Awhile back, this forum, Honda ATVs, carried a discussion on gasoline octane. Discussion quickly degenerated into an insult-fest, and the topic was locked down.

I hope for a less inflamatory (pun?) discussion.

Fact: A gallon of regular gasoline contains more absolute total specific heat energy than a gallon of high-octane gasoline. The difference is marginal, but consistent. (Readers are invited to prove otherwise.)

What advantage, if any, then, lies in burning higher octane fuel? More about that, later.

Premise: A properly-tuned engine, operating without engine knock/pre-ignition/detonation on regular gasoline, will not produce more power by burning higher octane fuel.

This premise was previously advanced by poster Bill Ciliberti. Unaccustomed as I am to defending Mr. Ciliberti (we've had many exquisite and delightful, but always respectful and civil, arguments!), he's absolutely correct in this instance: PROVIDED NO MODIFICATIONS OR ADJUSTMENTS ARE MADE TO THE ENGINE.

Now for the possible advantages of higher octane fuel. More power is possible from "high-test" if one or more of the following adjustments/modifications are made to the engine:

1) Spark timing is advanced. As every reader knows, gasoline engines typically fire their spark plugs a few degrees of crankshaft rotation before the piston reaches top dead center on the compression stroke. The analog used in an ancient training film watched in the garage was of a hunter leading a flying bird with his shotgun (not the best or most accurate example, perhaps, but it got the point across). The ignition characteristics of higher octane fuel permit more spark advance without the annoying, expensive sounds of engine knock or "pinging," permitting more optimum gas expansion as the fuel burns, delivering more power to the piston.

2) The compression ratio is raised (NOTE: Four-stroke cycle engine performance is discussed here; the principles are valid for two-stroke cycle engines also, but some two-stroke operating characteristics are beyond the scope of discussion.). Again, the flash point and flame front progression characteristics, ba-da, ba-da, ba-da of higher octane fuel, permits higher compression ratios without pre-detonation. Pre-detonation produces an inefficient and very fast combustion of the fuel-air mixture in the combustion chamber, essentially an explosion rather than a "smooth" burn with efficiently-timed gas expansion.

How does higher compression produce more power? Going back to analog city, half-fill a balloon with air, stick it with a pin, and you'll get a "pop." Fill another balloon with the same amount of air, but this time, squeeze it, compress it as much as you can, then stick it with a pin; you'll get a "POP!!!!!" Which puncture involves the release of more energy?

Summary: Higher octane fuel offers potential power increase only if the engine burning it is designed and/or adjusted to take advantage of the combution properties of the fuel. However, if you're convinced your totally stock Belchfire Behemoth ATV runs better with "race" fuel exceeding the manual's octane requirements, I wouldn't argue with you for the world. As a car salesman once told me, "A good deal's a state of mind." Sometimes, I'm convinced, improved performance (in MANY contexts) is a state of mind, also. I certainly hope so!

Now, flame (pun?) me if you must for the ideas expressed in this post; if you have additional or contradictory information, I want to revise and correct my concepts.

Tree Farmer

 

The voice from the top of the mountain in Tibet returns. Welcome!

As for the post, bang on! Those who insist that high octane fuel makes their "designed for regular octane fuel" vehicle (of any sort) more powerful and/or more fuel efficient, continue to amaze me. Glad its not my money being wasted on the expensive fuel! If it is designed for it, or as you state, modified accordingly, then it is required. Otherwise it is a waste of money.

Well stated.

DJ

 

Thanks for the support, Deej! However, I have my limitations.

For example, my "unassailable" logic isn't always persuasive. My wife insists her luxury car MUST have premium fuel; otherwise, she insists the engine "knocks" on the mid-range pushalene consistent with Mr. Ford's octane recommendations.

Never mind that I carefully explain her rig's advanced technology suite includes an "octane sensor," automatically adjusting ignition timing to the fuel used.

Now, when I'm tasked with filling up the sedan alone and unsupervised, do I slickee some mid-range into the tank, 'cause I know better? NOT ON YOUR LIFE! While she might not notice any difference in performance, she'd KNOW I switched fuels the moment I entered the front door. Wives are like that. "You DID remember to use premium, didn't you?" If I hadn't, my stammered answer, my failure to make eye contact, my shuffling feet, trying to dig my toes into the carpet, would give me away. Believe me, I know. I've been married a long time. And I'm too young to die.

