grade of fuel for n/a - Page 3 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 04-19-2004, 09:50 PM   #21
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Well here is my opinion on your thoughts.

Octane is about engine knock. Higher octane gas is designed for a higher compression engine. Premium gas burns hot under high compression.

With a lower compression engine the premium gas is not burned completely, leaving carbon buildup and fouling of spark plugs.

The only time I would switch to higher grade fuel would be with engine knocking.

From the gasoline FAQ on the newsgroups:

------------------------------------------------------------------------

6.4 Why are two ratings used to obtain the pump rating?

The correct name for the (RON+MON)/2 formula is the "antiknock index",
and it remains the most important quality criteria for motorists [25].

The initial octane method developed in the 1920s was the Motor Octane method
and, over several decades, a large number of octane test methods appeared.
These were variations to either the engine design, or the specified operating
conditions [65]. During the 1950-1960s attempts were made to internationally
standardise and reduce the number of Octane Rating test procedures.

During the late 1930s - mid 1960s, the Research method became the important
rating because it more closely represented the octane requirements of the
motorist using the fuels/vehicles/roads then available. In the late 1960s
German automakers discovered their engines were destroying themselves on
long Autobahn runs, even though the Research Octane was within specification.
They discovered that either the MON or the Sensitivity ( the numerical
difference between the RON and MON numbers ) also had to be specified. Today
it is accepted that no one octane rating covers all use. In fact, during
1994, there have been increasing concerns in Europe about the high
Sensitivity of some commercially-available unleaded fuels.

The design of the engine and car significantly affect the fuel octane
requirement for both RON and MON. In the 1930s, most vehicles would run on
the specified Research Octane fuel, almost regardless of the Motor Octane,
whereas most 1990s engines have a 'severity" of one, which means the engine
is unlikely to knock if a changes of one RON is matched by an equal and
opposite change of MON [19].

----------------------------------------------------------------------------

6.9 How is the Octane rating determined?

To rate a fuel, the engine is set to an appropriate compression ratio that
will produce a knock of about 50 on the knockmeter for the sample when the
air/fuel ratio is adjusted on the carburettor bowl to obtain maximum knock.
Normal heptane and iso-octane are known as primary reference fuels. Two
blends of these are made, one that is one octane number above the expected
rating, and another that is one octane number below the expected rating.
These are placed in different bowls, and are also rated with each air/fuel
ratio being adjusted for maximum knock. The higher octane reference fuel
should produce a reading around 30-40, and the lower reference fuel should
produce a reading of 60-70. The sample is again tested, and if it does not
fit between the reference fuels, further reference fuels are prepared, and
the engine readjusted to obtain the required knock. The actual fuel rating
is interpolated from the knockmeter readings [66,67].

--------------------------------------------------------------------------------------------

6.13 Can higher octane fuels give me more power?

Not if you are already using the proper octane fuel. The engine will be
already operating at optimum settings, and a higher octane should have no
effect on the management system. Your driveability and fuel economy will
remain the same. The higher octane fuel costs more, so you are just throwing
money away. If you are already using a fuel with an octane rating slightly
below the optimum, then using a higher octane fuel will cause the engine
management system to move to the optimum settings, possibly resulting in
both increased power and improved fuel economy. You may be able to change
octanes between seasons ( reduce octane in winter ) to obtain the most
cost-effective fuel without loss of driveability.

Once you have identified the fuel that keeps the engine at optimum settings,
there is no advantage in moving to an even higher octane fuel. The
manufacturer'* recommendation is conservative, so you may be able to
carefully reduce the fuel octane. The penalty for getting it badly wrong,
and not realising that you have, could be expensive engine damage.

---------------------------------------------------------------------------------------

6.14 Does low octane fuel increase engine wear?

Not if you are meeting the octane requirement of the engine. If you are not
meeting the octane requirement, the engine will rapidly suffer major damage
due to knock. You must not use fuels that produce sustained audible knock,
engine damage will occur. If the octane is just sufficient, the engine
management system will move settings to a less optimal position, and the
only major penalty will be increased costs due to poor fuel economy.
Whenever possible, engines should be operated at the optimum position for
long-term reliability. Engine wear is mainly related to design,
manufacturing, maintenance and lubrication factors. Once the octane and
run-on requirements of the engine are satisfied, increased octane will have
no beneficial effect on the engine. The quality of gasoline, and the
additive package used, would be more likely to affect the rate of engine
wear, rather than the octane rating.

