I finally finished reading all the automotive engineering manuals that I could get my hands on. It was tough going, but I know a heck of a lot more about engines than I ever did.

First things first.. Octane rating.
Octane rating is a measure of how stable a fuel is. There's nothing magical about the term. 87 Octane is the baseline for gasoline. That said, premium and super unleadeds are mostly 87 octane gas with anti-knock additives. Some fuels are better than others, it depends on the supplier and the source. 91 Octane gas burns no different than 87 octane gas, the only difference is when it burns. 87 octane gas in a normal good working engine works just fine.

Knocking is fuel that is pre-igniting in the cylinder from various factors (increased heat from pressure build up, hot spots in the chamber, etc) Knock robs the engine of power by burning some of the fuel before spark ignites the rest - the sound you hear is a pressure wave(s) hitting the chamber and piston and vibrating it. Knock can cause serious damage to the engine. 87 octane fuel is less stable at high tempretures because it contains chemicals that are less stable - not because it itself is unstable.

As for running higher octane fuel in your engine. 91 Octane fuel doesn't do anything for your engine, it doesn't give it more power, or mileage, or clean the engine. Unless you run a high compression engine it isn't doing anything for you - EXCEPT prevent knocking. That said it isn't hurting your engine either - as I said 91 octane fuel is essentially 87 octane fuel with some additives. Some fuels are better than others though. Now some gas companies put detergents in the fuel, or mix it with alcohol or other chemicals - but that's not for knocking.

Engines with higher miles on them can actually require higher octane fuel. No one in the industry says that you shouldn't use high octane gas, just that you shouldn't HAVE to use it. But as the years roll by engines can and do start to require higher octane gas to prevent knocking.

Modern engines have various components on them to prevent knocking as well - knock sensors, tempreture sensors, etc. When a modern engine detects knock one way it will compensate is by making the mixture richer - this cools down the combustion process and prevents knocking (it also gives the engine less power). Another way is to change the settings on the EGR valve. This all assumes that your engine can detect knock in the first place.

Back to my truck.

When all was said and done I put 300 miles on the truck with regular gas. Despite my griping, bitching, and moaning it turned out to be not so bad after all. It did knock on steep hills at highway speeds - but as I read from multiple sources, this is totally normal and expected. It got the same milage as the premium unleaded gas - as was also told by the engineering manuals. So will I run premium from now on - probably not. I'll stick with 89 octane mid grade gas just to keep the knocking to a minimum. I live in Seattle and there are a lot of hills here.

Now my sources.
I gleaned a ton of information from the 2000 Bosch field manual - an excellent resource by the way.
Some of the chemistry I got from a 1967 book on engine design Theory and Practice.
There were a few Dick and Jane books on how engines work and what the components were.

 

Just to add my two cents, the bottom line is that you need to run a fuel with a high enough octane to prevent knocking. Modern engines will adjust to the fuel by inceasing the mixture or adjusting timing or varying the amount of exhaust gas recirculated, but all these methods decrease power/fuel economy.

The best method to determine what you need to run is to listen to your vehicle. If you have knocking, move up a grade until it is gone. If you're running premium when your engine is designed for 87 or 89, consider trying a lower grade. In NC, it will save you about $.10 for each step down, which adds up.

 

That's a good summary, hits the high points well.

To elaborate a little ...neither the 2.5L, 3.0L nor the 4.0L (OHV) Ranger engine use a knock sensor. So there is no engine knock feedback to the PCM. What this means is that none of these Ranger engines is capable of retarding the timing or adjusting EGR flow when pinging rears its ugly head. It might be that Ford was worried that performance would suffer too much. Or maybe Ford honestly didn't realize that the 3.0L and even the 4.0L would be as prone to pinging as they are. Bottom line: it has pretty much been left up to Ranger owners themselves to address the issue of spark knock/pinging, if or when it becomes a problem, because the PCM isn't going to do it for us.

 

Good info. My 65 Mustang had a sticker on the air cleaner that said to use premium fuel. It had 10:1 compression.

 

Thanks for the research. And more important, thanks for sharing it with us.

 

Actually I was kind of wondering about the knock sensor. Interesting that Ford didn't choose to use it - from what I read the knock sensor is a part of a modern direct injection fuel system and an excellent diagnostic for the ODB II computer.

One more thing to go wrong on the truck I guess.