Fuel & Forced Induction Calculators

Fuel and forced induction are about matching air to fuel. More boost packs more air into each cylinder, and the injectors, pump and tune must supply enough fuel to match it. Running short of fuel under boost is how engines get damaged, so the sizing tools lean toward margin.

Which tool answers which question

• Boost to horsepower calculator: A quick estimate of what boost adds to a naturally aspirated engine. Pressure ratio does most of the work, and an efficiency allowance accounts for heat and...
• Carburetor CFM calculator: How much air your engine can use at peak rpm, and which common carburetor sizes fit it. If you've used the displacement calculator, your cubic inches are...
• Fuel injector size calculator: Injector size for your horsepower on gasoline or E85, naturally aspirated or boosted. It also corrects for fuel pressure and checks duty cycle from a datalog.
• Fuel pump flow calculator: How much fuel your engine burns at full power, in gallons and liters per hour, and the pump rating that covers it with a safety margin.
• Turbo pressure ratio and airflow calculator: The two numbers you need to read a compressor map: how hard the compressor has to squeeze (pressure ratio) and how much air it has to move (lb/min) for your...

Sizing injectors and pumps

Injectors are sized from the power you want, the fuel you run and the share of the time the injectors can be open. Many tuners keep the duty cycle at the top of the rev range below about 80 percent, which leaves headroom. A fuel pump has to deliver its rated flow at the pressure your system runs, and published flow figures usually assume a particular pressure and voltage, so check the rating at your conditions.

Boost, airflow and carburetors

Pressure ratio for a turbocharger is the absolute pressure out of the compressor divided by absolute pressure going in, so it includes atmospheric pressure. More pressure ratio means more airflow and also more heat, which is why intercooling matters. For a carburetor, a common estimate of the airflow an engine needs is displacement times rpm divided by 3,456, adjusted for volumetric efficiency, and the carburetor should be sized close to that and not much larger.

Common mistakes

• Sizing for peak power with no margin. Leave room for fuel, temperature and wear.
• Ignoring fuel pressure. Injector and pump flow both change with pressure.
• Adding boost without a tune. Air and fuel must be matched by a tuner.
• Oversizing the carburetor. Too big a carburetor can hurt response and low-speed driving.

Guides for this section

Want the background behind these tools? These guides explain the thinking and the numbers in more depth.

• Sizing Fuel Injectors and Fuel Pumps for Your Power Goal
• Turbo and Supercharger Basics: Boost, Pressure Ratio and Reliability
• Compression, Cam Timing and Pump Gas: Making the Combination Work
• All guides