-----------------------------------------------------------------------------------------

6.17 Can I tune the engine to use another octane fuel?

In general, modern engine management systems will compensate for fuel octane,
and once you have satisfied the optimum octane requirement, you are at the
optimum overall performance area of the engine map. Tuning changes to obtain
more power will probably adversely affect both fuel economy and emissions.
Unless you have access to good diagnostic equipment that can ensure
regulatory limits are complied with, it is likely that adjustments may be
regarded as illegal tampering by your local regulation enforcers. If you are
skilled, you will be able to legally wring slightly more performance from
your engine by using a dynamometer in conjunction with engine and exhaust gas
analyzers and a well-designed, retrofitted, performance engine management
chip.
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Old 04-19-2004, 09:55 PM   #22
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BTW, the gov't cited the oil companies (many years ago) to STOP advertising higher octane gas as giving you 'more power' as you used to see on so many commercials. Several oil companies have had to refrain from making misleading statements and to distribute information that you should use the grade of gasoline recommended in your owner'* manual.
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Old 04-19-2004, 09:58 PM   #23
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This is very interesting stuff.

I normally use 89 instead of 87, and things seem to be fine. I get good milage.
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Old 04-19-2004, 10:48 PM   #24
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While I dont dispute the historical facts concerning industry "standards" over the years, I will dispute some EPA motives that lay at the basis of todays "cleaner fuels". But that is an entirely diffrent subject, is it not? In my recent post I was stating REAL WORLD experiences over the years with engines. Alot of you guys run 180 degree thermos, and I do too. Do you know what that does??. The pcm sees the cooler runniing temps as a REQUIREMENT for more FUEL, and more importantly, An advancment Of TIMING. Hence, a cooler running engine resists DETONATION. This SAME computer sees 87 octane Diffrently than 93. Your saying that a particular engine that is optimum at 88 is not detonating. This is false. {Unless your running a flathead Briggs}. It WILL DETONATE at some point relative to timing.If its not pinging, the pcm has it under control, but not nessasarily optimum. 93 could be a waste of money only if you know what octane is required to keep the timing advanced maximum. This takes some expeirmenting with your own hardware.
You turbo guys that force a volume increase also increase comppression ratios, which require more octane. This is a no brainer, However,the pcm is still increasing timing untill it gets knock. the same principle applys to non turbos. If you drop octane you drop timing.
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Old 04-19-2004, 10:57 PM   #25
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Amen.
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Old 04-19-2004, 11:16 PM   #26
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Cfoote, this is a response to the other post you made. Your right about the oil companies claims being snubed by the gov. The old advertisment claims of "more power" with higher octane fuel was half truth, and half false. In those days with MECHANICAL DISTRIBUTERS, adding octane was only a 'potential benifit'. It did little unless you advanced the distributer. This allowed you to increase power without knock. Of course theres a limit to the power gain, and this varied from engine to engine. Today we have a pcm to do that for us. this is an intresting subject........
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Old 04-20-2004, 12:18 AM   #27
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I question a lot of your comments

First you say that a 180 t-stat will deliver more fuel. That makes sense, as the engine is still thinking that it is not correctly warmed up (if it is indeed running at 180, since we know the t-stat doesn't control engine operating temp, just the flow of coolant). BUT who is to say the engine burns all that fuel up? Who is to say the engine does not run rich? Certainly a engine running rich does not produce more power. There is much more to the equation, and that equation needs to be balanced.

Also, you say the engine will advance the timing. Well, have you examined a Bonneville PCM? Who is to say the Bonneville PCM (I'm talking naturally aspirated here -- not blown) has these mappings? And, why are there performance PCMs available if this is the case?

I find this really hard to believe because I had a Grand Cherokee that had a Mopar Performance (i.e. Chrysler) Performance PCM in it. It was tuned for 91 octane and above. The PCM was a OBD 2 unit. If thats the case, why did I have to change the PCM? Simple. The factory PCM didn't have the correct mappings in it for 91+ octane.

If a manufacturer could get away with getting another 10HP out of a engine because it used 'premium' gas, than it would have tuned it accordingly. Believe me, the marketing people would have gone crazy to edge another 10HP out of their engine as they want the car to be 'the best'.

