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Old 11-04-2005, 06:40 PM   #1
kgb
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Lightbulb Timing and AFR revisited - when the wideband reads > 14.7, and Qs on misfire

As long as the engine didn't misfire and the wideband is working, if the wideband reads > 14.7, the mixture is lean period, regardless of timing? If the engine misfired, how would the wideband read? Based on my reasoning, could I then use a lean reading (based on wideband logs) to tune to stoich, and adjust timing based on that, for the low load situation (specifically I'm looking at 20kPa and 60kPa which is where the Link is centered)? Of course this is all IAT dependent, but if I tune late at night (as IAT is cooler, more air flows = more fuel required), I would be richer than I need when IAT is higher.
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Old 11-04-2005, 07:18 PM   #2
Jon [in CT]
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As far as I know, there's only a weak relationship between misfire and spark advance. As you move the spark advance closer to TDC, the A/F mixture tends to ignite more readily because the additional compression imparts more heat to the A/F mixture. Thus, for example, it's possible that a particular A/F mixture which ignites when spark advance is 10° BTDC might not ignite when spark advance is 30° BTDC.

A/F ratio sensors respond to the partial pressure of O2 in the exhaust. When a misfire occurs, much (or all) of the O2 present in the intake charge will be flushed out the exhaust ports. The A/F ratio sensor will sense that as a very lean spike.

As for your basic question, I'm afraid I don't understand it.
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Old 11-04-2005, 07:25 PM   #3
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I am not sure I understand the question either, in regards to tuning while colder that is what you need to do if you do not have temp corrections. If you are talking about tuning for 14.7, that is not lean at the load levels you have talked about 20-60kpa you should be running around 14.7, at most prob 14.4 at midrange or higher rpm, and richening up as you increase in load.

oh, i see it was reading greater than 14.7, after rereading, haha. not really sure exactly what you are asking.

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Old 11-05-2005, 02:22 AM   #4
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I guess I was too vague, to a point that only I understand it...

The reason I ask is this - if the O2 is reading slightly lean (not a lean spike under misfire), you can't really have the mixture wrong from the wideband reading (meaning that particular mixture pretty much has to read that lean). I would richen that point up to right at stoich, at that point, with that particular IAT, I would have a perfect mixture - I can't go any leaner regardless of timing. There are times where if you need to retard timing, your wideband reading could change but of course that might only happen when the timing is far too retarded.

At low loads, the chance of knock should be minimal, to a point where playing with the mixture is safe enough to find stoich by leaning out first, then richen. With that in mind, knowing the mixture is exactly at stoich, you can adjust the timing a bit - but of course, this only really works for area where I want stoich (probably somewhere below 4K rpm, and should still be fine up the 60kPa center)

I don't know if this actually clarifies the situation I'm experimenting - with the way Link fuel map works, it's always a crap shoot with the fuel map because you can't adjust the RPM split points and the interpolation gets in the way (hence need to set 2 rows exactly the same, sometimes the adjacent zones exactly the same, just to find out what my mixture would be with those particular values)
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Old 11-05-2005, 02:26 AM   #5
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As for tuning at night with cooler IAT - my Link doesn't compensate for IAT and I want to tune for stoich at low loads, for as wide a range in the low RPMs as possible. Once I get that, as the IAT rises, the mixture would be richer and at low loads, should never be in danger of knock. It's backwards alright - but I have a good enough fuel map up at 100kPa and 140kPa, it's down low that I throwing way too much fuel (richer than stoich below 4K)
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Old 11-05-2005, 07:41 AM   #6
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Quote:
Originally Posted by kgb
There are times where if you need to retard timing, your wideband reading could change but of course that might only happen when the timing is far too retarded.
If you aren't misfiring and you retard ingnition timing, your wideband reading will not change. If you are misfiring and then retard ignition timing, there's a small chance that you would eliminate the misfire, in which case the wideband would read richer after retarding ignition timing.

Last edited by Jon [in CT]; 11-05-2005 at 07:48 AM.
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Old 11-05-2005, 08:38 PM   #7
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I guess in low RPM, the change in timing is usually not drastic enough to cause a change in the combustion time, and in high RPM, because the gas temperature is much higher, it's that much easier to burn the whole mixture before it hits the wideband O2.

I guess the fuel adjustment after setting the timing, is strictly to combust the tendency of knock in case there wasn't enough fuel - I'm thinking of running 13.3 down low on boost and somewhere in the 12s after 4000 or would that be too late?
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Old 11-05-2005, 09:03 PM   #8
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Quote:
Originally Posted by kgb
... it's that much easier to burn the whole mixture before it hits the wideband O2.
It's not as if your wideband is mounted just outside the exhaust ports. Isn't it way downstream, behind the turbo?
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Old 11-07-2005, 01:25 PM   #9
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Wideband is about 12-18" on the downpipe after the turbo - exhaust ports are far too hot
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Old 11-07-2005, 02:17 PM   #10
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Unless you are experienceing misfires at that level of vacuum, I would not be terrible concerned running leaner than 14.7.

Why are you running Link anyway. Have you fitted a turbo?
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Old 11-07-2005, 05:47 PM   #11
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Link came with the kit that I bought and I'm trying to maximize the Link instead of going to another EM, junking the Link in the process. I don't really care for leaner than 14.7, but really just as a method to eliminate the timing effects in AFR (which apparently has very little effect)
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Old 11-07-2005, 10:01 PM   #12
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Quote:
I'm thinking of running 13.3 down low on boost and somewhere in the 12s after 4000 or would that be too late?
thats really reallly lean for a turbo charged car
maybe somthing like 12.0 till .5 bar then 11.1-11.5 depending on egt (assuming running 1 bar) 14.7 is only good for fuel economy not power

do you set target A/F in the link for closed loop
in my car
closed loop at 65mph -3400rpm -target a/f 14.64 i am running like 40deg of advance.. ill get an egt of ~650C 3" from #3 .. at 32 deg it was 750 (too hot)
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Old 11-08-2005, 01:57 AM   #13
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Well, the 14.7 is really for emission... basically, I don't care for much power anything below 100kPa absolute (not boosting) and wants to lower emission as much as possible. On boost, Is 13.3 too low even below 3000rpm?

With the Zeitronix simulated output for closed loop - it's really only useful for targeting stoich as anything outside of 14.7+/-0.2, the reading go either way low or way high anyway.

And of course, not having the AFR nailed down in between zones when I had it nailed right at the center of the zone doesn't help at all
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