posted by Zaphod58
I worked at a flight school. A lot of people come in and buy 2-3 gallons of avgas. It's 100 octane. They'd mix it 10-1. 10 gallons [87 octane]
gas to 1 gallon avgas. According to them the engine ran better and they got a bit better gas mileage. Especially with older cars. [Edited by Don W]
There used to be hucksters at the annual State Fair who’d demonstrate some kind of fuel or oil additive and then sell it to dutifully impressed
motorists. Some people believed it helped. As a young “pump jockey” I used to sell such items as Marvel Mystery Oil, Wynn’s Friction Proofing
Oil Additive, Liqui-Moly and finally Andy Granitelli’s STP. I was alive and well when Shell introduced their X-100 brand of motor oil. Top of the
line, then, and an extra cost item.
Roger Huntington OTOH, an ME who wrote a monthly article in Road and Track, never had much regard for those additives. He explained that Pennsylvania
crude was especially well suited for lubricating engines. That was why Quaker State and Pennzoil and Valvoline were so popular. SOCal - Chevron -
developed special refining processes and chemical additives for motor oil to counter the almost total domination of the lubricating oil field by small
Pennsylvania companies. Anathema to Standard Oil types.
Sun Oil Company of Philadelphia, later Sunoco, was a small competitor in my neighborhood. Their claim to fame was the single octane grade they sold as
Blue Sunoco. Dyed blue. Where most gasolines were regular at 76 octane, and premium at 80 octane, Blue Sunoco was right between. 78 octane. They knew
many cars that called for premium would work ok on the lower octane. They priced their gas 1 penny over the other companies regular but a penny or 2
under the other's premiums.
A lot of people believed the higher octane made their car run better. Roger Huntington explained why that was not so. Gasoline is truly fungible.
Unless you dye it blue. It all has appx. 35,000 btu per gallon. Octane rating relates to knock resistance. Knock was a popular name for pre-ignition.
That is, the mixed air and gas charge fires before you intend for it to fire. Ping! That noise you hear is the connecting rod slamming against the
crankshaft bearing. Or sometimes you just burn a hole in the piston. Knock or ping is very destructive. And to be avoided.
During the mid 1960s, Sunoco offered its “260" brand and it was the highest octane available at the pump. Cars were sold with 10 to 1 and even as
high as 12 to 1 compression ratios. It takes a lot of octane to resist knock at those high comp ratios. OTOH, if you’re running 7 to 1 or even 8 to
1, regular grade gasoline at 87 octane won’t knock. So will it run better on 89 or 93 octane? They all tell me “No.”
Regardless of its octane rating, all gas has 35,000 btu per gallon. An internal combustion engine is a heat conversion device. It takes the latent
heat in gasoline and convents it into motion at the crankshaft, ready to be harnessed to do work. Q. How much octane do you need? A. Just enough to
prevent knock which is sometimes called “spark knock.” That is because another way to reduce knock is to retard the spark, that is, to slow the
timing. Or alternatively, you can enrich the mixture by “choking” the air flow, but now, all chocking is done automatically.
GM and Dupont learned adding lead to gasoline also retarded spark knock or ping. They created the Ethyl Corporation which marketed tetraethyl lead for
75 years. before we discovered the hazards of lead. Octane is now mainly determined by the refining process. I believe Standard Oil of Indiana's
American Oil Co - Amoco - was the lone holdout to lead as an octane booster.
[edit on 7/8/2006 by donwhite]