"jmk" wrote in message
oups.com...
Something to be aware of here... it was the "Coke bottle test" that got
George into all the GAMIjectors to begin with. He didn't like the way
the engine ran. So, although of questionable legality, played with the
"Coke bottle test" until he got all the injectors to put out the same
amount of fuel. ... And the engine ran WORSE. He figured he'd spend
the rest of the day figuring out why... and a couple of years later, he
had! G

Do you have a source for this?


What you want is the fuel/air mixture to be the same for ALL cylinders.
Ideally this would equate to the same amount of fuel in each, but in
the real world it does not. When set up right, since all cylinders
have the same FA mix, they will all peak at the same time. Differences
in probe positions, and differences in cooling air flow, will generate
different measured EGT's (as well as condition of plugs, mags, etc.).

http://www.avweb.com/news/reviews/182558-1.html (Mike Busch)
"The first theory advanced was that the front cylinders had to breathe their
induction air through a longer path than the rear cylinders and therefore
were getting less air, while the rear cylinders had a shorter induction path
and got more air. If the rear cylinders got more air but the same amount of
fuel, that would explain why they ran leaner and reached peak EGT first.
Conversely, if the front cylinders got less air but the same amount of fuel,
that would explain why they ran richer and reached peak EGT last.

It was an appealing theory. But it was wrong! In fact, all cylinders in
these engines breathe very nearly the same amount of air.

After investigating further and consulting with a very smart ex-TCM
engineer, Braly discovered the real reason why the mixture distribution was
skewed. The TCM fuel injection system is a continuous-flow system, which
means that each injector nozzle sprays fuel into the intake port of its
cylinder all the time...even when the intake valve is closed. During those
valve-closed periods, a certain amount of atomized fuel is sucked into the
induction manifold beneath the cylinders and is carried forward by the
airflow through the induction system.

What this means is that a bit of the fuel sprayed by the rear cylinders'
nozzles wind up in the middle and front cylinders. Likewise, the middle
cylinders "help" the front cylinders with a bit of their fuel. As a result,
the rear cylinders actually get a bit less fuel than what their injector
nozzles deliver, and the front cylinders actually get a bit more!"



With standard injectors just rotating them should be okay (and legal)
-- all you are doing is using the really terrible QC from the
manufacturer to your benefit. Traditionally the Coke bottle test
involves also ordering sets of different orfice dimensions form the
mfr. and adding them into the mix. This messes up the measured fuel
flow readings and required recalibration of the fuel metering system.
[Hence the reason the FAA frowns on it.]


"Once Braly understood the reason for the mixture maldistribution, the
solution was obvious: vary the orifices in the injector nozzles to
compensate for the "borrowing" of fuel between cylinders. The lean-running
rear cylinders need larger-orifice injectors that deliver a bit more fuel,
while the rich-running front cylinders need smaller-orifice injectors that
deliver a bit less.
Of course, George wasn't the first to come up with this idea. Knowlegable
A&Ps had been quietly playing "musical injectors" on their big-bore TCM
engines for years. I say "quietly" because the use of different-sized
injectors on a TCM engine wasn't exactly legal: the engine's type
certificate data sheet specifies that all injectors are to be the same size.
So this is the sort of thing that mechanics would usually do only on their
own airplanes, and it usually wouldn't show up in the logbooks or be spoken
of in public. Generally, this injector swapping was done on a hit-or-miss
basis without engineering discipline or instrumentation. Sometimes it
worked, sometimes not."


Frankly, 110 max difference in EGT isn't meaningful. As someone else
suggested, look at the spread in FF vs. peak temp for each cylinder.
THAT will tell you if you have a problem or not.

Don't think so...more like 25-50 max...