[RUBE]

I have read of erroneous readings from leaks and valve overlap or bad wb placement.
does anyone know enough about exhaust dynamics to say what affects very short systems might have on these readings?
my exhaust from manifold to atmoshere is about 30 inches long. on the previous engine as well as the present both carry black soot in the outlets. smells rich at least at idle. not always a steady idle. consistant 25mpg below 60mph and town driving. 2 inch bores on pipes and glasspacks duals. this is a 99 3.8 in a 1500#car.
now im thinking the sytem is so short there is absolutely no back pressure leading the sensors to say lean and cause it to run rich. also have read to work properly they need some back pressure, if so should i add some sort of restriction to the pipes?
any ideas out there
lets hear them

[cgrey8]

There may be no back pressure, but that doesn't mean there isn't back-flow. Even narrow duration cams can produce a fair amount of exhaust back-flow. Larger cams will do so even worse at lower RPMs because the exhaust valve is still slightly open when the piston is dropping in the bore. At higher RPMs, this is a chance for more exhaust to get pulled out of the cylinders even as the piston drops. But at lower RPMs, it's a chance for exhaust to literally get sucked back into the cylinder which has the effect of sucking fresh air back up the exhaust pipe. If the WB is near the outlet of the exhaust, it could be picking up oxygen from the outside due to back flow. I don't know that this is your problem, but it's something to consider...

[decipha]

i wonder if this had anything to do with Henry's logic of hanging the rpms until a complete stop on the older ecu's

just the first thing that came to mind

[cgrey8]

BTW, I also edited the title to not be in ALL CAPS.

[86GT]

I would be more concerned with temperatures. The HEGOS need to be at a minimum of 650 and if it is not the readings will be inaccurate.

[RUBE]

O2's are in the manifold just above the flange, wb will be 3in after them. dont think heat should be an issue.
am in the process of verifying mpg increase. took off the glass packs and put on a couple of 13in long turbo mufflers i had laying around. internals added some more length to exhaust stream. they hang down too far for permanent instal but seem to show better mpg. have a pair of lawson ind. glass pack style mufflers coming. they are baffled and 2 tubes inside like a reg muffler. they will fit better in my limited space.
i suppose if need be i could turn a 90 after each muffler and shoot the exhaust across to the opposite side. would look funky but would add another 3 feet of length.
Thanks for the insight.

[decipha]

its pretty easy to figure out if your chasing your tail in closed loop, just look at the hego's switching, if they don't switch as much at low loads or idle as they do at higher loads then you know you can't allow closed loop at those lower loads

[RUBE]

Lawson mufflers installed. i can hear the motor run without drone on the road. if you have limited space i would recommend them. super cheap and they work. they also have other models.
FYI

[decipha]

swell and the verdict for this threads title?

[RUBE]

jury is still out. but in this case i would say as long as the pipe is long enough to keep air from being pulled back up to the sensor, makes no difference. naturally exhaust leaks pre sensor would be bad.
mpg has not changed, so the extra foot and a half length due to the different mufflers had no affect.
for me, question resolved.
soon as i put some meat in the freezer, on with the tuning
Thanks to all for your thoughts.