white97T/A

I have been looking for a write up on how to determine the proper sized fuel pump. I know that Racetronix is well received and makes nice products but I like to understand what I need to get for my application.

I can find a lot of calculators for fuel injector sizes but I have not found much for fuel pumps.

Injuneer

Fuel demand is determined by HP at the flywheel. For normally aspirated setups, the conservative brake specific fuel consumption (BSFC) is 0.50 #/HR/HP. That means a 500 (flywheel) HP engine needs 500 x 0.50 = 250 #/HR of gasoline.

How much does a gallon of gasoline weigh? Let's be consrvative again, and assume the specific gravity is 0.70. That means compared to water (specific gravity = 1.00, and density = 8.345 #/GAL), a gallon of fuel weighs 8.345 x 0.70 = 5.85 #/GAL.

Divide 250 #/HR by 5.85 #/GAL and you need 250 / 5.85 = 42.75 GAL/HR. A gallon is 3.78 liters. 3.78 x 42.75 = 161.6 LITER/HR.

Let's be conservative, and assume that we want to maintain 20% return to the tank at max fuel demand. 161.6 / (1.00 - 0.20) = 161.6 / 0.80 = 202 LITER/HR to support 500HP at the flywheel, normally aspirated.

That works out good, because the shop that set up my fuel system used two 205 LPH Bosch fuel pumps. They normally use one 205 LPH pump for a 500 HP normally aspirated engine. In my case, the second pump supports a 300HP shot of nitrous, which has a much higher BSFC.

Let's reduce the above calculation to a simple rule-of-thumb to get you in the right ball park. Multiply normally aspirated flywheel HP by 0.40 to get the pump size in LPH. 500 HP x 0.40 = 200 LPH.

Flow is one consideration.... pressure is the other. A fuel pump has a flow vs. pressure "curve". As pressure goes up, flow goes down. So if you determine that you need a 200 LPH pump, and you want to operate your injectors at 43.5psi (3 bar), you want to make sure the pump can flow 200 LPH or more at 43.5psi. I operate my fuel system at 4 bar (58psi). I have to make sure the Bosch 205 LPH pump can flow at least 200 LPH at 58psi, and it can. Many pumps that will meet capacity requirements at 43.5psi may drop flow dramatically as pressure increases. Hence you would need a pump designated as a "high pressure" pump to operate at 58psi or above. You have to check the flow vs. pressure curve.

white97T/A

Thank you for the info, this is exactly what I was wanting to know. I am working on setting my car up with the 21 degree TFS heads and a cam, on the stock short block. The folks that I have been talking with on cam selection are estimating that I should make about 400-410 HP at the wheels.

Out of curiosity what do stock fuel pumps flow/ what kind of power can they support?

Injuneer

I really don't know much about the stock pump.

Remember you have to base pump size on flywheel HP, not rear wheel HP.