

 

I always understood the cold air induction theory and how it affect compression, but from what you tell me, It sounds as if your air coming into your engine is warmer, then you need higher octane gas for it to make a difference. If you have colder air, your compression gets better, and you don't necessarily need higher octane gas for an engine that wasn't specifically designed for it. I guess those extra 3 1" holes I drilled in the back of my air box to allow more air passing through my K@N filter probably is causing more hot air to be sucked back in and making things worse. I thought maybe the extra air going into the engine would compensate for the higher temperature of the air and it would equal out.