manual boost controllers, and ECU functions

[isham-research.freeserve.co.uk at pop.pol.net.uk]

> On Mon, 29 Jan 2001, Ken Keith wrote:

>> I was wondering what it is that the ECU on, for example, and MC-1
>> motor really does.  I think it basically controls timing, by retarding
>> it if it detects knock, and also controls the frequency valve (?, or
>> something) to keep the mixture as close to stoich as possible.

> Yes, from what I've read thus far.  It also controls boost via a solenoid
> valve (not sure the name of the valve).

It also determines when the mixture is to be changed from stochiometric
to 12.4:1 for wide open throttle operation, determined by the throttle
body switch.

The solenoid valve doesn't have an official name in English.  We refer
to it as the wastegate frequency valve.

> Under normal ops, the wastegate opens under a certain pressure set by the
> spring.  There's some vacuum tubing that comes into play here, but it's
> constant.

The MC-1 has three variable inputs to the wastegate - exhaust pressure
on the plunger, boost pressure to the centre chamber, and whatever is
fed to the top chamber.

> When warranted, the ECU opens a solenoid valve that allows manifold
> pressure to help hold the wastegate closed- thereby assisting the spring
> and raising the boost.

It modulates the valve, but only during wide open throttle operation.
This is why a healthy WoT switch on the throttle body (if yours has a
centre seam running round the sides it's probably broken) and good
vacuum tubing is essential for stock boost performance.

> When the specified level is reached, the valve is closed, the assist goes
> away, and the wastegate opens.

If the wastegate frequency valve's modulation by the ECU stops at any
time with the throttle fully open, you have another problem somewhere.

> The ECU can modulate the valve to hold any boost level it wants.

Actually it's quite a narrow range.  The spring is the major
determinant.  One of the major functions is to hold the wastegate shut
for a short period to develop short-term overboost.  Scott Mockry
once captured some fascinating oscillograph traces of this phenomenon.

>> I guess my question is, what does a chip do besides raising the
>> overboost fuel cutoff?

> It can raise the boost.  Other details I'll leave to Audi experts..

No.  All 'chipping' mods are accompanied by replacement wastegate
springs.  This is what actually does most of the work.  'Chipping' is
done in a number of ways including simply disabling the overboost
cutoff, moving the point at which it occurs with a resistor bridge,
doing this in a slightly more sophisticated way with a zener diode, or
full remapping.

The ECU contains a number of 16 x 16 tables that control initial timing
values.  These are indexed vertically by boost and horizontally by rpm.
If you raise the maximum boost the engine will see, you have to move the
timing values down and insert new ones at the top.  This involves
redigitizing the boost level input to divide (e.g.) 1.85 by 16 rather
than 1.4 by 16 to give the row index boost increment.  Then new values
have to be determined for the new rows from 1.4 to 1.85.  Etc.

Then there's the fueling - which is done in code, not tables.

>> Another question, would an SJM chip affect how quickly target
>> boost is reached?

> I doubt it, becasue a stock ECU should do this:

A chipped ECU reaches target boost slower than an unchipped one - the
inertia of the turbo is unchanged and it has to spin up further.  The
stiffer spring means you reach the previous maximum more quickly, but
it takes a little while longer to get to the top.

--
 Phil Payne
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