Fuel pressure / Flow rate formula

[Greg Hermann]

>What's the formula for figuring out the flow rate of an injector if you
>bump up the fuel pressure?  I've found 2 different ones that give *huge*
>differences in the result.
>
>The first is from an MSD fuel management catalog/brochure I have.  It
>states that
>
>
>F2 = sqrt(P2 / P1) * F1
>
>
>For me, I have a 20 lb injector at 40 psi and I want to check the flow
>at a maximum of 70 psi, so
>
>F2 = sqrt(70 / 40) * 20  =  26.4 lbs/hr
>
>
>Not as much as I had hoped.
>
>I then found a different formula on the RC engineering page
>http://www.rceng.com/technical.htm.  It's almost the same, except that it
>uses a strait ratio between the pressures instead the square root of them,
>so according to them, I have:
>
>
>F2 = (70 / 40) * 20  =  35 lbs/hr
>
>
>
>This is a very large difference.  Which one is right?

Surprising that RC doesn't know better than that--leads one to wonder about
their prowess (just ask them--they will tell you they have it) at
"blueprinting injectors.
>
MSD's formula is correct, as is the calc that give you 26.4 lbs./hr. Basic
fluid dynamics relationship.

And strongly recommend that you go no higher than 70 or maybe 75 psi, as
the pintle in the injector will begin to bounce on closing, and give you
non-repeatable fuel amounts, above that pressure. To get more fuel than the
26.4, you need a bigger injector. And remember, what counts is the delta P
across the injector--the difference in the absolute fuel pressure in the
fuel rail and the absolute pressure in the manifold. A simple gauge reading
off the fuel rail does not tell all you need to know!!

Regards, Greg

>
>thanks
>--Dan