Putting on the Tune.
Author/s: Ray T. Bohacz
Issue: March, 2000
Optimizing The Ignition and Fuel System In An Older Engine
Even though the vehicle in this story is a '70 Vette with a 350, it is not about
that. The voyage we are about to embark on is theoretical
and will be just as successful on any engine, regardless of make, number of
cylinders, or displacement. All too-often, the participants in
this hobby fall into the replacement mode, not wanting to apply theory;
switching components in lieu of tuning. Even after a pads swap in
most instances, there is horsepower locked away that can be finagled with a turn
of a screw or the bending of the float. The first step is to
fully grasp what needs to be accomplished, and that is how the engine responds
to spark advance, ignition, and fuel--the areas the we
can easily alter. Here, the lost art of tuning will be presented.
Air/Fuel Ratio
Some think that the more fuel you feed an engine, the higher its output will be.
This is only a half-truth; an engine makes the most power
when there is a sufficient amount of fuel to burn all of the oxygen in the bore.
If there is more fuel than oxygen to support it, or if the flame
extinguishes prematurely, leaving residual oxygen, the output of the engine
diminishes. The old-fashioned method of tuning by reading the
spark plug color is not very accurate. The problem is that only extremes in
mixture can be detected, since plug color is relatively
indifferent to slight changes in the air/fuel ratio. Modern technology provides
more accurate, real-time methods of reading mixture utilizing
linear or wide-band air/fuel meters. These instruments can identify an actual
ratio of fuel to air by measuring the oxygen content in the
exhaust with an oxygen sensor. These sensors differ from the ones used on
production vehicles in that they can only accurately identify a
ratio of 14.7:1. Air/fuel ratio meters vary in acc uracy, with a price spread
from $200 to over $10,000. It must be noted that any of the
current oxygen sensors will be degraded and eventually rendered inaccurate from
exposure to leaded fuel. For our test session, we used
a custom meter that employs a Bosch extended-range four-wire sensor and ran the
engine on 92-octane unleaded fuel. It is accurate to
two-tenths of an air/fuel ratio when exposed to a mixture richer than
stoichiometric, or 14.7:1.
When we apply numbers, an engine requires the most fuel at peak torque and then
needs to be leaned out at peak horse-power. The
volumetric efficiency of the engine is the highest at peak torque, since the
bore is the fullest with charge, thus it requires the greatest
amount of fuel. Historically, most engines want an air/fuel ratio of 12.7:1 to
12.9:1 at peak torque, then lean out to 12.9:1 to 13.1:1 at the
power peak. Herein lies the importance of using an air/fuel-ratio meter, since
an acceptable deviation is only 1.5 percent. Being a tuner
requires more than just reading a meter and shooting for a certain number. Fuel
can also be used as a coolant, quenching the cylinder and
warding off detonation. In many instances, an experienced tuner may chose to run
the mixture slightly rich, giving away a minute amount
of power, but then initiating a more aggressive spark advance curve due to the
cooler combustion chamber. This logic often leads to a
positive power gain. Even with this technology, do not dismiss spa rk plug
reading. It can be used for other diagnostic purposes. Mixture
variation from cylinder to cylinder and the beginnings of detonation can be
verified with dose examination of a spark plug. Edelbrock
provided two different secondary metering rod combinations for our Q-Jet to
allow altering of the air/fuel ratio. Our carburetor was
originally equipped with secondary metering rods BG while the Edelbrock rods
were CE and CC, in theory, one step richer and leaner
than our base setting. ACCEL offers an oxygen sensor bung that we welded to the
engine pipe exiting the exhaust manifold for
attachment of the air/fuel meter.
Continued from page 1
Ignition Output
With the popularity of nostalgia-based cars and engine compartments, it may be
deemed undesirable to add an OE-style electronic
ignition to an early vehicle. This forces function to follow form, unless a
little ingenuity is applied. We choose to incorporate ACCEL's
new 300+ digital CD ignition system and trigger it from the stock tach drive GM
points distributor. The 300+ offers the latest in digital
electronics in lieu of slower responding analog controls while offering the
power of an arc welder. Since the points will now be used only
as a trigger and will not carry any amperage, their life span will increase
dramatically, only to be diminished from wear of the rubbing
block.
