By: Henry P. Olsen
The
definition of drivability: the general qualitative
evaluation of a powertrain's operating qualities,
including idle smoothness, cold and hot
starting, throttle response, power delivery,
and tolerance to altitude and weather changes.
To
me drivability is the result of tuning an
engine to have both the proper air/fuel
mixture and the correct ignition spark timing
in order to get the most power out of the
engine in the rpm range being used. Many
tuners will only tune to get the maximum
power out of an engine on a dyno and do
not even think about tuning the engine for
the way it is driven. A drag racing engine
should produce its power at wide open throttle
but if you have to lift or feather the throttle
the engine still must be supplied with the
correct air/fuel mixture and the proper
amount of ignition timing so the driver
can actually drive the car thru a problem
with out a stumble or miss. A race engine
used on an oval or road race situation also
needs to produce power at part and full
throttle and the driver that has an engine
that not only produces good power but is
drivable at any throttle position can often
drive around another car that has a engine
that has not been tuned to produce power
at all engine rpm and load conditions that
the engine may see. A well-tuned drivable
engine can help a good driver win races
especially on a loose or slick track. T
his drivability does not only help win you
races, it can help a engine run cooler,
get better fuel mileage, and help in the
production of maximum power at all engine
rpm's.
What
we are going to try to show our readers
is how to tune a engine to get the most
power out of a engine at all the load conditions
the engine will be used at, and show the
tools we are using to allow you to see how
we are achieving great results.
Before
any tuning is done the engine must be in
good shape and have been designed for the
type of use it is intended for, a high rpm
race engine will not make a good engine
for a vehicle used as a every day commute
vehicle (always pick or build a engine designed
for the way it will be used). Fuel pressure,
spark plug gap and all tune up related items
must first be checked and set, and then
the initial timing and advance curve must
be checked and set for the type of fuel
being used.
The
tools used for checking ignition advance
curve are both a dial-back or advance timing
light such as the unit we use from OTC/SPX
and a distributor machine are very necessary
to obtain good results. The distributor
machine we use is a Sun model 506 which
we have modified to check the advance curves
on electronic distributors.
The
air/fuel mixture curve may now be addressed;
the method of preference for me is by using
an infrared exhaust gas analyzer such as
the unit that has made it possible for us
to obtain great results, the Performance
Gas and MicroGas exhaust gas analyzers from
O T C/SPX. These exhaust gas analyzers can
accurately supply readings that can indicate
the air/fuel ratio by looking at the C.O.
(carbon monoxide), the amount of engine
misfire by looking at the H.C. (hydrocarbons)
reading, and indicate if the engine has
to much ignition timing advance by looking
at the NOx (nitrous oxide) reading and/or
high H.C. readings.
Checking
the Air/Fuel Mixture
A
lean fuel mixture (too little fuel for the
amount of air in the cylinder) can cause
an engine to stumble or give a rough idle
as well as to run too hot, overheat, and
cause a lack of power. A rich fuel mixture
(too much fuel for the amount of air in
the cylinder) can cause an engine to "load
up" at idle, foul the spark plugs, and also
run sluggish. There are several different
methods to determine if the air/fuel mixture
is correct, among them are:
• High-energy
ignition systems have made this method much
harder because very little color is seen
on the spark plug and thus a job for an
expert.
• The
second method is by using timed acceleration
runs or top speed for the power system,
this involves using trial and error jetting
changes to obtain the best results. Obtaining
the correct cruise mixture is not as easy,
since this involves jetting the carburetor
to get the highest vacuum, then trial and
error to get the best engine feel. When
setting the power and cruise mixtures, it's
always advisable to stay a little rich in
order to avoid engine damage. The idle mixture
is set using a tachometer to get the max
speed from each idle screw and the go leaner
to get a 20-rpm drop in speed, this is know
as the lean drop method.
The
easiest and most accurate method that we
have found is by using an infrared exhaust
gas analyzer like the unit we use from OTC/SPX
Tool Company called a MicroGas to "read"
the exhaust gases. By using the MicroGas
the carburetor's jetting (air/fuel mixture)
curve can be checked at idle, cruise, or
power loads and then be tailored to what
your engine needs to run at its best at
all driving conditions.
