By: Henry P. Olsen
Authors
note: This is an article about how to tune
a carbureted engine. It will apply to any
engine from stock to high performance and
will help a tuner with tuning an engine
to run with today's reformulated gasoline.
In
this technical tuning article will explain
how to diagnose and tune an engine�s ignition
and fuel systems so you can get all the
power built into an engine while getting
as many miles of driving fun per gallon
of gasoline. These tuning methods can be
used to tune any engine from a stock flat
head Ford V-8 to a high performance engine
such as the 427R Roush performance engine
that is in the 36 Ford we will be using
as basis to illustrate this story. We will
first explain the tuning process and then
show how we apply this process to the engine
in the 36 Ford street rod we are working
on.
The
427 cubic inch displacement - 550 horsepower
Roush built small block Ford engine that
is in a 1936 Ford street rod definitely
had that killer look to match the power
the engine was advertised to have, but the
engine did not run as well as it was expected
it to. The performance related complaints
were: a hesitation when you hit the throttle
under hard acceleration, the engine had
a audible ping under hard acceleration,
the engine run good to about 4,000 rpm but
above 4,000 rpm the engine lost power &
you could hear the engine backfire thru
the carburetor, the last complaint was the
exhaust fumes made your eyes water &
also made it hard to breathe, Since the
Roush Performance engine had been dyno tested
at the factory before it was shipped, the
cars builder decided to have John Bishop�s
Hot Rod Tuning Service check the 36 Ford
out and find out what was causing the engine
performance problem.
The
Tuning Challenge
Tuning
a high performance to run its best with
race (leaded) gasoline is easier than tuning
the same engine to perform its best with
the unleaded reformulated gasoline that
most of us run in high performance engines.
This unleaded reformulated gasoline is designed
for use in a modern computer controlled
electronic fuel injected engine with the
goal of lowest possible exhaust emissions.
The computer that operates with a modern
fuel injected engine automatically adjusts
the air/fuel mixture and the ignition spark
timing in order to obtain the lowest possible
exhaust emissions along with the best possible
power and drivability. A street rod with
a carburetor must have both the ignition
spark timing and air/fuel mixture tuned
for the blend gasoline that is sold in your
part of the country.
Race gas and
the leaded gasoline that was sold at your
corner gas station until the mid 1970�s
was easier to tune for because a tuner could
�read� the spark plugs and look at the tailpipe
�color� to determine the air/fuel mixture.
Today�s reformulated gasoline does not leave
any �color� on the spark plug unless the
air/mixture is very rich and the tail pipe
color is almost meaningless. Most people
think their engine is running too rich because
the exhaust fumes burn their eyes; this
is not true. An engine with a rich air/fuel
mixture will have excessive carbon monoxide
(CO) in the exhaust; carbon monoxide is
an odorless gas that is quite deadly if
you breathe too much of it. A engine that
has a misfire from things such as: a rich
air/fuel mixture, a lean air/fuel mixture
or incorrect ignition spark timing will
have a lot of unburned hydrocarbons (HC)
in the exhaust. Exhaust gas with a high
unburned hydrocarbon (HC) content will burn
your eyes and make it difficult to breathe.
Checking
Basic Engine Condition
Before
any tuning is done you should confirm the
engine is in good shape by checking the
cylinder compression, cylinder leak down,
oil pressure, fuel pressure, and all the
tune-up related parts such as spark plugs,
wires, points are all in good shape. The
next step in the tuning process is to check
ignition timing and ignition spark advance
systems are functioning and set to factory
specifications. The ignition spark output
and the condition of the spark plug wires
should also be checked out with an ignition
scope.
Ignition
Spark Timing
Reformulated
unleaded gasoline burns different at a different
rate than leaded gasoline so ignition spark
advance curves will need to be tuned for
the gasoline you will be using. The first
step in the process of tuning an engine
designed for leaded gasoline to use today�s
reformulated unleaded gasoline is to modify
the ignition spark advance. The proper tuning
of the initial and the ignition spark timing
advance curve(s) may be the easiest, cheapest
method to unlock the power that was built
into your engine.
