Gurus,

Got a question regarding a strange phenomenon that I've recently discovered that may or may not apply to my '92 LT1/A4 coupe. I've seen a few posts on other forums, mostly the F-body sites, talking about "mystery timing" or what is essentially erroneous spark advance/retardation under acceleration that some OBD1-equipped vehicles have been prone to have. 

About a year and a half ago, I put a set of 23 lb. Bosch III injectors from FIC in my '92 coupe. I have a scan tool, and with the 23# units, my BLMs were a bit on the lean side (135 - 140). Last spring, I had a guy on the web do an "e-mail tune" for me where I was sending him Datamaster logs from my car, and he was sending me .bin files that I burned to a chip and installed in my ECM. We got the BLMs down to normal range (128 +/- 10) and the car ran fine all season, but last year at our club's track day, my car ran a best E.T. of a 14.36 which I felt was way off from what a healthy LT1 coupe should run. It was a fairly warm day that day, but other LT1 C4s were in the mid-to-high 13's. I could feel my car's power falling off around mid-track, and to me it almost had the feeling of the transmission slipping.

Over the winter, I've been teaching myself how to use TunerPro RT and going back and looking at the tune that the tuner provided me and comparing it to a stock tune.  I've also been going over the Datamaster logs that I took last spring and summer that had the new tune in place. In addition to the fueling changes, the tuner also played around a bit with the timing tables in the tune, giving me several degrees of additional advance throughout the mid-to-upper RPM ranges. In a couple of the datalogs, I can see where I started to register some knock right around 4,000 RPMs, and saw the ECM making the corresponding adjustment and pulling out anywhere from 3 to 5 degrees of advance. Putting 2 and 2 together now, I'm thinking that the drop in power I was feeling wasn't the trans slipping, but the ECM pulling timing out as the car was under WOT.

So my first inclination was that the tuner just had the timing tables too far advanced. Then I started reading about this "mystery timing" that some folks have come across where the ECM simply adds and/or pulls several degrees of timing for no apparent reason at all. I've learned that the early Y and F-body LT1 ECMs had tunes that transferred timing adjustments to a secondary "extended" table at exactly 4,000 RPMs, and a couple folks that I've talked to locally say that's where the issue might be coming from. In looking at my datalogs, it's right about at the 4,000 RPM point where the knock readings start and my timing gets pulled back.

Personally, I think this "mystery timing" is a bunch of malarky, and I think that my situation is really due to timing tables that are just too advanced for an otherwise stock LT1. This spring, I'm planning to go back to my original 24 lb. Lucas OEM-style injectors, and back to the factory tune and plan to do some more logging to see if there is a difference between the two setups. I was just wondering if anyone else had ever heard about this "mystery timing" thing.

Thanks!

Ron