Newsgroups: rec.autos.vw
Subject: Re: what octane rating to use?
From: jan@camhpp12.mdcbbs.com (Jan Vandenbrande)
Date: 9 Oct 92 15:07:42 PDT
Organization: M&E (Division of EDS), Cypress CA

In article <1992Oct8.133918.7594@jhunix.hcf.jhu.edu> nils@jhunix.hcf.jhu.edu (Nils Gokemeijer) writes:
>Nobody I have talked to can give any reason for using lower or higher
>octane gas. Can anybody give a good argument?

Hmmm, another potential FAQ. See also the FAQ in r.a

Leading assumption throughout this thread: You have a car that
runs fine on 87 Octane (US), and you want to use 92.
If however your car requires 92 Octane => 87 may damage your
engine because of knock.

>-assumption 1.  Less knock with higher octane  (true or not )
>		but that does not matter because of the knock sensor
>		in my GTI  (true or not)

 Less knock with higher = True (BTW: High Octane = resistance to knock).

 Matters even with a knock sensor: True. It can only adjust a little bit.

>-assumption 2.  Higher octane is cleaner or behaves better, 
>		so your car will accellerate faster  (true or not )

High Octane != Clean (depends on what you mean by clean)
Behaves Better =? What do you mean by behaves better?
Accelerate Faster = True if you have a knock sensor or change
		    your timing to exploit hi-test, otherwise no.

It used to be that High Octane gasses also had more detergents
to keep your fuel system, valves, & engine cleaner.
However, nowadays the amount of detergent in many of the better
brands is the same for high and regular octane gasses.

>-assumption 3.  Higher octane contains more energy, so your 
>		car will get more MPG.	(true or not)

Higher != More Energy. The way it burns is different so you can exploit
the combustion better.

>-assumption 4.  Lower octane contains more pentane and other inferior
>		fuels, but does that not mean it contains more
>		energy, so lower octane will give you better MPG
>		(true or not)

This is too much of a sweeping generalization.
Gasoline as we know it is really a cocktail of many types of hydro
carbon molecules.
The composition changes between brands, seasonally, geographically,
and between batches. 
Because of distribution costs, many refineries have agreements and 
actually share the same gas, especially in more remote areas.
So don't be surpised to see the same truck deliver fuel at one brand
and then deliver at another station with another brand name.

>-assumption 5.  High octane gas is cleaner, so your engine will last
>		longer (true or not)

Depends. With leaded gas that used to be somewhat true => more lead =>
more valve lubrication (with lead) => last longer.
You can also say: Higher octane => less knock => less damage to your
engine => last longer.
But other than that I don't think it'll make any difference.

>-assumption 6.  Because you get better mileage with more expensive gas
>		It all works out even in the end.(T/F)

Not generally.

>-assumption 7.  Low octane gas will do fine, if you use high octane
>		once in a while to clean your engine.(T/F)
>
See above on detergents. If your car runs fine with a good quality,
high detergent low octane gas, than there is no reason to use
high octane gas. If you have a knock sensor, you can exploit
its combustion characteristics to advance your ignition.

>Thanks, Nils ( a proud, and happy, GTI '85 owner)

You of course know about changing to the later map on your ecu
& advancing your ignition by several degrees to get alot more
power out of your GTI?

Jan
-- 
Jan

jan@camhpp12.mdcbbs.com		(work)
jan@lipari.usc.edu		(school, usually works, forwards)
Newsgroups: rec.autos.vw