Re:: Safe Boost

[ptimmerm@mashtun.JPL.NASA.GOV]

Allan asks:

The fix Peter Wales outlined seems quite easy to implement.  But others say
that overboost protection is gone.  I'm not clear if this is sensor
saturation or A/D saturation or something all together different.  

Here is a little tutorial

1. Safety

The point we were trying to make is that the ECU normally shuts down
the fuel pump when the boost sense reaches a preordained value,
to try to stop us hackers.  The least sophisticated means of fixing
it is to wire the fuel pump ON.  This defeats the most important
safety feature.  If you crash you new hot rod, the pump will keep
on ticking, giving you a licking, a flame licking that is.  The 
clamp circuit allows for shutdown due to crashs.  The ECU would see
that that the motor is not turning over and kill the pump.  It would
not see the less likely event of a stuck wastegate.  The signal is
clamped to a lesser value, so it doesn't know about it.  The voltage
divider would allow the ECU to see a errant boost level, albiet a higher
than stock level.

2. Performance

The Hot Wire method offers no way to increase timing, so in addition
to being the ugliest, and most dangerous, it is also the slowest.
The clamp circuit may or may not increase you timing depending upon
how the voltage clamp rolls in.  Setting it low enough does give
some voltage advance, but only at high boost.  This method is safer,
and offers a bit more performance potential.  The voltage divider
offers a change to timing throughout the range of boost, all the way
down to and below atmospheric levels.  You can feel the difference
letting out the clutch at just above idle to cross a street.  You 
can also feel the improvement of midrange and really feel it at max
boost.

For an UrQuattro I-5 running 15 psi of boost, the first two options
will give about 200 HP and the later giving 225 HP, based on my own
testing.  This type of gain should be mimicked in other cars as well.

3. Difficulty

I think that there is some additional complexity in setting up
the faster mods.  Not in the wiring, but in the engineering.
The hot wire fix requires no testing.  The clamp requires little.
And the voltage divider requires quite a bit.  But it's worth it.

I hope this helps give a little more basis so all can follow the 
"discussion".

paul timmerman



 crank or
distribuitor sensors)