OCTANE

[Marc N.]

1. WHAT WAS THE RECOMMENDED OCTANE RATING STATED FROM GM FOR A 1967 427BB BACK IN 1967.

2. WHAT WAS THE RECOMMENDED OCTANE RATING STATED BY GM FOR A 1970 454BB BACK IN 1970.

CANT SEEM TO FIND THE ANSWERS FOR THESE TWO CARS IN MY BOOKS. NOTE: I KNOW IT WAS LEADED GAS BACK THEN. BUT OCTANE RATING RECOMMENDED....I DONT KNOW...HELP!

[Joe L.]

Re: OCTANE

Marc----

In all my years involved with cars(more than I'd want to say, but it includes the 60s), I rarely observed a specific octane rating specified for cars. Owner's manuals would just say "regular" or "premium". That made sense, too. During the 50s and 60s gasoline station fuel pumps NEVER specified an octane rating, with the POSSIBLE exception of SUNOCO. Since I lived on the west coast, I never saw a SUNOCO pump anyway.

During the late 70s, about the time of raging inflation, a government organization known as the Cost of Living Council came into being. For some reason or another, they required the octane ratings to be posted on filling pumps. As a matter of fact, the octane rating system found to this day on those yellow labels is called the "Cost of Living Council Formula" and is the average of the Research + Motor Octane ratings.

One "clue" as to required octane ratings, though, was provided for 67-69 L-88s. Those cars came with a label attached to the console which specified that the minimum octane rating of the fuel used must be 103 Research or 95 Motor. Using today's "Cost of Living Council Formula", that would be 99. However, bear in mind that L-88s with their 12.5:1 compression ratio would require a fuel with SIGNIFICANTLY higher octane than the 10.0 to 11.0:1 compression ratios of most other Corvette engines of the day.

One other thing to bear in mind is the fact that there are many different octane rating systems besides "research" and "method". As I recall from my flight school days, aircraft fuel uses a different rating system. Any octane "number" that you see has to be considered in the context of the octane system which it represents.

[Everett Ogilvie]

Re: OCTANE

All,

What are the risks, if any, of using "too high" octane fuel, say in 11:1 engines? There are racing fuels available, with lead, that advertise up to 110 octane (not sure of the rating method). Can too high of an octane rating cause damage to these engines, or is it a case of never enough? Will the ignition of the mixture and the flame propagation be "too much", or does more octane mean better burning? I have used some 110 fuel before, diluted with unleaded premium, and did not observe any adverse effects, but don't want to take any chances.

Everett

[Joe L.]

Re: OCTANE

Everett---

With one caveat, excess octane does no harm whatsoever to the engine or performance but it may do harm to your wallet. The caveat is that if your're using leaded avgas, these fuels, even the so-called "low lead" 100 octane, contain very large amounts of lead. As I've said before, lead is good for raising octane cheaply and protecting exhaust valve seats. It's bad for everything else in the engine. So, indirectly, if you're using too much leaded avgas to get high octane, the excessive lead, not the octane, could be bad for engine components. The 110 octane fuel that you're probably talking about is the unleaded variety. Use that type to your heart's content as long as cost is no object. Be advised, though, that if you "blend down" the 110 octane, which is most likely an R+M/2 rating, with an equal volume of 92 octane unleaded, that will yield 101 octane fuel. That should be PLENTY for an 11.0:1 engine.

As far as octane requirements go, the rule is VERY simple. If your engine suffers from significant pre-ignition under normal operating conditions, you need more octane. If your engine doesn't suffer from significant pre-ignition under normal operating conditions, you DON'T NEED more octane and you might be able to get by with less. More octane is not better or worse for an engine which doesn't suffer from significant pre-ignition under normal operating conditions.

Sometimes I think that some folks get the notion that having an engine that uses "high octane fuel" is a status symbol. So they use it even if they don't need it. That's fine if it makes one feel important, I guess. But, if your engine doesn't need it, your engine doesn't need it and it's NOT otherwise better for an engine. Chevron, Mobil, Texaco, Shell, Unocal, and others would like for you to believe that, though.

[Everett Ogilvie]

Re: OCTANE

Thanks for the info. The 110 fuel I referred to is Sunoco, and I was told it has some lead, but I do not know how much. It is a racing fuel, and I know the "roundy-round" (circle track) cars use it.

I did a rough weighted average to arrive at an estimated octane level for my car:

(5 gals(110 octane) + 10 gals (92 octane unleaded))/15 gals = 98 octane.

This is plenty even if the estimate is rough, especially at my one mile altitude.

The 110 price is sky high, but if diluted like above, it spreads the cost.

Everett

[Jack H.]

Re: OCTANE

Octane is a measure of fuel ignition consistency, not a measure of energy content. Given repeated elevation on the PVT curve, what is the variance of combustion flash? Higher octane rated fuels will ignite more consistently with a lower standard deviation than lower octane rated fuels.

As Joe says, neglecting the additive issue (lead, Etc.) putting a higher quality fuel into your tank than your engine really needs does no harm. You're simply tossing $$$ out the tail pipe.

But, you should be able to run an 11:1 CR engine safely at 5000 ft. elevation on standard high test pump gas if your engine hasn't been 'tricked' (static/dynamic advance changed, aggressive cam, Etc.). I have no problem around Denver with my 11:1 SB or my '65 396 BB....