100 octane is the most popular piston engine aircraft fuel. The vast majority of these planes run just fine on 87 octane car gas, or even the old 80 octane aviation fuel. Most of them are big, low compression, conservatively timed engines. Simply no need for a high octane rating.
It isn’t necessarily true that if an engine has a low compression ratio, high octane fuel isn’t needed. For example, large radial engines have great, big cylinders. In fact, they are so big that two spark plugs are used to get ignition in two places so as to get even burning of the fuel. When the fuel first ignites and burns, the temperature and pressure of the part that has ignited shoot way up. It immediately begins to expand thus further compressing the unburned fuel which, if the octane rating is too low, can easily detonate.
This detonation is quite destructive because the cylinders and pistons are aluminum and pieces of metal are knocked off. The same erosion happens to aluminum automobile cylinder heads if pinging (detonation) is persistent.
David Simmons, you may be interested to know that Porsche once pulled a significant (as in maybe a 5%) increase in performance on some of their cars back in the late-80s/early-90s by porting an extra spark plug per cylinder. They’re using essentially the same incremental design principle on the new engine blocks and I wouldn’t be surprised by the introduction of the same theory here soon.
But as far as my own test goes, this is where I, as the driver, meet that psychological barrier which I probably cannot control no matter how objective I pretend I am.
If I expect an increase in performance, there’s no way I’m gonna notice that extra millimeter or two that I put on the pedal in similar conditions. When I was mashing it to the floor, I can’t guarantee that I wasn’t pushing harder on the firewall, and getting a little extra out of the flex of that. Whatever difference I noticed is probably best articulated as “something was different,” but even that might be ascribed to the fact that I was paying closer attention. It certainly wasn’t worth the extra price, except that Premium is cheaper in West Virginia than Regular is in the Playground of the Damned.
(True story: My old VW Rabbit had a half-inch tall restrictor bolt under the accellerator pedal. Removing that was the finest performance increase I’ve ever pulled out of a car I’ve owned. Hmm. Something to add to my TTD list tomorrow…)
What interests me is the perceived low-end loss of power that I thought I experienced. Is this even possible? And even if it is, would it be noticeable? I have lots of reasons to doubt my own impressions. Sobriety, unusually, is not one of them.
110 octace exists. I’ve got a friend who drag races his 70 Charger competitively, whenever they race, they use Airplane gasoline at 110 octane. Just an FYI 
I know, but Brad_d said he thought that 110 octane was the commonly used aircraft fuel. I just wanted to point out that perhaps he meant 100, since 110 is relatively rare.
I have to disagree with this a tiny bit. More complete and even burning is a big side benefit of dual plugs, but the actual purpose is safety. Is a Rotax 912s great, big cylinders the reason it has dual plugs?
I’m glad you mentioned this, I haven’t seen anyone else bring it up in this discussion yet.
They aren’t the only ones. The mighty 1989-1997 Ford Rangers had dual plugs in their 2.3 liter engines. A few Nissans did too.
Noone said it didn’t exist. It’s just not the commonly used aviation fuel.
We’ll have to get an engine specialist in here. When I was taking ground school on “Aircraft Engines” as an Aviation Cadet in 1943 we were taught that the two spark plugs were not for redundancy but were necessary to prevent detonation. I can’t think of any reason why the Army would have a reason to mislead us about that point.
I never have heard of a Rotax 912 so I don’t know anything about it.
Interesting… Maybe the reasons have changed? During your ground school in 1943, airplanes had huge radial engines, and needed better combustion MORE than they needed the safety factor (which was often provided by having more than one engine).
I took most of my reciprocating engine classes in 2000 (still working on my aerospace degree) and I was taught that dual plugs were for safety. This is probably because newer airplanes have smaller cylinders and are more likely to have only one engine.
Seems likely to me that dual plugs started as a performance measure (with safety being a side benefit) and are now a safety item (with better performance being a side benefit).
The FTC has forced oil companies to stop advertising that higher octane gas than recommended for your car will give you better performance. The last link from the FTC says:
Some notes
1> Octane number is measured in ASTM-CFR (knock) engines (http://www.mech.kuleuven.ac.be/tme/museum/engines/other/default_en.phtml#05) to establish octane values of various fuels. When blended together, the octane value is linear.
http://www.cpchem.com/specialtyfuels/reference/applications.html
2> Technically, there’s no such thing as Octane Number >100. However, you can say “a gasoline with a performance number of 110.” http://www.prime-mover.org/Engines/GArticles/octane.html
3> Octane number measurement for Aviation and Automobile fuels are different. Especially, when the performance number is greater than 110.
What gazpacho say’s above is true, with special emphasis on this part of the quote;
“Your best bet: listen to your owner’s manual.”
Which doesn’t mean you should use low octane gas in a engine designed for higher octane. You can harm your engine if it doesn’t compensate. And even if it does, you will lose performance.
All this guessing, and the answer is right there in your manual.
Peace,
mangeorge
Thanks for the correction, Joey G - I was digging into some vague memories, so I’m not to surprised that I was not correct.
I drive a 1997 Mazda B2300 (which is made by Ford and is identical to the Ranger); you’re right about the dual spark plugs, too. It’s got a 2.3-liter 4-cylinder engine with two plugs per cylinder.