Gotta love the internet, Facebook group rant Options

[IronHillRestorations]

The first time we had a Renegade Festival in Hot Springs (2nd mid west event), I lucked up and got a tech rep from BG Petroleum Products to give us a tech session. BG (was Wynn's Spitfire) makes fuel and oil additives. Their fuel additive is very effective for helping remove excess carbon build up. This is especially a problem in Japanese cars, or at least it was 20 years ago. He had quite a bit of technical data and, was there for a Toyota, or Honda tech summit of sorts. I saw him getting stuff out of the truck with a big BG on the side and started asking questions, and he graciously agreed to give us a tech session.

The guy did a cool "Mr. Science" kind of demonstration showing the difference between regular and premium gasoline. The demo showed that regular gas burns faster and cleaner than premium. He put a small eye dropper of each fuel on their own white Corell saucer. The regular burned without a lot of smoke and left a little residue. The premium burned slower, made noticeably more waxy like smoke, and left much more residue on the plate. Part of the point is to demonstrate that gasoline octane is the opposite of what most people think, "high test" or "the good stuff" is less volatile and has more additives. High octane fuel is the same fuel as regular, but with an additive package to lower the volatility, raising the octane for higher compression engines. He went on to say that you should use whatever your auto manufacturer recommends, and if no solid recommendation the general rule of thumb is to use the lowest octane you can, without getting pinging, or pre-ignition. Using high octane fuel in an engine that doesn't require it, can result in more unburned fuel, that can cause carbon build up in the combustion chamber, raising compression, and thereby requiring you to use a higher octane fuel that adds to the problem. Make sense? Are you tracking with me?

Most all of the guys there had questions, as well as their minds changed about the best fuel to use, in any car.

Someone posted on one of the 914 FB groups asking "what octane fuel should I use". My reply was based on my own experience, but mostly on the info from the BG rep: "Unless the mfr or engine builder says otherwise, you should run the lowest octane you can as long as you don't get pinging, or run on. Octane additives typically translate into carbon build up in the combustion chambers." Good grief, you'd have thought I said there's no Santa Claus, or our 914 is just a VW.

I've been around these cars for a while, and cringe whenever I get called an "expert" or "guru". Part of the great thing about our internet 914 community is we get to share insight and experiences both helpful and otherwise. Most everyone of us that's been elbows deep in a project has something good to share, even if it's "man I really messed up" But some people have to tell you how wrong you are, even if you aren't far off the mark, I don't get it (IMG:style_emoticons/default/blink.gif)

[Chi-town]

Combustion chamber design, spark plugs location, intake runner design, spark timing, spark length, etc.

All of these things come into play with combustion, detonation, and all the wonderful things that happen in our motors.

Saying "you should always run the lowest octane possible" is just a dumb statement without knowing the specifics of an engine or tuning done to it even on a basic production vehicle.

Modern cars are always advancing ignition till knock occurs and then making corrections to achieve optimum performance. This is a constant process and the ECU is always learning.

In an older static timing setup like a 914, your timing can only be advanced as far as the fuel will support. Hence why putting in 100 octane in a car tuned for 91 will usually net you no gain and may actually cost you power.

Then there's additive packages and oxygenation of fuel and what they do to burn rates and deposit buildup.

There's far too many variables to make blanket statements like he did

[bbrock]

Saying "you should always run the lowest octane possible" is just a dumb statement without knowing the specifics of an engine or tuning done to it even on a basic production vehicle.

Is there an example of when running higher octane than is needed to prevent pinging for that particular engine would be advantageous? I thought the sole purpose of octane was to retard detonation for the purpose of controlling pinging. Not trying to be argumentative, just want to learn.

[jd74914]

Saying "you should always run the lowest octane possible" is just a dumb statement without knowing the specifics of an engine or tuning done to it even on a basic production vehicle.

Is there an example of when running higher octane than is needed to prevent pinging for that particular engine would be advantageous? I thought the sole purpose of octane was to retard detonation for the purpose of controlling pinging. Not trying to be argumentative, just want to learn.

That's not quite the right way to think about. Modern engine controls can operate up to the bleeding edge of what the supplied fuel can provide to maximize performance, efficiency, etc. It really depends where you are in the map (ie: low load, low torque demand, high engine speed you may be shooting for max efficiency vs. high load/torque demand where you want power). They maximize performance based on the fuel characteristics (and air, engine temperature, etc. characteristics for that matter)

When I say operate at the bleeding edge this means that they manipulate spark timing up until the point detonation occurs and then back off. That means that you could put 87 into your STi instead of 93, but it will retard timing to prevent knocking. Just pulling timing might be a somewhat simplistic way of looking at it-the controls may be changing ignition timing, valve timing/lift to modify cylinder pressure (higher pressure lowers autoignition delay times), boost levels, etc. This could all be based on a measured knock limit, with changes happening after one knock event is detected. Or a misfire, etc.

Hopefully that answered your question in a somewhat roundabout way.

Edit: Just some food for thought: When modern engines have a single misfire, they have "missed" emissions requirements for literally the next tens of minutes to hours. Just one. That's how tight the controls are. I wish I could find the presentation that showed these figures-fascinating for people who like numbers.

[bbrock]

Hopefully that answered your question in a somewhat roundabout way.

Well not quite. If I'm understanding, that's a great explanation of why modern engines are much more flexible in that they can perform optimally with a wider range of fuels. In other words, modern engines are able to adjust to maximize performance for whatever fuel they are supplied, within limits of course. I'm wondering specifically about when the advice to run the lowest octane possible would be wrong. Seems like that would mean that in some cases, adding octane above the lower limit needed to prevent knocking for a given engine would increase performance, efficiency, longevity, or reliability or reduce emissions. Given that lower octane gas is almost always the cheaper option, is there anything else that octane does besides reduce knocking?

[jd74914]

Well not quite. If I'm understanding, that's a great explanation of why modern engines are much more flexible in that they can perform optimally with a wider range of fuels.

The issue is that it's a local optimum; not a global. The global optimum exists around their design point

I'm wondering specifically about when the advice to run the lowest octane possible would be wrong. Seems like that would mean that in some cases, adding octane above the lower limit needed to prevent knocking for a given engine would increase performance, efficiency, longevity, or reliability or reduce emissions. Given that lower octane gas is almost always the cheaper option, is there anything else that octane does besides reduce knocking?

Octane is literally a measure of knock resistance (how this is referenced changes when talking about MON or RON). Assuming the same fuel type, each gallon has the same amount of energy in it. There can be some variance is how they ignite (single stage, dual stage ignition), but that's getting really complicated.

It's really all about what the manufacturer recommends as they've designed around it. Their recommendation in many cases is likely the minimum octane required to statistically prevent knock without intervention for their given baseline engine design. They decided a certain fuel was necessary either a) because they absolutely need it, or b) because it allows some engine performance characteristic which is worth more than the difference in cost.