BAD XXX

The Stoich Ratio on LS1 Edit is 14.737. Can you lean out the engine by adjusting this number for closed loop operation? My O2's do not appear to be accurate, since the headers and ORP went in, so they always force my LTFT's positive, 15-25%, even with large changes to the IFR table. I would like to get an opinion before I do this. BTW, I looked for exhaust leaks, none.

66ImpalaLT1

I would not change this unless you have some scientific means of determining the Stoich ratio of your O2 sensors.

BAD XXX

So what are my options?

HumpinSS

iTrader: (1)

maybe a vacuum leak

Cal

iTrader: (1)

OK, here's the way I understand it: LTFT's typically go positve after a header install, because the headers help the engine process more air. The PCM senses more air is going through the MAF, so it adds more fuel by making the LTFT's larger. Basically you have increased the volumetric efficientcy of the engine. So to bring them back negative again, you can either scale your MAF table or your IFR table. Anyone feel free to jump in here if any of this is inaccurate or you can add to it.

Bink

Yup I think Cal has it.
Changinging stoich is a global change I believe. If your MAF is stock -> IFR.


Hey Cal!

Richard@WCCH

iTrader: (3)

If you want to tune differently than stoich, You'll need to change one of the following: As Cal suggested, modify IFR Table (most effective mod), alter MVE Table (a bit more tricky, but still effective), disconnect the MAF and alter the MVE Table to target a different AFR. I'm afraid that as long as the engine is run in "Closed Loop" your have to deal with the target AFR to be 14.7:1.
BTW, nice to see board members who live in the same town as me. Welcome, friend.

Richard

Cal

iTrader: (1)

Post deleted by Cal

ChrisB

I would think it's more likely that the O2's have "shifted" their response function slightly due to differing temperatures from the new header location. The heater circuit doesn't have any kind of feedback loop (resistance sensing, etc.) so there is no "good" temperature control.

The stoich ration is essentially going to set the a/f ration at 1.00 lambda. If we assume that the pcm targets a certain a/f ration then changing this would alter the mV setpoint that the pcm "swings" around.

If you are going to "play" with this value you would probably want at least a good wideband hooked up so you can see what your a/f ratio actually is - from that you can to a certain extent determine if your changes are appropriate. If you alter it too much then you could shift the swing point out of the sensors "range" and start causing other problems.


Also the fact that you made "large IFR changes" but still have large l-trim values would lead me to believe there is some other issue (dead/bad O2's, wiring, coolant, exhaust leaks, etc.)

billc5

iTrader: (1)

I have been thinking about this myself, I really should just go and raise the value and see what happens. It makes alot of sense to lean out this O2 sensor commanded ratio when the flow through the engine is improved. By the strictness of the definition for VE we have to add more fuel in the same cylinder air charge and hence the stock calibration is no longer valid. My first impression was that the narrow band sensor would want to swing symmetrically around its designed reading which by definition is a narrow band, and hence you would need another sensor designed to be symmetrical at your prescribes value. However in retrospect The curve seams to suggest that some calibration (asymmetrical) should be available which the designers could use to adjust for clean air standards. After all there is a value that can be edited.

My answer to your question is, I dont know with a definite maybe.

ChrisB

No, actually you have a smaller amount of "losses" vs. theoretical. You get ve by dividing actual airflow by theoretical. So if you make the engine more effecient you move in more air. VE *doesn't* measure how effecient the engine is with the air it pumps, only how much it pumps. Brake specific air consumption would give you an idea of how effecient the engine is with the air it injests.

If you increase the VE then the maf sensor reading will increase also, so you should maintain an a/f ratio. Now if you actually change your charge quality, etc. you could effect how well the charge burns - which would bring your actual pre-mix a/f ration closer to theoretical (perfect atomization).

Realistically I would still think most of the effect seen here is from the difference in o2 temperature - if you look at a narrowband response curve temperature makes a *large* difference.