Quote:
Originally Posted by Blueclyde
I have been following the discussion on this thread and am struggling quite a bit with some of the information. It is hard to argue with the results of the stop watch so it does appear that some of this is valid. I know a little bit about engines and engine controls strategy but do not have any experience with the GM controller on the L99 or LS3 engines. As I see this, both the L99 and LS3 engines should be susceptible to this problem. I have an LS3 and I know for a fact that my dim-witted dealer put 87 octane in the tank prior to deliver. They were not even aware of the fact that it called for 93 octane. I questioned them about it at delivery and advised them to read the manual.
Back to the issue of spark tables and "clearing" the ECM. Most current automotive controls systems employ what is called "Adaptive" control strategy. The primary purpose of this strategy is to allow the engine controller to learn and re-adjust as the engine/vehicle goes through its normal use/wear life cycle. This strategy is one of the ways that the auto companies insure their vehicles comply with the EPA & CARB emission useful life periods legislated for on-highway vehicles (no trivial task here). This is nothing really new. This has been around for a while. One point that I would like to make is that these systems general learn bi-directionally. Meaning, if an engine subsystem has a problem (like a plugged air filter), the system will adapt to that. Once a problem is corrected the system will adapt back. Yes, resetting adaptive parameters back to zero (or "clearing" the ECM) will generally speed that process up, but it will also occur on its own over the course of normal vehicle operation.
With respect to spark calibration tables and "93 Octane Tables" and "87 Octane Tables" I would be very surprised to find more than one spark calibration table in the control strategy. Again, I am no expert on the GM controller but I would be surprised if there are multiple tables. Generally there is one spark table that is calibrated across the speed and load range using the recommended fuel. I would think in this case with this being a performance vehicle the base spark calibration was done with high octane fuel. This engine is equipped with a knock sensor that is calibrated to detect and report occurrences of spark knock back to the engine controller. "Generally" engine control strategies use this knock sensor input data to "derate" or pull back spark advance from the base spark table. Calibrations are created to pull back the appropriate amount of spark for the input signal received from the knock sensor. There are probably other sensor based spark derate systems employed in addition to knock. Parameters such as intake air temperature and engine coolant temperature probably also have spark derates associated with them. If this is in fact the case, then none of this makes any sense, unless there are also other fueling corrections being made when the spark is being pulled out.
My point to this is do not assume what is being written hear is all correct. There is obviously something to this as again the stop watch doesn't lie. I would also submit that the folks at GM's CAC probably do not even fully understand how this all works. It would be really good to hear from a true GM expert (probably somebody from GM Powertrain calibration) to enlighten us all a little bit.
|
You know a "little bit about engines?" You sound like a "G." Nice post.
Still, I'm calling the dealership tomorrow to ask what kind of fuel they use to fill up new Camaros. And if they say 87 or 89, I'm going right to my local dealer to ask them if they know anything about this fuel table mess.