PA32-260 Auto Fuel STC

I know anecdotally from my IA that some of the Cherokees are somewhat
marginal in their ability to handle the high vapor pressures of winter
autofuel. They would stumble on takeoff & climbout the first hot days
of spring.

The symptoms that I had heard sounded more like flooding rather than
fuel starvation. It could be that the floats were sinking in the carb
bowl as the fuel was boiling. The front of the bowl is close to very
hot exhaust pipes, and the bowl vent is back directly into the air
intake. Any one else know any more than this?

I had heard some of the Grummans have had the autofuel STC pulled for
similar reasons.

Those of us using the autofuel STCs have to be cognizant of its
possible limitations. It has worked well so far in my 172M (about 15
years), but I avoid flying in very hot weather and avoid parking bright
sun (being concerned about fuel tank heating) just on principle. I'll
let someone else verify that experiment.

Pilots talk about the vapor pressure as though it is an absolute
measure. It is not as it doubles about every 15 degreesF. Raising a
fuel temp 15 degrees F will increase the actual vapor pressure of avgas
enough to eliminate the measured reduced pressure benefit of avgas. In
other words, autofuel has a 15 deg F handicap in handling high fuel
temps.

Octane requirements are a separate issue. The 235 HP Cherokee is an 80
octane engine, the 260 requires higher octane than regular.
(Petersen is no relation to me)

nrp wrote:
: I know anecdotally from my IA that some of the Cherokees are somewhat
: marginal in their ability to handle the high vapor pressures of winter
: autofuel. They would stumble on takeoff & climbout the first hot days
: of spring.

True enough of any aircraft, but the Cherokees' stock fuel system is a bit
marginal to begin with. We got the STC for our 180 HP O-360 Cherokee and it required
electric boost pump replacement. After that, we never got reduced fuel pressure at
full power, full-rich, max nose-high attitude like we did with the stock pump. To
give you another piece of info. If you have an O-320 *low-compression* (150hp), you
get the 87 AKI autogas STC and keep the stock fuel pump. If you do *NOTHING* but swap
pistons in the engine and turn it into a 160 hp, you need to swap out the fuel pumps
due to "flow" issues. There's no difference in fuel flow by putting in
high-compression pistons since everything else is kept constant.

: The symptoms that I had heard sounded more like flooding rather than
: fuel starvation. It could be that the floats were sinking in the carb
: bowl as the fuel was boiling. The front of the bowl is close to very
: hot exhaust pipes, and the bowl vent is back directly into the air
: intake. Any one else know any more than this?

Sounds reasonable. I talked with Petersen about this and he said that the
PA-24 (Commanche) failed due to "hydro-locking." Boiling fuel in the carb sinking the
float would certainly cause flooding and hydro-locking.

: I had heard some of the Grummans have had the autofuel STC pulled for
: similar reasons.

Quite possible. They're a bit tigher cowled than the PA-28's.

: Those of us using the autofuel STCs have to be cognizant of its
: possible limitations. It has worked well so far in my 172M (about 15
: years), but I avoid flying in very hot weather and avoid parking bright
: sun (being concerned about fuel tank heating) just on principle. I'll
: let someone else verify that experiment.

Absolutely. I'm especially leary of the octane rating for the
high-compression STC (like I have). Most autogas with a 93 rating only has an 88 or
so *motor* rating... which is similar to the avgas lean rating. As such, I like to
have some 100LL in at least the takeoff tank... particularly on hot days.

Also, autogas tends to have trace amounts of gunk/water in it. A screw-on
water-separating filter on the gascan lids took care of that.

: Pilots talk about the vapor pressure as though it is an absolute
: measure. It is not as it doubles about every 15 degreesF. Raising a
: fuel temp 15 degrees F will increase the actual vapor pressure of avgas
: enough to eliminate the measured reduced pressure benefit of avgas. In
: other words, autofuel has a 15 deg F handicap in handling high fuel
: temps.

True. Petersen sells a vapor pressure tester. Basically just a syringe
with a screw-on gauage that you can pull a vacuum on a fuel sample until it boils.
I always see 100LL about 1" higher than autogas, but on all but the ugliest days the
autogas reads in the green on the gauge.

: Octane requirements are a separate issue. The 235 HP Cherokee is an 80
: octane engine, the 260 requires higher octane than regular.
: (Petersen is no relation to me)

I think the PA-32 can have the O-540-E4B5... that's on the 91 engines list. No PA-32
that I can see on the airframe list.

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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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