MINI 1st Gen Talking Injectors

Discussion in 'General Technical Questions and Answers' started by Whine not Walnuts, Oct 24, 2018.

  1. Whine not Walnuts

    Whine not Walnuts Active Member
    Supporting Member

    Mar 14, 2017
    waitin for my check
    Fuquay Varina, NC
    +250 / 0 / -0
    The modern MINI is a fuel injected vehicle and anytime that an engine modification is performed that increases the horsepower at the crank it is important to evaluate the injector size.

    First lets talk about some simple terms related to a Gen1 MINI.

    1.) SIze - the size of an injector is based on how many cubic centimeters (cc) of fuel is distributed in one minute. A 380 injector can be called a 380 cc injector, a 550 a 550 cc injector. The actual amount of fuel an injector will provided is based upon the pressure of the fuel system. Most non high-pressure fuel pumps operate in a 45 to 66 PSI range. As the MINI is a European vehicle, the metric system is used relative to pressure. The fuel system for the Gen1 is rated at 3.5 bars that equates to 50.7 PSI. A note, remember that pressure is based upon sea level. Just as in your tires the pressure will drop as you go up in altitude the correlation is applicable here. The higher the fuel pressure, the more fuel will be provided.

    2.) Brake Horse Power or BHP - this is the horsepower measured at the crankshaft.

    3.) Wheel Horse Power or WHP - this is the horsepower measured at the wheel. Horsepower is consumed/lost between the crankshaft and the wheels via the drivetrain. On a rear wheel drive (RWD) car the rule of thumb is approximately 85% of the crankshaft horsepower reaches the wheel. In a front wheel drive (FWD) car as the MINI this lose is approximately 11%. In calculating fuel injector sizes the BHP must be used. To convert your WHP into BHP you can divide by 0.89 (89%).

    4.) BSFC or Brake-Specific Fuel Consumption - BSFC is the term used to reflect the amount of fuel used per unit of power created. For a MINI Justa, a BSFC of 0.45 to 0.55 is appropriate with the 0.55 being used when there are modifications in place. For a MINI S/JCW that is supercharged a 0.55 to 0.65 BSFC is appropriate with 0.65 being used when there are modifications in place. The more modifications, the higher the BSFC. The BSFCs noted here are based upon gasoline consumption. An E85 ethanol/gasoline mix will have higher BSFC numbers as it requires more alcohol to produce the same energy as gasoline.

    5.) Injector Duty Cycle - The Duty Cycle of an injector is the percentage of time that injector is open providing fuel between spark plug detonation. Most will advise that a safe Duty Cycle or percentage is from 80% to 90% (0.80 to 0.90). At issue here is that engine requires a certain amount of fuel for a good Air/Fuel (AF) Mixture. The more fuel provided, the "richer" the AF, the less fuel provided the "leaner" the AF. A lean AF produces higher temperatures and a very lean AF can result in temperatures so high that a hole can be burned in the piston.

    With these basic terms, the formula for calculating the amount of fuel required is - (BHP x BSFC)/(The number of injectors x the Injector Duty Cycle). Using a 200 BHP value on a stock supercharger engine we would have - (200 x 0.55)/(4 injectors x 0.80) = 110/3.2 = 34.37 pounds of gasoline per hour. This 34.37 is based upon a fuel pressure of 45 PSI so as the MINI has a fuel pressure of 50.7 PSI we have 45/50.7 = 88.7%. So we multiply 34.37 lbs x 0.887 to come up with 30.48 lbs. As gasoline weighs approximately 6 lbs per gallon we have 30.48/6 = 5.08 gallons. Next we need to convert gallons of gas into cubic centimeters. There are 3,785.41 cubic centimeters to a gallon of gas - 5.08 x 3785.41 = 19,229.88. We now divide this number by the 60 (number of minutes in an hour) - 19,229.88/60 = 320. So for a 200 BHP engine, an injector rated for at least 320 cc is required. The Gen1 MINI injector size is 380, the S injector size is 300. This is why you will see many comments that if a 15% pulley is installed that the S injectors are above a safe duty cycle number.

    I am not going to go into spray patterns at this time.

    If you do not want to go back to school, or ask your children/grandchildren to figure this out for you, below is a website that has prepared input parameters for you to use.
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