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In Reply to: Re: Tom. let me take a try at this......... posted by tom colitt on March 03, 2002 at 13:23:51:
Everything makes sense. So I should be able to go with slightly larger injectors now that I souped up my 218hp stock engine with a cam and exhaust header. Thanks again, Tom
Does anyone know how fuel injector delivery translates from ccm/min into lbs/min? I have 185cc/min injectors and am thinking about getting cheaper Ford/Bosch injectors rated in lbs/min. Why do they use such an odd unit that depends o variables like fuel density, temperature, etc?
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
then 22 pound per hour would be the selected and rated flow for the injector at 30 psi of fuel pressure. Maybe a bit to high, but will not hurt if you plan to use the high end of the RPM's of the engine. Remember the 22 pound per hour is the max rate not the normal useage of the injector. So if you hold to the 30 psi on the fuel pressure regulator then for the same amount of firing time for the injectors you will flow more fuel (the injector is larger, more gasoline can go through it). This is where the fuel map from the EPROM comes in to play.
Another thing to think about is fuel pressure. If you raise the pressure you will flow more fuel when the injector is open.
I would suggest using the more conservative flow rate and adding an adjustable fuel pressure regulator.
The formula for determining how much more fuel will flow in the eixiting injector by raising the fuel pressure is: The square root of new fuel pressure divided by the old fuel pressure X rated injector flow rate. Assuming you use the 18 pound per hour injector and raise the fuel pressure from 30 psi to 35 psi, it would be the square root of 35 divided by 30 which = 1.080 X 18 which then =19.44. This is the new flow rate of the injector at the new fuel pressure.
As you can see the adjustable fuel regulator is an easier way to adjust the amount of fuel that the injectors can flow. If your existing injectors are in good working condition add the adjustable fuel regulator to control the amount of fuel flow across the injector.
I think a Ford Mustang uses Bosch injectors. Any good auto store can tell you what those injectors are rated for.
Good luck.
Grant
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