06/06/2003
Lets draw a graph.  On the vertical axis will be spark advance - from TDC at
the bottom to some lead or advance at the time.

On the horizontal axis will be mixture - in terms of excess.  The center will
be lambda (1) where air and fuel are in chemical balance ( stoichiometric ).
Left side will be around .8 (air to fuel or 25% excess fuel ) and the right
will be about 1.25 or about 25% excess air.

Somewhere around a 12.5 to one fuel air (around 20% excess fuel) will be the
point for best power ( most power per unit mass of air ).  Note - for a single
cylinder NA engine as base.  Somewhere between 16 and 17 to 1 - 15% to 20%
excess air - will be Lean Best Power - ( most power per unit mass of fuel ).

Somewhere between 5% and 10% excess air, the charge burns fastest because the
additional oxygen speeds combustion and the additional nitrogen is not yet
enough to slow or stop combustion.  This spot is MEASURABLE by lowest Exhaust
Gas Temperature.  And yes, it varies engine to engine by almost every know
mechanism - but stays in this region.

Somewhere between 5% and 10% excess fuel, the induction period is the
shortest, and the combustion temperature is the highest.  Again, measurable by
EGT as the Highest EGT at this load and rpm.

It is critical on EGT to know which side of stoic you are on.  Going lean past
lowest EGT will raise the temperature rapidly - as the charge is still burning
when the exhaust opens, and if leaned further ( slowing more ) will still be
burning when the intake opens and you will get lean backfire.

Slow burning - from lean or other conditions - can be disastrous to valve.  As
long as the exhaust valve is on its seat, it could basically care less about
chamber temperature - its heat is being carried off thru the seat.  Once off
the seat - it has no cooling until its reseated.  The longer the charge burns
after the valve is off the seat - the hotter the valve becomes.  With good
effective combustion, the heating should be over by the time the valve comes
off the side.  If you've leaned enough to lean backfire, the valve has been
subjected to 4500f + combustion temperature for at least another 180 degrees.

Back to timing.  Draw a parabolic curve with the dip pointing downward.
Center this curve not at stoic - but at the point of highest egt.  This is the
point of least ignition lead needed.

Combustion has two relevant phases.  First the induction period around 500c
when as fast as the energy is being released, its absorbed by the other
particles in the charge.  This continues until the reaction is large enough to
release energy faster than the remaining charge needs it for reaction.

As it gets thru "cold flame" or "blue flame", it erupts into full on flame and
normal combustion.

This now consumes the charge, and has the flame characteristics associated
with it.

Peak energy release gives us highest egt.  Richer - the excess fuel is
strictly an internal coolant and more time is needed to bring the charge up to
temperature.

Leaner - the main charge is burning faster - but there is less hydrogen and
less chemical reaction from the fuel, so the induction period tends to
increase - requiring more advance.

An EGO can not find the points of peak power and peak economy.  It only reads
the mixture.  Other methods must be used to find the peaks.  Once found, the
EGO is one of the best possible means to RETURN to this peak spots.
Arbitrarily picking numbers and getting anal about them means you haven't a
clue about tuning.  

An EGT will help you find the peaks and the valleys of exhaust temperature.
Again, this does not take you all the way - but it will take you to the
crossover points.

Keep in mind ON A SINGLE CYLINDER NA ENGINE that the power output is about the
same at 10 to 1 air fuel as it is at 14.7 (stoic) output.  That peak output at
around 12.5 to one is only about 5-7% higher than stoic, and that at peak
temperature, you have picked up about 60-70% up the power increase of
richening the mixture.

Going lean to the lowest EGT will give you about the same increase ( 60% to
70% ) of the possible increase in going to lean best power.

Multi cylinder engines will behave exactly the same - to the extend that the
mixture is identical at each cylinder.

The key of understand is how what you are doing will affect either the
induction period or the charge burning period.  

Raising compression ratio - no other changes - will always result in
shortening the induction period and little changes on the charge combustion.

Increasing the swirl/tumble will up to a point - reduce the induction and the
charge times.

Keep absolutely in mind that combustion effectively starts with the intake
valve closes and the piston is moving upwards.  Heat is being added to the
process and the action has began.  A spark is not necessary for combustion to
occur.  Anyone who has seen an engine "diesel" and keep running after the
ignition is turned off knows that it can and does happen.

Combustion is temperature controlled - more heat - faster combustion.
Typically, the engine runs out of time before combustion occurs.  The spark
ionizes a portion of the charge - increasing nicely the number of free
radicals and provide a localized hot spot that carries a small portion of the
charge to enflamement.  Then the rest of the charge does its thing.  The
safest way to thing of the spark is not ignition, but as avalanche.  It does't
do anything about putting the snow on the mountains, it just triggers the
release.

Optimum NA tuning has the cylinder at very light knock.  Which translates to
constant volume combustion or similar to Homogeneous Charge Compression
Ignition.  What happens is that the main charge is one RCH away from
"dieselling" when the spark avalanched it just barely in front of it doing
itself uncontrolled.

--------------------------------

Depending on which side of peak egt you are on.  If you are richer than peak
egt, dropping some fuel would move you closer to peak egt, and you would need
less advance to compensate for the excess fuel internal cooling.

In this case, then reducing the fuel will increase the power because of higher
temperature in the chamber - provided that the reduction does not bring you to
the detonation limit.

A side note on EGT.  At any given mixture at any given power level and rpm,
timing should be to get the LOWEST Peak EGT.  Consider that the peak flame and
combustion temperatures are closely constant.  The cooler the EGT the more
power the engine is making from that fuel.

Yet we talk about looking for peak EGT.  At any given load, the mixture ratio
determines the peak flame temperature.  Finding that point is critical.  After
that point, excess fuel simply is an internal coolant.  Theoretically, should
not power peak at this point?

Yepp, cept that the internal cooling of the excess fuel reduces the
compression load on the engine as the amount of excess goes up, and the charge
cooling effects of the excess fuel go up with the excess.  They combined
result allows power to increase moderately with these affects until you are
about 20% excess fuel.

Never make the dweebdom mistake of assuming the mixture numbers in any report,
text or manual are absolutely correct.  They are correct for the engine tested
on and your results will vary - because you have different valve timing,
chamber and piston shape, compression etc. Use them as mental guideposts and
do the tuning with what the engine is happiest with.

Many turbo charged engines are running with 30 to 40 or even more excess fuel
and they keep making power.  Lots of dweebs anally extracting fact about it.
This is simplicity.  The excess fuel increased the engines detonation
resistance faster than the small drop in power the mixture imposes.  10 to 1
air fuel has about the same output as stoic.  Peak is 5 to 7% over stoic.
With the small "efficiency" loss at 10 to 1, they can run 50% or more pressure
and make not the small peak increase but 50% or higher power from the
increased pressure.



On Mon, 2 Jun 2003 22:51:30 -0400, you wrote:

> >I am not going lean, but rather less rich.  I use the fuel to help
> >cool, and feel I could still use a bit less, and maybe gain a HP or 2.
> >My expectation was to decrease timing when dropping a jet size (still
> >rich of stoich though).  From what you said I would be wrong.