Switching from an inductance-based ignition system, which is what a points or
factory HEI is to a capacitive-discharge design, offers
many benefits. The ACCEL ignition offers multistrike capability below 3,000 rpm
and higher current and voltage outputs. The design
criteria for an ignition system is to supply the highest voltage and amperage
for as long in the crankshaft's rotation as possible. This will
allow the most efficient conversion of fuel to energy. Ignition bum time is
referenced in degrees of crankshaft rotation at engineering levels;
a layperson may be more familiar with milliseconds of arc duration. ACCEL has a
companion epoxy core ignition coil, but we decided on
the cylindrical oil-filled Super Stock coil to retain the original mounting
bracket and look.
Spark-plug gap is a concern; it is desirable to have the widest gap possible so
that the ignition system can fire without degrading burn
time. Our subject 350 had older cylinder heads that required a short-reach
13/16-inch plug. With this ignition, it is beneficial to open the
plug gap to at least .040 inch on high-compression engines and to .050 inch on a
street application. Our spark plug did not offer enough
side electrode length to go beyond the original setting of 0.035 inch, limiting
the affect of the added output. A wide gap serves the
purpose of exposing a larger ionization window and increases the energy required
to create a path to ground.
Spark Advance and Rate of Gain
In an engine, the piston travels faster than the flame expands across the bore.
This necessitates giving the flame a head start, meaning that
it is initiated prior to the piston reaching top dead center. Start the flame
too early and the cylinder pressure will start to rise while the
piston is still traveling toward TDC, creating abnormal combustion and
decreasing engine output; too late, and peak cylinder pressure is
reached later in the crankshaft's arc of rotation, killing power and raising
exhaust gas temperatures.
Simply put, anything that will affect the flame speed will determine the rate of
spark advance that the engine will want to produce
maximum brake torque (MBT). Combustion chamber shape and design, spark plug
location in relation to the bore center, mixture
motion, compression ratio and piston-crown design are only some of the
considerations.
Continued from page 2
For ignition tuning, ACCEL supplied an adjustable vacuum advance and mechanical
weight and spring kit. Using a dial-back timing light,
the spark advance curve would be documented.
Quantifying the Changes
Ida Automotive was enlisted to do the tuning and installation of the parts, as
well as provide the use of their DynoJet chassis dyno.
Owner Bob Ida has years of experience extracting the most from any engine, and
his facility is fully equipped with the necessary
equipment. As an authorized ACCEL EMIC, he is well-versed on not only ignition,
but EFI systems as well. At press time, Ida was the
only facility on the East Coast with the new DynoJet friction brake. Prior to
this option, as it is an inertia dyno, only wide-open throttle
tuning could be performed. This left the part-throttle functions of spark
advance and mixture to be done seat-of-the-pants style. With the
friction mechanism, the engine could be loaded through the drive wheels,
simulating various driving scenarios.
It must be noted that all results at the drive wheels are approximately 80
percent of the flywheel rating. This is the loss from the drivetrain
and volumetric efficiency changes that occur when the engine resides in the
frame of the vehicle. When tuning an engine, the percent of
gain is what needs to be recognized more than the total power number. We all
like to see big dyno numbers, but a tuner looks at the
delta, or change, in output. As an example, if an engine produced 100 hp at the
drive wheels, and through tuning that figure was raised to
135 hp, at first it may not seem that impressive. Mathematically, though, that
is a 35-percent gain in power without changing any hard
parts. In contrast, a 400hp engine tuned to 435 ponies is only a 9-percent gain.
The Real World
After a day of thrashing Jon Racich's beautiful Vette on the dyno, we made some
interesting discoveries. For accuracy, each change was
verified by making multiple pulls and then accepting the repeatable results. Due
to space constraints, we will represent the changes and
how they affected the engine, but in actuality we made 43 pulls in total. With
all of that said, let's move into the chain of events and see
how we used our heads to tune for power.
RPM BASELINE ACCEL KIT
700 10 10
1,500 17 25
2,000 22.5 26.5
2,500 24 31
3,000 24 34
3,500 24 36
Final curve with vacuum advance
at part throttle, light load
RPM Total Advance
700 33.5
1,500 37
2,000 42
Even though we used the $2,000 linear air/fuel ratio meter (left) for our tuning
session, it would be overkill for the enthusiast who wants to
tune his own engine. To meet the need for an inexpensive mixture analyzer,
Edelbrock has brought to market its low-cost air/fuel monitor.
Working with an OE-style oxygen sensor, the kit comes complete with the
appropriate bung to mount the sensor, wiring harness, plus
instructions, Instead of a digital readout with an actual ratio, the Edelbrock
meter uses a series of 7 LEDs to indicate an approximate
mixture. This unit can be affixed to the dashboard with the supplied Velcro
strips or be installed temporarily for tuning, being used as a
tool. No longer will you have to guess at carb jet sizing or how the weather is
impacting air/fuel ratio.
