Hello Jos
Sorry I did understand exact wording of your question, but will describe
what you need to know and do.
The 914 uses a turbocharger to squeeze air molecules together at a greater
than ambient pressure. Unlike a normal aspirated engine, if you were to
vent the float bowl to ambient pressure on a turbo charged engine and
began to boost above ambient pressure, air would in fact flow the wrong
way through the carb jets and starve the motor for fuel.
Thus on a 914 the float bowl is vented instead of to ambient pressure, to
airbox pressure. When not in boost airbox will be close to ambient, when
boosting it will go above ambient pressure.
OK so now we have at least bowl pressure following turbo output.
Now you need greater fuel pressure than float bowl pressure (which is the
same as airbox pressure) to be able to fill float bowl with fuel. This is
the job of the fuel pressure regulator. You want between approx 2 and 5
PSI fuel pressure (absolute) over airbox pressure (absolute).
In absolute terms, you need your fuel pumps to work harder in thinner air
than in thicker air. It is very possible that filters and pumps can
provide at 2K, but not at 8K. If pressure can not be kept up, you are in
danger of having your motor playing silent night. Turning on second pump
can help, going down in altitude can help, and dropping manifold pressure,
which will drop airbox pressure will help. Remember a stuck wastegate can
overboost a 914. The fuel pressure regulator can not keep up if you go too
much over redline boost pressure. Thus you can run out of fuel on take off
in a very bad position. If you are not between 2 and 5 when rolling, abort
take off. If you get airborne, and have engine quit, see if reducing
manifold pressure helps get back differential of at least 2 PSI, if it
does,you can probably fly pretty safe at a reduced power till you can
land.
An awful lot of words. Simple way to follow is to have a differential
gauge and keep it between 2 and 5.
I hope the following will answer your question:
You need to plumb 2 lines to your differential sender, 1) fuel pressure
and 2) airbox pressure.
1) Fuel pressure. You need to connect to one of your carburetor fuel
supply lines that run between the fuel pressure regulator and the
carburetors. You can purchase a double length banjo bolt from Rotax and
extra crush washer and stack another banjo (run to your sender) on the
output of the fuel pressure regulator. Don't reuse old crush washers.
(that said when you you are stuck somewhere you can heat real hot with
blow torch or cigarette lighter and quench crush washers in oil to soften
them, then do figure 8s on a piece of wet dry sandpaper on a flat surface
to make them flat, get to 600 grit should do it). I am using Earls -3
teflon hose and will fill lines with 100LL. Don't want 20 years old mogas
in there after a while
2) Now for the other side of the sender. Airbox pressure. You can do this
2 ways. You can purchase an extra nipple for a spare plug in your airbox
and run that to sender. If you don't have an extra hole, you can T to the
fuel pressure regulator line or the actual airbox pressure side of the
enrichment solenoid.
Or you could do as I am going to do. There are 2 pressures in the airbox.
Actual pressure (Port most side of airbox) and Impact air pressure ( Think
starboard most side of airbox). Impact air pressure is a bit more than
Actual airbox pressure. It is used to increase the float bowl pressure a
bit more to in fact allow the engine to run a bit richer. If TCU
determines that boost is OK to apply, above 108% power the wastegate will
close, and the enrichment solenoid will in fact plumb impact air to the
carburetor float bowls, instead of plumbing actual airbox air. This will
give a richer mixture that will greatly aid in cooling engine that is
huffing and puffing quite a bit harder than if run at 100% power.
I will use a injected molded 316 Stainless steel "T" from www.mcmaster.com
to take instead of actual airbox air, the pressure going to the
floatbowls. This pressure will be actual airbox air up to ~108% throttle,
and impact air if the solenoid is in fact working. I don't think there
will be a tremendous difference between the 2, but I am hoping I can see a
flicker of the differential needle when the solenoid switches from actual
to impact. This "T" needs to go somewhere between output of the enrichment
solenoid and the carb float bowls. Be very careful not to break the nipple
on the enrichment solenoid, it is plastic. If there was a leak going to
carb, this T should indicate higher than normal differential pressure that
you probably would not see if you were plumbed to the airbox. Running lean
in during war Emergency Power (115%)is not very conducive for long engine
life. Just a note, I don't like the white plastic Ts Rotax uses and
changing to the stainless ones as described, and since I need to re wrap
the hoses, will not put back the cheap stuff Rotax provides but Teflon
spiral wrap also obtainable from www.mcmaster.com
Try burning a piece of the Rotax supplied spiral wrap, then try teflon.
Bob as far as your question, you want as little restriction on return flow
flow of the fuel pressure regulator. Do not consider a restrictor. If
restrict a bit too much you could easily exceed the max 5PSI especial in
thick air. Parallel pump design best check that when both pumps are
running in thick air you do not exceed 5PSI. Rotax in their manual tells
you how to measure restriction of return.
Differential pressure is what you are interested in on a 914. A restricted
filter or gasculator, or plugged pre pump filter will restrict flow. If
you put your thumb over output of pump in one of the mentioned conditions
you probably have great pressure. The way the fuel pressure regulator
works, is it needs a specific flow for its internal designed restrictor to
restrict enough for "Crack" pressure to be achieved and begin flowing
fuel to carbs. Differential gauge can give you heads up to fuel
starvation. Thinner air, pumps worked harder, failure will most like be
not enough pressure. Thick air and 2 pumps running in parallel, or
restricting return line will most like fail in that more than 5PSI going
to carbs.
Ron P.
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