AZ455

What octane do you guys run in your 8.5:1 455s? I noticed the sticker in the glove box says low octane unleaded, but also mentions the use of 91.

I have the timing a bit over stock, with initial around 14° @ 1100 rpm. I have been using the 91 it requests for a while now, but was thinking of trying a tank of 87. After all, if it will run without pinging on 87, I may even get a bit more mileage & power out of the 87.

I know most will say listen for pinging, if you hear it, retard the timing or get higher octane fuel. However, what I am wondering about is mild detonation that cannot be heard. Anyone run 87 on a stock low compression 455 for a while without issue? I usually drive it pretty conservatively, but I do get on it from time to time (freeway ramps, etc.).

Again, the engine is stock, except for timing being advanced. I notice the initial timing spec for the 455 varies greatly in the service manual depending on the vehicle it is in, or the engine code. Anywhere from 8° to 14°. The vehicles are all 1974, so I assume all low compression engines. Is this difference due to different tire sizes, gear ratios and vehicle weight that are behind the motor (lower initial for the heavy, highway geared cars)?

Thanks!

Allan R

Actually 87 octane now is the equivalent of the 91 in the owners manual. You should be able to run it no problem. Be careful to stay away from E85 though. Personally, I run 87 for 2 tanks then run 93 or 94 for a tank. Don't see any difference in performance except at the pocketbook. If your car is tuned for 87, it should run just fine. You know the drill obviously if it starts pinging. I drive my car conservatively most of the time and there are no issues with any engine noise (except the throaty pleasant sound of dual exhausts)

AZ455

Wow, I had no idea that todays 87 was equivalent to 91 then. That is good to know I guess. Fortunately we don't even have E85 pumps around here, but I would avoid it like the plague anyway. What do you have your initial timing at? The dual exhaust sound is definitely nice.

Allan R

Well I've also changed out my distributor to an HEI with a radical timing curve, so I am only running at 8° and it runs just fine. If I retard the timing to 12? It starts the car shaking and running like it has a lumpy cam + the exhaust gets a really heavy smell to it.

Can't remember offhand, but I thought HEI didn't come into the Olds line till 75. Is your's regular distributor or HEI? I know it's a 74.

AZ455

Mine is still the old points style distributor. I think HEI may have been available on some Olds in 1974? Not sure though, you may be right about 75.

I wouldn't mind making the switch to HEI sometime, but no rush. I don't mind messing with points, and dwell, and them wearing out. Anyway, I will see what happens running some 87 with 14° initial.

One other question for you. I assume your 8° initial is without vacuum advance? Also, if you have vacuum advance, is it on ported or manifold vacuum? Mine is on manifold vac for the extra advance at idle to help create a smoother and cooler idle. My initial 14° is with vacuum advance disconnected.

shattuck

this is the same problem even i am facing waiting for the solution please help me and thanks in advance.

Allan R

That would be correct. I do have vacuum advance and it goes directly to the TCS port screwed into the manifold. The idea is the same as your description. Only 71/72 had the TCS terminal. I suspect that when I reinstall the EGR I will likely have to change timing. I've heard lots of guys on this site advocating that 14° or more works well for them.

FWIW, I am planning to put the original points distributor back in the car. Am I nuts? I don't think so. Properly maintained, it should run just as well. One of the reasons I'm going back to the OEM housing is so I can install my OEM twin snorkel air cleaner. The air can I have on now is stock, but I had to modify the back side so it would fit. The HEI takes about 1/4" too much space for it to fit right.

When the OEM distributor goes back though, I'm seriously thinking of putting in a pertronix unit to replace the points/condensor. That will give the efficiency of HEI while maintaining the stock look.

Does this help at all?

coltsneckbob

What exactly do you mean equivalent? How so? I've never heard this before.

Allan R

Octane ratings back in the old days was based on using tetraethyl lead (TEL) number. Tetraethyl lead was extensively as an antiknock agent and prevented exhaust valve and seat wear. (you also know that earlier production engines didn't have hardened valve seats, and relied on lead additives in the gas) The more TEL that was used resulted in higher octane ratings. For the most part higher octane was only required for higher compression engines; there is no advantage to running high test in a car that is designed for regular. Starting in 1972, legislation was passed to eliminate TEL from gasoline because of the harmful emissions it added to the air.

