: The autogas STC is undoubtedly the best thing that has ever happened to
: aircraft owners. If you've got the STC, but aren't using it, you are
: literally flushing thousands of dollars down the toilet.
: --

I agree on that. I will add one more datapoint to the mix here. We put the
Petersen high-compression autogas STC on our Cherokee-180 (requires 91 A.K.I or
better) about two years ago. Due to the difference in fuel ratings, I was concerned
about detonation margin. We've got 93 A.K.I pump-gas here, which is what I use.
Basically, that means the motor octane is probably about 88... less than the 91 the
O-360-A3A was type-certificated at.

I generally run at least 20% 100LL on the takeoff (right) tank, and 100% autogas on
the left for local cruises. I've done enough local flying with long climbs, hot
takeoffs, different mixtures, etc and haven't noticed any ill effects. I'm sure that
you cannot forget to enrichen at *all* when operating on mogas, but I haven't had any
issues. For the record, Petersen said when they did the vapor lock/detonation testing
(in some ridiculously hot desert place at 100+ degrees IIRC), they couldn't get it to
detonate on 89 mid-grade either. The FAA guys insisted on a little "safety-margin"
and made it 91.

On thing I did (rather accidentally) find out about that initially disturbed
me. I shut down the engine with the mags after a flight (I just had to jump out and
get something from my car and didn't want to have to prime it to start). It *almost*
died, but started to diesel at a ridiculously low RPM (100 or so). Dieseling =
preignition = BAD... BUT, the big problems are these:

- Extremely low RPM makes for a *LONG TIME* (20x that of takeoff time) that the
mixture is in the hot cylinders. It's got extra time to decide to light off.
- Extremely low RPM makes even the idle throttle setting "full-throttle." The MP
gauge said basically atmospheric (26-28"), even at idle setting of the throttle. Each
cylinder gets a full-throttle gulp of mixture then which can slow-bake in the jugs.
- Idle mixture is generally set for slightly rich (not super-rich)... best mixture for
preignition.

Concerned, I tried it in a friend's PA-28-150 running 87 autogas. Same thing,
and that engine combo config is identical to Jay's... just 4 rather than 6. Nobody
ever has had issues with octane on the low-compression engines.

Anyway, I'd be interested to hear if you can do the same trick, Jay. I'm
pretty convinced (due to the above reasons) that it's a non-issue for normal
operation. Interesting, though.

-Cory


--

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