Motorhead, you say,

"Ultimately any engine will perform better with higher octane gas."

Really? ANY engine? Without adjustment (e.g., spark advance, manual or automatic) or modification (e.g., higher compression/greater volumetric efficiency)?

Interesting. While I have some idea how higher octane fuel's burning characteristics can develop more power with optimum ignition timing and higher compression operation, I don't understand the mechanism universally improving perfomance in any and all engines; surely a logical explanation exists.

Tree Farmer

 

How about throwing this one into the pot. This is how it was explained to me by an engineer. Yet another variable... the long stroke engine will also derive more benefit from high octane fuel.
For comparison lets look at Blackpowder VS modern Gunpowder. Blackpowder burns slow, hence the longer the gunbarrel the more thrust the bullet will receive until the powder is fully spent. The bullet is pushed all the way out of the barrel as the powder keeps burning. Modern Gunpowder burns very quickly and thus gunbarrel is not as important for muzzle velocity. It is an instantaneous blast that propels the bullet out of the gun barrel.
High octane gas, burning slower, actually pushes the piston to the bottom of the cylinder as it burns(more torque, similar to deisel fuel which is even slower burning) where as lower octane gas burns faster giving the piston a blast to send it on its way (higher velocity maybe, but less power).
I hope this helps.
I can only go by my experience and the facts that I know to be true. My old (77)truck with a simple electronic ignition system, 8.5:1 compression and a carburetor will show the best results of premium fuel, over regular, when pulling a large trailer especially up hills. 98 Dodge Ram owners manual also specifies using Premium fuels when pulling heavy loads.

Jeff

 

How about one more variable to throw into the mix! What if the "octane rating" of fuel is raised by the use of "oxygenates" which do not contribute to the energy of the fuel. Wouldnt this result in a negative impact on power output? Many places require the use of these additives and seem to lessen power and mileage while supposedly improving emissions. What are your thoughts on this?
Just some more fuel on the fire!

 

Trailboss450
The high octane fuel I am refering to does have oxygenates, (10% ethanol, which is a type of grain alcohol I beleive) at a octane rating of 94 for premium, and 90 for regular with ethanol where regular without ethanol is 87. These oxygenates do not show any power loss over regular premium fuels, but they burn cleaner which to me would mean less waste and a better burn.

A point I forgot to ask about was that someone posted that high octane fuels burn cooler. Is this true? I have seen engines, that were run on avgas or with too much octane boost, which melted valves because the fuel was too hot. If high octane runs cooler what then would cause this situation? (We use to use a percentage of avgas or octane boost in our street rods especially on drag night even with stock engines)

Jeff

 

I've got a Ford Bronco with a 460, those are two things that were never put together at the factory. The 460 is carbureted (Edelbrock), with a Mallory vacuum advandce distributor, no electronic ignition box, no EFI, just a good 'ol carbureted big block. You've had two people on this forum state that higher octane gas gave them better fuel economy and more power, in REAL WORLD tests. You haven't cited any real world tests so far. I think that alone prooves my point.

 

Bigred450 explained it before I got the chance. Higher octane usually translates to the fuel contains more long hydrocarbons, which take longer to burn. This causes the fuel to burn, producing power, through more of the power stroke of the engine. You talk as if there is a maximum octane rating for a given engine, and a sharp cutoff at that point, where no more power can be made. This is NEVER true in real world situations. The curve is more of a logrithmic curve (increases sharply in the beginning then slowly levels off), with the x-axis being octane level and the y-axis being power produced. The curve begins to level off as the octane gets above 90 on a normal engine, but it's not a sharp cutoff to a horizontal line, it's a gentle rolloff, meaning higher octane will yeild less improvement, but an improvement all the same.

I'm sorry, I didn't realize you had already talked to those "guys in the white coats at the motor fuels lab", so they told you that you were right and I was wrong? Bull****. And don't get all cocky. This has, thus far, been a civilized conversation. Don't ruin it. Until you go out and try it for yourself, you have no idea what your talking about. Once you try running higher octane gas, THEN you can talk all you want.