The car was engineered to run on 87. I need to see some proof that a stock PCM will advance the timing (as you say it will!) and increase power until it senses knock. Again, this is on a car that has been designed to run on 87.

Your whole assumption is that the Pontiac PCM has advance timing for higher octane fuel. I say it doesn't. We can ask a tuner who programs Pontiac PCMs for more info. Anyone care to contact one?

I have a BMW M5, and that car definitely has the mappings it needs to run on higher octane fuel (such as 94 which we used to have here)

I just don't buy it....sorry!
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Old 04-20-2004, 01:33 AM   #28
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I've been running 85 octane for the past four years, it works great - at high altitude. My car doesn't seem to run any differently if I burn 91. It'* pretty hard to find anything higher than that out here.
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Old 04-20-2004, 03:08 AM   #29
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The car was not "engineered" with JUST 87 in mind no more than it was engineered with JUST 5w-30 in mind. The engineers dont set minimum octane ratings, other people do. Gm has to build an engine that will reliably operate on that minimum. Thin oils are for epa ratings. they dont recommend 20-50 simply because of a stamp on the bottle that omits the words ENERGY CONSERVING. If you use 20-50, you can expect a 1-2 mile per gallon drop in your fuel consumption. This is why gm does not recommend that oil. To meet federal cafe standards, engineers will do everything short of runny 3 in 1 oil in tests to get mileage numbers up to make epa people happy. Thats why they run 195 stats- to lean the sucker out and get those mileage numbers. Melt the motor? you bet.But hey, the mileage numbers increased by one in the process. Some new stuff is running205-210 now. Overdrive units with a 1 to 1 final ratio that engages at 40 mph and beats the tran to death is another all out attempt to get those numbers up. Its gonna hit the pocket book one way or another. I just assume to burn more gas to keep it runnin
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Old 04-20-2004, 06:20 AM   #30
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Quote:
Originally Posted by Drifter420
And Alcohol powered car.. the Alcohol is their to control heat/cooling affect of Alcohol powered car cause they actually use Nitrous as main source of power.. Now you Chemist.. chime in and let me know whats what and if it'* right or not??
I'm not sure this is correct, Drifter. Modern racecars use alcohol because it has a higher flash point (and octane rating) than gasoline. Methanol, or wood alcohol (CH3OH) is used in Formula 1 racecars, top fuel dragsters, and small-scale remote controlled vehicles. If I recall, methanol actually contains less energy per unit volume than gasoline (which can produce roughly 129,000BTU'* of heat per gallon). The main point of using methanol is that it is highly compressible before it will spontaneously ignite. I believe the same can be said of ethanol (CH3CH2OH), which is the same stuff you'll find in a pint of whiskey. Both types of alcohol can be used as oxygenates and octane boosters in regular gasoline, though methanol (which is used to make methyl tertiary butyl ether - MTBE) is highly carcinogenic. You don't want to be drinking that stuff.

If you remember from your high school chemistry, Charles' and Boyle'* Laws both relate the temperature of a system compared with the pressure of a system. Thus, if you raise the pressure of the system (say, in our discussion, the cylinder of an engine), the temperature will rise as well. High performance engines have high compression ratios in order to generate a lot of power, but as you can see, high compression means high cylinder pressure, and high cylinder pressure means a temperature rise. Low octane gasoline will detonate under these conditions, high octane gasoline is less susceptible, aviation fuel is a little better, and alcohol takes the cake for compressibility.

Since alcohol does have a lower vapor pressure, it tends to evaporate more easily than, say, water, and thus does make a very good evaporative cooling agent, though the statement about "using nitrous as a main source of power" seems erroneous to me since nitrous oxide CANNOT burn on its own (remember, to create a flame, you need three components: heat, oxygen, and fuel. N2O on its own contains no fuel - the nitrogen is inert). Nitrous, up to a point, also provides its own form of intake charge cooling since the the process of vaporization is endothermic, and hence absorbs energy (makes it cooler). But too much nitrous, and the oxygen content in the cylinders gets a little too high, and when burned with fuel, raises the temperature of the cylinder well beyond safe levels.

But I'm studying electrical engineering, not mechanical engineering or chemistry, so somebody please correct me if my information is wrong. I can't be spreading lies now, can I?
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