In place of lead, there has been a continual stream of new R&D that modified the detonation ranges for gasoline. The method of determining octane nowadays uses other chemicals and additives to determine octane and is referred to as the (R&M) ratings. Research Octane number + Motor octane /2 = new octane rating. RON is the rating given to an engine running at 600 rpms, and MON is for engines that are under heavy load. Add the 2 values, divide by 2 and you get the octane rating. Most cars with low combustion engines actually need a lower octane rating to run better. Higher compression performance engines requiring more octane get rated the same way. I have to find the website, but there is documentation that shows comparitive chards that today's 87 is the equivalent octane of 91 octane in 1972. It's been discussed here before. Some of the resident experts can explain the difference in how octane was calculated back then better than I can.

coltsneckbob

I knew about the use of lead for valve and seat wear and I suspected it had something to do with knock for whenever I put Amoco "white gas" (as we called it) in my 69 LeMans (in the year 1971) I experienced knock. However, I still don't see how 87 octane today is the same as 91 rating in 1970. Maybe I am just being dumb today....not an unusual Saturday occurrence!!!!

chrisneu68olds

I was going to ask the same question but there is the answer. If you can find the URL I would like to read more. Thanks Allan!

AZ455

This thread definitely has some good information now! Doing some searching on Google, I found verification about the current Octane formula vs. strictly going by the RON number like when these cars were new.

Think that is a great idea, nothing wrong with points! The twin snorkel air cleaner is worth it in my opinion.

I am considering possibly getting one of the Pertronix points/condenser replacement units as well. Is this the "IGNITOR" I have heard of? Seems like a good solution to me, but I still have some research to do. I have heard of these units, but don't know anything about them.

MDchanic

Your question is the same as saying you don't understand how it can be 20° in Windsor, ON, and 68° in Detroit, MI at the same time.

Research Octane Numbers (RON) are measured at a lower RPM and load on the test engine, and tend to be higher.

Motor Octane Numbers (MON) are measured at a higher RPM and load on the test engine and end to be higher.

US fuel used to be measured on the RON scale (as most of the rest of the world, including Europe and Au/NZ still is), but in the mid-'70s, some government genius decided that since both the RON and MON numbers have their weaknesses, a more accurate measurement of octane could be obtained by averaging the two (thus RON+MON÷2), which is called the Anti Knock Index (or AKI).

Since the RON tends to be about 8-10 points higher than the MON, the average of the two will be about 4-5 points lower than the RON, which is to say that an AKI of 87 is roughly equal to a RON of 91, and an AKI of 94 is roughly equal to a RON of 99.

- Eric

Allan R

No, it's not dumb.

Try looking at it this way:
Remember when 100 octane was common? Sunoco260 gasoline was listed at the pump at 102 octane. Those octane ratings sound very high compared to today's octane ratings, but they were not as much higher. Why? Because there are 2 ways to rate octane. One is called the "research" octane number RON and the other is called the "motor" octane number. MON

The research octane number is ALWAYS a higher number than the motor octane number, so when oil companies used to advertise their Hi-Test gasoline, they used the more impressive research octane number. This gave the average consumer the impression that the gas had more power than it actually did. Ah yes, the days of "fill'er up with hi test'. And at only 2cents a gallon difference? Why would you settle for regular, right?

But "motor" octane is a number about 10 points lower than the Research number, because that was measured when the engine was under load. The oil companies didn't advertise that lower number.

In 1972, government legislated oil companies to post an average of the 2 different octane ratings, which lowers the octane rating number for the same fuel that had higher numbers before. That's no doubt why GM included - even the 72 owners manual - to use the gasoline Research octane number in their fuel recommendations. The books were already printed or in print when the legislation was passed. And it takes a while to filter down to production.....

So when you look on a gas pump, you'll see a sticker for the octane rating that says "R+M/2", which stands for Research octane plus Motor octane divided by 2, which is the average of both rating numbers.

Using that information, if you look at the old 100 Research octane, it also had a Motor octane rating of about 90. Today that is posted as the 'average' of about 95

That's still a little higher than the 93 or 94 octane of today's Premium, but not as big a difference as most people think. The conversion from 91 octane in the old days to 87 now is the result of 91 research octane added to 84 motor octane averaged to 87. So you're still getting about the same octane rating, but with with ethanol additives to give the octane rating instead of tetraethyl lead.

