Thanks, GoNavy for a great post!

Don

I personally ruined three engines in the mid 80's and early 90's by putting super into them. What happened was carbon deposit buildup inside the cylinders because higher octane is a slower burning fuel and leaves more residue in an engine not designed for slower burning fuel. Carbon deposits are a major cause of pinging or knocking (also known as detonation). The carbon deposits heat up and retain heat, like an ember, and ignite the fuel prematurely.

In case you didn't know, detonation is very bad for an engine. What normally happens is a clean burn along a smooth wavefront that generates nice even pressure on the pistons. Detonation is not smooth, usually has multiple uneven wavefronts, and puts tremendous stress on the engine because you don't get smooth even pressure on the pistons.

Every car I owned (and they were all bought new) developed real bad pinging after about four years of having super run through them (it takes a long time for those deposits to build up and I am a low mileage driver). After my third car in a row with serious pinging problems I began to suspect it was something I was doing and started asking questions.

I found a petro-chemical engineer 's website that explained to me what happened and why I should generally use the grade recommended for the engine by the manufacturer. He said people like me were victims of oil company advertising (and he worked for an oil company ironically).

Carbon deposits actually reach a state of equilibrium in all cars after about 20k miles according that that engineer. The deposits flake off and are replaced by new deposit material. What you want is a steady state with a low level of deposits, well below the threshold of causing knocking.

I switched to regular after that and my cars have had no knocking problems.

In an interesting side note:
Those cars when they entered the knock phase became very sensitive to brands of gasoline. One brand of gas (and I only tried super) would knock much more than another. Shell was the worst and knocked like crazy. Chevron was the best, the least knocking brand that I could find here in CA. That engineer went on at length about different formulations of gas that could yield the same octane rating. In a nutshell he basically claimed that cheap gas was just that, cheap gas. Other than that, it was way over my head.

Things I remember that will cause carbon deposit buildup:
o - too high an octane rating in your gas.
o - cheap gasoline (lack of or poor detergent and additive packages), virtually all major brands of gas today have decent packages so this is a carryover from the old days when not all of them did.
o - an engine that burns oil will usually have high carbon deposits, usually these are old engines anyway, and using a grade of gas higher makes sense in older engines to nurse a bit more life out of them before a rebuild.
o - regularly not letting the engine warm up fully.
o - driving a car too gently. A little leadfoot and high rpm's every once in a while helps the carbon deposits to flake off. Only helpful after the engine warms up.

In retrospect I was guilty of three of the above, wrong grade of gas, I drove very gently, and my commute was under three miles, resulting in a rarely warmed up engine. Merely switching gas was enough to cure the problem however, I still drove gently and my commute was still short for a long time afterwards.

It doesn't get that cold here (40's at worst) and my HCH-I has never knocked. I use Chevron regular exclusively. It is the most expensive gas yes, but I learned my lesson the hard way, and my HCH doesn't need much gas anyway. :-)

Interesting Thread and good information.

If i read between the lines the general concensus is that Regular 87 Octane gasoline is quite suitable and increasing the octane will not create a noticeable difference in the FE and furthermore there is some risk of actually hurting the engine rather than helping.

Is that a fair assessment?

I was wondering if I should use Esso Extra instead of the regular gas and was trying to research the implications but found no real answers.

When would someone want to use higher octane? Only in high performance vehicles?

More thoughts would be welcome!

To sum up, you use high-octane gas only when you're told to. The Civic Si, for instance, is supposed to be filled with premium. The other Civics are regular 87-grade.

No. Only if the manual recommends it. Or if you know that your engine is knocking, which a modern engine (read: electronic fuel injected) should rarely if ever do.

I've never heard of kmh's situation, but it does make sense, particularly if the octane booster was not ethanol/ fuel had no detergent and since he has such a short drive/cold engine. Bad coincidence of circumstances.

If you do some web searching on octaine, you'll find it's basically there to keep the gas from exploding under pressure for high-compression engines. It's all about the compression ratio of your engine. High compression engines need higher octaine ratings so the gas/air mixture can be under more pressure and not explode. Because these are usually high-performance cars, higher octaine gets associated with high performance, but that's not the case. Putting 91 in an engine that calls for 87 is not a good idea over the long run.

It's been discussed quite a bit. A lot of us have experienced bad gas (from the same station we use all the time), or bad gas from a station we don't normally go to. A lot goes into it - water, age, etc. I think your best bet is following manufacturer's recommendations. A lot of folks here hypermile on regular old 87 octaine gas. Your technique and environment will matter a whole lot more.

*** Retired after 65,000 outstanding miles ***

thanks gonavy and kmh3 ...

it is nice to learn more technically what is happening.

I will be sure to use proper gas from now on, I never thought about it in the way you had described and i have heard of the high rpms occasionally to 'clean' the combustion chamber, usually just taking a short run on an interstate from one exit to the next would help.

Thinking about that article again ... lubricants in fuel? to help reduce wear and increase efficiency? Wouldn't there be a problem with your piston rings if there was enough blow-by for those to have effect? And wouldn't the general properties plus the cleaning additives of fuel counteract any beneficial qualities of the fuel lubricant? I have found that fuel in your oil will definately break down your viscosity ...
(side note) and why it is also imperitive to change oil regulary in a rotary because some inevitably gets in much more often than a piston engine as well as designed to burn a little oil at a consistant rate at certain rpms to keep the apex seals lubricated. RX (rotary) owners who don't read thier manual will run the engines low or get upset with this quality ... which also is the reason RX's have had problems in the past with reliability ... not the engine's fault, but the owner's ignorance/naivety ... lack of proper maintenance with ANY engine will cause failure - especially bad/low oil.

So you say Chevron is an overall better fuel for you personally ... I have always liked thier fuel myself, but Shell has been rather good as well, and I do think they are all the same except for maybe the lowest priced place may have the lowest quality. Even though states test the fuel quality and pumps for accuracy, it is only once a year, so I always wondered if they put what they like and charge you the same lol I know MOST places, mid-grade fuel is a mix of low and premium (except for the ethonol 89/90 E10) ... you have to trust the owner to set the ratio correctly.
It is nice to know they don't depend on the middle east, the more sources of crude oil the better for us all.
Speaking of the E10 gas ... So that also would be bad compared to 87? I don't have the opportunity to buy it any longer, Florida doesn't have it, I did use it when I was in Kansas. It was cheaper than the 'staight' 87 and they claimed it was better for my car, better mileage, and better for the enviroment. Just curious.
Also, isn't the fuels in general in other countries like (Japan) uses overall levels higher octane than we do here? I was just curious about some things

Hoo boy, tell me about it! Just the other day, when I ... huh? Oh. Never mind.

Uhhh, was an elevator, or the bed-covers involved in the story you almost told?



Don

Wow - I walked into that one!

*** Retired after 65,000 outstanding miles ***