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 ofbuilding 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 willflow 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 cansiphon 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 houafter 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 fueldelivery 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, ovaltrack 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 fuelsystems. Today’s state-of-the-art engines produce a lot more power than a race engine of ten years ago. The process ofproducing horsepower revolves around the conversion of fuel into energy. The more pounds of fuel an engine can burnefficiently per hour, the more horsepower it produces. Even though your car may not miss, pop, bang, skip or do anythingelse 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 isspecified. These pumps offer the highest fuel delivery volume of any mechanical pump yet maintains low fuel pressure at lowengine speeds. This feature alleviates “loading up” of the spark plugs. The BG Six-valve and Super Speedway mechanical fuelpumps 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 “laysdown” 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, fuelpressure 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 manyapplications. 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 bypasswithout an idle bleed is recommended when constant fuel pressure is needed from an electrical or mechanical pump. A beltdriven 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.

 

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