Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
= math constant
As an example, a 403-ci engine produces peak power at a maximum of 6,500 rpm. Using the formula:
[(403 ÷ 2) × (6,500 ÷ 1,728)] 201.5 × 3.76 = 757.64
Here, a carburetor size of 700 to 750 cfm is appropriate.
A vacuum-secondary carburetor is somewhat forgiving, allowing you to use a slightly larger carburetor. A carburetor with mechanical secondaries is not so forgiving. If in doubt when deciding between two sizes in the required range, it’s often better to choose the smaller carburetor. Selecting a mechanical secondary carburetor that features a double-pumper design that is too large for the application can result in sags or bogs upon acceleration if the engine uses up the pump shot before the main fuel shot is delivered.
Forced-induction engines utilizing either supercharging or turbocharging can take advantage of larger carburetors, since forced-induction can typically increase VE to a point well over 100 percent.
EFI is an active system that adjusts fuel delivery according to the operating conditions of the engine while a carburetor is a passive system that controls both air and fuel intake. The engine control unit (ECU) manages an electronic fuel-injection system’s fuel delivery, and it precisely controls fuel delivery timing and duration via the injectors. I could refer to a throttle body as the engine’s air valve. The throttle body’s only job is to allow air to enter the engine. In theory, the more air delivered, the more power the engine can produce; of course, when mixed with the appropriate ratio of fuel. Changing an original equipment manufacturer’s (OEM) throttle body for a larger aftermarket throttle body is necessary in order to introduce more air, but there is a point of diminishing returns. Going with the largest throttle body available isn’t necessarily the best move.
Factors of Carburetor Size
Listed here are generalizations of relative carburetor size (cfm) applications.
Smaller Carburetor
• Suited for more torque at a lower RPM range
• Better for automatic transmissions and lower stall-speed torque converter
• Appropriate for lower compression ratios
• Accommodates less camshaft duration
• Suited for lower drive-gear ratio
Larger Carburetor
• Suited for higher horsepower engines at higher RPM range
• Suited for manual transmission or automatic with higher stall-speed converter
• Appropriate for greater camshaft duration
• Suited for higher engine compression ratio
• Suited for higher drive-gear ratio
Throttle body size, in terms of throat diameter, needs to be properly sized to the displacement of the engine. If the throttle body is too small, air velocity is faster, but volume is often insufficient. If the throttle body is too large, the air velocity is typically too slow, but more air is drawn into the engine.
An accepted formula for determining throttle body size in millimeters is based on engine displacement, measured in cubic inches of displacement (ci):
Throttle Body Size (mm) = √[(ci × 196.3 × RPM at max. hp) ÷ 67,547]
Where:
196.3 = math constant
67,547 = math constant
For instance, let’s say that the engine features 408 ci, and this engine is expected to reach maximum horsepower at 6,500 rpm. Using the formula:
For this naturally aspirated engine example, a throttle body featuring an 88- to 90-mm throttle body is approximately the correct size. Slightly increasing the size provides a small cushion for maximum horsepower. In this example, a throttle body size of around 90 mm does not restrict the air needed for this engine at a speed of 6,500 rpm. While you could go with an even larger throttle body, air speed (velocity) may slow down and not fill the chambers quickly enough.
Throttle bodies for electronic fuel injection systems manage incoming air only. Moving to a larger-volume throttle body can often improve performance, as long as the engine can use the increased air volume. (Photo Courtesy BBK Performance)
If you grossly oversize the throttle body, you introduce a lot of air very quickly, which may be too much air for the engine to handle during quick or snap-throttle activation, possibly making the vehicle undrivable on the street, while at the same time not improving horsepower.
If you’re running a forced-induction system, throttle body size becomes slightly less critical, since the supercharger or turbocharger forces air into the engine. A Roots-style supercharger sucks air into the engine through the throttle body, while a turbocharger pushes air through the throttle body. The goal here is not to create a too-small bottleneck restriction for airflow.
Air pressure, as well as air density, is approximately twice as high in a turbo setup as compared to a naturally aspirated engine, so in theory you can use a smaller throttle body for a turbo as compared to a naturally aspirated setup.
A variety of methods of monitoring airflow have been used in production vehicles over the years, including now-outdated van airflow meters, Karman Vortex airflow meters, and speed density systems that rely on a host of engine sensors to determine airflow. The most common systems today use either a hot-wire or a hot-film mass airflow (MAF) sensor. A hot-wire or hot-film MAF sensor allows the electronic control module (ECU) to directly measure intake air temperature, humidity, and air density. A hot-wire MAF sensor features a thin platinum wire located in the airstream that is heated to a programmed temperature using a reference voltage of around 5V supplied from the ECU. As intake air passes the wire, the wire is cooled, resulting in a change in resistance. As the wire temperature drops, the voltage changes. The ECU sees this drop and manages the voltage signal for operating conditions. Based on the required voltage changes, the ECU is able to determine the quality of the air mass.
This MAF sensor is mounted to a section of air intake ducting. A hot-wire or hot-film MAF sensor allows the ECU to directly measure intake air temperature, humidity, and air density. A hot-wire MAF features a thin platinum wire located in the airstream that is heated to a programmed temperature using a reference voltage of around 5V supplied from the ECU. As intake air passes the wire, the wire is cooled, resulting in a change in resistance. As the wire temperature drops, the voltage changes. The ECU sees this drop and manages the voltage signal for operating conditions. Based on the required voltage changes, the ECU is able to determine the quality of the air mass.