If you are pinging....your timing might also be off.
The newer engines (2000 up? Not sure when) have anti-knock/ion sensing in them and this doesn't apply, unless the computer is bad.

If you have a cap and rotor, you DEFINITLY don't have anti-knocking technology.

Higher octane is made to resist the "Dieseling" pre-ignition found in high-compression engines. Just like a diesel, if you have a high compression engine
ò Hot engine + extra heat due to extra high compression in performance cars = self-ignition.
Ignition is delayed with the higher octane.

I read a good review of it I'm too lazy to look up again...but I did learn that "R+M/2" means "Reasearch Test Octane Level + Mechanical Test Octane Level / 2" "Bad Gas" might have an R octane rating of 100, but the Mechanic tested it and found it knocked at 80.....(R+M)/2 gives it an octane of 90. But mechanical is the test your engine does, so it's acually pretty crummy gas.

If you have a "regular" engine run regular in it and check your timing. Things that can throw it off include heavy load, heavy load uphill, steep uphill, and high temperature. And in some cases, wear/old age.

When I'm in phoenix and pulling a load (and know I'll be hitting hills) I use premium. In general, I also advance the timing slightly in the summer to prevent pinging/knocking.

That's a 93 Amigo, 160k miles, almost always regular unleaded.

I don't want people getting the wrong idea here, Premium Octane gasoline is NOT detrimental and will not cause carbon build up as compared to a lower octane gasoline.
We use more of the "premium" components when making "Premium Gasoline" aka Alkylate. And thus less of the "junk" components. It is better with gum, carbon build up, and pinging. It has better heat stability and so on.
[color:"red"] Now the bottom line. Away from the lab.[/color]
If your engine is not pinging;
Do you need to run "Premium"? [color:"red"] NO! [/color]
Are you wasting your money? [color:"red"] YES! [/color]
Like many others stated run the lowest octane rating that doesn't ping in your area.

Knockman-

I assume you must work for one of the large greedy oil cartels...oops, I mean philanthropic oil companies. <img src="/forums/images/graemlins/grin.gif" alt="" />

While I would say that it may not and is probably not drastically detrimental, it remains that higher octane burns slower and has a lower amount of energy per unit. By burning it in an engine that does not require it, it is an inefficient (or less than possibility efficient) process. Agreed? Therefore, there is unburned or incompletely burned percentage of fuel remaining after the combustion cycle. This will deposit somewhere. It can end in or out of the combustion chamber. Over time, this may produce some negative effects.

Now, before I start sounding like chicken little, I must clarify that this is not an instant occurrence. One tank of 104 octane gas in your 84 Escort (unless it is a diesel Escort) will not cause it to blow up. Use of the higher octane gas is not the preferred practice, and it may even contribute to a lower performance of the engine because of the slower burn. i.e. The air/fuel mix burns too late in the compression/ignition timing cycle.

As for additives. I would have to disagree that higher octane gas is "better" or "cleaner." All gas gets a set amount of detergents and additives as required by the gov. There are a few proprietary additives out there that may be added to the upper level gasoline mixes. I believe that Techron appears in all grades of Chevron gas now. If you are using a well known brand, from a well volumed station on a regular basis, there is little to no benefit from using a higher octane gas.

Here is a bit of interesting reading. There is a ton more out there.
http://theserviceadvisor.com/octane.htm
http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm

Michael

I am 100% sure that when I use 89 gas I have noticably
better gas milage than with 87 gas. But I see no benefits
when I move to 91. Seems like my engine uses 89 gasoline
more efficiently somehow. My engine is new and has no
deposits.

Oleg
Rodeo S V6 4WD 2002

Ive noticed the same thing
i have a 98 Rodeo LS with 88k and the 3.2 DOHC
and I get about from 16-16.9mpg on 87octane (depending on how hard i push her) to 17.1-17.8 on 91 octane but i also get 17.1-17.8 on 93 so I have chosen to stick with 91. <img src="/forums/images/graemlins/smile.gif" alt="" />
good luck

Sorry Michael,
I should have gone into a little more detail about my situation. I test all different grades of fuels from the "mountain" 85 octane to the "sea level premium" of 94 octane to racing fuels at >110 octane. These fuels are "pipeline samples" meaning there are no additives per brand these are put in the truck from the terminal to the station. So when I stated the Higher Octane fuels are cleaner/better quality it is before any additives. I haven't heard of any detergents or additives "required by the gov". Nearly all gasoline is taken out of a spongeable pipeline system to terminals then treated with the brand specific additives and sold at the stations. Most un-branded stations use no additives at all.

I meet with the "experts" twice a year in regional and national meetings. These are the ASTM meetings for gasoline and octane. I called two of them to verify my statements about the higher octane not being in anyway detrimental. They stated the same thing; The higher octane fuel (unleaded) does not burn slower or not as completely as the lower octane and does not cause carbon build up. They did mention that any higher octane fuel branded as racing fuel (airplane gas) has small amounts of lead in it, and will if ran in a normal engine will burn your valves by.
So, IMHO it is OK to run 91 Premium instead of 87 Regular gasoline in your engine, But the one thing we all agree on is that: it is a waste of your money to buy any fuel above the minimum octane required to make your engine not ping.

Just to twist the topic a little, and since there are experts in the room:

Aside from proprietary additives like Techron, is there a functional difference between different brands of gas?

I was told that ARCO gas contains more ethanol (I think it was ethanol?). The informant went on to explain that the ethanol stripped the gunk from the inside of your tank and sent it through the engine.

[color:"red"] 85 [/color] <img src="/forums/images/graemlins/kewl.gif" alt="" />

As far as the additives, we test the fuels before they are put in. I try to use a "Major" station that contains an additive. I wish there was an industry comparison on which additive was best, but I can't find one. Anyone?

As far as the Ethanol, the general opinion is to use no more than 10% in gasoline. The station selling the ethanol mix is required to tell you it contains ethanol, but unfortunately often doesn't give the percentage.
[color:"blue"] Use caution, [/color] If you have a good old Isuzu with say 100k or more and decide to try a tank of ethanol mix. Be prepared to change the fuel filter on the side of the road. Over time your gasoline tank will accumulate moisture, and everything will be working ok until you introduce the ethanol, which will be soluble with the water/dirt/rust mix and send it down the line plugging your filter.
IMHO, the perfect time to switch and start using some ethanol mix is right after you have changed the fuel pump/cleaning/flushing the fuel tank. Then use the ethanol mixed fuel at least every third or fourth tank to assure there is no major build up of moisture. I have followed this idea for the last 100k on my 89 after changing the fuel pump. <img src="/forums/images/graemlins/kewl.gif" alt="" />
Good Luck.

Just one last thing about gasoline and certain stations brands being better than others. Any of you live near a tank farm? I do... less than a mile. I see ever kind of truck from Shell, Marathon, QT, BP, Sunoco, among many many others come in and out of that tank farm. They all get their gas from the same place. The Exxon station gets its gas from the exact same tank farm that the mom and pop no name station gets theirs. Where the quality of the gas that you put in your tank comes in to affect is how the underground tanks at the individual stations are taken care of and what kind of tanks they use. Some stations used steel tanks buried in the ground. They tend to collect condensation and guess where that goes? Right in your tank... the better stations have insulated tanks and this doesn't present a problem. Also, the busier the station is, the faster the gasoline gets restocked. This means that the gas isn't sitting in a tank collecting moisture. Some place like QuickTrip is undoubtably going to have less moisture and contaminates in their gas than the little mom and pop gas stations that only refill their tanks every week... whereas QuickTrip refills theirs daily.