CCS 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 HP, 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 installing a hood scoop or outside air intake whenever rules allow. Under hood air is heated by the engine and headers and reduces the amount of power produced. A reduction of 10 degrees Fahrenheit is approximately equal to a one percent power gain.
There should be a minimum of 3 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 a 4-inch element preferred for racing engines over 500 HP.
It is highly recommended that the carburetor be sealed to the hood scoop or external air intake when it is employed. Otherwise, instead of flowing into the air horn, air will travel across the top of the carb and out of the intake tract. If air is driven through the carburetor, it can draw fuel from the engine, causing it to run low.
Unless the back of the snorkel is sealed, windshield snorkels are infamous for siphoning. Air pan kits or manufactured air pans for sealing the carburetor to the scoop are available. To enhance air flow into the carburetor, an air bell or radiused intake should be employed wherever practical.
After installing a sealed scoop, it's not uncommon for a drag race car's ETs to improve by 0.3 seconds and peak speed to increase by as much as seven miles per hour. If the scoop is too short or the fuel supply system is insufficient, a car will not pick up ET when it is sealed off.
The same is true for oval track cars. Paying attention to the design of the inlet tract will pay benefits. Oval track vehicles will generally improve lap times by 0.1 to 0.5 seconds if an appropriate air intake system is implemented, depending on track length.
Fuel Delivery
Many racers have fuel supply issues because of fuel system malfunctions without even realizing it. Today's state-of-the-art engines provide far more power than a race engine made a decade ago. The conversion of fuel into energy is important to the production of HP.
The more HP an engine generates is proportional to the amount of fuel it can burn each hour effectively. Even if your car does not miss, pop, bang, skip or do anything else out of the ordinary, it may not be getting enough gasoline to produce optimum power.
Where a mechanical fuel pump is specified on an oval track, a Belt Drive or Hex Drive Fuel Pump is suggested. These pumps supply the most gasoline of any mechanical pump while maintaining low fuel pressure at low speeds. This function prevents the spark plugs from "loading up."
When utilized as directed, the Carter Racing Six-valve and Super Speedway mechanical pumps will also provide adequate fuel volume. When a mechanical fuel pump is required, a Hex Drive Fuel Pump is ideal. These pumps supply the most gasoline of any mechanical pump while maintaining low fuel pressure at low speeds. This function prevents the spark plugs from "loading up."
An electric fuel pump is the greatest technique to prevent fuel starvation in drag race cars. The engine may be experiencing periodic fuel starvation if a car is "lazy" or "lays down" in the middle of the track yet pulls well in a higher gear.
Why? Typically, the carburetor bowls are full at the start line, so the car accelerates quickly, but the bowls are drained dry. The vehicle accelerates briskly in lower ratios, with the engine quickly ramping up rpm. The demand for fuels rises as a result of the fast acceleration. When the float bowl fuel level falls below a certain level, the vehicle "lays down" due to starvation. Engine speed increases more slowly in high gear, enabling the bowls to fill again.
The Fuel Can Test
There have been plenty of well-researched and well-written articles and books detailing how the fuel system works. However, many new and veteran racers are unaware of the fundamentals of fuel flow and HP. A combination of air and fuel is required to create torque and HP. It takes about .5 lbs. of fuel to make 1 HP for 1 hour.
If you weighed the gasoline tank before and after running a single-cylinder engine, such as the one in your lawnmower, under a load of one HP for one hour, the tank would weigh approximately 0.5 lb. (five-tenths of a pound) less. As a result, the fuel flow equation is 1 HP = 0.5 lb. of fuel per hour.
On a dyno sheet, this is written as B.S.F.C. (Brake Specific Fuel Consumption). Racing engines that have been fine-tuned can occasionally be more efficient, with B.S.F.C. numbers of roughly #.40, or four-tenths of 1 lb. of fuel per HP per hour. In addition, the alcohol formula is about 1-lb. of fuel per HP every hour, which needs the operation of a belt-drive pump, but that's another tale.
A 600-HP engine will typically consume 300 lbs. of gasoline per hour, whereas an 800-HP engine would consume 400 lbs. per hour. Remember that these amounts of gasoline must pass via the needles and seats, as well as the fuel pressure regulator. Consider that the gasoline distribution system must contend with G-forces: massive loads that might cause the fuel to stall 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 brings us to the region where the most confusion exists.
