GAUGING YOUR GEARING
This article was written by Bill Peacock and is reproduced from the American KART-TEC H magazine
Getting 'dialled' in for a track you've never raced on before can be tough. And the
toughest part is finding the right gear. One shortcut you can use is to judge whether
or not the carb settings are normal. To explain this theory, we'll go back over some
old ground because the points in question are so important to learn.
To give you an example, let's say that at your home track you normally run the high speed
jet at one turn open and the cylinder head temperature at about 300 degrees. However
you've decided to race today at a track you've never even seen before. On the advice of a
friend, you've changed the axle gear from a 68 tooth to a 72 tooth. Your friend assures you
that, because the track is much tighter than your home track, the much lower gear is
necessary.
When you get enough laps on the new track to find your way around, you start trying to
turn some good laps. However, you find that you are way off of what normal times for your
class on this track should be. Also, in order to get the head temp up to something near
normal, you've had to lean the high speed jet down to almost 1/2 turn open. Even with the
carb that lean, you still can't seem to get the temp up to normal. An examination of the
spark plug shows that the engine is apparently still running too rich. At this point, you may
resort to shortening the pipe in an attempt to find something near normal heat. Stop - hold
the phone, George. You're doing the wrong thing.
If you continue to lean the carb down in an attempt to get the temp up, you will probably
stick the motor. In reality, what the carb and plug are trying to tell you is that you are
geared so darn low that the engine isn't having to work hard enough to use the fuel you're
trying to give it. Quick Henry - take off some teeth.
The Magic Feel
If you aren't making the engine work hard while towing you around, it will run cooler than
normal even if you lean the carb down below your normal settings. When the engine is
trying to run in this situation, it also has a lazy feel and won't produce the sharp, crisp
exhaust note that it does when its running as it should.
There is a visceral sensation when the engine is sharp and the setup is right. we call this
feel "hard" rather than "soft" and there is an analogous feeling when driving your car on the
street. From that experience you can learn to relate to the message that your kart engine
is trying to send you.
When you drive your car on level ground at 100kph there is a visceral feeling that the engine
is just loafing along. This is an example of the soft feel. However when you start to climb a
fairly steep grade you will have to mash the throttle down harder and the engine will start
sending you both audible and mechanical signals that it is really having to work hard to get
you up the hill. This is an example of the "hard" feel. When the engine is having to strain
to do the job its putting out as much horsepower as it can manage.
To get you around a track fast your kart engine will also have to provide all the horsepower
it can and when you're dialled in right it will produce noises and feelings similar to your car
engine when climbing that steep grade. You almost want it to feel like it's trying to pull
itself apart in the process of hauling you around the track.
Hard Versus Soft
The feel of your engine is directly associated with cylinder head temp and spark plug
reading. If you're geared right, the engine will need the jet settings you normally run to
provide enough fuel to satisfy its needs.
When the engine is running like this, the "hard"
feel will be evident. In other words the right gear makes the engine work hard.
On the other hand, if you're geared way too low, the engine doesn't need much fuel and you
will have to lean it way down to prevent four-stroking and rich plug readings. Running like
this, the engine will exhibit the "soft" feeling.
The opposite side of this particular coin is when you are geared too high (not enough
teeth). In this situation, you can hardly get the carb rich enough to keep the temp down
near normal numbers. However, being geared too high is somehow easier to detect than the
too-low-a-gear/too-cool problem.
Tips and Tricks
Your kart/engine package (like any other internal combustion engine) makes its maximum
power at a specific RPM figure. The trick is to manage to keep the engine in that range over
as much of the track as is possible. Remember that all gear ratios on non-gearbox karts are
a compromise. The gear ratio that yields the best lap times is really a little too high (not
enough axle teeth) for the best possible acceleration and a little to low (too many axle
teeth) for the maximum top speed. However, because you don't have a gearbox, you gear
the kart to provide the best overall performance.
Although it may sound too simplistic to be true, its been our experience in Roadracing that
you usually run all the gear you can gat away with. In other words, we take teeth off the
axle until the kart slows down. Once that happens, we usually put one tooth back on and
run it that way. This setup requires that you drive the kart very smoothly and learn to carry
all the momentum possible into the corners. There are many people who mat disagree with
us on this one but we feel that there is validity to the idea. The problem with the very low
gear is that it usually goes with a very short pipe setup and the short pipe usually makes
lots of heat in the engine.