 

Ok, here is my view on this subject. First, the differences between a lower octane fuel and a higher octane fuel. A lower octane fuel is designed to be run in stock or even modified engines. Just because the motor isn't stock doesn't mean you can't use it in a built motor. Take for example a 351 Windsor stroker engine, if you build it with a different length rod and piston combination, aftermarket heads and even with 11:1 compression, you could run 87 octane through it with no problem, and easily put out over 300 hp. I am going to agree with Tree Farmer with his side of the story. A motor will not need to run a higher octane fuel if it runs fine with no pinging on a lower octane, but you also can't hear engine knock at times, a sensor on the vehicle could pick it up, and retard the timing, without you knowing, and with a higher octane fuel, it would keep the timing where it was or even increase it. This if course is on a more sophisticated ignition system, unlike the antiquated systems found on atv's. Now there are many variables to this, none etched in stone, and always exceptions to the rule. The temperature of the engine, the air temp, even the fuel temp could make or keep an engine from knocking if it is on the threshold of more or less octane fuel required. A higher octane fuel, needs more heat and pressure to ignite than a lower grade of fuel. Your ignition system better be up to the task of firing all this "stable" fuel to make full use of it, or else it will just be blowing out your tailpipe. Now, I know the benefits or running higher octane fuel, if the engine requires it, but I would not put it in any of my vehicles just because. I own one vehicle that will run with 87, and it will never see 89 as long as I own it, doesn't need it. Someone mentioned, that a engine runs best with a little pinging, well that guy must rebuild his bike after every ride. A engine does make the most HP at the leanest mixture possible WITHOUT knocking, and every engine has a different bottom line. Now, I have heard of the vehicles sensing the octane, but I don't think it was explained correctly or fully on here. I may be wrong with this, but this is what I think I have had my cars apart more times for racing and I know there isn't a sensor which actually detects the octane in the gas. I do however have a octane rod in my distributor, which can control timing, I am assuming by way of input from the oxygen sensors (showing either a rich or lean exhaust) and adjusting the timing curve in the distributor, with a higher octane fuel, it is possible to run a more agressive timing curve. Which could hurt or help boost parts of the powerband. Here is one exception to what I just said, my girlfriends Camaro. She has a 84 Camaro with a 350 in it, Holley carb. I hate it and we are selling it (Getting a FORD!!!) Now since I hate this car so much, I neglect it, and only fix it if it undrivable. She is able to get free gas from her work, which is 87 octane and that was great. But the car runs like **** on it, when you turn it off it will continue to "run on". Probably due to excess carbon build up on the piston and valves, which cause the left over gas in the cyclinder to ignite when the car is off. Well, she doesn't use 87 anymore, but instead she uses 93. With 93, the problem is gone, less volatile. Later

 

Here's a nice little post on AVGAS in a non aviation engine... FWIW

 

Extraordinary, Motorhead!

You tell ME, "Bull****. And don't get all cocky. This has, thus far, been a civilized conversation. Don't ruin it."

I'm "ruining" the conversation? You're the one resorting to profanity in contrast to logical argument.

Also, "Until you go out and try it for yourself, you have no idea what your talking about. Once you try running higher octane gas, THEN you can talk all you want."

Motorhead, I may talk all I want without your permission, thanks. Regardless of your assumptions, you've no idea what I may have tried and experienced.

Further, if you genuinely believe your sweeping generality that ALL engines produce additional power without limit by increasing fuel octane with no adjustments or modifications because of unspecified and undocumented "real world" tests, and vague "logarithmic" (asymtotic?) curves, you're welcome to the notion.

Maybe you're right!

Tree Farmer

 

Thanks for the link, we can kill two birds with one stone here:

"As you can see, by using straight Avgas or by mixing various types of fuel together you are modifying a number of important fuel design parameters. You may hit on a combination that works well , but more likely you'll have an engine that doesn't detonate, but doesn't accelerate very well either. So avgas is SAFE, but not a very good choice"

So, we've determined two things: Avgas is SAFE, and it may or may not run better. Now, according to every experience I've had with Avgas, it HAS run better, so either I'm a pretty lucky guy or I'm right in what I've been saying this whole time. Either way, I will continue to use it, and benefit from it, and you guys can take that however you want.