Re: 93 octane for 164

[Scott Fisher <sfisher71@domain.elided>]

Before everyone starts to hyperventilate about buying
98 octane gas at the pump, Australian octane and US
octane appear to have about as much in common as
Australian dollars and US dollars -- the spelling. :-)

Seriously, there are at least three commonly used
definitions for octane -- the research octane number,
the motor octane number, and (in the U.S.) the average
of the two.  Both RON and MON are determined by
comparing a sample of gasoline in a special engine
with a variable compression ratio, and comparing the
point at which knock of a known amplitude occurs
against a control blend.  The research octane number
(RON) is typically higher because it's determined
under less severe conditions than the motor octane
number (MON), according to Exxon:

http://www.exxon.com/exxon_productdata/lube_encyclopedia/octane_number.html

Typically, U.S. pumps (since 1972) list the average of
the two, which usually work out to four to five digits
lower than RON.

Note also that octane is EXCLUSIVELY linked to
detonation, and does not intrinsically provide more
power.  Higher octane fuel permits design
modifications in an engine that in their turn may
provide more power; in particular, turbocharged
engines can be tuned to provide more boost with
higher-octane fuel because there is less risk of
detonation (knock).  But a naturally-aspirated engine
of modest compression (say, 9:1 on an aluminum engine
with its better heat characteristics) and reasonable
timing doesn't need high octane fuel.

I'd guess that the Australian figures that John quotes
are most likely the higher RON figures, not the
average.  Not that there's anything wrong with that,
as long as it's what everybody expects.  It's like
telling someone it's 35 degrees outside -- do you put
on a down jacket or shorts and sandals? 

--Scott Fisher
  Tualatin (down jacket, definitely), Oregon

--- John Wiltshire
<John.Wiltshire@domain.elided> wrote:
> Looking forward to what Fred has to say on the
> octane rating for a 164, but
> in the meantime, here's my two bob's worth from the
> Australian perspective:
> 
> *89 Octane Standard ULP:  Performance is ordinary,
> pinging is common under
> load.  About 380km per tank (city driving, and
> filling when the fuel light
> comes on).
> *95 Octane Premium ULP:  Much smoother performance,
> no pinging, and about
> 420km per tank.  Sometimes would get attacks of
> pinging, so not all 95
> octane created equal.
> *98 Octane Premium ULP:  A bit smoother than 95,
> noticeable that the engine
> copes better with stop/start driving, backing off
> throttle etc.  No pinging
> ever while using 98 Octane.  No better fuel economy
> than 95.
> 
> With 98 Octane now widely available in Australia
> from Shell, Mobil, BP and
> Caltex, it seems to be replacing 95 Octane as the
> "Premium" fuel (This only
> holds for capital cities - 98 Octane out of these
> area can still be
> difficult to get).
> 
> Can't get to my owner's manual at present to check,
> but I think that it
> specifies 95RON minimum for the 164.
> 
> John Wiltshire
> Sydney Australia
> 
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