Wow. Great reply. Thanks. Most of the concerns sound like they won't be
of much concern with my Chevy 350 / Holley carb installation. (I should
have probably changed the email subject title.) I plan on using gravity
feed for fuel pressure with an electric backup pump in case I do encounter
any vapor lock problems in fuel lines too close to hot things. My
compression ratio is 9.7:1. It remains to be seen how low an octane rating
I can use without knocking. Anybody know if this is likely to be low enough
for regular unleaded MOGAS? Seems like I'll probably be just a bit too high
a compression for regular.
Mike Kimball
SR #044
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com]On Behalf Of
dmp@radbsd.mcg.edu
Sent: Thursday, August 26, 2004 12:53 PM
To:
rebel-builders@dcsol.com
Subject: RE: Mogas in the M-14P?
On 26 Aug 2004 at 9:25, Mike Kimball wrote:
What's the difference between automobile and airplane valve guides? How
come valve guide lubrication isn't a problem with unleaded in cars?
Cars, for the last thirty, forty odd years, have had hard valve seats
call stellite that don't require the thin film of lead to prevent pressure
welding between the valve and seat. Interesting if aero engines still
aren't using hard seats when Briggs and Stratton has for years.
Also, is there any reason why 100LL and MOGAS shouldn't be mixed? (Other
than unknown octane rating and vapor pressure and the usual rubber parts
incompatibility with one type or the other.)
The EAA did a huge amount of research on mogas years ago.
I don't see why you couldn't mix them, but there are things you have
to remember.
First, Auto gas octane rating is the average of the motor and
research octane ratings, (MON & RON) while avgas is based on the
tougher MON rating. Since typical mogas has a ten point spread,
it's MON is usually five points lover than the pump number. So an
87 mogas would be equivalent to a 82 MON avgas, which is why
low compression av engines like it. They're not designed for the
amount of lead in 100LL. (But you still should use some leaded
because of the valve seat thing.) You _might_ get by with 97 mogas
(92 MON) in a 100LL engine, but there's a lot of ifs in that
statement.
Second, mogas tends to have a higher vapor pressure, and that
value tends to vary from region to region and season to season.
That could be a problem on hot days and altitude, or if the carb or
fuel lines are exposed to excess heat. (The EAA looked at that very
closely.)
Third, mogas with _any_ alcohol is forbidden under STCs, and
sadly, alcohol content isn't always listed. The most common, ethyl
alcohol, shouldn't be a problem with most modern fuel systems.
(Certain older rubbers tended to soften in alcohol.) Methyl alcohol is
much more aggressive in attacking components, but isn't a
common mogas component.
As to a Moose with a MP14, Proseal should handle mogas just fine,
and the MP14 doesn't need high octane fuel. The only question
would be the rubber components within the MP14's carb. But
considering how nasty some of the fuels probably are (were) in the
USSR or third world soviet satellites, it's something to check into,
but not worry too much about.
---
David Parrish
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