More musings on pump capacity...

[Charles]

Lots of Luck Eric wrote:
>2- Why would you wonder for one minute if your fuel pump is adequate =
when it
>is so easy to add a simple ashcroft type guage and some nylon line to
>measure the fuel pressure under any condition you care to evaluate.  =
That is
>what regulators are for e to take care of variations such as voltage and=
 >load.

	Well...I have, in fact, done so. But you are assuming that the
gauge responds fast enough to see momentary short-duration changes,
and that any flickering of the needle isn't due to vibration and bumps
on the road. If some bubbles are being formed in the pump inlet due to
cavitation, cylinders will not fire when they pass through the
injectors.
	A regulator won't fix this. They are also not intended to make
extra fuel pump capacity where none exists. That's why I've been
wondering (for several minutes).

	Also, I've done a little more math.

	Stock injectors: wide-open 273 cc/min @ 36 psi.
	Assume density of gasoline =3D 0.75 g/cc
	6*273=3D1638 cc/min =3D 1228 g/min =3D 163 lb/hr with all 6 open.

Original engine rating: 252 hp. Assuming 0.5 lb/hp hr BSFC, this
requires 126 lb/hr. So the injectors should be open 126/163 or 77%
duty cycle, which is about right - most systems are designed to max
out at 80-85%.

	Now, I'm running 43 psi fuel pressure, so the above flow goes up
by sqrt(43/36) =3D 1.093:1 =3D 138 lb/hr.

	My two additional injectors are rated at 61 lb/hr each at 43 psi
(I won't bore you with the calculations, but if anyone has data for a
Siemens # 8213A for a Lamborghini, that's what they are).
	My fuel map has them open about 60% at 15 psi boost, so 2*61*0.6
=3D 73 lb/hr.

	The unknown is the amount of fuel going through the nitrous
setup. My "Nitrous Oxide Injection" book shows anywhere from 40 to 60
lb/hr for a 100 hp plate system. If this is proportionate, my 40 hp
nozzle should be consuming about 20 lb/hr.

So the total consumption is 138 + 73 + 20 =3D 231 lb/hr.

	From last weekend's measurement, the pump puts out 1350 cc in 30
sec (at the simulated 15 psi boost level). This is 268 lb/hr at .75
density. On paper, at least, I should have enough pump. But it
wouldn't take a lot of error in some of my assumptions to be exceeding
this. Any constructive thoughts welcome...

-Charles
'81 745i