Re: Winter wondering

[Orin Eman <orin@WOLFENET.com>]

> 1.  Fuel octane. The owners and Bentley manuals suggest 91 (CLC) octane, but that 87 is OK if you are willing to suffer lower performance.  Over the past 10+ years, I have tried every possible combination of fuels, and have never seen any lower performance (or fuel economy) from using the 87 octane (I have compulsively recorded 95%+ of the fuel purchases).  This experience has included a range of temperatures, loads, and altitudes.  As a validation of this empirical data, the max. boost (1.3 bar) is the same with 87 or 91/92.  If the computer were to decide that it does not like 87, I think the first thing that it would do would be to cut the boost (e.g., to 1.1 or 1.2) to control pre-ignition.  Since it does not do this, it must be happy with 87.  Which leads to the second question ........  

Actually, cutting the boost is the last thing the computer does to control
detonation.  It has two timing maps, one for premium fuel and one
for regular fuel.  It starts on the premium fuel map.  If it detects
knock, it starts retarding the timing.  If it gets to its retard limit,
and the engine is still knocking, it switches to the regular fuel map.  
Now it will retard from the regular fuel map if it detects any more
knock.  If it reaches its retard limit again, it will cut the
boost, but only by about .1 bar - the computer has a very limited
range of control with stock programming.

> 2.  Boost pressure.  The highest (and commonly seen) boost pressure on the display is 1.3 bar.  Is this normal (for a stock vehicle)?   

It's normal.  The display is usually inaccurate and the max boost allowed
by the computer is 1.43 bar, so a display of 1.3 is normal...  however,
it is possible, though unlikely that your display is accurate and
your WOT switch isn't working.  This would prevent the computer from
allowing maximum boost.  What does your display read with the ignition
on, before you start the car?  0.9 or 1.0?

Orin.