Fuel Pump Requirements Based on Injector Size

February 01, 2011, 02:49:48 PM

I have read a few threads with concerns over what size replacement fuel pump to buy and thought there should be an easy way to calculate the required pump volume (l/hr) based on the injector mass flow rate (lb/hr) and the number of injectors (unitless).

I did a quick search online and found the average weight of a gallon of gasoline to be about 6.2 lb/gal. The basic conversion for one gallon to liters is 3.785 l/gal. With that knowledge you can make an equation using the following:

Injector Mass Flow Rate = X lb/hr (the number will vary with the rate of the chosen injector)
Number of Injectors = Y unitless (the number will vary with the engine application)
Weight of a Gallon of Gasoline = 6.2 lb/gal (this is pretty much fixed)
Number of Liters to a Gallon of Gasoline = 3.785 l/gal (fixed number)

[(X lb/hr) x Y x (3.785 l/gal)] / (6.2 lb/gal) = volumetric flow rate of fuel pump needed at 100% injector duty.

Once you do the unit cancellation you end up with l/hr so I am pretty sure I got the equation right, if not please feel free to correct. I ran the numbers for a V-8 car and came up with the following:

(19lb/hr x 8 x 3.785 l/gal) / 6.2 lb/gal = 92.7935 l/hr (makes sense as a stock 5.0 car comes with a 90 l/hr pump)

24 lb/hr = 117.2129 l/hr
30 lb/hr = 146.5161 l/hr
36 lb/hr = 175.8194 l/hr
42 lb/hr = 205.1226 l/hr

How much hp the injectors will support can be calculated here if you are curious:

http://www.witchhunter.com/injectorcalc1.php4

I used these guys to clean and flow test my old 24 lb/hr injectors and it was well worth the cost to get them back withing a few percent of each other. It solved the lean/rich problem I was battling on the old motor.

Anyhow, maybe this will help someone and hopefully I did my math correct.

Darren