My opinion on this is if they say use 93, then use 93.  They say it for a
reason.  My son put 87 in his Volvo 850 and it ran like crap.  Oxygen sensor
issues, power issues, etc.  Once he went back to 93, it ran much better.  I had
a similar problem with a Toyota Cressida several years ago.  

mmm

drives ok on 95 octane, like a rocket ship on 98 but 87 and 93??? sits
quicker to walk?

andy

940 2.3 fpt

I use 83 octane in my 2000 S40.  It is a little peppier when I put in the
recommended 93 octane, but the performance difference isn't worth the price
difference to me.  I did the same thing with my 1987 SAAB 9000, and it never
hurt the engine.

I did some miles-per-gallon tesing recently.

'86 240, non-turbo, auto trans.  Tires at abotu 34-35 p.s.i. inflation
pressure (sorry, I don't have the metric conversion, but I use close to
the max pressure allowed for the tires).  I drive 75 mi. each way to
work daily, about 2/3 highway and 1/3 on smaller roads.  I get 25-26 mpg
on 87 octane and about 28 mpg on 89 octane.  The percent improvement in
fuel economy seems to be greater than the percent increase in cost.
Further testing over the next week or so will give more data for better
accuracy.  I definitely feel the engine is running more smoothly.

A Volvo mechanic I know described to me that the anti-knock sensor
retards the ignition timing when knock is detected, retarding by a
variable amount as needed to correct the knock.  This gives reduced
performance and economy at those times.  Higher octane will activate the
sensor less often, allowing greater ignition timing advance with more
mechanical energy delivered per drop of fuel.  I suspect that best
overall economy occurs when the octane has been raised just enough so
that the knock sensor rarely retards the timing.  Further increase in
octane will not result in more advance, thus no further increase in
energy realized from the fuel.
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