Here's another installment in the continuing air filtration saga.
As Richard Notton mentioned, changes in air flow restriction can affect
fuel mixture. Whether this will be a significant enough factor to prompt
you to re-jet your carburetor depends on how drastic the change is.
The way to find out could involve a combination of 1) measuring the
pressure drop (or rise, as the case may be) of air pressure inside the
air cleaner when changing to a new configuration; 2) analyzing the
resulting change in combustion factors with various instruments, such as
an exhaust gas analyzer, etc.; and 3) noting any physical symptoms, like
overheating or pinging (lean mixture), black smoke or fouled spark plugs
(rich mixture), etc.
Here's the rub: Depending on the vehicle and environmental factors, such
as the mean elevation above sea level at which the vehicle is normally
operated, a less restrictive air cleaner may actually improve the
fuel-air mixture. For example, my normal operating altitude range is
between 4,200 and 10,000 feet above sea level, with most of my driving
around 5,000 feet elevation (my home's altitude). If the vehicle is
jetted for lower-altitude operation, as most of them are, it means that
the mixture at higher elevations will be too rich. If using a less
restrictive air cleaner makes the mixture slightly leaner, it would be
to my benefit. In most cases, going to a less restrictive air cleaner
would not make a full jet size difference (varies with carb manufacturer
and available jet increments), but it would be a step in the right
direction.
In terms of performance, a less restrictive air cleaner is good. You may
have to fine-tune the carburetor jetting (both main jet and step-up jet
or metering rod), but that just goes with the territory.
Radiusing (or eliminating) some of the sharp corners in the air intake
circuit would also go a long way toward improving air flow, particularly
in and above the air horn of the carburetor.
Strainer-type air cleaners, such as paper, can quickly reduce air flow
as they load up with dirt, making the mixture too rich. Here, as
mentioned, the old oil bath cleaner has an advantage, being able to hold
more dirt without a significant decrease in flow efficiency.
I'd also like to expand on another interesting point that Richard
brought up: The hermetic seal that is formed when the air cleaner oil
level goes up when the engine is shut off. This may work either for or
against you in terms of corrosion, depending on ambient atmospheric
conditions. When you shut off an engine, there is always some
combination of air, fuel, and exhaust gas left in the engine. Exhaust
gas contains a large proportion of water vapor, being one of the
principal combustion products of hydrocarbon fuels. If the air cleaner
seal prevents air from circulating through the engine, it will tend to
maintain a certain level of humidity within the engine. This may be
either higher or lower than ambient humidity, depending on where you
live. (By the way, this factor will have a greater effect upon cylinders
whose valves are open at the time the engine stops.) Anyway, if you live
in an arid environment, it would be better to allow the air to
circulate. The amount of moisture trapped in an engine will also depend
on whether you shut off an engine when it is still cold vs. fully warmed
up. A cold engine will be running a richer mixture and the trapped
exhaust gas will tend to have more water vapor in it by mass.
Also, many people assume that hotter is synonymous with drier, but high
temperature in and of itself doesn't automatically mean drier. Hotter
air can hold more moisture than colder air. Hot air at say 50% relative
humidity contains more moisture by mass than cold air at 50% relative
humidity. As hot air containing a certain percentage of water vapor
cools down, relative humidity rises and may cause condensation if it
rises high enough. Therefore shutting off a hot engine doesn't guarantee
a dry engine. The main factor would be the fuel mixture in effect at the
time the engine was shut off, rather than the temperature of the engine.
As it happens, however, a colder engine will normally be running a
richer mixture, so it's usually better to fully warm up an engine prior
to shutting it down for storage. If you are going to park a vehicle for
a long time, a really good technique is to warm it up, then let it idle
for a minute or so, then close down the idle mixture screw on the
carburetor until the engine dies from fuel starvation. This
substantially reduces the amount of moisture trapped in the engine.
Inserting dessicant gel cartridges in the end of the tailpipe may also
help. You can also remove the spark plugs and insert special dessicant
gel holders in place of the plugs. Synthetic oil is also better for
storage because of its longer lasting, clingy film.
Back to the subject of fuel mixture for a moment. If one is serious
about experimenting with fuel mixture, there are several relatively
simple instruments that are useful for monitoring combustion conditions
while driving and providing data that will help select proper jet sizes.
An exhaust gas temperature sensor (EGT) and cylinder head temperature
sensor (CHT) are quite useful and not generally too expensive. Fuel flow
sensors/totalizers, oxygen sensors, and CO sensors are very nice, but
more pricey. A manifold vacuum/pressure gauge is cheap and will
complement the above instruments. Using these instruments effectively
involves keeping careful records and plotting changes in sensor readings
against modifications to the fuel system and different driving
conditions. It's very educational, since it clearly points out various
interrelationships in the entire power production process.
Some people swear by spark plug deposit color changes as an indicator,
but I don't believe these are very reliable. For one thing, you have to
shut off the engine under the exact conditions that you want to check,
such as at full load going up a hill, then coast to a stop and remove
the plugs for inspection. If you take your foot off the accelerator and
continue to drive the vehicle under a different load, the deposit color
will change, making the reading useless.
There is also a cute gadget called a ColorTune or something to that
effect. It is a clear, pyrex spark plug that lets you watch the
combustion flame color while the engine is running. As the colors vary
from blue through colorless through orange, it indicates a leaner or
richer mixture. However, this is primarily for setting your idle
mixture. Even if you could watch it under normal driving conditions, it
would be confusing because of constantly changing loads. Maybe on a
dynamometer, this would be useful. Of course, if you have access to a
dynamometer, you probably would have access to much more sophisticated
instruments than the ColorTune thing.
Oh, well, it's time to wake up and shove off in the direction of the
salt mine.
Happy new year to all,
Alan
(Salt Lake City, Utah)
===
To unsubscribe from the mil-veh mailing list, send the single word
UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.