I got the following quote from a chevy forum. (Yeah I know, but they also run the same gas issues as we do) The writeup starts out as a tribute to the Camaro, but goes on to talk about the burdens GM faced with performance when congress introduced the Clean Air Act in 1970, and then legislated lead free gasoline in 72. I also found out that AMOCO actually produced lead free gas from earlier than 1920!!

Allan R

That's the one. Not cheap either. Supposed to have an idiot proof installation guide. Sign me up!

MDchanic

This is a pretty good description of octane testing.

I could have sworn I saw a more detailed one recently, but I can't for the life of me remember where.

- Eric

coltsneckbob

OK, thanks for the explanation. I get it. However, I have one quibble. If the octane rating is shown exactly as you showed then the government screwed up their math. Per the associative rules of math it should be (R+M)/2 since division takes precedence over addition. Guess it just the scientist in me!!!!!

MDchanic

Here's another bit of information:

"Gasoline consists of a complex mixture of hydrocarbons. Most of these are alkanes with 4-10 carbon atoms per molecule. Smaller amounts of aromatic compounds are present. Virtually no alkenes or alkynes are present in gasoline.

"Gasoline is most often produced by the fractional distillation of petroleum, also known as crude oil (it is also produced from coal and oil shale). The crude oil is separated according to different boiling points into fractions. This fractional distillation process yields approximately 250 mL of straight-run gasoline for each liter of crude oil. The yield of gasoline may be doubled by converting higher or lower boiling point fractions into hydrocarbons in the gasoline range. Two of the main processes used to perform this conversion are cracking and isomerization.

"In cracking, high molecular weight fractions and catalysts are heated to the point where the carbon-carbon bonds break. Products of the reaction include alkenes and alkanes of lower molecular weight than were present in the original fraction. The alkanes from the cracking reaction are added to the straight-run gasoline to increase the gasoline yield from the crude oil. An example of a cracking reaction is:
alkane C13H28*(l) --> alkane C8H18*(l) + alkene C2H4*(g) + alkene C3H6*(g)
In the isomerization process, straight chain alkanes are converted into branched chain isomers, which burn more efficiently. For example, pentane and a catalyst may react to yield 2-methylbutane and 2,2-dimethylpropane. Also, some isomerization occurs during the cracking process, which increases the gasoline quality.

"In internal combustion engines, the compressed gasoline-air mixtures have a tendency to ignite prematurely rather than burning smoothly. This creates engine knock, a characteristic rattling or pinging sound in one or more cylinders. The octane number of gasoline is a measure of its resistance to knock. The octane number is determined by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Isooctane is assigned an octane number of 100. It is a highly branched compound that burns smoothly, with little knock. On the other hand, heptane is given an octane rating of zero. It is an unbranched compound and knocks badly.

"Straight-run gasoline has an octane number of about 70. In other words, straight-run gasoline has the same knocking properties as a mixture of 70% isooctane and 30% heptane. Cracking, isomerization, and other processes can be used to increase the octane rating of gasoline to about 90. Anti-knock agents may be added to further increase the octane rating. Tetraethyl lead, Pb(C2H5)4, was one such agent, which was added to gas at the rate of up to 2.4 grams per gallon of gasoline. The switch to unleaded gasoline has required the addition of more expensive compounds, such as aromatics and highly branched alkanes, to maintain high octane numbers.

"Gasoline pumps typically post octane numbers as an average of two different values. Often you may see the octane rating quoted as (R+M)/2. One value is the research octane number (RON), which is determined with a test engine running at a low speed of 600 rpm. The other value is the motor octane number (MON), which is determined with a test engine running at a higher speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of 90, then the posted octane number would be the average of the two values or 94.

"High octane gasoline does not outperform regular octane gasoline in preventing engine deposits from forming, in removing them, or in cleaning the engine. Consumers should select the lowest octane grade at which the car's engine runs without knocking. Occasional light knocking or pinging won't harm the engine, and doesn't indicate a need for higher octane. On the other hand, a heavy or persistent knock may result in engine damage."