The
engine takes a mixture of air and fuel mixed
together, the piston then compresses the
air/fuel mixture into the combustion chamber,
then at the ideal time a spark is then supplied
by the ignition coil thru the distributor
and spark plug wires to the spark plug.
This spark creates a controlled burn in
the combustion chamber that pushes the piston
down and thus creates the power to send
your street rod accelerating down the road.
If you want your engine to perform it�s
best, the proper air/fuel mixture must be
ignited by the spark plug at the correct
time so the engine can turn the energy in
the gasoline into power for it�s driver.
The
distributor ignition advance curve that
is comes in most distributors must be designed
to allow the engine to perform well with
many different drivers with widely varying
driving styles and operating conditions,
this means the ignition advance curve in
most cases is very conservative to avoid
the possibility of engine damage from detonation.
Detonation or pinging can result if the
ignition spark advance is too advanced for
the octane of the fuel and the needs of
the engine. The variables that need to be
taken into account when you pick a ignition
advance curve are the weight of the vehicle,
how the driver will operate the engine (the
rpm range of engine operation or if the
driver tends to lug the engine), octane
of the fuel, compression of the engine,
altitude of the vehicles main usage and
the heat of the air. The distributor�s mechanical
and vacuum advance curves must be correct
for the engine and the fuel being used or
the engine�s performance will suffer as
well as the possibility of engine damage
from detonation
Ignition
Spark Advance Curves
The
ideal ignition timing for power and fuel
economy is just short of the point where
detonation or pinging occurs, the correct
ignition timing (the initial timing plus
the advance from both mechanical and vacuum
advance mechanisms) will cause the pressure
created by the fuel being burned in the
combustion chamber, that is pushing the
piston down to be at its peak / maximum
when the piston is at about 12 and 15 degrees
ATDC (after top dead center). If the peak
cylinder pressure is reached before the
12 to 15 degrees ATDC you will lose power
as the piston fights to compress the burning
air/fuel mixture and detonation may also
be experienced which can lead to engine
failure. If peak cylinder pressure is reached
much after the 12 to 15 degrees ATDC, the
engine will just be wasting the energy the
combustion process creates as wasted heat
out thru the exhaust system.
The
amount of ignition advance needed changes
with engine speed, engine load, engine temperature,
air temperature, compression ratio, the
octane of fuel being used and the air/fuel
mixture. A fast burning - rich air/fuel
mixture (12.5 to 1 air/fuel mixture) that
will produce the maximum power and will
require less advance than the slower burning
- leaner air/fuel mixture (14 to 1 air/fuel
mixture) that is used for cruise speeds
to properly burn all the fuel. A vacuum
advance adds a little more ignition advance
to allow the engine to fully burn this slower
burning - leaner air/fuel mixture that is
seen at part throttle conditions such as
a 65 mph cruise. A 12.5 to 1 air/fuel mixture
burns the fastest and will supply the best
power, but a leaner air/fuel mixture of
14 to 1 is much better for fuel economy
and also will not tend to foul spark plugs
like the richer 12.5 to 1 air/fuel mixture
will tend to do.
If
you do not have enough ignition spark advance
the engine may lack power and the engine
may tend to run hot or overheat. If you
have too much ignition spark advance the
engine will also want to run hot and lack
power, if excessive pinging or detonation
is present you are also risking engine damage.
Any distributor, whether it is original
or performance replacement should have the
mechanical and vacuum advance curves checked
to confirm they are correct for use with
your driving style, engine package and today�s
reformulated unleaded gasoline.
The
best way to check both the vacuum and mechanical
advance curves of a distributor is in a
distributor test stand; this is because
you can check the spark advance curve at
any rpm without fear of over-revving the
engine. Back in the 60�s and 70�s almost
every quality engine tuner had a distributor
test stand in their shop so they could check
the advance systems in the distributor,
but now days it is getting hard to find
a shop that has one.
