reg. unleaded 85 vs. unleaded plus 87

[badazzbuick]

is there any wat that using regular unleaded with an octain level of 85 would cause my reatta to run rough and even stall every once in a while? because my car has past history of dieing a lot about every week. for the past few weeks i have been using unleaded plus /87 and i havnt had a problem yet? im doind a prosses of elimination to find the problem, and the only variables that have changed with the car is the gas, and the wheather has goten pretty cold here in the last couple of weeks too which may also be helping out. but just didnt know about the gas thing and if it would cause it to die..why? .thanx

[Guest Buick Mike]

85 octane!? Will anything run on that? Seriously, I'd use the 87. I think that is the octane the Reatta was designed to use.

Mike

[padgett]

85 is regular in Denver - is related to altitude/max cyl pressure.

[Shelby]

I use the 90 or 91 in the Summer The mileage is better to cover the high cost .

I use the med grade in the winter.

[Filobedo]

Regular Unleaded is 87, Unleaded Plus is 89 & Premium varies from 91 to 93. So, Where would you get, or even want 85 octane???

[Filobedo]

Oh, Denver. My Bad...

[Guest imported_dantm4]

The lower the octane, the lower the amount of carbons in the gas, which can sometimes lead to predetonation (before the spark) when the air and fuel is compressed together by the cylinder. (at least my understanding by doing some research on octane) Probably why your car isn't running well, but if it isn't knocking, good luck. Carbons burn to make the explosions happen in the engine.

Hence when you buy higher level (priced, octane) gas - it seems peppier and is less likely to knock, but you have to decide what kind of performance you want and how much you're willing to pay for it. The research suggests that there isn't much of a mileage benefit to buying higher priced gas. (price/mile) I'm guessing Denver's high altitude means that the thinner air can be mixed with a lower octane without knock. I would also guess that since colder air is denser than warmer air your car would probably run better on a higher octane in the colder months of the year, but this is just a guess, and is still a price/performance issue.

Just my two cents....hope my memory and my chemistry/physics is accurate.

Dan

90' Black/Tan Coupe

[padgett]

The PON (pump octane number which is a combination of the research method and the motor method) is just a measure of how fast the gas burns and at what temperature/pressure the burn will start. squeeze any fuel enough and it will begin to oxidize rapidly (combust) which is why Diesels are known as Compression Ignition (CI) engines and ours are Spark Ignition (SI).

In the range we are talking about (87-93 at sea level and 85-91 for "High Altitudes") a gallon of regular has almost exactly the same btus/pound as hi-test. The difference is all in the burn rate and ignition point. All start from the same base (preferably SA sweet and not Alaska bunker) the octane is determined by how far it is refined (highest octane is drawn off first and there is less per gallon) and how many octane enhancers are added (for many years tetraethyline lead was the preferred additive hence the name "Ethyl Gas". This permitted use of lower grade petrolium as a base so more could be produced per barrel or crude.

Aside- in the '20s when lead was introduced it was common for a salesman to demonstrate on a badly knocking engine by waving their necktie which had been dipped in pure TEL over the air intake. The engine would immediately smooth out on the fumes. The effect on the life expectancy of the salesmen is undocumented.

So there is no difference in power if you can control the burn rate and starting point. Detonation, knock, or more accurately pre-ignition are terms for either combustion ignition occuring before the spark or the same factor initiating a second flame front in the chamber. All of my street cars are tuned to run on 87 PON at sea level for economic reasons.

For max efficiency (power, mpg, whatever) you want the peak chamber pressure to occur at approximately 10 degrees ATDC (varies slightly according to chamber design & is found with dynos and not math) with a smooth increase. *Everything* we do is to achieve that with the minimum amount of fuel. Repetitively.

The interesting thing is that the more vaccuum you are pulling (highest at low load cruise, zero at WOT), the lower the octane requirement so an engine that likes premium around town on a hot day will probably run very nicely on the cheapest gas available on the Interstate (particularly I-95).

Could go on for a few hours. Won't.

[Tinker]

More than you want to know about octane.

ALL about Octane

[padgett]

Interesting but is really a layman's discussion (and I think the table at the end of 7.2 is really Research method which averages about 4 points higher than the number on gas pumps today).

In The Beginning (see the earlier pages) there were two ways to measure the octane number of gasoline, the Motor method which simulated high loads and the Research method which was more "normal operation". Enter the marketters. The gas companies, understanding the American "more is better" concept all rated their gasoline by the Research methiod because it was higher (Phillips 66 once referred to the octane number).

When the gov got involved, they looked at the two different rating mechanisms and in their infinite wisdom decreed that both would be used so what we see today is the average, literally (MM + RM)/2. Note also that octane is measured using a standard undersquare engine (3.5" bore, 4.5" stoke) created in the 1920s. Oh well, just numbers.

If you really want to know what is going on, see H. R. Ricardo's "High Speed Internal Combustion Engine, "The Internal Combustion Engine in Theory and Practise" (think last edition was around 1968), C. F. Taylor; MIT Press; 1985, or books on the subject from E. F. Obert.

[badazzbuick]

awsome thanx to every one for their input..it will help me along my way to finding my problem:)!!

[Guest DTerry]

I've always believed that as long as it didn't clatter, the lowest octane (price) gas was okay; that there was no performance gain from higher octane fuel. Now I'm wondering if acceleration may be affected. In the less sophisticated cars (without knock sensor), I'm sure it makes no difference, but maybe there is an advantage in not having the knock sensor retard the ignition timing during WOT. Obviously steady state cruise wouldn't suffer, but do you suppose acceleration would be greater with higher octane? Y'know, my life was a lot simpler before you folks began confusing me with facts.

[2seater]

If you watch your knock numbers, and particularly the spark retard, you will know if you are losing performance. It is not unusual to see a few knock counts right after a shift during wide open throttle acceleration, with the stock chip and regular fuel. To me that means it is optimized for the fuel type being used and typically will turn in the best economy also. I have said this before but I will repeat this suggestion. Try a full throttle start with the fuel you are using now. A good running car should spin the tires for several feet. The next time you fill up, use the highest octane available, preferably from the same vendor. Try the test again. My observation is the car may be less responsive and wheelspin, if any, will be reduced. I have tested this pretty extensively, and higher octane is only useful if you have knock problems, or have other modifications to take advantage of it. Padgett has done a good job of putting this is numbers, and I suspect the additional time to peak cylinder pressure, (with hi octane), reduces the power making potential. It may feel smoother, but overall performance does not seem to increase.