[Diy_efi] boosted compression ratio

[Mike]

At 01:35 PM 23/12/2002 +0800, you wrote:
>I'm not looking for maximum economy - I'm pretty much getting that
>anyway. The aim is to reduce pinging and possibly knock that causes
>the ECU to retard timing from it's target, resulting in a drop in
>output torque. An improvement in economy is expected as a
>side-effect.

mmmm OK, I take it you mean the ECU reaches its own max advance
by virtue of a knock sensor - which we assume is operating correctly
(I say that as these are high impedance devices and dont need much
in way of connector contamination to go a little haywire), though
its weird an ECU would be arranged to keep advancing until knock
on a production car ! <phew>

>No modifications inside the head. The engine was stock-finnicky.
>ECU is stock - very difficult to get any modifications done for
>ADR-spec'ed Digifant. The only changes on the car are a modified
>filter box, K&N filter element, a slightly warmer camshaft and a
>tweak on the spring pressure of the air flow meter.

Eeeek - You get much better performance going for a hot wire type
AFM than the vane types ! If I recall the factory AFM on my VL
turbo is a pretty much generic Bosch type in that it was intended
to replace many vane types - I have a spare which you are welcome
to try (you are in PErth?)  - the pin configuration may well
be similar, you only need to put in a burn cycle ?

>Engine management is mostly closed-loop so even changing the fuel
>pressure regulator didn't increase fuelling.

So why did you tweak the spring on the AFM, you did make the
spring a little weaker - right ?

Bear in mind most ECU's are designed to have only minimal shift
from initial settings so as to avoid wild fluctuations affecting
overall stability. So if you start out quite lean then the ECU
may not have enough opportuntiy to make it richer, hence you
are hearing ping ! I've messed with mine to some degree and found
that it won't change fueling by more than about 8% from the
base settings - ie. It will try then hits an aysmptote. So
as long as I set it up to a median then it can go a little
lean or rich as needed but wont go far from the base settings.

There is an adjustment pot on my AFM (which goes only to the
ECU) which is a rich/lean trim and this has about a 15% affect
(or maybe a little mroe) on ECU injection DC - its a 22 turn pot.

>Changing the base timing by up to 10 degrees changes nothing as far
>as detonation is concerned; it'll still advance timing until the
>onset of knock.

Thats a little strange, you are saying the ECU is designed/programmed
to keep advancing timing until it detects knock - how long does it
keep it there, or does it 'try again' a bit more etc etc ?

>I've spent the last 5 years of my spare time looking and waiting for
>extractors for my car... no luck. Looks like I'll have to go
>overseas, buy them, put them on a pallet and ship them back home.

hrrrmmm, they are not difficult to make, I had a 3" full exhaust
system made just a few months ago for my car by 'Exhausts r us'
in Osborne Park - ended up with full stainless steel and new cat
along with dump pipe etc etc. The main guy there is Dale who often
makes custom extractors for several non Australian cars. In about
6 - 8 months I'll be looking seriously at a twin turbo setup and
he'll likely be doing the manfiold. They are a bit idiosyncratic
but the best people seem to share that trait but, he wont 'jerk
you around' and I had maintenance done on it (free) after damage from
a 1800km drive when going to/from Monkey Mia with a full load
of gear and 4 passengers!

>If I use normal ULP, the 7c/litre saving is offset by a 10 to 15%
>increase in fuel consumption. And then there's the crap in the fuel
>that gums up injectors to they need a "dose" every 15,000km as
>well... not been required since 99% of the fuel used has been 98.

Did you do a double blind trial ;-)

Its amazing how much of the economy gain is defined by rate of
change of foot pressure, know anyone with a dyno and fuel
consumption test setup ?

I 'seem' to get about a 5 - 10% improvement in economy when
going from ULP to Premium but havent noticed much at all when
going from Premium to BP 98 - havent tried optimax yet - partly
because here in Perth there arent many outlets.

>Well, the car's been "pinging for 267,000km" and so far there's no
>problem to be seen. When the head was removed after I blew a gasket
>(selected 2nd instead of 4th gear approaching Shell) at around
>160,000km, the tops of the pistons and the head showed no evidence
>of damage.

Nice bit of irony there, no problems after 267,000K. But blew
a head gasket - huh ?

The head gasket is the weakest - did you retorque it after it
was first put in - I did mine after 1000K of a new head some
2 years ago and yes all the head bolts were showing different
settings - trick was to release by 1/2 turn before retorquing.

CLearly you are not getting major ping thats enough to damage
pistons (or rings perhaps), but enough to weaken the gasket
seal - so I 'feel' its something else thats happening here !

>> I'm curious what your engine configuration is and what your
>> objective re ignition advance is ?
>
>To stop the knock/ping and let the ECU reach it's nominal target for
>advance. It seems to do that at certain times anyway; depending on
>conditions.

I wonder if there isnt something else thats occuring here,

i mean you've 'tweaked' the spring in your vane AFM and assume
the ECU has wide range over which to control fueling.

What you may have here is a combination that results in the
ECU not being able to fuel because its base is out of its
median control range ?

Do you have a TPS - with functioning acceleration sense,
ie Rate of change.

Have you looked at the injector duty cycle change when
in stead state cruise and how this changes between fuels
for the same cruise conditions ?

What sort of plugs are you using ?

I used to use the 'split-fire' until I melted the ends off
those in chambers 5 and 6, went to bosch 4-point and noticed
not only an improvement in economy but much less ping. The
metal hanging off the Splitfire seems like a disaster waiting
to happen for preignition issues, ie. Sharp pointy ends
where its split in comparison with the bosch 4-point which
I can run 10.5psi in winter versus no more than 9.5psi
boost on the splitfires when I tried a new set in last July...

Just occured to me, have you changed/upgraded the capacitor
which is near the ignition coil, ie The one which goes from
coil +ve to ground. I had an odd problem for a few months
with weird ping, octane didnt change a dman thing, ended
up taking out the 470pF suppressor - changed it for an 8uF
Bosch can cap in the same position - problem solved like
chalk and cheese. The old cap however didnt show any leakage
but did change its capacity by 30% when heated !

In fact the more I think of it - this could well be one
of your problems, because if this cap is flakey then it
might well be enough to push the ECU over the end so
to speak into high retard territory - how olds the cap ?

Even though that part of the circuit only really gets ignition
voltage, I put in a 250v AC rated 8uF industrial cap and the
difference was more than marked, much more stable at 4000-
6000 rpm then it ever was - even to break traction at WOT
on a hill in 2nd in an automatic !


rgds

mike


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