From: nils@jhunix.hcf.jhu.edu (Nils Gokemeijer)
Newsgroups: rec.autos.vw
Subject: Summary: What octane rating to use?
Date: 16 Oct 92 17:11:30 GMT
Organization: Johns Hopkins University - Homewood Academic Computing

This is a summary about gas, and different octane ratings.

In a nutshell,  

Octane ratings used to give the percentage octane (C8H18) in the
gasoline. Other products is gas burn faster and more irregular, and
may preignite.  Currently there are two ways of figuring out the 
octane contents.  a) put it in an engine that does not knock with
a reference gasoline. Does the new gas knock, octane rating is lower,
does it not knock octane rating is higher.
b) do a chemical analysis to find the octane contents. 
The rating used in gasstations is the average of the two: (M+R)/2

You should put recommended gas in your car,  Lower octane 
will overheat combustion room, higher octane is a waste of money.

If the engine knocks, fix the timing!
To be able to set ignition earlier you can use a higher octane.

octane ratings only refer to the resistance against
preignition, and that is it.  Now with higher octane, you can get 
earlier ignition, without knock. 

Earlier ignition can give better performance (i.e. more power),
better accelaration and better mileage.  It is much more important
to have good timing than good gas. 

To keep your engine clean, use Chevron's Techron FI once in a while.

The replies I got:

> -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)

Not exactly.  High octane fuel is more resistant to pre-detonation. When
the air/fuel mixture is compressed during the compression stroke, it gets
very hot from the process of compression, possibly hot enough to ignite it
even before the spark plug fires.  This is pre-detonation, and high-octane
fuel won't do it as much.

When pre-detonation happens, the best that can be done is to retard the
ignition timing.  This is an imperfect solution, but it's all we can do at
the moment.  Using higher octane fuel, we can advance the ignition timing
further, thereby getting more power.

You need a car with a knock sensor to properly take advantage of this.  If
you have a knock sensor, or if you set your own ignition timing, you can get
more power from higher octane fuel.

Also, pre-detonation is rarely a problem with low-compression engines, like
turbocharged and supercharged motors.
=
True.  To an extent. 
This is true.  Your knock sensor deals with lower-octane fuels by retarding
the spark, which also lowers your power.

true, only if knock occurs.  Once knock is gone, higher octane has no
effect, EXCEPT IF:(knock sensor is available) in which case higer octane allows 
the electronics to advance
spark up to its design limit.
True, if ignition signifcantly advanced.
Somewhat true. I had a ping that went away with higher octane gas, but the
real culprit was the timing. Your car should not ping with recommended fuels.

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

Completely untrue.
False.  Depends on the brand name.
Cleaner?  I don't think so.  You WILL be able to accelerate faster because
of being able to run with the spark more advanced.
false
True, only if you adjust your ignition timing to make use of higher
knock resistance or have an ignition with knock sensor feedback.
I don't believe that it gives you more power, but it can burn cleaner thus 
increasing the life of your cat. (F)

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

Completely untrue.
False
More MPG: True.  Same reason (spark timing)  More energy: False.
false
False, octane is only a measure of anti-knock capabilities.

> -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)

I believe this is false, but I'm not 100% sure.
False
false
False

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

False.
False
false
Generally false
No real answer here. Depends on what your car recommends. (F)

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

False.
True, to an extent. 
I agree with this.  I used to get 21 MPG on my Vanagon, but it couldn't
handle regular gas.  Then I timed it, and found that it had been running
about 5 degrees too advanced.  After timing it to factory spec, now it
runs great on regular, but only gets 18 MPG.  Miles per dollar works out
the same.  I'm tempted to go back to the old timing and premium fuel just
to increase driving range!
false
False, higher octane alone does not will give better gas mileage.

> -assumption 7.  Low octane gas will do fine, if you use high octane
> 		once in a while to clean your engine.(T/F)

False.  However, marketing being what it is, many brands only offer the
"good" detergents in their expensive fuels.  Some offer the same detergents
with all octane levels, though, so if you're using a low-octane fuel with
good detergent, then there's no advantage to going with high-octane fuel.
False.  Use better brand name of gas to clean up your engine.  I'm told that
one tank in 3 of Chevron (that Techroline stuff) will help clean your engine. 
I don't know how much truth there is to that.
I think that all of these "premium is cleaner" beliefs are old wive's tales.
As I am led to understand it, "gasoline" is pretty generic when it leaves
the refinery.  The distributers add various compounds (ethanol, xylene, et al.)
to adjust the octanes as required.  The octane number we see is in no way
related to the percentage of octane in the fuel, except that the fuel we
buy "should behave as if it contained n% octane."
false
False, higher octane does not insure other qualities.

> OTher comments:

One thing that you missed is that low-octane fuel burns faster than high
octane fuel.  Therefore, especially at high revs, you are more likely to get
a complete burn with low-octane fuel, and therefore should reduce
hydrocarbon emissions.  You may also get more power with low octane fuel,
but the ignition timing advantages of high octane fuel probably outweigh it.

Generally, you should use the lowest-octane fuel you can get away with
without pinging, and with good detergents.

