Re: 88 MB Urq questions

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <199809131842.VAA29067@vtoy.fi> "Jouko Haapanen" writes:

> It is going to be necessary for me to do something with the turbo itself.
> My choices come down to three different approaches: a)factory exchange
> turbo (simple and guaranteed to work) b) rebuild the turbo (cheaper than a,
> but not as mind-easing) or c) an improved turbocharger design, hybrid or
> otherwise.  Option C has me interested, but there is so little (read none)
> past experience here in Finland (at LMS) with modifying MB's that I have to
> research this one on my own.  Is there a shop somewhere that has experience
> with modified MB's, and can someone recommend them?

There's very little information _anywhere_ on the MB, as I'm sure you're
aware having dealer access to the microfiche.

The problem is that the MB is a cobbled-up abortion.  For those who
don't know the engine, it's a derivative of the 1986 hydraulic tappet
engines fitted to the 200TQ (1B), the 5000TQ (MC) and even the lowly
Passat (JS) uses the same head from 1986.

The MB's weakness is mixture control.  It has no OXS sensor and obviously
no cats - whereas the MC engine uses closed-loop A/F ratio controls,
the MB uses a computerised guessing algorithm.

At present, we don't understand exactly how full throttle enrichment
works on the MB engine.  The mechanisms are quite clear and understood -
it's exactly how they are used that is the mystery.  Detailed and fairly
careful measurements using a VAG 1348/1 to calibrate the potentiometer
on the air mass sensor and test driving the engine suggest that (on all
three cars tested) the air mass sensor 'tops out' at 4500 rpm or so
in the top three gears.  If this is true (and Scott Mockry has some
well-founded theoretical objections) then some other mechanism is
responsible for fuel enrichment from this point onwards.  Orin Eman
has produced an excellent little duty cycle display that runs directly
off Pin 8 of the ECU (the wire is resistive - another problem solved
by Scott) showing that the duty cycle is _violently_ altered by the
ECU under acceleration.

The problem is - these duty cycle values are calculated, not derived
from tables (as with ignition and boost values).  The code is complex
and not yet understood - but it _must_ make assumptions about the
engine's geometry.  Especially - if our experimental results are valid -
air intake volume above 4500 rpm is being guessed at.   Temperature
and boost are still measured, but volume _might_ be unknown and
assumed.  Change the air inlet duct (K&N?) or the cam, and you _might_
disturb the whole house of cards.

We think this is what happened to John Robinson's car, which has had
some major 'tuning' surgery and now stops at 4500 rpm.  It's utterly
weird - the car accelerates like a rocket to 4500 - and then stops
accelerating.  Period.

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
 (The contents of this post will _NOT_ appear in the UK Newsletter.)