Several things have always aggrovated me about the GenIII pcms. Not having source code for the os's, certain things are either unknown or you have to take someone's else word on how it works. Unlike the days of old when it was reasonable for a professional like myself or even a hobbiest to break apart a 16k bin and know what every byte did, things are to big and complicated to handle like that anymore. Thats why we pay companies like HPT the big bucks to do it for us.
It may not be reasonable for me to spend the time to reverse engineer the thousands of gm os's that exist, but I decided that it was for me to build a benchtop stimultor to simulate a running engine and function as a digital scope to watch outputs.
Anyway, with the racing year slowing down I finally found a little time to devote to my project. Took a CStamp packing a 40mhz PIC18F6520 and layed out and built all my hardware. Wrote all the software for the microcontroller and then wrote an app to control and communicate with it on the PC side. Couldn't find any data on the event timing on the 24x/1 signals so I took an pigtail wired with a led and read the timing patterns off an engine that we just got back from the machine shop. Here is the data I read incase anyone else would like to have it.
24X-1X.xls
I'd never seen a definative answer on what the units for eoit tables really meant, so that was the first thing I wanted to tackle. I'd heard everything from 15,33,45 on the reference periods, much a less what starting point the bondaries were relavent from. So after much flashing and observing, I compiled a data sample only to discover that a reference period is actually 90 degrees. The normal and boundary are added together like we were told. but the starting point appears to be some non natural point long before the engine cycle. Also the makeup numbers seem to have no effect. If the code is actually functional, there must be a master switch that has it disabled because no matter were I put the makeup numbers I never observed more than one fuel pulse per engine cycle. Here is the data I recorded.
Observed_FT.xls
The only formula that I could come up with that fit all recorded data was -784 + ((Boundary + Normal) * 90). That puts the stock settings yeilding an eoit of 300 degrees ATDC compression, which makes sence. That also means the max that it can be retarded is 656 ATDC if viewed as a 720 engine cycle. Which is all the range we need since for somthing like an G5X3 the exhaust is closed by 390 and the intake by 610. Which before I was always conserned that if the units were 45 degrees or less than 8 + 8 wouldn't allow enough range to delay any of the fuel after the exhaust had closed.
Don't know if this helps anyone, but it makes me feel better about what I'm changing and tying to rationalize the results I've had from boundary changes.