Cherokee exhaust repair?

On Aug 23, 7:51 pm, wrote:
150flivver wrote:

: On Aug 22, 10:03 pm, wrote:
: ... Besides, I'm pretty sure that if we changed exhaust, we'd lose our
: autogas STC.
:
: -Cory
: ************************************************** ***********************
: * Cory Papenfuss, Ph.D., PPSEL-IA *
: * Electrical Engineering *
: * Virginia Polytechnic Institute and State University *
: ************************************************** ***********************

: I don't see how installing a Powerflow exhaust will invalidate an
: autofuel STC.

The autofuel STC is very complicated for -160 and -180 hp PA-28's. Unlike for 150 hp -140's or -150's, the Petersen
PA-28 high-compression STC is more than just a sticker for the fuel filler on the tank. Basically, the ones that are even
applicable must have the dual exhausts. If they have the aluminum, flip-up cowling they are not applicable due to vapor-lock
issues with the muffler near the firewall. Even the ones applicable require replacement of the electric fuel pump with dual
(redundant) electric fuel pumps and some re-plumbing.

Bottom line is that with a different exhaust, the autofuel STC would most likely have to be re-approved, which would
amount to doing a whole 'nother autofuel STC certification.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

The autofuel STCs I've seen are for airframe and engines with no
reference to the type of exhaust system. In general, higher
compression engines don't have an STC for autofuel because of the
increased chance of detonation. Still don't see how a different
exhaust would increase the likelihood for detonation in a low
compression engine.

: The autofuel STCs I've seen are for airframe and engines with no
: reference to the type of exhaust system. In general, higher
: compression engines don't have an STC for autofuel because of the
: increased chance of detonation. Still don't see how a different
: exhaust would increase the likelihood for detonation in a low
: compression engine.

http://www.autofuelstc.com/autofuelstc/pa/wa.html
http://www.autofuelstc.com/autofuels...alnumbers.html

There are a number of issues. Detonation is only one. Others include vapor lock (exacerbated by
exhaust location and configuration), hydro-lock (boiling fuel in the carb... that's why the PA-24 isn't
applicable, for instance), etc. I agree that for *almost all* autofuel stc's, (high compression or low
compression) there isn't anything to it.

For the PA28 with -160 or -180, there is a lot to it, and exhaust was an important reason why
some are applicable and some are not.

-Cory


--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote in message
...


For the PA28 with -160 or -180, there is a lot to it, and exhaust
was an important reason why
some are applicable and some are not.

-Cory

What I found interesting is that when I overhauled my engine, I upgraded
from a 150HP to 160HP. It involved different pistons and wrist pins.
Everything else stayed the same, including fuel burn.

This upgrade invalidated my autogas STC. I called Petersen, and they said I
needed the one with the fuel pump. I think that's a little strange, as the
fuel system did not change.

Steve Foley wrote:
: For the PA28 with -160 or -180, there is a lot to it, and exhaust
: was an important reason why
: some are applicable and some are not.
:
: -Cory

: What I found interesting is that when I overhauled my engine, I upgraded
: from a 150HP to 160HP. It involved different pistons and wrist pins.
: Everything else stayed the same, including fuel burn.

: This upgrade invalidated my autogas STC. I called Petersen, and they said I
: needed the one with the fuel pump. I think that's a little strange, as the
: fuel system did not change.

Don't even get me going on that. It's a bull**** rule that basically stems from the fact the
Piper's PA-28 fuel system was marginal to begin with. Before we bought the STC, I talked with Petersen
directly about octane, why the STC was the way it was, etc. He said that the problem was "inadequate
fuel flow," with the stock Piper pump and it wouldn't pass what he called the "ditch test." Full-power,
with the plane in a very nose-high attitude (i.e. tail in a ditch) and the fuel pump didn't maintain
enough pressure. Also, on his test plane, the right tank flowed a little better than the left, so the
autofuel STC requires takeoff and landing on the right tank.

It was exactly my point that swapping pistons to make a -150 a -160 doesn't change the fuel flow
at all... yet by some magic, the pump is no longer adequate. It's bull****, but unfortunately that's
the way the legality of the system requires.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************