Correct idle mixture/AFR steps:

[wicks]

Setting my mixture recently after rebuilding FD and many other maintenance bits, I placed an AFR meter sensor approx 12 inches into the right exhaust, and after a warm-up drive, set the AFR to 14.5 at 950RPM. I did not do any air hose/vacuum/o2 sensor or other steps.

Reviewing the K-Jetronic manual, noted the page on the procedure has additional steps. Wondering if I should have also taken any of these other steps:
- Plug secondary air pump check valve
- Plug vacuum limiter hose connecting to T above throttle plate
- Install CO sensor (in this case I'm using IM AFR meter) ahead of catalytic converter instead of tailpipe. Does AFR meter in tailpipe not really work?

My air injection pump has no belt, so it's not contributing to the exhaust but for any vacuum based movement of air.

[Black_Hat]

is this on your 94 c2 Turbo ?
for idle mixture screw setting, you need to unplug the stock o2 sensor to set
the plug is on the drivers side of the engine

[wicks]

Yes the 3.6T. OK I shall do that as well. No plugging of vacuum lines? And can AFR (wideband/LM-2) in the tailpipe work for an actual good reading (is there an approximation of how much the cat changes the reading and take that into account)? Or wait, can I screw the LM-2 sensor into the bung for the car's sensor?

[Black_Hat]

no wide band before the cat

[wicks]

OK...still googling, hopefully will be able to find out if the AFR meter sensor in the tailpipe is useless even though it seems people use them that way...and of course then why did I buy this clamp...

[mark houghton]

Originally Posted by wicks

OK...still googling, hopefully will be able to find out if the AFR meter sensor in the tailpipe is useless even though it seems people use them that way...and of course then why did I buy this clamp...

Back when I was running with an AFR gauge I installed the sensor in the tailpipe as far in and as close as possible to the muffler....in my case maybe 10" or so. I was not running with a CAT so can't tell you anything about how that might affect AFR readings, but can tell you that the tailpipe location worked fine for me.

I suppose there is the possibility of a slightly O2-diluted exhaust stream during idle since the sensor is so close to "seeing" atmospheric O2 intruding from the tailpipe tip, but not the case when on the throttle as the exhaust gas quantity and velocity would (presumably and logically, my opinion) preclude any interference from outside air. With that said, the only downside would be a potentially leaner than actual reading at idle, which is not necessarily a bad thing if you're base tuning is being done at idle.

This....on my last 930, not a 964...so take that for what it's worth.

[Alan L]

My O2 sensor a few inches inside my zork works fine - even at idle - stable readings. So 10" inside an exhaust would be plenty.
Alan

[flightlead404]

Originally Posted by Black_Hat

no wide band before the cat

sticking the wideband sensor into the bung for the narrow band sensor will be just fine for this purpose (temporary idle mixture check).

Generally a WBO2 sensor before the turbo is not recommended because the pressure increases prior to the turbo as the engine loads up which will skew the O2 reading. However, this mixture test is at idle so increased back pressure from the turbo is not really an issue.

My understanding is that the sensor change due to pressure increase is linear and I've seen conversion charts (not sure if they are sensor-specific or broadly applicable) so in a pinch you could put it there permanently if you have a way to implement the conversion.

Its fine sticking it in the tail pipe as well as long as its far enough up that there isn't any fresh air getting to it, a few inches is fine.

As far as the catalytic converter, that will definitely affect the ppm CO measurement, which from memory is actually the measurement called out in the specs (not AFR). However, AFR to CO is a straight conversion (charts available from Google), and I don't think the cat will affect AFR measurement. I'm not as sure on this bit however. <<<<---Yes it will, I checked.

[Rawknees'Turbo]

Originally Posted by flightlead404

. . .

Its fine sticking it in the tail pipe as well as long as its far enough up . . . a few inches is fine.

. . .

???

!!!



Just a few?!?!?!

[flightlead404]

Originally Posted by Rawknees'Turbo

???

!!!



Just a few?!?!?!

Well, you know, according to popular lore some of us have a Porsche to compensate

[wicks]

Haha! Well, so is the opinion that using an AFR meter (LM-2) adequately into the tail pipe, then, would produce a usable measurement of engine mixture? Not just at idle but testing cruise and WOT, etc?

[flightlead404]

Its fine if you don't have a cat. If you have a cat you are in a catch 22 situation. The cat will affect the O2 reading in an unknown way. Placing the WBO2 before the cat also places it before the turbo where the increase back pressure will affect mixture reading but in a known and predictable way.

You should really tune for output, not a specific AFR

[wicks]

I.e. horsepower on a Dyno? Max HP means the engine is running at the correct and safe mixture? I reveal how little I know about actual tuning...

[flightlead404]

Originally Posted by wicks

I.e. horsepower on a Dyno? Max HP means the engine is running at the correct and safe mixture? I reveal how little I know about actual tuning...

Just my 2c. I'm just a shade tree mechanic.

Then you should work with someone who does. AFR will give you a sort of "safety limit" by using it as a minimum steady state mixture at a given load {rpm,MAP}. But this is really just an analog for detection of detonation.

Theoretical best power is achieved at a given AFR but this is for a steady state condition.

In theory, theory and practice are the same. In practice, they rarely are.

Automobile engines are rarely in a steady state condition. Its the way the tune handles change of conditions that makes an engine driveable or not, like when you open the throttle do you get a lean stumble? Go overly rich? What about when you close the throttle?

Internal combustion engines are really just air pumps. Intake and exhaust systems effect the efficiency of the pump and are usually optimized for a specific set of engine conditions, at the expense of other conditions. Air and fuel are both fluids with inertia. Air and fuel take time and energy to speed up, slow down, and change direction. Atomized fuel in the air stream might pool or collect in certain conditions. None of these things are really measured by AFR, although some are measured by Volumetric Efficiency.

AFR might get you a base tune, and the logic built into some EFI systems for auto-tuning (essentially calculating VE for you based on a set of operating conditions you generate by driving around), will help. But blindly setting an AFR will never give you optimum results.

All that said, your initial post was about setting the AFR at a single, specific point - idle. That's an entirely different question than tuning for AFR.

Assuming you have a relatively stock setup, all the tuning is already done for you either in electronics, or by a combination of hydraulics and the shape of the inside of the air flow meter and the movement of its arm. When you set the idle AFR you are just giving this pre-set set of functions and pre-calculated formulae a starting point.

[wicks]

Yes, indeed, thanks for the detailed thoughts. One poster said that the idle mixture screw does change the fuel ratio throughout the rev/flow range...maybe just ever so slightly, or something once the flow is 10x what it is at idle or something, the amount of fuel adjusted in or out by the idle screw becomes immaterial?

[Alan L]

At idle you are sucking in 1-2% of your air flow. The fuel is set accordingly for idle, at this air flow. It has a minute effect (<1%) at WOT. It has proportionally more as you approach idle. So it will have a 'noticeable' effect just above idle. Beyond that you won't notice it.
Alan