Tech | Carb Tuning Secrets
For nearly 100 years, a carburetor has been at the heart of almost all internal combustion engines. Only in the last 20 years or so have car makers used electronic fuel injection exclusively on gasoline engines. There have been examples of mechanical fuel injection for automotive applications in the past, and of course diesel engines use fuel injection. However, the classic Ford cars we love were all carburetor-equipped, from the 170-cubic inch six-cylinder with a one-barrel Autolite carb to an R-code 427 with eight barrels of Holley induction in the form of dual-quads. Carburetors still compete effectively with fuel injection on engines when it comes to making power and when we do an EFI conversion, horsepower production is not the single largest motivation for doing so. The fact is that when the time comes to run the quarter-mile or make a few pulls on the rollers, a carburetor gets the job done just about as well as fuel injection. Even though simple in principal, in that a carburetor functions as an atomizer, there are many subtle variables to be considered and adjustments that can be made to get your engine running its best.
For this discussion we're...
For this discussion we're going to look at carburetors that have a central body and separate metering blocks and float bowls such as those from Barry Grant, Quick Fuel Technology, and Holley; and we'll save the Carter and other style units for another overview. This exploded view of a Barry Grant four-barrel shows the base plate (4), central body (3), two metering blocks (2), and float bowls (1).
Of course, once engine modifications begin then things become a little more involved. When one enthusiast decides on a mild 289 with single exhaust, while another equips his K-code with long-tube headers then carburetion requirements will differ. However, two rules of thumb apply across the board when it comes to carburetor tuning and selection. The first is sizing the carburetor appropriately to the displacement or flow capacity of the engine. For example a 750cfm carb would be too large a choice for an A-code 289 that the factory equipped with a 480cfm unit. However, adding compression, cylinder heads, and a higher-lift camshaft increases the flow capacity of the engine and then a larger carburetor might be called for. A 600cfm unit is a good bet for a healthy short-deck engine while a 650 to 750 cfm would be a good size for a stock or stroked Windsor engine. Save the 850 size for your 428 or larger displacements or otherwise radical engines. According to the experts at Pony Carburetors the following table lists approximate flow capacities of different Ford engines in their stock state. These numbers are calculated at 90 percent volumetric efficiency.
|289/302ci ||453 cfm|
|351ci ||550 cfm|
|390ci ||609 cfm|
|428ci ||668 cfm|
This illustration is also...
This illustration is also courtesy of Barry Grant and it shows how the fuel (in red) is transformed from a liquid state to an atomized state as it passes through the carburetor, picking up air form the air bleed passageways.
When we spoke to various experts in the carburetor field about the most common problems that folks encounter with a carburetor on a street performance car they cited stumble off idle, bogging at wide open throttle application, secondary engagement issues, and lean or rich operating conditions. These problems can be addressed with adjustments to the float bowl fuel level, accelerator pump, secondary opening systems, and jet settings. In this short overview let's examine these common engine performance complaints that can be addressed by simple carburetor adjustments and then look at a dyno-documented example of the benefits achieved when carburetor size and engine timing are optimized for the engine.
Matching the carburetor size to the capacity of the engine ensures good throttle response throughout the rpm range and also the best economy of operation.
This diagram illustrates Bernoulli's...
This diagram illustrates Bernoulli's principle and shows how fuel is drawn into the slipstream by the low pressure created because of the diameter restriction at the center of the venturi.
The second rule of thumb that the experts we spoke to stressed over and over again is that a properly tuned carburetor will be complemented most in terms of performance by the correct engine timing. Correct initial timing for most stock V-8 engines occurs at 10 to 14 degrees before TDC. On engines with high-
performance components added, the initial ignition timing should be advanced. For example, high-performance camshafts often have a longer duration for valve events and so they generate less depression or signal at the carburetor at idle. As a result air-fuel droplets are bigger, atomization is poorer, the burn rate is slower, and the combustion more incomplete. To overcome these tendencies initial timing should be increased.