Regardless of theory, the bottom line is still a matter of which setup gets you around the
track fastest. However we have found that the tallish gear setup (fewer axle teeth) will
usually run just as good - if not better - lap times than a lower gear (more axle teeth)
setup. If you can get good lap times with a tall gear, you have the bonus of reduced engine
RPM, which saves some wear and tear on the engine. Its amazing how much difference
1,000 RPM makes to the life of the engine.
When the Numbers Fool You
The temp gauge is a useful tool for engine tuning but it can also lead you astray. At a new
track, most of us aim for that magical temperature reading that has always told us we're on
target - at home. To get that number back when we're at a new track we'll play with carb
settings, pipe length and gearing. And yet, that strange reading will probably be telling you
that the gear you've got is wrong. Try leaving your engine/pipe adjustments where you
know they work and see if just a few teeth more or less won't pull your engine back into its
ideal operating temperature range.
Home Setting :-
80 Teeth
In this example, you have found your
engine runs its best at your home
track with 80 teeth, a 5/8 turn high
speed jet setting and a temp gauge
reading of 325 de grees.
The stop
watch and reading of the spark plug
and piston crown have consistently
confirmed this.
Now you move to a new track
Jet Setting 5/8 turn = 325 degrees
Geared too low :-
84 Teeth
Too many teeth on the back when you
don't need them can keep the engine
from working as hard as it should, thus
causing it to run too cool.
Without
considering the gearing, you might find
yourself leaning
the carb down
dangerously low to get the temp
reading back up to 'normal'. this risks
a stuck piston.
Jet Setting 3/4 turn = 250 degrees
Jet Setting 5/8 turn = 275 degrees
Jet Setting 1/2 turn = 300 degrees
Geared too high :-
76 Teeth
Too few teeth on the back when you
actually need more can overheat the
engine and cause it to run hot. You
might find yourself richening the carb
way past normal in a vain attempt to
bring down the temp gauge reading,
but the engine will feel lazy and loose
acceleration.
Jet Setting 7/8 turn = 325 degrees
Jet Setting 3/4 turn = 350 degrees
Jet Setting 5/8 turn = 375 degrees
A Touch of Ambience
When you're thrashing away in the pits pulling plugs and changing gears to figure out what
your temp gauge is trying to tell you, don't forget to consider the air. as the sun beats
down on your engine, all that metal is going to heat up - even sitting in the pits. as a rule
of thumb, the engine will heat up and cool down about 50 percent as much as the ambient
temperature of the air surrounding it. Because a cylinder head temp gauge reading is taken
from a ring around the spark plug on the outside of the head, the gauge displays the
temperature of the metal outside rather than the direst temperature of the combustion
chamber inside. An exhaust gas temperature sensor will also be affected but to a lesser
degree. Also, EGT numbers are much higher than CHT readings and so the percent of
change is too small to consider.
Odds and Ends
Interestingly, it's been our experience that when you are geared too low (too many axle
teeth) the engine doesn't seem to turn hardly any more RPM than normal. This can throw
you off because it seems logical to assume that too low a gear will create more than normal
RPM. What's happening is you're just not getting the signal your engine is sending you.
The reason is that the breathing limitations of any given engine/pipe setup has a limiting
effect which acts rather like a governor. Remember that any internal combustion engine is
essentially an air pump. The more air that any engine can pump through itself the more HP
it will make. Getting more fuel into the engine is easy, getting more air in is harder.
In this particular instance, the engine won't turn any more RPM because it simply can't pump
any more air. Therefore, the engines breathing limitations dictate a maximum RPM figure
that the engine simply won't go far beyond, regardless of how you're geared.
The other thing that we've experienced more than once is the odd sensation of the engine
feeling like it wants to die when you get on the throttle or it four strokes after executing a
corner where you were off the throttle. If you stay on the throttle the engine picks up and
starts running again. The answer - way too rich.
The problem is that when you get off the throttle the engine is so rich that it loads up with
excess fuel. When you get back on the throttle it takes a while for the engine to pump
enough of the excess fuel out the exhaust to start running properly again.
Final Thoughts
They may not always be aware of it but the best at-track tuners are the ones that always
seem to get there the quickest - they have developed the feel for how their engines are
running. They're the same ones who can tell you exactly what is wrong when the chassis
isn't handling right either.
Getting dialled in is a complex game and you need all the help you can get. Spend some
time at practice to develop this feel for your own engine and relate it to the visual
information from your temp gauge. See if you don't get to the right gearing and better lap
times faster than before.