Isn't it supposed to be easier to feed one carburetor rather than two? Wrong! In a tunnel-ram engine, the needle and seat area, as well as the float bowl capacity, have increased. The single four-barrel car, which is the most common nowadays, has a significantly more difficult time keeping the fuel bowls filled. A 700-HP tunnel-ram engine consumes 350 lbs. of fuel every hour or little more than 85 lbs. per float bowl. A 700-HP engine with a single four-barrel (not unusual these days) requires 175 lbs. each float bowl, but a 1200-HP Pro Stock engine requires 600 lbs. max and 150 lbs. per bowl.
So, what happens if the fuel supply is inadequate? It may not miss or "burn up" parts. It may not live up to expectations. It's possible that the new camshaft, race carburetor, or flowed-heads that didn't help you out overworked an already stressed fuel delivery system. Carburetors can't disseminate the best air/fuel mixture unless the fuel system can keep the float bowl levels in check.
Low fuel levels may not cause the motor to miss or "burn" a piston, but they will restrict fuel flow and cause performance to suffer. It's fairly unusual to gain 1- to 4-tenths of a second after updating a fuel system with a single four-barrel carburetor. ETs have been found to drop by as much as a second in severe circumstances!
Is it possible for an overly large fuel system to degrade performance? No, it ensures that your combo will realize its maximum potential: when the float bowls are full, the needles and seats will close. If your fuel system isn't up to par, fluctuations in battery voltage will cause fuel flow adjustments from run to run, affecting the float levels in the carburetors, and your consistency will be thrown out the window. So, how can you tell whether your volume is sufficient?
Obtain a 1-gallon gas can to test your fuel system (do not use a molded-plastic gas container, marked super-jug, or antifreeze-jug as you will not get accurate readings). Open the lid of the tin can and insert the two or four carburetor fuel lines from your regulator, then turn the system on and time how long it takes to fill it.
High-speed cars will need to pump a gallon in 25 seconds or less. A 9-second one should fill the can in 20 seconds or less, an 8-second car in 15 seconds, and a 7-second vehicle in under 12 seconds. Important Note: During the test, you must follow two tight guidelines. One, have a fire extinguisher nearby, and two, do not perform the test alone.
How do you know you're getting the most out of your vehicle? Even if it is functioning well, you have nothing to lose and everything to gain by doing the gas can test: including the possibility of boosting your ET and achieving more consistency.
When your vehicle isn't working properly, always do the gas can test first; it's one of the cheapest diagnostic tools you'll come across. Keep in mind that valve springs, ignition systems, torque converters, and even engines have all been replaced when the fuel system was the source of the problem.
So, your car failed the test; what should I do now? CCS Fuel Systems will help you choose the right fuel pumps, regulators, and pipes for your HP level. You can rely on The CCS Powerfuel Advantage to help put you in the winner’s circle.
Fuel Filters
Only filters built for racing, such as the QFT 400 GPH or QFT Inline filter, should be fitted for the same reason. Filters are generally recommended as long as they do not limit fuel flow. Before the pump, the filter should be inserted into the line. This filters the fuel and keeps harmful materials out of the pump and the rest of the system.
Controlling Fuel Pressure Settings
For a gasoline carburetor, the fuel pressure should be set between 6 and 8 psi. An alcohol carburetor is a whole different species with entirely distinct needs. At idle, the Alky carburetor will require 4 to 5 psi, and at wide-open throttle, 9 to 12 psi.
Remember that fuel pressure isn't the same as volume. The pressure is worthless if the fuel bowls are not filled. Fuel pressure is merely a measurement of the degree of restriction in the fuel system.
Regulators and Bypasses
A fuel pressure regulator is required by most electrical fuel pump systems. In many situations, a single QFT regulator is adequate. When utilizing different types of regulators or in large HP engines, two regulators are suggested to avoid excessive fuel limitation and give enough volume.
A bypass is preferable over a regulator for mechanical fuel pumps and some electrical pumps. A diaphragm bypass without an idle bleed is recommended when consistent fuel pressure is required from an electrical or mechanical pump. A diaphragm bypass with an idle bleed is required for a belt-driven fuel pump that uses gasoline or alcohol. Higher-pressure mechanical fuel pumps that dispense alcohol, such as the Carter Six-15 valve's psi, require a throttle bypass to provide the carburetor's variable fuel pressure.