JETTING
WITH AN INFRARED GAS ANALYZER
On
Car Testing
After
the basic engine condition and tune-up (fuel
pressure, timing curve, etc) is confirmed
to be correct, as well as checking to be
sure there are no vacuum leaks, the next
step is determine what the air/fuel mixture
is at idle thru 3000rpm. If the cruise mixture
is off, first change the jets in order to
get the air/fuel mixture correct at the
2500-3000 cruise rpm range. Then check and
set the idle mixture. If the air/fuel mixture
is too lean at idle or part throttle and
the idle mixture screws do not provide enough
adjustment, the correction may involve enlarging
the idle "jet". If the mixture is still
lean at 1000 thru 1800 rpm, the idle channel
restriction, if used, may have to be slightly
enlarged to allow more fuel to be delivered
at part throttle.
This
lean condition at part throttle condition
will cause the engine to miss or stumble
at part throttle due to a lean air/fuel
mixture. This problem is very common on
many carbs. ( The notable exception is the
Demon carburetors, which have used the MicroGas
to do exhaust gas analysis along with dyno,
track and drivability testing to establish
the factory fuel curve that allows them
to come with a stumble-free guarantee.)
If the air/fuel mixture is too rich at idle
and part throttle, the idle jet/restriction
may be too big and may need to be replaced
with a smaller one.
The
next step is a road test using a portable
infrared gas analyzer to check the cruise
speed air/fuel mixture-main jetting, followed
by checking the power air/fuel mixture under
load. During a road test you are able to
read and correct the jetting to have the
correct mixtures at idle, cruise and power/wide
open throttle.
The
starting point for air/fuel mixtures that
we look for from a stock engine is:
Idle:
1 to 3 % CO or a 14.1-13.4 to 1 air/fuel
mixture
Cruise
rpm: 0.75 to 1.25- % CO or a 14.2 -14 to
1 air/fuel mixture (hot cam engine 1 to
3% CO)
Power
mixture and acceleration 6.6% CO or a 12-1
air/fuel mixture for a "normal" engine,
a high performance engine with improved
combustion chamber design such as a Pro-Stock
or a Winston Cup engine can use a power
mixture of 4% CO or a 13 to 1 air/fuel ratio.
The
CO reading from an infrared gas analyzer
is the reading that we use to determine
the air to fuel ratio. (Note: CO is partially
burned fuel)
The
other readings that exhaust analyzers provide
are:
HC
(hydrocarbons): The amount of unburned fuel
or an indicator of an engine misfire, the
best mixture gives you the lowest HC.
CO2
(carbon dioxide): The product of complete
combustion, the best mixture gives you the
highest Co2 reading
O2
(oxygen): A high O2 reading indicates a
lean mixture or an exhaust leak.
Note:
if O2 is above 2 to 3% any CO readings will
not be accurate.
NOx
a high reading can indicate detonation from
excessive ignition advance or the gasoline
does not have enough octane for the engines
needs.
The
best power and cruise air/fuel mixtures
(CO) will burn all the o2 in the cylinder
and create the lowest HC reading (misfire)
and will also cause the co2 reading to be
its highest.
Note:
all infrared readings assume there is no
air pump or catalytic converter. If used,
the air pump must be blocked off and any
readings taken must be in front of the catalytic
converter.
A properly tuned
fuel and ignition system will allow your
hot rod perform up to it's potential and
give you a better running, more reliable
ride!
Source
Ole's
Carburetor & Electric Inc.
120
El Camino Real
San
Bruno , CA 94066
650.589.7377
olescarbs@sanbrunocable.com
John
Bishop
Hot
Rod Tuning
808
Burlway, #2
Burlingame
, CA 94010
650.343.4860
jfb396@aol.com
OTC/SPX
Corporation
655
Eisenhower Dr .
Owatonna
, MN 55060
800.533.6127
www.otctools.com
Barry
Grant Inc.
1450
McDonald Road
Dahlonega
, GA 30533
706.864.2206
www.barrygrant.com
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