If
you do not have access to a distributor
test stand, an optional method that you
can use to check the vacuum and mechanical
advance curves is the use of a dial-back
timing light. A dial-back timing can allow
you to read the advance curve of an engine
at different engine speeds even if it does
not have a degreed vibration dampner, just
take care not to over-rev the unloaded engine.
The vacuum advance curve can also be checked
with the use of a hand vacuum pump to vary
the vacuum supplied to the vacuum advance,
just use the timing light to read the amount
of advance given by the vacuum advance at
different amounts of vacuum from 1 to 23
inches of vacuum.
Ignition
Spark Advance Guidelines
The
�hot-rod� advance curve used most on a 9.5
to 1 compression engine with a mild camshaft
(duration less than 220 degrees at 0.050�)
is 10-12 degrees initial timing plus 22-24
engine degrees of additional advance from
the mechanical advance mechanism. In most
cases, full advance (32-36 degrees) is in
by 3500 rpm. An engine with camshaft duration
above 220 degrees at 0.050� will like more
initial timing; however, the total timing
will stay the same (32-36 degrees). In most
cases when you are using a �hot rod� mechanical
advance curve, the amount of additional
advance from the vacuum advance should not
exceed 10 degrees and not be in before 10
inches of vacuum, or performance may suffer
from too much advance.
The
mechanical advance should not start advancing
until just above the base idle speed, too
much advance with the engine speed too low
may cause a ping or detonation problem,
which can lead to engine damage. The best
guideline we have found for determining
what initial timing is best for a gasoline
engine is in the Barry Grant Inc. catalog
and/or web site in the Demon carburetor
selection guide; they recommend 10 to 12
degrees of initial timing when the camshaft
duration is less than 220 deg. @ .050 valve
lift, 14 to 16 degrees of initial timing
with less than 240 deg @ .050 and 18 to
20 degrees of initial timing with a cam
with less than 260 @ .050 valve lift. The
total ignition mechanical advance from the
distributor must be reduced when you increase
the initial timing because engine damage
will result if the total advance is excessive
for the engine compression and fuel being
used. A modern 9.5 to 1 compression Chevrolet,
Ford or Chrysler engine will in most cases
will use 36 degrees of total ignition advance
including the initial timing set plus a
maximum of 10 additional degrees of advance
from the vacuum advance (if used).
A
engine with a high performance camshaft
designed to create power above 3000 rpm
will respond well to 18 degrees of initial
timing because the air/fuel mixture is not
uniformly mixed at lower engine rpm, so
the additional initial timing allows more
time for this air/fuel mixture to burn in
the cylinder; the same theory also applies
to the use of a supercharger or a race designed
intake manifold such as a Edelbrock Victor
or any of the new air-gap style intake manifold.
A high performance cam and/or intake manifold
will decrease the velocity of the air/fuel
mixture at low rpm�s, this low velocity
causes the air/fuel mixture to not be as
well mixed as a engine package that has
been designed for low rpm operation.
Tuning
the Ignition for Fuel Economy
The
ignition spark advance curve an engine needs
for maximum power with the rich air/fuel
mixture the engine has during wide open
throttle acceleration is different than
the ignition spark advance it needs for
the leaner air/fuel mixture the engine has
when you are cruising at 65-75 mph on the
highway. In almost every case we would prefer
to use a distributor with a vacuum advance
tuned to provide an additional 10 degrees
of advance when the engine vacuum is above
10 inches. The extra 10 degrees of ignition
spark advance that a properly tuned vacuum
advance can provide will give the extra
spark advance a engine will need to burn
the leaner air/fuel mixtures an engine will
use a light throttle while driving your
street rod down the road at normal highway
speeds. The extra timing from a vacuum advance
will also help the engine run a little cooler
and give you better fuel mileage when you
are driving at normal highway speeds. Many
of the original Ford and Chrysler distributors
have adjustable vacuum advances, for General
Motors and MSD replacement distributors
we use a vacuum advance we buy from Ole�s
Auto parts that have been limited to 10
degrees of advance.
