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# Re: 88 MB Urq questions

```(I've not snipped any of the discussion - I want a complete copy to
go to John - via fax as he isn't on email.)

In message <199809140201.TAA06002@gonzo.wolfenet.com> Orin Eman writes:
>
> Phil said:
> > 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.
>
> It seems that it turns into a MAP system once the air flow meter
> tops out.  Given MAP (manifold pressure), RPM, intake air temperature
> and a table of the engine's Volumetric Efficiency (VE), you can
> calculate the fuel requirement of the engine.  There are some
> equations somewhere on the the diy_efi site.
>
> Now volumetric efficiency is a measure of how much air gets stuffed
> into the cylinders.  It depends on just about everything...  cam,
> exhaust, manifolds, intake/exhaust ports, combustion chamber shape
> etc..  The effect of a lot of engine tuning is to increase volumetric
> efficiency.  Higher volumetric efficiency means more air gets
> into the cylinders at a given manifold pressure/RPM.

Exactly what's happened with John's car.  K&N filter, some porting, a a
new cam and a different turbo.  We have the stock cam back in at the
moment.

> So, if you increase volumentric efficiency, you've got to find
> some way of getting more fuel!  IF the air flow meter has topped
> out and the ECU is doing some simple calculations based on its
> idea of VE, then you won't be getting enough fuel!  Yes indeed,
> the house of cards falls over.

Scott is of the opinion that mechanical enrichment continues way past
4500 rpm.  Is it possible that the VE on John's engine has been changed
enough to produce full paddle deflection this early?

> > 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.
>
> How does it do at less than full throttle?  Better?

It seems (to me) to be noticably better at full throttle as long as the
revs are kept down.

> As for the US MC engine, it forgets about the O2 sensor pretty much
> as soon as you go on boost and uses one of two RPM based duty cycle tables
> for the frequency valve... (one table for low boost, one for
> not so low boost levels).  There is no carry forword of anything learnt
> by the O2 sensor to on-boost behavior.

As you showed, the MAC12D is a cut-down MAC11.  I would not expect any
carry-forward here.

> On my car, I haven't seen any evidence of it leaning out at higher boost
> or RPMs using a simple LED bargraph on the O2 sensor, nor using
> the voltages from the O2 sensor on my data aquisition board.
> However, I have noticed that the MC tends to detect knock
> around 4800 RPM...  We have a code change for the MAC11 along with
> a simple 1 transistor and LED circuit which will indicate when the ECU
> detects knock and retards timing.  There may be a code change soon
> that displays this on the check engine light!  The knock LED tends
> to come on between 4500 and 5000 RPM on the MC with not much above
> stock boost levels.  It takes quite some timing retard to stop this.

The MB is a single knock sensor engine, like the MC-1.  If the fixing
for the second knock sensor is present on the engine, would bolting
a second one on and hooking it up to some instrumentation help?  I note
your comments in the code about noise from these devices.

> (I'll try to find the MAC12 listing and look into doing the knock
> LED.  Got to do the MAC14 first tho'.)
>
> Again, if you are getting more mixture in the cylinder than the ECU
> expects, its timing tables are going to be wrong too... more likely
> to knock... and once it starts retarding the timing, it does
> feel like you stopped accelerating!

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

```