Better mileage at 87 octane compared to 91

[Clay350]

I do not see how 91 would give anyone less MPG or hurt performance. I know there are other variables but I always understood that higher octane would burn cooler which is important for high compression or high strung engines. To me Yamaha Recommends 91 octane looking at the manual?

 

[fb40dash5]

I think this might be true as a general rule, but when I think of the many chemical species that go into gasoline, there are lots of exceptions, so you can't bank on it. It just depends what the refiner is blending.

As far as ethanol goes, think of it this way: We get energy out of fuel by reacting it with oxygen to form carbon dioxide and water. Since ethanol already has some oxygen in it, you can think of it as being partly burned, so there's not as much energy left to extract.

The advantage of burning ethanol and other oxygenates is that since they carry some oxygen along with them, they reduce the amount of carbon monoxide formed during combustion.

I always heard the "higher octane, lower energy" rule explained as higher octane fuels being further refined vs. lower octane, and of course you can't refine more energy into the fuel, you can only reduce it. If they're increasing octane with different chemicals rather than refining, that could be out the window.

Either way, higher octane than required doesn't help anything at all. An engine is designed not to knock on "x" octane gas, and unless you alter effective compression, there's no point in exceeding it.

Ethanol is stupid as a mass-market fuel. But, I used to have a WRX that I'd run half premium and half E-85 in at the dragstrip. Poor man's unleaded race gas, if you've got the fuel system to spray enough of it!

 

[Dave in Houston]

I always heard the "higher octane, lower energy" rule explained as higher octane fuels being further refined vs. lower octane, and of course you can't refine more energy into the fuel, you can only reduce it. If they're increasing octane with different chemicals rather than refining, that could be out the window.

Either way, higher octane than required doesn't help anything at all. An engine is designed not to knock on "x" octane gas, and unless you alter effective compression, there's no point in exceeding it.

Ethanol is stupid as a mass-market fuel. But, I used to have a WRX that I'd run half premium and half E-85 in at the dragstrip. Poor man's unleaded race gas, if you've got the fuel system to spray enough of it!

In general, if you make the material more dense, you increase its energy content on a volumetric basis. A refinery has a wide range of reaction and distillation processes. Some of them will make the materials more dense and some will make them less dense.

Crude oil is made up of certain chemical species. Many of them are low octane, so they have to be put through some reaction processes in order to make them suitable for use in gasoline. Then they become other chemical species. That's part of the refining process. And everything that goes into gasoline is a chemical of some kind.

Most people don't realize it, but another one of the real disadvantages of using ethanol in gasoline is that ethanol is hygroscopic, meaning that it soaks up water. Because of that undesirable characteristic, ethanol can't be blended into gasoline at the refineries.

Instead, the refiners make something called "RBOB" (regular base for oxygenate blending) and PBOB (premium base for oxygenate blending). These will meet gasoline specs when 10% ethanol is added. The RBOB and PBOB are sent to terminals, usually by pipeline. The ethanol is shipped by truck or rail because it would suck up every drop of water in the pipeline.

Then RBOB or PBOB and the ethanol are "splash blended" right in the tank truck.

Since the ethanol has to be shipped by rail or truck, isolated shortages can occur, causing short-term price spikes.

So you are right about ethanol. For many reasons, it makes crummy gasoline. It does reduce CO emissions, but it also makes billions of dollars for the farm conglomerates like Archer Daniels Midland.

My understanding is that the refiners would be perfectly happy if the requirement to blend ethanol was revoked.