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In Reply to: injectors- cc/min into lbs/min? posted by tom colitt on March 03, 2002 at 01:34:42:
Regards, Tom.
and all you engineers on the board can check me to be sure I got it right.
We know that gasoline weighs 6.2 pounds per gallon. So if we convert cubic centimeters in gallons and then multiply by the time (in this case minutes) we can make the conversion.
According to my Pocket Ref Book to convert cubic centimeters into gallons multiply by 0.00026. So 185 ccm X 0.00026= .0481 gallons. So in your case the injector is rated for .0481 gallons per minute.
However we need to get to pounds per minute. Since we know that there is 6.2 pounds of gasoline in one gallon we can mulitply 6.2 pounds per gallon X .0481 pounds = 0.29822 pounds.
In you case the injector is rated for 0.29822 pounds per minute or 17.89 pounds per hour (multiply by 60).
This seems to be pretty close, as a typical factory Chevrolet 350 engine uses 19 pound per hour rated injectors.
Remember the only way to change flow rate across the injector is to change injector "open time" and fuel pressure. Since the fuel pressure is contstant at the injector (i.e. 30 psi in your case) then "open time" is the only variable. And that is what the ECM does on the BMW engine.
Another way to determine the correct size injector is: (Engine HP X BSFC) divided by the (number of injectors X 0.8) = injector flow rate.
In your case engine HP is 180
Brake specific fuel consumption (BSFC) is 0.50
Number of cyliners is 6
0.8 is 80% duty cycle for the injector.
The result is 18.75 pounds per hour for the injector. Very close to your existing injector rate. So try to find a injector that is rated between 18 and 19 pounds per hour and you will have a good set of injectors.
Be sure to check my math!
Sources: Fuel Injector Basics, 1999 Petersen Publishing and Fuel Injection 1978-1985, Hayes and Bosch Fuel Injection and Engine Management, Charles O. Probst, SAE, Bentley Publishers.
Grant 1985 735i 5 speed
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