The theory:
The idea is that the engine compresses the air/gas mixture in such a manner
that it burns when (and only when) the spark plug fires.  The problem is that
this burn point is very close to flash point of gas.  The engine may already be
hot from running, and/or there may be glowing bits of carbon in the combustion
chamber (acting in effect like another spark plug).  This will cause the
gas/air mixture to explode spontaneously at the wrong time, resulting in
knocking (pinging, diesling, detonation, preigintion).  

The problems:
Back in the days of leaded gasoline, We had good quality gas.  Tetraethyl lead
was an excellent anti-knock agent.  Unfortunately, it also messes up the air,
so they had to stop selling it.  

About the same time, they developed ethanol (gas blended with ethyl alcohol). 
This is supposed to help the environment since alcohol is a renewable resource.
However, gas engines only work well when the alcohol is <= 10%.  After that,
you may get poor running and/or vapor lock, where the alcohol evaporates out of
solution while in the fuel system.  Vapor lock has been beaten to death, so
I'll leave that one alone.  Since alcohol has less chemical potential energy
per volume, ethanol is also not as powerful as plain gasoline.  Therefore, you
will not get the mileage from it as you would from plain gas.  

They also use methyl alcohol.  The problem there is that methanol is corrosive
to ordinary rubber and plastic in all but the smallest percentages.  What does
this have to do with knocking?  Keep reading.

Alcohol and the other anti-knock agents are not as good as lead.  Therefore,
the potential for knocking is greater.  But the gas companies dump in alcohol
because it is a cheap way to maintain the octane number of their gas.  When an
increased load is put on the engine (starting from a stop sign, going up a
steep hill, performance driving), the compression is raised, again increasing
potential for knock.  On my older cars, you can change the timing yourself. 
Increasing the timing (to a point) will give better performance, but with the
above problems.  Decreasing the timing will allow me to use cheap gas without
knocking.  Once again, this has to do with compression of the mixture. 
Generally, the higher the octane, the more you can compress the gas before it
detonates.

The newer cars check for knock and change the mixture and/or timing as needed
to compensate.  When this happens, you get less performance.  So if you can run
harder, faster, etc before your computer takes over, you get less performance. 
Nice theory, but I've known people who've tried a can of "octane boost" in
their tanks, and their cars ran like shit.  

The problem is that most mfgs put more cleaners into higher price gas.
It is these cleaners that clean the injectors (not that the gas is
cleaner) that gives better long term performance.

If you use good gas low octane gas with cleaners, you don't need high
octane.  If you use cheap gas with dubious cleaner content, you can
either use good low octane gas or Chevron's Techron FI cleaner every
once in a while to maintain long term performance.

One general theme you have in your questions is that higher octane fuel
is better in all respects.  The octane rating has no correlation to the
others.  It is merely a measure of knock resistance.

The best approach is to find a good brand of fuel that is well refined
to remove impurities (like sulfur) and filtered to removed dirt.  I 
would use the lowest octane grade of their fuel on a regular basis that
will not ping with your normal tuning.  If you have a knock controlled
ignition, you might want to use a higher octane only when you will be
demanding more performance (i.e. autocross, high-altitude, etc.).

Well, on some of the fuel-injected varieties of VWs (like mine) it
recommends a certain octane.  Here are my experiences with it.

	It costs WAY TOO MUCH, as a matter of fact, so much that any savings
	from mileage can be deemed negligible.

	Around here, gas testing isn't very good.  I have gotten gas with
	water in it before (not the lower octanes, either-the higher octanes.)

	From the business point of the gas station owner this makes sense-it's
	cheaper to water down those higher octanes than have to keep spending
	on them.

	People generally regard the lowest octane to really cheap gas.  What I
	would challenge these people is, if it is the same gas that has kept
	their lawn mower going for five to ten years longer than their car, then
	I should HOPE TO THINK that it is good enough for their car.

	Just some thoughts.

On air-cooled engines, lower octane gas burns hotter at the stock compression
ratios (7.0:1 and up) in VW Type 1 engines.  They were designed for 91 RON
(which works out to a higher value of R+M/2 octane, though I'm not sure what)
gas and the heads will overheat if run on lower-octane fuel.  This doesn't
always show up as the entire engine overheating (e.g. oil thinning, etc)
since it's localized in the heads, but you'll see signs of overheating
under the rocker covers, siezed adjuster screws, and eventually damaged valves.

Gene Berg has a whole treatise on octane and compression ratios that he
sells as part of his technical literature.  Basically, the rule is to
use the octane appropriate to your compression ratio, which means 92-93
premium gas in stock old VW's like a 73 squareback.

From: Mark Sirota <msirota@ee.rochester.edu>
From: pH Fred == pgheit01@ulkyvx.louisville.edu
From: James Thompson <jimt@u.washington.edu>
From: "Andrew J. Huang" <andyh@chaos.cs.brandeis.edu>
From: Mark Shaw <mark@wdc.sps.mot.com>
From: James Matthew Kokernak <kokerj@rpi.edu>
From: bsu-ucs!01ksdavis (Rebel without a clue.)
From: lcs@icad.COM (Larry Stone)

-Nils ( GTi '85 owner)
Newsgroups: rec.autos.vw