Spade

Hello again, it’s been a while hasn’t it. Okay, so I found these formulas a long time ago, hidden in the back of a fairly useless website and thought it’d be great to add this to the FAQ. I mean if I’m going to spend all this silly time researching this, I might as well share.

First things first, selecting an injector.

Injector flow figures are taken at a static rating which is the equivalent of 100% duty cycle and is used for injector flow rate comparison purposes only. Injectors are not intended to be operated without cycling (ie 100% duty cycle). Choose injectors by factoring in an operation range of 80-90% of their static value at your intended peak horsepower level with the following corresponding BSFC in mind.

If you are monitoring the injector duty cycle on your final engine configuration, a value of up to 90% is “acceptable”. If you are trying to calculate the required injector for a completely new engine build, use 80% as a good safety factor. I’m going to generalize to 85% for examples.

MATH: CC/Min = Lbs/Hr x 10.5 (NOTE: This factor can vary from 9.7 to 10.6 depending on the grade and formula of the gasoline produced for different seasons, 10.5 being the best “safe” figure).

Example: 240cc/min = lbs/hr x 10.5 or 240/10.5 = lbs/hr which means, 240cc injectors = a whopping 23LBS/HR. (22.857142857142857142857142857143 for the literal freaks)

Brake Specific Fuel Consumption (BSFC)
Brake Specific Fuel Consumption is the ratio of fuel consumed (in lbs. per hour) to horsepower produced. This ratio is a direct indicator of how efficiently the engine converts fuel into power. Most factory gasoline type engines run approximately a .50 to .55 Brake Specific Fuel consumption (BSFC) range while a highly efficient normally aspirated race engine operates at approximately a .40-.45 BSFC. (No, factory Honda engines are not “highly efficient race engines”.)
Most common turbocharged or supercharged engine configurations run in the .55 to .60 BSFC range.
For Methanol powered applications the BSFC is doubled (i.e. turbo/methanol: 1.10-1.20).
These factors should be considered when sizing & selecting injectors for your particular application.

To find the Max. HP value of the injectors from (Lbs/hr). on a N/A 4 cylinder engine fueled by gasoline.
HP = ([Injector size (lb/hr) x Duty Cycle] /BSFC) x (# of Injectors)

Example: HP rating of 4 each of 23 Lb/Hr injectors @ 85 % duty cycle and .50 BSFC.
[(23Lb/Hr injectors X 85% Duty Cycle) / .50 BSFC] X 4 injectors.
[(23 X .85)/.50] X 4 = 156.4 engine hp. in your stock 240cc injectors with the stock fuel pressure (we’ll tackle that shortly).

Quick and easy:
Injector Size Per Horsepower for a 4 banger: LBS/HR = HP/6.8

Background: lbs / hr = [BSFC ÷ (# of Cylinders)] x HP) ÷ Peak Inj. Duty Cycle
In general if you use BSFC = .50 and an acceptable duty cycle of 85% on a four cylinder engine, then:
Lbs/hr = HP ÷ 6.8
Example: You plan to make 240 HP. What size injector should you use?
Lbs/hr = 240 ÷ 6.8
Lbs/hr = 35 lb/hr

Flow Rate Change
By Pressure Regulator Change (CRAP more math!)
F2 = (√P2/√P1) x F1

F2 = New flow rate (lbs/hour or cc/min)
F1 = Old flow rate (lbs/hr or cc/min)
P2 = New Pressure
P1 = Old Pressure
An adjustable fuel pressure regulator allows you to change the amount of fuel delivered per unit time from the injector. Any changes affect the fuel curve globally, so re-mapping the idle and light load conditions is usually necessary. The mass flow is proportional to the square root of the pressure ratio:
Example: You have a 300 cc/min (@45 psi) injector. How much will it flow at 60 psi?
F2 = (√60 / √45 ) x 300
F2 = (7.746 / 6.708) x 300
F2 = 346 cc/min

To convert lbs/hr to GPH
Divide LBS/HR by 6

To convert CC/Min to GPH
Multiply CC/MIN by .01585

Pump Flow Conversion Factors
GPM = lbs/hr ÷ 360

To convert from lbs/hr to gals per minute (of gasoline) use: GPM = lbs/hr ÷ 360


Example: Your engine uses 221 lbs per hour of fuel. What are the fuel system requirements?

GPM = 221÷ 360
GPM = 0.61 gal / minute


So, what’s all this mean…….RESEARCH before someone tells you that you need 450cc injectors because your turbo’d to push 200whp. When in reality all you’d need is 370cc or 35lb @ 85% duty cycle!

thisisntjared

good stuff. i hope people realize this is good info good tech

94hatchling

i'm actually starting to cover some of this in my engine's class. props on the hard work and research. great info.