Sorry, I had this on my computer, but I appear to have not bothered to record the source.

- Eric

Highwayman

This looks like the source.

http://chemistry.about.com/cs/howthi.../aa070401a.htm

coltsneckbob

That is very interesting. However, the last line in the article says "consumers should use the lowest octane gas that doesn't cause knock" and that may be true for older cars, but I wonder about newer ones. I drive a new E350 MB. The manual specifically says use premium gas. It adds that if premium is not available I could use regular in emergencies, ostensively to get to a pump with premium, but then to fill with premium before I do extensive driving. The manual further states that continued use of regular would violate my warranty terms. So, this begs the question are there other properties of premium (or the octane) that are not explained in these articles?

brown7373

The math as explained is correct, but there is no way today's premium is anywhere close to what it was during the 60's. I drove many of these cars when they were new (66 GTO, 69 GTO, 69 GP, 1970 Bonneville 455, 70 442, 67,68,69 Corvettes) with compression ratios 10.5:1 up to 11.0:1 and they almost never pinged unless they had too much timing advance. Even with A/C running and 95 degree temperatures, they didn't ping. When gas really changed was in the late 70s to early 80s, when 93 or 92 was as high as you could get. Suddenly, everything with over 10:1 would ping under acceleration, and the timing needed playing with, which usually meant less. Even freshly rebuilt engines with no carbon build-up couldn't run according to specs and the correct timing.

87 may be equivalent to 91 from the early 70s, but no way 93 is equivalent to 95 to 98 or Sunoco 102, any more than todays plastic Tonka trucks are equivalent to the older metal ones. Gas today is designed for todays cars, not those of the 60s or even the 70s. And unfortunately, that makes sense, since newer cars probably make up 99 percent of all miles driven.

MDchanic

I agree, Bob.

Modern cars are constantly adjusting timing based on input from their knock sensors, so you'll never hear them knock.

I would apply that statement to cars without knock sensors, and follow the manufacturers' recommendations for cars with knock sensors.

- Eric

Rickman48

Allen R - I remember my Father, who was a Ford mechanic, telling me never to use Amoco [Standard] fuel, in anything, including the lawnmower!
Apparently it was eating carb gaskets and causing carbon build-up in the higher compression 'Thunderbird' motors - but then too, they were Fords!
Although, that was advise I heed to this day, especially after that gulf fiasco!!

Allan R

Your presentation is exactly right. Way to go you mad scientest!! FWIW as far as math goes, I can still add 1+1 and get 3 almost every time . So I don't 'associate' with math skills very well. I just try to 'communitate' the idea

DeltaPace77

Think most of the above’s got you covered, but lot's of the older gas was the exaggerated research octane, while today's is a mix between the more real world motor octane. While leads really got nothing to do with it, was a cheap way to get the octane up. Today’s 93 is more expensive than 87, as requires further refining, yielding less gasoline per giving crude input. California limits premium to 91, supposedly saving a little crude, to refining a little more to 93. Our government at work on important issues.

Railguy

I have to disagree with this because today's octane rating is just that a rating.which means an 87 octane rating behaves like 87.So if it behaves like 87 why would it be equal to 91?Octane is a hydrocarbon the more octane "8 carbon molecules in a chain,get it 8 oct" gas has the less likely it is to ignite prematurely but this is more costly to make.less gas per barrel of crude.So oil company's looked for other additives to make the oil go farther more gas per barrel.thats why it's a rating not an actual octane.Right?
raight

MDchanic

Wrong.

Please read the information I posted above.

It's an octane rating. Octane is used as one comparison compound to provide a measurement scale for anti-knock properties.

Actual octane is not used in gasoline, or even in the octane testing process (except perhaps for calibration of the equipment). The scale of measurement is based on the properties of octane, as a yardstick.

Octane was never commonly used as an anti-knock additive - the first common anti-knock additive was tetraethyl lead, which was used because it was cheap and very effective.

This system was devised a long time ago, in the teens or twenties, I believe, long before modern internal combustion engines existed.
If they were to design this system today, I'm sure it would take the measurement differently, use different comparisons, and have a different name. It is used now only because it is universally understood and provides a common reference point through time, which is useful to people like us.

- Eric