A few questions have come up regarding fuel pressure while
flight testing Ralph Love's Legacy, N122PT. It has an IO-550 with MT
prop. We've had indications of high fuel pressure when the throttle is
retarded (38-40 psi) in the pattern which is accompanied by engine
roughness. All gets better when the mixture is leaned. Testing has
been done in Redmond - so patterns about 4-5000 ft. So is this
"normal" and just explained by the overly rich state due to altitude
and leaning is the solution or is something else going on? Where should
the fuel pressure be measured from? The unmetered side of the fuel
metering unit or the metered side? and why. And lastly,
where have you mounted the fuel pressure transducer? EI suggests the
firewall, but that leads to a long fuel line and ? does that make any
difference in the readings, dampening etc. Can you mount it closer or are
the vibrations going to cause failure of the fitting and fuel leaking over hot
cylinders?
Dan,
I’ll offer that engine roughness that accompanies the high
fuel pressure you note at low power settings almost certainly indicates too
rich a mixture. During testing/set up of my TSIO-550E, I once got the mixture
so rich that the engine quit on takeoff. Easy fix after a couple of gulps, but
makes one appreciate a good set up, and some attention to these details.
I also have an MVP-50 and think it’s one of the best/most
reliable instruments in my airplane. I use it to monitor lots of stuff besides
the basic engine and it’s been flawless. Like you, I have seen high fuel
pressures during rapid throttle movements during testing, but they were of very
short duration, and slowing the throttle movement rate has completely eliminated
them…after I got the boost and fuel flow dialed in as directed by
Continental SIT 97-3E.
I typically fly very lean (100+ LOP) in the high teens to low
flight levels, and increase the fuel flow about 2 GPH when I start down from
altitude. During testing, I tried descents to pattern altitude while remaining
very lean, and had no problems at all, but I had a Cherokee engine quit
momentarily on short final many years ago because I forgot to richen the
mixture, so I’m being a bit conservative here. I don’t go to full
rich to descend because I’m typically 20+ minutes out and the fuel burn difference
is significant. I’m not an instructor, and am reluctant to recommend this
as a technique because you still need to get the engine set up for a go around
(RPM up and mixture in) before landing...but it works well for me.
The “book” says to measure the fuel pressure on the
un-metered side, so that’s what I did...although the Continental bulletin
has you measure metered pressure during set up. Might be a question for a good Continental
expert? I mounted my pressure transducer on the firewall with a long line.
Since the flow through this line is effectively zero, the pressure is the same
at the manifold as at the transducer end. A very long hose will provide some “dampening,”
but for the size and length, I doubt even a highly precise instrumentation
package could measure it. For an analogy, you don’t notice any “dampening”
of the brakes, although the lines are the same diameter…or smaller…
and the length is several times more.
I did mount my flow transducer on top of the cylinders, just
forward of the baffle that separates the cylinders from the accessory case and the
back of the engine. I built a rubber shock mount to hold it, insulated it well
from radiant heat from the cylinders below, and focused cooling air from the
inlets over the top of the transducer. Has worked well for 400+ hours.
Fuel leaking over a hot engine is always a problem; more so if
you have a turbo. Best quality hoses, fire sleeved and replaced periodically,
is good insurance. I also installed a “standard” EI temperature
probe in the engine compartment exhaust tunnels (one on each side) and hooked
them to the MVP-50 as “fire lights.” The normal air temp in this
area is 120-130 degrees, so I alarmed them at 150 degrees. Never a false alarm…so
far…and I think this will give the first indication of anything flammable
getting to one of the turbos. In flight these are red hot, and will combust
almost anything…oil leak, fuel leak, etc.
Hope this helps.
Bob