| Basic Carburetor and Fuel System Maintenance and Tuning Tips |
| There is no substitute for a
well-designed fuel and air delivery system. By ignoring
these two critical areas, all the work of building
a strong powerplant is wasted. |
| |
| Air
Delivery |
| For maximum horsepower, the coolest,
most dense air possible should be available at the
carburetor inlet. Keeping restriction in the inlet
path to a minimum - or better yet, pressurizing the air - is
also desirable. |
| The denser the air, the more
you can get into the cylinders. This allows the engine
to burn more fuel and make more power. We recommend
that a hood scoop or outside air intake should be installed
wherever rules allow. Under hood air is heated by the engine
and headers and reduces the amount of power that can be produced.
A reduction in temperature of 10 degrees F. is approximately
equal to a one percent power gain. |
| There should be a minimum of
three inches of clearance between the top of the venturis
and a hood scoop. If an air cleaner is installed, the
tallest possible element is preferred with four-inch
element preferred for racing engines over 500 HP. |
| When a hood scoop or external
air intake is used, it is highly suggested that the
carburetor be sealed to it. Otherwise, air will flow
across the top of the carb and out of the inlet tract rather than
into the air horn. If air is forced past the carburetor it can siphon
fuel, causing the engine to run lean. Windshield snorkels are
especially notorious for siphoning unless the rear is sealed. Air
pan kits for sealing the carburetor to the scoop are available or
they can be fabricated. An air bell or radiused intake should be
used whenever possible to increase air flow into the
carburetor. |
| It is not unusual for a drag
race car to improve ETs by 0.3 second and increase
top speed by as much as seven miles an hou after installation
of a sealed scoop. A car will not pick up ET after
the scoop is sealed off if the scoop is too short or the fuel delivery
system is inadequate. |
| On oval track cars, the same
is true. Paying attention to the inle tract design
will pay off. Depending upon track length, oval track
cars will typically improve lap times by 0.1 to 0.5 second once
an optimized air intake system is installed. |
| |
| Fuel
Delivery |
| Many racers experience fuel delivery
problems without ever being aware that something is
wrong in their race car’s fuel systems. Today’s
state-of-the-art engines produce a lot more power than
a race engine of ten years ago. The process of producing
horsepower revolves around the conversion of fuel into
energy. The more pounds of fuel an engine can burn efficiently
per hour, the more horsepower it produces. Even though
your car may not miss, pop, bang, skip or do anything else
peculiar, it may not be getting all the fuel it needs to make maximum
power. |
| In oval track applications, a
BG Belt Drive or Hex Drive Fuel Pump is preferred where
use of a mechanical fuel pump is specified. These pumps
offer the highest fuel delivery volume of any mechanical pump yet
maintains low fuel pressure at low engine speeds. This
feature alleviates “loading up” of the spark plugs.
The BG Six-valve and Super Speedway mechanical fuel pumps
will also deliver ample fuel volume when used according to
recommendations. |
| For drag
race cars, a BG400-2 Electric Fuel Pump is the best way
to guard against fuel starvation. If a car is “lazy” or
“lays down” at mid-track then pulls well
in a higher gear, the engine may be experiencing intermittent
fuel starvation. |
| Why? Typically, the carburetor
bowls are full at the starting line so the car leaves
hard but in the process, drains the bowls dry. In the
lower gears, the car accelerates rapidly with the engine picking up
rpm very quickly. This rapid acceleration increases the demand for
fuel. When the float bowl fuel level drops, the car “lays down”
because of fuel starvation. In high gear, engine speed
increases more slowly allowing the bowls to fill again.
|
| |
| The Fuel Can Test |
| There's been no shortage of well-researched and well-written articles and books explaining
the workings of the fuel system. However, many racers, both novice and experienced alike do
not fully understand the physics of fuel flow and horsepower. To produce torque and horsepower
requires a mixture of air and fuel. To produce 1-horsepower for 1-hour requires approximately
.5-lbs of gasoline. If you ran a single-cylinder engine, like the one in your lawnmower, under
a load of 1-horsepower for 1-hour and weighed the fuel tank before and after, the tank would
weigh approximately .5-lb (five-tenths of a pound) lighter. Therefore the equation for fuel
flow is 1-H.P. = .5-lb of fuel, per hour.
|
 |
| This is expressed on a dyno sheet as B.S.F.C. (Brake Specific Fuel Consumption). Highly-tuned
racing engines can sometimes by more efficient, yielding B.S.F.C. figures of around #.40 which
means 4-tenths of a lb of fuel, per h.p., per hour. Incidentally, the formula for Alcohol is
approximately 1-lb of fuel, per h.p. per hour which, as a consequence, necessitates the running
of a belt-drive pump, but that's another story.
|
|
| Typically, a 600-HP engine will require 300-lbs of gasoline per hour and, by the same formula,
an 800-HP engine needs 400-lbs per hour. Remember, these quantities of fuel have to be delivered
past the needles and seats and the fuel pressure regulator. Consider also, the fuel delivery
system has to combat 'G' Forces: loadings that are so formidable they can threaten to stall the
fuel in the line (this may also give a clue as to why a fuel line that is too large in diameter
can be as harmful as one that is too small). This leads us to the area that is least understood.
|
| When you have only one carburetor it should be easier to feed than two, right? Wrong, in an engine
with a tunnel-ram layout, both the needle and seat area and the float bowl capacity have doubled!