Continued from page 3
Bob Ida documented the timing curve and found that the vacuum advance was not
working, while the centrifugal advance was issuing
only 14 degrees, for a total spark lead of 24 degrees. The baseline dyno pull
netted some disappointing numbers all around. Peak
horsepower was 169.7 and the torque came in at 235.9 lb-ft. Of greater concern,
the engine was dangerously lean, with a wide-open
throtte (WOT) air/fuel ratio of 14.3:1. We needed to fatten it up before we
added any spark advance. With this mixture ratio, the lack of
spark advance was what saved this engine from detonating.
To guarantee valid results, we installed new ACCEL ignition components,
including the vacuum advance unit, and adjusted it with the
supplied 3/32-inch hex wrench. Tuned through the vacuum port, we seated the
adjustment fully counter-clockwise as a starting point.
The unit was set using the dyno brake to provide a total timing curve of 42-
degrees BTDC. This provided the best throttle response
without getting detonation. Since there is no vacuum signal present at WOT, it
would not affect this part of the test.
Edelbrock supplied the two choices for secondary enrichment, one step richer and
leaner than we already had. After installing a set of
smaller-diameter Edelbrock metering rods, we made another dyno pull, concerned
only with mixture and found the same problem --
14.3:1 air/fuel ratio, no change at all. Since the engine had a new fuel pump
and filter, we decided that the fuel supply problem had to be
in the float bowl. After removing the air horn, we discovered that the float
level was 3/32-inch too low. The richer metering rods could do
nothing if we were running the bowl out of fuel. We raised the float level
1/32-inch above specification and readjusted the secondary air
valve tension, since it was too slow to respond. We were rewarded with a reading
of 12.7:1 at peak torque and 12.9:1 at peak power.
The dyno readings were up 6 hp and 7 lb-ft of torque.
We went back inside the distributor for a weight and spring change. Before we
installed any new parts, we sprayed brake cleaner on the
breaker plate, checked its movement, and lubricated the slides with a
high-temperature grease. The ACCEL kit offers three spring
choices; we choose the blue medium-tension one, which in theory should give full
advance by 2,800 rpm. On the chart, the advance rate
for the three spring choices is referenced in distributor rpm, which is one-half
the crankshaft speed. ACCEL also includes a new travel
stop, but we wanted to check the timing curve with the changes we had already
made. But, we were wrong; it didn't work. We still had
the same total advance, it just came in sooner. The distributor would have to
come out and the shaft removed to install the new stop.
It didn't take long to find the problem; someone had added a braze to the stop,
making it thicker and limiting the travel of the advance
mechanism. American ingenuity took hold and Bob Ida filed down the braze to a
dimension small enough to accept the ACCEL bushing.
We now had 36 degrees of total advance and found 23 hp and 22 lb-ft of torque!
An ignition coil has no moving parts, but it does wear
from being charged and discharged. To be complete, we also swapped to a new
ACCEL Super Stock coil and gained 5 more ponies
and 5 lb-ft of twist.
Continued from page 4
Car owner Jon Racich found the perfect hiding place for the 300+ box, in the
right-hand kick panel where an optional factory speaker
would have resided. Remember, when installing the ACCEL 300+ on a GM points
ignition, the resistance wire from the starter needs to
be removed to provide full charging voltage to the primary circuit. With the new
ignition connected, some interesting things happened.
The air/fuel ratio went rich, down to 11.9:1 and 12.2:1, respectively we lost
power. This indicated that the 300+ was working, burning
fuel that we were wasting before. The engine was now too rich with our original
choice of metering rods. Ironically, these same rods
provided the proper mixture ratio with the weaker ignition. We swapped in the
other set of Edelbrock metering rods, brought the air/fuel
ratio back up to 12.9:1 and 13.2:1, and found 7 hp and 5 lb-ft.
As a final touch, we added the high-flow ACCEL washable air filter element and
came away 3hp and 3 lb-ft.
Master tuner Bob Ida is pleased with the results. He found 44 hp and 42 lb-ft of
additional torque for totals of 213.7 and 277.9 for
torque and horsepower respectively. Other than the ACCEL ignition and air
filter, the engine is the same as when it rolled in. Fuel
mileage, throttle response and idle quality have all improved.