We reached out to the guys in Summit Racing’s renowned tech department—the largest full-time staff in the industry—to identify and answer the five common tech questions they hear on a consistent basis. With help from Summit Racing’s tech advisors and other industry experts, we’ll answer those questions here at OnAllCylinders. Today’s topic: fuel pumps.
When we refer to fuel pump size here, we’re talking about the size or amount of fuel flow—not the pump’s physical size. More accurately, we’re talking about the fuel flow volume as measured in gallons-per-hour (GPH).
So here’s the real question (and it’s one of the most common questions asked of the Summit Racing tech department):
What is the optimum GPH for my engine?
There’s a simple formula to calculate GPH or LPH (liters per hour), but there are some variables to this formula, depending on your application. Norm Koval, a team leader in the Summit Racing technical department, typically follows this formula as laid out by the air/fuel experts at Holley:
Optimal GPH = (Max. HP x BSFC) / 6
Optimal LPH = (Max. HP x BSFC) / 1.585
In order to make the proper calculation, you need to determine your variables for this equation, starting with your vehicle’s maximum horsepower (Max. HP). You can determine this by having your vehicle dyno tested or by making a reasonable horsepower estimate based on the manufacturer’s advertised horsepower plus a conservative estimate of power gains made by aftermarket modifications.
The second variable in the equation is BSFC, or brake-specific fuel consumption. BSFC is a measurement of the amount of fuel consumed per unit of power produced. According to Holley, an engine typically requires .5 pounds of fuel per horsepower every hour at wide open throttle. However, this number generally applies to naturally aspirated engines.
“There are exceptions,” Koval said. “What if it’s supercharged or turbocharged? Then I might give that number a fudge factor to bump up the GPH.” If you have a highly modified, naturally aspirated engine, for example, you can tweak the BSFC number up to .55 pounds. Aeromotive recommends the following adjustments for power adders:
- Nitrous combinations: .5 to .6 pounds
- Forced induction engines .6 to .75 pounds
Once you’ve determined your realistic horsepower and BSFC numbers, multiply them together and divide the product by six (the typical weight of a gallon of gasoline). For example, if you have a stock engine that makes 350 horsepower, your equation will look like this:
(350HP x .5 lbs) / 6 lbs =
175 lbs/hr of fuel / 6 lbs = 29 GPH
In our example, you’d need a fuel pump that delivers 29 GPH of fuel.
A Word About Fuel Pressure
Whether you choose a mechanical fuel pump or an electric fuel pump, you’ll also need consider fuel pressure and its relationship with fuel volume. This relationship is inversely proportional—that is, as pressure increases the volume will decrease with everything else being equal. A certain amount of fuel pressure is always required to maintain engine performance by assuring that fuel is available on demand. As engine demand goes up, as is often the case in forced induction applications, the amount of required fuel pressure will increase, causing the fuel volume to the engine to go down. Other factors and conditions, such as acceleration G-forces and friction within the fuel system itself, will also affect fuel pressure.
A basic understanding of this critical pressure/volume relationship is needed when designing the proper fuel supply system for your vehicle. You need to select a fuel pump that will meet your engine’s fuel volume AND fuel pressure demands. Many of the top fuel pump manufacturers offer charts that show fuel volume across a reasonable pressure range, so you can get the right balance for your vehicle’s fuel demands. You can also reduce sudden pressure rise and help maintain or increase fuel flow by modifying your fuel system with larger fuel injectors or other aftermarket upgrades.