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Injecors by Razor_Wing
Started on: 12-19-2003 04:49 PM
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Last post by: Solo2 on 12-19-2003 08:10 PM
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Report this Post12-19-2003 04:49 PM Click Here to See the Profile for Razor_WingClick Here to Email Razor_WingSend a Private Message to Razor_WingDirect Link to This Post
Ok, I'm building my engine up for a turbo, and I'm going to put new injectors in (I know stock kinda work, but mine are bad annywaise). It's going to be strokered and the goal is 9-10 psi constant, and attainable of 14psi for "race" condentions. I'm going to get ACCEL injectors. I need to know what lb/hr rating I need. Thx for the help!


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Report this Post12-19-2003 08:10 PM Click Here to See the Profile for Solo2Send a Private Message to Solo2Direct Link to This Post
Choosing The Correct Fuel Injector For Your Application
Fuel requirement in lbs./hr = (Max HP x BSFC) / (number of injectors x duty cycle)
Note: to convert from lbs./hr to the Metric measurement of cc/min, use this equation: [(lbs./hr) x 60] / 6.177 = cc/min
Max HP is a realistic horsepower estimate at the crankshaft or known value from engine dyno testing. Chassis dyno horsepower figures can only be used once
you factor in the drive train losses, which can vary from vehicle to vehicle. Ask your chassis dyno operator to calculate the drive train horse power loss for
your vehicle. Add the drive train horse power loss to the drive wheel horsepower to closely estimate crankshaft horsepower.
BSFC or brake-specific fuel consumption is the amount of fuel consumed per unit of power produced. It is an indication of the efficiency of the engine
configuration and calibration. Actual BSFC is a function of compression, camshaft timing, cylinder head design, tune, ambient conditions, etc. The lower the
BSFC number, the more efficiently the engine is making power. Engine dyno testing can provide exact BSFC data. To estimate the fuel requirements of your
engine, use the examples below that best match your engine type. The reason we use a higher BSFC value to calculate fueling requirements for a supercharged
engine is because of the parasitic losses or the power required to driving the supercharger that is never seen at the crank. In other words, a supercharged
engine that dyno tests 450 hp at the crank, may actually be making 490 hp, but the supercharger and drive assembly is absorbing 40 hp, so you net out 450 hp.
Also, the heating effect of pressurizing the intake charge in a non-intercooled system also increases the fueling requirement of a super/turbocharged engine.
Always remember that too lean of a mixture can result in spark knock, high combustion temperatures and engine damage. Itís smart to be slightly on the rich
or safe side.
Engine type Gasoline Alcohol
High compression 0.45 to 0.55 0.90 to 1.10
Low compression 0.50 to 0.60 1.00 to 1.20
Super/Turbocharged 0.55 to 0.65 1.10 to 1.30
There is one other parameter involved in properly sizing fuel injectors: duty cycle. This is the percent of time that the injector is actually open (which is also
referred to as pulse width) vs. total time between firing events. When an injector is open 100% of that time, the injector is in what is called a static condition.
For road-racing engines that are at maximum power for extended periods of time, the desired maximum safe duty cycle is 0.85. This ensures that the injector
is closed a sufficient time to keep it from overheating. For a typical street engine that spends less than 1% of its time at maximum power, you could argue that
a higher duty cycle could be used to calculate fueling needs.Typically we would not do this because again we want to error on the safe side. Some may ask why
not just install the biggest injector you can find. Well itís the same analogy of putting an 850cfm carburetor on a Chevette motor, overkill at best, more like a
controlled leak. One other thing to remember is that an injector can only open and close so fast, this is called minimum dynamic flow range. If the ECU, in an
attempt to lean out a rich mixture, selects a pulse width that is shorter than the injectorís minimum dynamic flow range, the injector becomes inconsistent in
its ability to supply the required fuel. This results in poor engine performance, surging and stumbling. In other words bigger isnít always better.
Letís calculate the fueling requirements of a few engines to illustrate what we have been talking about.
For the first example letís take a stock Ford 5.0L Mustang motor that makes an advertised 215 hp and look a the very conservative approach Ford used to
calculate the injector size for the factory engine by using the O.E. typically safe 0.80 duty cycle limit.
Fuel injector size = (215 hp x 0.55) / (8 x 0.80) = 18.5 lbs./hr or the ACCEL p/n 150119 injector
Now letís upgraded the engine with more efficient GT-40 type components that will lower the BSFC and use a more realistic 0.85 duty cycle limit. Ford says
this combination of GT-40 parts will produce about 275 hp.What injector size is required to support this?
Fuel injector size = (275 hp x.50) / (8 x 0.85) = 20.1 lbs./hr or the ACCEL p/n 150121 injector
Until now your only choice would have been to go with a 24 lbs./hr unit, which would be fine if the engine was making about 325 hp, but not ideal for 275 hp.
Remember the comment about realistic horsepower; donít kid yourself! Now letís factor in an adjustable fuel pressure regulator as a tuning tool for this setup.
By adjusting fuel pressure you can change the flow rating of a given injector. The calculation is simple, as long as you know the static flow rating of an injector at
a specific pressure. For example ACCEL p/n 150121 flows 20.0 lbs./hr at 2.7 BAR or 39.6 PSI, which just happens to be where the stock Ford non-adjustable
fuel pressure regulators are preset. As a point of reference, most GM factory fuel pressure regulators are preset at 3.0 BAR or 44.1 PSI. If we were to increase
the fuel pressure from 39.6 PSI to 45 PSI, what will be the new flow rating of the ACCEL p/n 150121 injector?
New flow rating = [square root of (new pressure /old pressure)] x old flow rating
New flow rating = [square root of (45 PSI / 39.6 PSI)] x 20.0 lbs./hr = 21.3 lbs./hr
This increase in flow rating would support about 15 additional horsepower on our GT-40 engine. An adjustable fuel pressure regulator is an excellent tuning
tool as long as the fuel pressure does not exceed 55 PSI, which is the limit that the stock fuel line fittings are designed to handle. So letís say we increase the
fuel pressure up to 55 PSI, then the ACCEL p/n 150121 injector would be flowing 23.6 lbs./hr. But because ACCEL offers p/n 150123 that flows 23.1 lbs./hr at
39.6 PSI and 150124 that flows 24.3 lbs./hr at 39.6 PSI, radical increases in fuel pressure are not required to find the perfect match for your engine. The key
is to make power efficiently, choosing the correct injector for your intended needs and using the adjustable pressure regulator as a fine tuning tool.
For the third example letís use Fordís new 392 crate motor p/n M-6007-A392. Out of the crate, using a 750cfm carburetor, this engine dyno tested at 453 hp
with a .454 BSFC. Letís calculate the injector size you would need if the 392 were to be fuel injected.
Fuel injector size = (453 hp x 0.454) / (8 x 0.85) = 30.2 lbs./hr units or the ACCEL p/n 150130 injector.
As a point of reference, this same 392 crate engine has made over 530 hp on a dyno with Air Flow Research 185cc heads vs. stock GT-40X heads. To support
this new-found power, using the same equation, larger 35.2 lbs./hr units or the ACCEL p/n 150136 would be needed. So when calculating injector size, if you
are planning on large power adders in the future, keep in mind that you may have to upgrade your injector size. Just like if you might have had to put a bigger carburetor on a modified motor in the past.
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