So it looks like the consensus on the "reboot" problem my car and others
are exhibiting is related to the knock computer malfunctioning.
My understanding of this system (which is based purely on understanding in
general how one might work, not this specific car) is that the knock sensor
feeds back data to the ignition computer, the ignition computer uses this
information to modulate vacuum to the distributor in order to control
vacuum advance/retard. In other words, it's not changing the timing
electronically, it's changing the vacuum signal which is routed to the
distributor.
Can someone verify if this is correct?
In this case, it would seem that if the car is having this kind of problem,
then disconnecting the vac. to the vac. advance and retard, and tuning the
ignition timing the old fashioned way, "by ear" (drive it, advance it, if
it knocks, back it off, if not, advance some more, etc.) would result in
"fixing" the problem. At least removing the symptom of the timing being
retarded whenever the ignition computer has such a whim. The ignition
computer might decide to retard the timing, but it can't, since i
disconnected its vac line. In this case, the ignition computer would
function no differently than a regular ignition module.
So, I know one of the "hot" mods on Z cars and old Datsuns, for reliability
and performance, was to remove the vacuum advance and instead recurve the
distributor for more and later centrifugal advance. I understand there are
a lot of air-cooled Porsches which run this way as well. The idea is that
the vacuum advance advances the timing under light throttle and idle only
(when vacuum is high), and you just run the static advance at WOT. This is
great for emissions, since it tunes it at idle and light throttle, but bad
for performance. With fully mechanical advance, the advance is determined
solely by RPM. So you get the right amount of advance regardless of
throttle position. Only drawback is you get no advance at idle, but heck,
you don't need the extra power then. This was good for quite a few HP and
a couple of tenths on the autocross track for the Z-car guys.
Has anyone done this, attempted it, on an A2 16V? I can't see why it
wouldn't bear the same kind of fruit, since the A2 16V is really tuned
similarly to a Z engine (hot cams, high-revver, peak power is way up
high). Not only that, it's highly reliable. Only caveat is, you have to
set the ignition timing more accurately, since the ignition computer with
knock sensor won't bail you out. Usually on a Z, we'd run peak advance at
like 20-25 degrees, peak from about 3k on up. So it's static at idle,
logarithmic up to 3k where it's +22 or something like that, then +22 all
the way up. To get the advance back at idle which you lose from the vac
advance, you just set the static timing way hotter than spec (like +15-20
degrees on a Z, compared with +5-7 spec). This would be for an 11:1 240Z,
idle about 1100 rpm, max power about 6500 rpm.
To get it even better, this would enable you to run an MSD or other
aftermarket, performance ignition. No knock sensor, but no need if you
consistently run the same gas, tune the car yourself and pay attention to
the pinging.
Later-
-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Josh Karnes <>< "As long as the devil gives you slack in Austin TX
your chain, you think you are free."
- Dick Brown
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