OBD-II Observations
Thread Starter
Burning Brakes
Joined: Nov 2001
Posts: 1,241
Likes: 9
From: Houston TX
OBD-II Observations
i finally spent some dyno time with a wideband, on a '96 LT4, and thought i'd share some observations...
a/f ratio: actually not too consistent. it varied from pull to pull, but (at WOT) never went outside the range from 12.2:1 to 12.7:1. i thought it would be richer, since it smoked pretty hard from the pipes.
i also noticed that it went richer from 5k rpm onward. the stock programming does indeed add fuel from then on. i'd previously assumed this was in compensation to some dynamic of the engine, but now see GM wanted it that way.
made some changes and got it running between 12.6:1 and 13:1, again, varying a little from pull to pull. i was surprised that such a small amount of leaning out was enough to eliminate smoking from the exhaust.
timing: with stock timing, it would just barely ping from 5.5K onward, and drop some timing. i disabled knock retard to play with timing, and used a real-time scanner to confirm what was happening. i settled on 1.5 degrees less than stock, which did not ping at all. i then re-enabled the knock retard, and checked to make sure no false knock was read.
amazingly, with all of the fueling and timing tweaks, the power varied more just in pull to pull variation than it did from the tuning changes. i'd read of a few people claiming they got really good gains with aftermarket tuning. i can only guess that their cars were less sensitive to detonation, therefore allowing more timing than stock. all pulls were within about a 6hp window.
on the road, the lower timing made it such that knock retard never hits anymore. so the car feels amazingly consistent, the same amount of pull every time i floor it. also, (due to the fueling changes) the pipes are already less black than before, taking on a lighter, greyer tint.
the part that was really a success was the mods to ASR settings. you have to really overdo it to feel the throttle pedal kickback. it will allow low levels of slippage, i can just feel the timing retard as the slip starts getting bigger (then finally kicking the throttle back if i push the limits even harder). almost good enough that i'd consider auto-xing with it on. if nothing else, it won't totally kill the run now if i forget to disable it prior to starting.
not much to report other than that...
a/f ratio: actually not too consistent. it varied from pull to pull, but (at WOT) never went outside the range from 12.2:1 to 12.7:1. i thought it would be richer, since it smoked pretty hard from the pipes.
i also noticed that it went richer from 5k rpm onward. the stock programming does indeed add fuel from then on. i'd previously assumed this was in compensation to some dynamic of the engine, but now see GM wanted it that way.
made some changes and got it running between 12.6:1 and 13:1, again, varying a little from pull to pull. i was surprised that such a small amount of leaning out was enough to eliminate smoking from the exhaust.
timing: with stock timing, it would just barely ping from 5.5K onward, and drop some timing. i disabled knock retard to play with timing, and used a real-time scanner to confirm what was happening. i settled on 1.5 degrees less than stock, which did not ping at all. i then re-enabled the knock retard, and checked to make sure no false knock was read.
amazingly, with all of the fueling and timing tweaks, the power varied more just in pull to pull variation than it did from the tuning changes. i'd read of a few people claiming they got really good gains with aftermarket tuning. i can only guess that their cars were less sensitive to detonation, therefore allowing more timing than stock. all pulls were within about a 6hp window.
on the road, the lower timing made it such that knock retard never hits anymore. so the car feels amazingly consistent, the same amount of pull every time i floor it. also, (due to the fueling changes) the pipes are already less black than before, taking on a lighter, greyer tint.
the part that was really a success was the mods to ASR settings. you have to really overdo it to feel the throttle pedal kickback. it will allow low levels of slippage, i can just feel the timing retard as the slip starts getting bigger (then finally kicking the throttle back if i push the limits even harder). almost good enough that i'd consider auto-xing with it on. if nothing else, it won't totally kill the run now if i forget to disable it prior to starting.
not much to report other than that...
