Fuel for thought

[C M Smith <cmsmith@domain.elided>]

Higher octane gas does nothing but waste your money. The only 
qualifications are if your engine needs the extra octane to avoid 
detonation, pinging, knocking, pinking, or whatever you wish to call it, if 
your engine management system has a knock sensor which allows the turbo 
boost or ignition advance to be modified to raise the bmep in the cylinders 
so that detonation could occur were it not for the higher octane (usually 
this is a feedback loop through the knock sensor and the ECU), or you have 
a strange need to get the extra cleaning power of the higher "detergent" 
action of premium fuels. I doubt any street car, and probably no streetable 
car, would benefit from octane much above 94.

BTW, in winter climes the formulation of gasoline sold at retail is altered 
every spring and fall. Generally, winter formulations have higher 
volatility so as to reduce wetting out of fuel due to extremely cold metal 
surfaces inside the engine. FI engines are relatively insensitive to this 
effect, but at minus 30 Celsius, even the spark plugs can become wet with 
fuel on  a cold start (often a cold non start as a result). This higher 
volatility can cause vapour lock problems in warmer conditions, so every 
spring the fuel is adjusted back to summer formulation. Again, FI engines 
with high pressure fuel delivery do not suffer from vapour lock nearly as 
much as carburretted engines
.
However, this may explain some odd problems experienced by some drivers as 
the seasons change, especially if the car is driven infrequently. My SAAB 
Turbo runs better on summer formulation fuel and my ALFA 164 prefers winter 
formulation. In fact, at about minus 5 Celsius with winter fuel my 164 just 
gets up and goes...until the engine gets really warm (nearly never in 
winter here!).


Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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