Whereas the single four-barrel car that is most prevalent today, has a much harder task in keeping
the fuel bowls full! A 700-HP tunnel-ram engine needs 350-lbs of fuel per hour which equates to a
little over 85-lbs per float bowl. A 700-HP engine running
a single four-barrel (not so uncommon these days) needs 175-lbs
per float bowl, compared to a 1200-HP Pro Stock engine with demands of 600-lbs max, 150-lbs
per bowl.
|
| So what happens if fuel delivery is weak? Your engine may not miss or "burn-up" parts. It may
just not perform to expectations. The new camshaft, racing-carburetor, or flowed-heads that
didn't pick-you-up may have overstressed an already taxed fuel delivery system. Carburetors
cannot disperse the optimum air/fuel mixture unless the fuel system has the ability to maintain
correct float bowl levels. Fuel levels that are two low may not cause the motor to miss or "burn"
a piston, but they will reduce fuel flow and performance will suffer. It is not uncommon after
upgrading a fuel system with a single four-barrel carburetor to pick up 1- to 4-tenths of a
second. In extreme cases, E.T.s have been known to decrease by as much as 1 second!
|
| Can a fuel system that is too large hurt performance? No, it assures your combination will reach
its full potential: the needles and seats will shut when the float bowls are full. Conversely,
if your fuel system is marginal, fluctuations in battery voltage will cause fuel flow changes
from run-to-run which affects the float levels in the carburetors and out-the-window goes your
consistency! So how do you know if your volume is adequate?
|
| Test your fuel system by obtaining a 1-gallon gas-can (do not use a moulded-plactic gas container,
or marked super-jug, or antifreeze-jug as you will not get accurate readings). Open up the top of
the tin-can and insert the two or four carburetor fuel lines from your regulator, switch on the
system and carefully measure the time it takes to fill it. High 10-second cars will need to pump
1-gallon in 25-seconds or less. A 9-second car should fill the can in 20-seconds or less, 15-seconds
is all it takes for an 8-second car and under 12-seconds for 7-second vehicles.
Important Note: It's essential to observe two strict rules during
the test. One, keep a fire extinguisher handy and two, do not carry-out the test by yourself.
|
 |
| 1/4 Mile E.T. |
Time to fill a
1-Gallon Gas Can |
| 7 sec |
under 12 seconds |
| 8 sec |
15 seconds |
| 9 sec |
20 seconds |
| 10 sec |
25 seconds |
| 11 sec |
30 seconds |
| 12 sec |
35 seconds |
|
|
| How do you know you're getting all of the performance from your car? Perform the gas-can test even if
your car is running well - you have nothing to lose and everything to gain: including the prospects
of improving your E.T. and gaining increased consistency.
|
| When your car isn't performing, always carry out the gas-can test first - it's one of the least-expensive
diagnostic aids you'll encounter. Keep in mind that valve springs, ignitions systems, torque converters,
even engines have been changed, when all the time the fuel system was at fault.
|
| So, my car has failed the test - now what do I do? Call us at BG Fuel Systems: we have the correct fuel
pumps, regulators and plumbing for your particular H.P. level. Rely on The BG Powerfuel Advantage to
help put you in the winners circle.
|
| |
| Fuel
Filters |
| For the same reason, only filters
specifically designed for racing, such as the BG5000
or BG Inline filter, should be installed. Use of a filter
is strongly advised as long as it doesn’t restrict
fuel flow. The fuel filter should be installed in the line before
the fuel pump. This filters the fuel, preventing any damaging
material from entering the fuel pump or the rest of the system
|
| |
| Controlling
Fuel Pressure Settings |
| Fuel pressure should be set between
6 and 8 psi for a gasoline carburetor. An alcohol carburetor
is a different animal with very different requirements.
The Alky carburetor will require 4 to 5 psi at idle and
9 to 12 psi at wide open throttle. |
| Remember, fuel pressure is not
a substitute for volume! If the fuel bowls are not full,
the pressure is meaningless. In fact, fuel pressure is
simply an indication of the amount of restriction in the fuel
system. |
| |
| Regulators
and Bypasses |
| Most electrical fuel pump systems
require the use of a fuel pressure regulator. One BG
regulator is sufficient in many applications. The use
of two regulators is recommended when using other types
of regulators or in high horsepower engines to avoid
excessive fuel restriction and provide adequate volume. |
| With mechanical fuel pumps, and
some electrical pumps, a bypass is preferred rather than
a regulator. A diaphragm bypass without an idle bleed
is recommended when constant fuel pressure is needed
from an electrical or mechanical pump. A belt driven
fuel pump, using gasoline or alcohol, requires a diaphragm
bypass with an idle bleed. Higher pressure mechanical
fuel pumps delivering alcohol, such as the 15-psi BG Six-valve,
require a throttle bypass to supply the variable fuel pressure
required by the carburetor. |