Ignition
System Upgrades
In
most cases your engine will perform its
best with electronic ignition, but an engine
can perform quite well with point-triggered
ignition. The original equipment electronic
ignition systems that Chrysler, Ford and
General Motors have for most engines are
a very good and reliable ignition system
that can work very well in a street rod.
These original equipment electronic systems
have the advantage if you happen to have
an ignition failure while on a road trip
you can find parts to repair it at any auto
parts store; this is not always the case
with many of the aftermarket electronic
systems you may buy.
Any
original or high performance aftermarket
replacement distributor must have the ignition
spark advance curve(s) tuned for your engine
and the gasoline you will be using. Most
of the high performance replacement ignition
systems such as a MSD distributor come with
a very conservative ignition advance curve
installed in the distributor. A new MSD
distributor comes with a selection of advance
springs & bushings so you can set the
advance curve you want, however most vacuum
advance equipped distributors have too much
advance from the vacuum advance for the
reformulated gasoline of today.
Spark
plug wire condition & maintenance is
very important consideration in the proper
tuning process on any engine, as the spark
plug wires age the insulation material will
break down allowing the spark that is being
sent to the spark plug to take a shortcut
to ground. Many of the aftermarket spark
plug wire suppliers offer heat resistant
spark plug wires with improved insulation
materials such as silicone, these improved
spark plug wires are available in 7mm or
you can upgrade to 8mm spark plug wires.
Tuning
the Roush Performance 427R Ignition System
The
ignition timing that we read when we checked
it with a timing light was 30 degrees BTDC
(before top dead center) total all in by
2000 rpm, the initial timing that varied
from 15 to 22 degrees BTDC. The next step
was to remove the distributor from the engine
and check the advance curve on a distributor
test stand. The actual advance readings
when the distributor was checked on the
distributor test stand show us the distributor
had 14.5 degrees of mechanical advance was
all in by 1800 engine rpm. The fact that
entire spark advance curve was in by 1800
rpm was most likely the reason for the ping
problem the engine and the reason the timing
was varying at idle was the advance springs
were so light that the advance was starting
at idle speeds.
Using
the Barry Grant initial timing guide we
decided that 18 degrees of initial timing
would work quite well with the camshaft
that is in this engine. We then recurved
the distributor so it would give us 16 degrees
of mechanical advance that starts advancing
at 800 rpm and is all in by 3000 rpm to
give us 34 degrees of total mechanical advance.
The MSD distributor that is in this engine
did not come equipped with a vacuum advance
and since the reason the customer selected
this engine package is maximum power the
vacuum advance was determined to not be
a priority. We then reinstalled the recurved
distributor into the engine, set the initial
timing at 18 degrees BTDC and confirmed
the total advance to be 34 degrees BTDC.
The tech literature that came with the engine
specified the total timing was not to exceed
35 BTDC.
The
changes with the initial timing and spark
advance curves made an improvement in how
the engine in this 36 Ford performed, the
idle was smoother and the engine had more
power at low and mid range rpm driving conditions.
The engine ping problem the engine had when
we first started the tuning process was
gone but the throttle response is still
not up to what an engine like this should
have. The idle quality still left a lot
to be desired and the problem with backfiring
at engine speeds above 4000-rpm still needs
to be addressed. Now that the ignition is
tuned for the unleaded reformulated gasoline
it is using, we can go to the next in the
tuning process, which is tuning the air/fuel
mixture for power and fuel economy.
To
be continued in Part 2: Fine Tuning the
Fuel System.
Sources:
Ole�s
Carburetor & Electric, Inc.
Ole�s
Carburetor & Electric Inc.
120
El Camino Real
San
Bruno , CA 94066
650.589.7377
olescarb@sanbrunocable.com
John
Bishop/Hot Rod Tuning
Burlingame,
CA
Ph#650
343 4860
jfb396@sbcglobal.net
Demon
Carburetion
Barry
Grant, Inc.
706-864-8544
www.barrygrant.com
OTC/SPX
Corporation
1-800-533-6127
www.otctools.com
|
