Gasoline Octane Comparison

[John L.]

For years I have been adding about 2.5 gallons of Cam 2 110 octane racing fuel and topping it off with 93 octane pump gas when filling up my 66 425hp vette. Just found out that the local gas station discontinued carrying the Cam 2 and its not sold anywhere else in the area. So, I have spent some time researching the DB Archives about gasoline recommendations and one take away is that pump gas should be okay to use in my vette, the engine of which was rebuilt years ago with 11 to 1 compression ratio. One other thing that came up a number of times is that 93 octane gas sold today is equivalent to 98 octane gas sold in the 60's due to the fact that the method utilized to calculate octane has changed. Is this 93 octane to 98 octane comparison correct? If so then, I assume 100 octane racing fuel available today to be equivalent to 105+ octane fuel based on the older method utilized in calculating octane? Lastly, did GM provide minimum octane requirements for C2's when they were sold new, especially related to vettes with higher compression engines, I know they did for the L88?

[Robert M.]

Re: Gasoline Octane Comparison

John, I stopped using Cam II. First at $10.00 a gallon it got to be too expensive. All I use year round in my 67 435 is Av Gas from the local airport. I go over and fill three 5 gallon safety cans. It is now $6+ a gallon. It is 100LL( 100 octane low lead) Car runs great on it and ther is no ethynol in it. It is also great to sit in the car all winter. It does not deteriorate like auto gas. Try it, you will like it.

[Joe L.]

Re: Gasoline Octane Comparison

For years I have been adding about 2.5 gallons of Cam 2 110 octane racing fuel and topping it off with 93 octane pump gas when filling up my 66 425hp vette. Just found out that the local gas station discontinued carrying the Cam 2 and its not sold anywhere else in the area. So, I have spent some time researching the DB Archives about gasoline recommendations and one take away is that pump gas should be okay to use in my vette, the engine of which was rebuilt years ago with 11 to 1 compression ratio. One other thing that came up a number of times is that 93 octane gas sold today is equivalent to 98 octane gas sold in the 60's due to the fact that the method utilized to calculate octane has changed. Is this 93 octane to 98 octane comparison correct? If so then, I assume 100 octane racing fuel available today to be equivalent to 105+ octane fuel based on the older method utilized in calculating octane? Lastly, did GM provide minimum octane requirements for C2's when they were sold new, especially related to vettes with higher compression engines, I know they did for the L88?

John-------


The 93 current-to-98 old comparison is about right. Perhaps not exactly correct but in the ballpark.

I don't know of specific octane number requirements for 60's cars. I'm sure they existed but I don't recall seeing them published anywhere. If they were published, I would expect to find them in the AMA specifications which can be found on the GM Heritage web-site.

Here's what I suggest: fill your tank with 93 octane pump gasoline. If your engine doesn't ping on this, then it's a waste of money to use any higher octane or octane additives. If it does ping, try reducing the initial advance by a few degrees and see if that solves it.

[Michael W.]

Re: Gasoline Octane Comparison

John, the spread between the old and new ratings numbers is typically 4 points. 'Old' 98 is now 94. Most modern fuels typically have a difference of 8 points between their RON and MON numbers.

North America changed from advertising the RON numbers on the pump to the present AKI numbers in the mid-70s. By and large, the rest of the world still uses RON numbers which gives rise to the legend that 'their' gas is better than 'ours'.

[Duke W.]

Re: Gasoline Octane Comparison

When purchasing "race gas" one must determine the octane number methodology. Is it RON? MON? PON? Aviation Method? Marketers are often vague about the actual methodology used.

Fuel requirements are listed in AMA specs and owner's manuals. In the fifties, actual RON is specified, but this changed to more vague terms such as "Premium" in later years - probably because of the fact that there are too many variables to specify a specific octane number for all driving environments and conditions.

You'll find an explanation and simple (approximate) conversion formulas along with ways to quell detonation in the following pdf:

DukeWilliamsEngineTuningPerformance&EconomySeminar .pdf (application/pdf Object)

Duke