Tree Farmer

 

I go along with everything stated as far as no benefit with premium on an engine that does not require it. However, I believe that octane level drops as gasoline ages. Thus, if your ATV sits for a while and you have regular grade gas in it, the octane at the time of use may be less than desirable. Use of premium may postpone this possiblity a few weeks.

 

Treefarmer,
I have heard (30th or 40th hand) that the higher the octane of a fuel the harder it actually is to ignite. This is how it corrects the pinging and knocking from an over timed or under tuned engine. Any response? I really do not know.
(Yes i run super in my wifes car to. I tell her its because she deserves the best. Hehe, Hey she's happy and I to am too young to die that slooowww death of an unhappy wife.)

 

I've said it before, and I'll say it again, stop philosiphizing (sp?) and just buy a single tank full of higher-octane fuel (like 100 aviation fuel, it's only about $2.50/gal right now) and run it for yourself. You will tell a difference, just as I do.

I've got an '86 Ford Bronco with a 460 engine that is slightly modified, and only about 9:1 compression, which is anything but high. I can run unleaded gas with a small amount of pinging. Premium completely gets rid of the pinging, hence according to your logic, since it doesn't ping anymore, I must not be able to use higher octane, right? I've got a G-Tech accelerometer, which can measure horsepower with 1hp resolution and (according to the manual) +-5% accuracy. On Premium gas an average run would show 240hp, while on Avgas it would average 255hp. That's a real-world significant difference, not a philosophical difference.

You are right in saying that higher compression engines will see more benefit from high-octane gas, but you still see SOME increase even in normal engines. You can try and dispute me all you want, but I know the truth, I've tested it myself, and you can't convince me otherwise.

 

Not to cause any trouble here, but is it possible that both of you guys are right? I cannot remember Tree Farmer ever posting something erroneously, or that was just not true, so I will go with his thoughts on the issue, as they also seem to make sense. However, I also have to agree with Motorhead. I had a '80 Chevy Shortbox with a 350 bored out, highrise manifold, cam, etc, etc. It was cranking out around 340 hp with regular unleaded gas (87 oct.), and seemed to do allright. My mechanic told me to try running premium (92 oct.) through it, that I'd get better mileage. Well, I tested two full tanks of regular for gas mileage, and came up with around 10-12 mpg, regardless of city or highway driving. Now, when I switched to running premium, after 3 tanks, I was consistently averaging between 15-17 mpg. My engine had no problems with the regular, and pinging noises at all, I just wanted to see if there was a difference, and if premium was better. In this case, I did get more mileage with the premium, and it was actually cheaper for me to burn premium, than to keep re-filling with regular. Our Suburban is the same way, with a completely stock engine, as is my Dad's Toyota, and my Honda. Now, my Moms Oldsmobile, and my Sisters Pontiac have no difference at all, so they run regular.

As for the 'Cat, the dealer had it filled with regular gas. After I think two tanks of regular, I put in premium on accident, as I was filling the suburban too. Well, both 'Cat's seemed to run smoother, and were a little "peppier", seemed to be more responsive, so We've been sticking with premium since. I've also seen a few guys in the AC forum say that the 'Cats do seem to run better with higher octane gas. Don't know why, that's just what I've heard. So to get to the point, I think Tree Farmer is right with his "general rule", yet as Motorhead states, there are varying cases-engine to engine differences.

Just my thoughts.

Mike

 

I read in dirtbike mag that you will get the most power with a very slight amount of spark knock, so I thought hmm, ok and bought 5 gal of 105 octane low lead avation fuel, this is what i found when I threw it in my 86'200SX
1. cool smell
2. cooler running
3. bad idle when run hard (after full RPM hill climb)
4. "felt" like more power on the top end-
The only reason I could think of why it had more power on top end was that it was runing cooler and the fuel-air mix was more correct than with the 87 octane fuel.. but the difference was very slight.. OR maybe it was makeing less power everywhere except top end? I dunno but I have ridden with people with the same bike/quad as mine who use 105 octane and I can stay right with them. When I build a high comp engine I use just enough octane to keep her from pinging, otherwise 92 is enough. I am not bashing race fuel, there is no reason not to use it if you want to Just dont use the leaded stuff like me, I found that leaded avation fuel reacts with honda's special coating they put on the valve face and it will cause valve falure, I read this info in the climber manual and dirtbike mag. Joe

 

tell me if im wrong, but the higer the octane, the slower it burns, and thats why in 2 strokes it really does not matter unless you are running a stroker.

 

one more thing, as for avionic fuels. they are for airplanes, where there is less oxygen to burn, and they will basicaly melt your engine if ran at sea level. just a word of caution!