[Diy_efi] long term fuel trim

[Adam Wade]

--- Tim Marsteiner <tmarstei at yahoo.com> wrote:

> Do you have any good links to OBDII info? 

I don't right off, but it's been used on all US-model
cars since 1993 or so, so there's a ton of stuff out
there.  Should be good stuff in the archives, and
possibly the links area of the DIY_EFI site.

> How does one go about dialing in the ignition timing
> advance?

Well, timing is a trade-off.  The later you start the
burn, the less combustion pressure you have working
against the compression stroke, but the more room the
combustion gases will have during the power stroke,
and thus the lower amount of useful work extracted
through the crank.  Start it too early and you lose
power through resisting the compression stroke, and
what's worse is that you start to run into detonation
by raising the pressure at its peak (as well as
greatly increasing the rate of pressure increase near
TDC).  Ideal would be a very fast-burning fuel and a
later spark timing, but make it TOO fast-burning and
you have another high pressure peak, and you're back
to detonation again.

Finding the best intersection of those things is
tricky, since burn rate changes based on charge
density (which is related to compression ratio, cam
overlap, and throttle position) and charge composition
(EGR slows the burn; over-rich or over-lean conditions
also slow the burn).  This is why I recommend setting
the fueling first, and then dialing in the spark
timing, since optimum spark timing may change based on
fueling.

If you know what you are seeing on a four-gas, you can
watch for characteristic changes in exhaust gas
composition that correspond to burn rate and how
complete the burn is before the exhaust valve opens. 
As another poster noted, in high-compression engines
that are run lean, the detonation limit will come into
play before finding best power.

> Just tweek until it knocks a bit then back it
> off some for a reasonable margin?? Does this need to
> be done on a dyno?

I would do it on a dyno if you possibly can.  You will
probably find that in many cases (many combinations of
alpha and rpm), your best power timing will be
retarded more than "a bit" from the knock limit.

>> That's an interesting idea.  Have you been able to
>> find any data on how long it takes to "learn" a
>> trim?

> I believe it is reset by disconnecting the battery

That's consistent with the Triumph iteration, so no
surprises there.

> and I’ve heard numbers like 50 miles or so for it to
> “learn”...

Interesting.  I'll be interested to see if you can
determine anything more substantial from data-logging.

>> The only drawback I see is that you'd pretty much
>> need to keep the WBO2 in there all the time, or
>> eventually the ECU would "unlearn" the trim after
>> you reinstalled the original O2 sensor.

> Yes, I know this. However, I’ve actually got this
> comparator working with my narrow band sensor.

That'll get the job done, then, altohugh keep in mind
that running rich at cruise will shorten the life of
both O2 sensors over running at stoich.  Since I take
it you will be removing the WBO2 after tuning, your
only concern would be a 5-10% reduction in O2 sensor
life.

> If I want to keep the narrow band sensor in place
> AND monitor with the wideband, can I just lay the
> wideband sensor up in the exhaust a few inches or
> does it definitely need to be installed in a bung
> up near the engine?

The closer to the engine, the quicker it will respond
to changes, but you want to watch your EGT.  Too cold
and the sensor won't begin operation; too hot and it
could be damaged.  Modern sensors are a lot more
tolerant of temperature ranges, but I'd check the
specs on the sensor you have and maybe use an IR
thermometer to check header temp at various spots
during mid-throttle no-load running for a minute or
three while on the sidestand.  Add at least 50-100 F
to the number you see, and if you're within the range
for the sensor, you're good.

Somewhere near the stock sensor location should be
pretty adequate.  Some others here will probably have
some even better input on sensor location/

=====
| Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |

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