FOAK: Using low octane and leaded fuel

It's pretty simple- at altitude, the air is thinner, so it's
a like having a low compression engine. If that's not enough
of an explanation, just take it as given, out in the western
USA at high altitudes, the octane rating of regular unleaded
is commonly 85 (R+M)/2, whereas in lower elevation areas it's
87, exactly for this reason. You don't need the higher octane
so why pay for it.

 

Did you miss the part where it's Bolivia? It ain't exactly downtown.

 

You'er mixing two effects.

Octane rating is a fuel's ability to burn without knock; higher
combustion pressures increase the likeliehood of knock, so, as higher
altitude, means lower (maximum[1]) combustion pressure, this reduces
the chances of knock.

Mixture richness is affected by altitude as well; carbs will tend to
produce an over rich mixture at altitude but an FI system may or may
not, depending on what it measures. The old 8V BMW K series for
example, that used a flap valve to measure airflow, needed a special
plug inserting to tell the ECU to lean out the mixture if it was to be
normally used at high altitude.

[1] at peak BMEP with the throttle wide open.

 

<G>

 

Its a FI engine, an Aprilia Pegaso, same motor as Bonwick's Tenere.

 

At high altitude in the US, I could still buy 93.

At sea level in Mexico, sometimes only 87 is available.

 

Neither of which contradicts anything I said.

At sea level, 87 is plenty of octane for even relatively high
compression bike engines with optimally designed combustion
chamber shapes. E.g., my SV650 had 11.5:1 compression and
was designed to use 87 octane (R+M)/2.

Larger combustion chambers without a centrally located spark
plug or those with otherwise unfavorable shapes can require higher
octane than 87 even at significantly lower ratios.

Really, I wouldn't sweat it. Use higher octane if it makes you
happy, but as long as you can get 87 at sea level, and 85 at
high elevations, you're going to be just fine.

Forced induction engines obviously have different octane
requirements. But you don't have to take my word for it, just
read the owner's manual.

 

If it's got a knock sensor and decent fuelling map in the ECU, it'll
cope fine. I'd hazard a guess that these days, most FI engines are made
for the world market and don't require anything special in setup for
altitude or rubbishy local petrol.