The "special engine" mentioned is a single piston unit with an
adjustable cylinder head.  It is "screwed down" to increase the
compression to the point detonation occurs.  This is then compared to
the setting derived from the standard chemicals and the "octane rating"
is calculated.  Batches of fuel are blended with additives and tested
before shipping to your FLGS (favorite local gas station).  (I once sold
lab instruments to these folks.)

At least we can count on consistent octane "quality" in name brand gas
here in the US, and this is not the case in many other countries,
especially Mexico.

In my MGs the distributor has an external thumb wheel you can turn to
conveniently adjust the timing for the gas you have.  A clever European
invention from the post war period where each tank might be different.
Frankly, I still find it useful since I tweak my timing for the gas I'm
willing to pay for.

http://www.theglobeandmail.com/globemegawheels/20000427/octane.html

--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia w/2.1
'65 MGB (Daily driver since 1969)
'74 MGB GT (Restoring sloooowly)

Personal mechanic for:
'70 MGB GT (Daughter's)

Assisting on Restoration (and spending OPM):
'72 MGB GT (Was daughter's, now son's)
'64 MGB (Son's)

Stripped and gone but their parts live on:
'68 MGB, '73 MGB, '67 MGB GT