Opinions wanted ( analysis of the effects of spark advance variation on vacuum)

[Bruce]

Opinions wanted ( analysis of the effects of spark advance variation on
vacuum)
Did you disable the IAC, set the VE table so that for a given amount of
timing the fuel baseline was the same?.
Over what period of time was all this done?.
How long was the engine run for temps to stabilize.
Looking at the BLs, seems like the pcm was affecting the outcomes, by being
closed loop.

I'd suggest repeating things with the above considerations.
Nice to see someone putting in the effort to really tune something and share
the results.

I've found that making cruise corrections takes the ecm 15-20 mins for each
correction to really take effect.

While timing is a big concern, remember it's really about recovering heat
energy.  ie having the optimum timing of when the reaction will be pushing
the piston back down.  So plug gap will have a rather large effect also (in
response to you using a .040" plug gap).
Bruce







----- Original Message -----
From: cmillard at crutchfield.com
To: gmecm at diy-efi.org
Cc: Chipsbyal at aol.com ; ewright at silverback.gorilla.net ;
douglas.t.wood at lmco.com ; jll at edge.net
Sent: Friday, September 21, 2001 10:04 AM
Subject: Opinions wanted ( analysis of the effects of spark advance
variation on vacuum)


Friends, Programmers, Hacks, Lurkers-
.
I would like your feedback/ opinions on a little experiment I did.
The experiment was studying the effects of spark advance (closed throttle)
on idle vacuum.
.
Please take a look at it here:
http://cmillard.freeservers.com/Z28/PCM/Ignition/idle%20spark%20tuning.htm
.
My theory was based on for the same given RPM (900 rpm) the spark advance
which yielded the highest vacuum would give the best fuel economy. The MAP
sensor reads opposite vacuum (higher vacuum is lower MAP, and vise versa),
so I was shooting for minimum MAP. I plan to apply this to idle at first,
then the entire timing map, if enough data can be collected.
.
This is based on Bruce's tuning tips I read about two years ago on ECMGuy's
page, which just started to click in my short circuited brain (zap!)
.
The test vehicle uses a '8051 (as Daawagon and Dave H. called it). The
16188051 is factory in my 1995 Camaro Z28 running 11.4:1 compression with a
LT4 HotCam (219/228 .525/.525 112)
.
I did several excel pivot table charts based on data from about 12 flashes
with closed throttle spark advance varying between 12 and 34 degrees. Here's
what I found:
.
The higher the spark advance the higher the vacuum ( to a point, up to 36
degrees advance ) Optimum spark at idle was 34 degrees, factory was 20
degrees- whoa.
.
The higher the spark advance the lower the injector pulsewidths (matchs
vacuum), again to a point. This again would help to suggest better gas
mileage.
.
Sorry for the rant, it's Friday.
.
Your feedback is appreciated,
-Christian
(disclaimer: working on 2 hours sleep)
http://cmillard.freeservers.com
(Feel free to snarf the folders of my site for amusement)
Christian Millard
Application Developer
Crutchfield Corp
www.crutchfield.com


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