I used to be a chemist in a previous life and sold analytical equipment
to oil refiners, so here is my $.02 on this topic:

Dana Morphew wrote:
>
> Do all knock sensors adjust the timing through a range of degrees above
> and below the adjusted distributor setting?  Do you know what the range
> is?

Knock sensors are usually piezoelectric ceramic devices that can detect
the shock wave  (knock) set up in the engine when the air/fuel mixture
detonates all at once rather than burning evenly.  The ECU can retard
the timing electronically based on the signal from the knock sensor over
a fairly narrow range, but I don't know how much.

How much xylene or toluene would one add to a gallon of 87 octane
> to change it to 92 octane?

You have to determine this by trial and error.  This is how the oil
companies do it.

"Octane" is actually a pure aliphatic hydrocarbon, C8H18, and is
assigned the rating of 100.  The mixture of shorter and longer chain
hydrocarbons that result from the distillation of crude oil is then
measured against the "octane standard."  Since gasoline is a refined
natural product each distillation is different, and they use a test
engine with a variable compression ratio (head can be adjusted up and
down), to find the knock point for each batch of gas.  They will then
mix distillation fractions and additives (formerly tetraethyl lead;
anyone on the list remember when high octane gas used to be called
"ethyl?") to get the gas within spec.

I've been adding fuel conditioner at each
> fillup for my diesel Caddy, so this would not be an inconvenience at
> each fill of my Vanagon.  I'll have to check on the cost of xylene and
> toluene to see if there is money to be saved here.  Thanks for the very
> informative post, David.

There is no money to be saved, only more to spend to gain a minimal
boost in performance. Today's gas has a full additive package (including
detergents to keep injectors clean, thanks to the BMW gas spec which is
now a standard).  You really don't need to add anything  more unless you
get condensation in your tank, at which point you can add some Heet
(alcohol) to suspend the water and carry it out.

I also don't think it would be worth the risk to carry around gallons of
highly flammable xylene or toluene in your trunk!
>
> -Dana-
>
> David Marshall wrote:
> ...
> > all done using a 1982 Rabbit Distributor with vacuum advance and vacuum
> > retard - generally considered the best performance non-knock sensor
> > distributor.  If there was a knock sensing ignition present, there would
> > have been no knock as it would have retarded the timing until it
> > disappeared - thus giving you less power.  With 87 octane fuel in a high
> ...

--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext 208

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