People have mentioned cranking up timing at cruising RPM/CylAirmass in order to significantly increase gas mileage, but there were no real guidelines anywhere or ideas of what to watch out for when doing it. I'm just wondering what a ballpark advance range is for people who have tried this.

I really doubt A47 would be willing to share an example spark table, but if you'd at least chime in that'd be awesome.

 

bump, i wanna read this

 

me2... s

 

Most cars run a lot more timing advance when under light load conditions because there is less chance for detonation, but if you're looking for fuel economy changes I don't see how that would make any significant differences without also altering fueling. The automakers don't run cars too lean under light load cruising because it can cause the NOx emissions to skyrocket, but if you lean the car out you may see a small increase in economy. I'd have to think about what the computer would be doing in closed-loop operation for that to have an impact.

 

Well, as it stands my car idles and cruises around at 13.8-14.9 (and noone could tell me why it is so rich), so I don't have much to fear about leaning it out.

 

That really isn't too rich. Most cars from the factory are in that range. The stock narrowband sensor can only accurately read A/F at 14.7:1 which corresponds to ~450 millivolts (0.45V). That's the "switching" point and the computer essentially only knows if the car is richer or leaner than that by adjusting fueling under closed-loop operations, richening it up slightly until the AFR goes richer than that and leaning it back out until its goes leaner.

The reason why it isn't running exactly at 14.7:1 is for emissions. That's the optimal ratio for complete combustion of pure gasoline, but the nitrous oxide emissions start to increase a lot when you run leaner. If you try to command a leaner A/F ratio under closed-loop cruising I'd probably make sure to go back to a stock tune if you ever have to emissions test your car where you live.

 

That makes sense. My HPT and interceptor says the car is "commanding" 14.7 so I might try changing it to 14.9 and see if it does what I'd like it to. I understand what you're saying about emissions, I just don't follow why some of our cars seem to idle/cruise around 14.7 while the few I've seen personally tend to be around lower 14s. Luckily, where I live they only check OBD2 sensors (no pipe sniffer) so I can get away with it, but good call pointing that out.

I upped my timing at cruise a few more points, will see how my mileage was affected when I get home.

 

mine always stays around 13.9 - 14.9 at idle, regardless of what injectors i was running. other cars i tune like to hover around high 14s (14.5 - 15.5)

Dont change your stoich afr. it will throw off all of your other fueling calculations. there are other ways to lean out your idle/cruise afr

 

Such as? Everything I've done so far was just corrected by the primary O2 sensor as a fuel trim. I just assumed changing the stoich adjusts the voltage it looks for.

 

OL and CL eq ratio tables

 

Hmm, I think the previous tuner had those all set to 1 when I started tuning my car and the AFR was never up there, but I'll try that again and see what happens. On the OL/CL in gear-out of gear tables the one axis is ECT, do you know what the other axis is?

 

ambient temp.

and now that i look at it, theres no table for modifying the ratio when in closed loop, only open.

Some of the LSx guys change their stoich afr to get better cruise mpg but I would imagine they have to re map their other calculations based on this new number. I havent tried it so i have no idea how it will affect our fueling tables. I wouldnt go w0t without recalcluating first.

another method, although probly not a good one, would be to force PE in the low rpm and low map/maf areas and command what you want through the PE EQ table... This will probly have other side effects that i havent thought of yet.

another thing i just remembered, if your VE is off, and the PCM is referencing it against a tuned maf at those points during idle/cruise, it will have an effect on your afr.

 

Ambient temp?

The way I figure it, WOT has nothing to do with stoich anyhow. If I'm understanding the car right, stoich is checked by a voltage that the rear O2 is reading - when you got WOT it ignores the rear O2 completely. Ergo, it should only effect normal driving and not WOT at all.

I set the tables back to 1 beyond about 150* with no effect. I am currently running off MAF only above 1000 RPM so VE has nothing to do with it.

Tomorrow I reset the OL/CL eq tables back to the normal values and play with the stoich value.

 

there is no CL eq ratio modifier. only OL

your PE calculations are based on 14.7.
EQ = 14.7 / desired afr. (to command 11.5 you would do 14.7 / 11.5 and put the quotient 1.287 in the PE table).
you may have to recalculate PE based on hte new "stoich" value...

Try it and monitor your AFR's closely. Report your findings

 

Yeah, I saw that after your other post, but I had already loaded a tune with modified tables on my car the night before. Oh well, no harm done.

Ahh, I see what you're saying, I just assumed the engine calculated the number off a hardcoded 14.7 value, but I guess that's not a brilliant idea. I'll give it a shot.

I love discussions like these. If I could give you +rep I would. :-)

 

thanks.

waiting to hear the results...

 

Hmm, so not only did the AFR not seem to change, but when I went WOT it was around 11.6, which is right around what it would be if you took 14.7/1.262 (where 14.9/1.262 = 11.8).

I'm running out of ideas. Maybe I'll swap the primary O2 out again and see if there is some form of recalibration for it. Unplug the battery or something.

 

how much did the actual w0t afr change from what it was before? and what was your PE set to?
I must say I'm surprised if it didnt change your cruising / stoich afr... however you might have to get more agressive with it, say 15.1 or so which should really futs with your PE.

keep me posted.

 

The WOT before was 11.8 and after the adjustments was about 11.5-11.6. I think I may try a more aggressive adjustment and leave the WOT PE the same. My PE values were 1.245 across the board before, 1.262 after.

 

hmmm. leave your pe at 1.285 (11.4) and try 15.1

 

I'm going to leave my PE alone and see what happens if I set stoich to 15.2. Will report tomorrow.

 

do work son... lol sounds great i might have to mess with this some too

 

I changed the commanded to 15.2 (and the interceptor confirms it) but the AFR is still in the upper 13s/very low 14s.

Went WOT on the way home. AFR fluttered around 11.9 with stoich being 15.2 and the PE being 1.245. I'm thinking it's hardcoded or at best not going solely off the value we can change in HPT.

 

damnit. I have the exact same rich idle afr there's gotta be something we're missing in the tune...

 

Dainslaif, PM me on RLF... I'll send explain the details for LSJ timing for mileage.