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Re: timing and octane

>Date: Mon, 21 Jul 1997 14:17:41 -0400 (EDT)
>From: C1J1Miller@aol.com
>Question(s) for the list:  My '91 200q (3B Motronic dual knock sensors) lists
>octane requirements as 87, with 91 for maximum performance.  Later S4 and
>S6's list 91 octane required (manufacturer's brochure).  How does the knock
>sensor ignition (motronic) work?  is there a range of timing advance that is
>possible (max advance/retard); does gasoline with higher than 91 octane allow
>Does the ignition maximize advance? (i.e., keep advancing engine timing until
>it knocks, then back off), and if so, how often does it cycle through
>attempting to advance the spark?  
>chris miller, Windham, New Hampshire, c1j1miller@aol.com

i'm not dure what motornic your audi has.  the lh-jetronic has the air mass
sensor (hot wire), while the l-jetronic has the air flap unit.  easy to check,
the air flap is a alloy "L"-shaped housing just above the throttle.  motronic
is a l[h]-jetronic except with computerised control of ignition timing (hence
no distributor, or at the least no timing control on the distributor).

the knock sensors (motronic only) are just that; they listen for the peculiar
frequencies which signify pre-detonation and, using engine timing data, they
can figure out which cylinder is causing the probem.  timing for *that*
cylinder is then retarded.  the objective is to advance timing to the point
just before which knock can damage the engine.

the knock sensors/boost and timing control is completely automatic.

2 knock sensors are used - 1 sensor used solely for engine timing, the other is
used for (turbo) boost.

this system therefore compensates automatically for varying octane rating. 
generally, the higher the octane rating, the greater the power.  because timing
is retarded for lower octane (earlier knock), you will generally get less power
with lower octane gas.  for example, my rs2 is rated at 315hp on 98-octane
unleaded (which we don't get in nz, and so i run it on 96.  i've run on avgas
(100) once and not noticed a difference.  it will run on 91, but i'm not keen
to try it (it would get a lot less than 315hp i'd expect).

'95 rs2
'90 ur-q