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longtubes and closed loop fueling

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Old 05-23-2004, 11:06 AM   #1
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Default longtubes and closed loop fueling

Since our front o2's provide the information for fueling in closed loop when they are positioned farther downstream does it throw off the mixture?The reason I ask this is I am getting bad fuel mileage but my ltrms are good,and
wot os'2 read 890-900.During cruise and light throttle is the volume of exhaust reduced enough for the sensors to read improperly?Someone must know about this as I here of heads and cam cars getting 25+mpg when I can only manage 13.
thanks in advance for your help
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Old 05-23-2004, 05:55 PM   #2
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why dont you adjust the target a/f ratio from 14.7 to what you desire to get your mpg back? that should be a pretty solid test
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Old 05-24-2004, 10:31 AM   #3
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Ok enlighten me on this procedure.Will I need to hook up a wide band to see what it's really doing now and then adjust it back some way to stoich?hp tuners has been ordered but it has not arrived yet.
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Old 05-24-2004, 11:02 PM   #4
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yah there's a field that says desired a/f 14.7. I assume if you set it to 15.0 it will skew some closed loop values and lean it out. assuming headers make the system think its leaner than it is.

I'll give it a shot. You can always alter the a/f desired for long trips, since at low load the block can dissipate much higher EGT's than at high load . At cruise you can defo burn a little leaner than at wot since the heat goes away so fast.

I'll jack up my desired a/f to 14.9 and see if it makes any driveability issues and mpg gains while logging for knock.

i run aggressive timing so more fuel + more timing = more mpg in my eyes. but theres always the other tuners (less timing/less fuel = more mpg) ..
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Old 06-02-2004, 08:51 PM   #5
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Cool I'll try it also,thanks for the tip.
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Old 06-03-2004, 12:12 PM   #6
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any news on this? I'm experiencing the same issue with cam and longtubes.


Wait a sec, if the computer sees the car trying to run leaner, won't it compensate and dump even more fuel? I think that's what is happening now. When I autotap my car, the AF is 14.6, 14.7 constantly. Yet it's going through gas like mad. I don't think simply adjusting the stoich value will correct it.

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Old 06-13-2004, 06:00 PM   #7
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yes i got hp tuners and fooled around with the value for stoich while logging.I was able to change ltrms by doing this so its affecting the mixture.I am waiting for my rpm addapter for my wideband before I go any further.The milage has improved slightly
but not dramaticaly.Et streets with 20 psi probably has something to do wiith it.I am also adding timing as 2001 cars are 19 degrees max at high load and rpm.
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Old 06-13-2004, 07:21 PM   #8
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Well, the cold O2s is a pretty good theory
given the plumbing. It would apply more at
idle and low cruise. If you get O2 heater
or switching codes (and some folks do)
then believe it. But headers also increase
cylinder filling as they reduce backpressure,
the spent gas remnant in the cylinder is a
"poor man's EGR" and reduces fresh charge
draw. Increased draw reduces your mileage.
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Old 06-13-2004, 10:38 PM   #9
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LTs can actually over-scavenge the cylinders at low engine speed and can actually pull air/fuel right out of the motor and out the exhaust. I still get 20mpg average with all of my mods and a stock target a/f ratio. Remember though, stock o2 sensors are only accurate at 14.7:1 so I'm not sure how well changing the target will work. Worth a shot though.
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Old 06-14-2004, 08:57 AM   #10
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This is a good point too. I'm not entirely clear on
the true nature of oxygen sensor electrochemistry
but if you are getting shoot-through (like from a
strong scavenging vacuum bounce and a high
valve overlap cam) your exhaust will contain a
high oxygen (and fuel) level. Everything I've read
indicates the sensor works off exhaust gas oxygen
partial pressure and this sort of "assumes" fully
combusted air/fuel beforehand (for the 14.7:1 = 0.5V
ideal output notion). If the air/fuel >exhaust gas is
not burnt then you could be indicating the oxygen-
based (false) voltage and not the eventual (burned
down by the cats) true chemical balance like will
be seen by a downstream wideband.


Quote:
Originally Posted by MyLS1Hauls
LTs can actually over-scavenge the cylinders at low engine speed and can actually pull air/fuel right out of the motor and out the exhaust. I still get 20mpg average with all of my mods and a stock target a/f ratio. Remember though, stock o2 sensors are only accurate at 14.7:1 so I'm not sure how well changing the target will work. Worth a shot though.
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Old 06-16-2004, 05:34 PM   #11
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Ive got the slp long tubes coated stainless ,and yes i've had the slow switching codes.
I changed to denso sensors and checked for exhaust leaks until I was blue in the face.
A strange thing happened when I raised the value for stoich in hp tuners,my ltrms went positive.When I lowered the value for stoich and the ltrms returned to negative.
I am now in the process of bumping the timing to see if that helps the milage deal.The car still has a stock cam in it so overlap is not excessive.
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Old 06-24-2004, 06:04 PM   #12
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Well I pulled the cats today and stuck in the wideband to check air fuel ratio with the value for stoich at 13.8847.It idled at 15.6,crused at 14.7-14.8.Wot was 12.0 to 12.1 and that suprised me as I'm seeing some kr at 4000-5600 with timeing at 17.5 at 4000.Thats got me stumped as to why it would rattle the motor with it being that fat and so little advance.This was with 92 octane fuel which is the best we can readily obtain.This seems to support the theory that our o2's lie to us in long tubes at cruise.
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Old 06-25-2004, 12:15 AM   #13
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Quote:
Originally Posted by samz28
yah there's a field that says desired a/f 14.7. I assume if you set it to 15.0 it will skew some closed loop values and lean it out. assuming headers make the system think its leaner than it is.
I think it would be really surprising if the pcm used a desired value for a/f to control closed loop mode. From everything I've read the o2 sensors are only accurate for indicating "above 14.7" or "below 14.7", so it would be surprising if the pcm software could use any o2 voltage away from the .45V center point to control fuel trims. Does LS1Edit also have this field?
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Old 06-26-2004, 12:22 AM   #14
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I don't know what edit has but my mileage has returned to it's normal 300+ per tank now also.I didn't change anything else that would account for the increased mileage.
The 02's were reading around 960 when the wideband said 12 to 1,at wot.
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