Mixing Gasoline-Octane Result

[John L.]

For my 66 L72 with 11:1 compression ratio (or about that) I usually mix 5 gallons of Sunoco GT 260 (100 octane, unleaded) with 15 gallons of Sunoco 93 octane pump gas. Now, when comparing octane ratings of today vs octane ratings from the 60's, I understand today's posted octane ratings are about 4 octane lower when compared to the 60's, so 93 octane pump gas is equivalent to 97 octane pump gas of the 60's.

So, if I am doing the math correctly, 5 gallons of GT 260 plus 15 gallons of 93 pump gas results in a 1960's octane rating of 98.75 (25%x104+75%x97) Do this sound right? Thanks

[James W.]

Re: Mixing Gasoline-Octane Result

You octane would be 94.75

5 gal x 100 octane = 500
15 gal x 93 octane = 1395
500+1395=1895 / 20 gal = 94.75


James

[Duke W.]

Re: Mixing Gasoline-Octane Result

Is the Sunoco "100 octane" RON, MON, or PON? If it's PON then your calculation gives a good ballpark number. My 2012 San Diego tuning seminar that's an easy web search explains the different octane measurement techniques and how to approximately convert one to another.

However, since there's no way to calculate the required octane, the best method is to use decreasing quantities of the high octane blend with pump premium and find the least amount of high octane that will keep the engine out of detonation.

If it's an original Tonawanda-built engine the actual CR is likely closer to 10.5 than 11, and with the OE lazy centrifugal curve and conservative initial timing recommendation it may run detonation-free on 93 PON pump gas, which is 97-98 RON.

Back in the day, "premium" was not all 100 RON as was commonly believed. Oil companies only blended in enough octane to minimize customer detonation complaints, and it varied with season and altitude. Cold winter temperatures and high altitude reduces the octane requirement compared to hot sea level summer conditions.

Duke