fuel grades

[71chevylongbed]

is there really any difference between 87 and 89 octane gas. the guy who built my engine said to run 89 but how much difference would it make over 87

[Longhorn Man]

The octain reading is basicly the resistance to ignition.
Run your tank to almost empty, and put in no more than 5 gallons of 87. Drive it in variouse conditions...stop and go, freeway, performance, towing if you can, and if you hear anything that sounds like small rockes on your windshield or a more metalic ping, then you need to go to higher grade of fuel.

[71chevylongbed]

i'll try that and see if it makes a difference

[PureChevy]

i was having a problem with my truck dieseling when i turned it off. Someone on the board here told me to run 89 octane. I used it and the dieseling stopped. I run 89 only now.

[67Fleetsidedream]

I have to run 89 octane or better. My truck diesels bad with anything lower.

[shuttermutt]

Just my $0.02 on the dieseling problems...

Dieseling (a.k.a., "run-on") is caused by something in the combustion chamber being hot enough to ignite the fuel and air without the spark plug firing. There are a number of reasons why this may be happening, including:

- An idle that's too fast or an over-rich idle
- A lean condition or vac leak
- Carbon deposits in the combustion chamber
- Running a spark plug that's too "hot"

Essentially, anything that would cause the combustion chamber to heat up (considerably) more than normal is a candidate for root cause of dieseling.

Now, I'm not going to suggest to anyone that they not run a higher octane gasoline. That's up to each of us to decide based on our experience and how our engines are built. Most engines, however, that have a dieseling problem are trying to tell you something else is wrong. Fix the root cause and you'll eliminate the problem. Regardless, it's worth a shot.

[71chevylongbed]

before i had my motor rebuilt it dieseled and i used a higher octane gas and it stoped it for a while then it got worse i fried the motor a few weeks later and when the engine builder pulled the heads off every valve was burned

[lukecp]

When my truck had low-compression and a pair of open chamber heads on it, i got about half a mile gallon better fuel economy with 89 octane compared to 87. I saved like 50 cents a week running 89, lol.

It also seemed to run a little better. I also ran a tank of 91 through the same motor and got 10mpg, compared to the usualy 11.5mpg. It did seem to run better, but i had my foot in it the whole time, proably the reason for the lower gas mileage.

I now have about a point higher of compression....it will still run on 89 octane, but it will have a hint of spark knock at full throttle, so i only run 89 in it if i a little light in the pocket book, then i keep my foot out of it. It prefers 91, and runs really good on 93
Of course i could back off the timing a little bit and run 89 all the time...

[67Fleetsidedream]

Dieseling (a.k.a., "run-on") is caused by something in the combustion chamber being hot enough to ignite the fuel and air without the spark plug firing. There are a number of reasons why this may be happening, including:

One time I accidently put 5 gallons of diesel fuel in my 93 Mazda 626. ( the pump nozzle was black and it was supposed to be green )
It smoked and ran like a diesel for a couple of days. This was a totally unmolested stock engine and it diesled when I turn it engine off after driving. Explain that one.
Also it ran like brand new car after that.

[71-longbed]

deisel fuel has more detergants i think ... probaly cleaned out everything

[1972C10]

Im runnin some open chamber 993's i cant get away with runnin anything but 91 or better.


Timing has alot to do with it too you can bump the timing down to avoid clatter.

[Bobalouie]

Diesel doesnt have any more detergents than any other grade of fuel, it just burns hotter. Old hotrodders trick to clean the carbon deposits off of your heads and pistons is to pour diesel fuel into the carb with the engine running about 2K rpms. smokes like crazy and cleans everything out. Thats why it ran like a top after you did that.

[WRMZ71]

speaking about compression ratio how would somebody know what type of compression they have. i have to run 93 octane in mine otherwise the valves rattle under heavy acceleration.

[72 CustomCamper]

Diesel only has an octane of something like 42...

Chuck

[Javy]

Diesaling is detonation.
From what I've heard,back off your timing or run higher octane fuel.

[shuttermutt]

Dieseling (a.k.a., "Run-On") is when the engine continues to ignite the fuel/air mixture after the ignition is turned off. This condition is usually caused by an over-rich idle, an extreme lean condition, or carbon deposits in the combustion chamber. Timing is rarely a significant contributing factor here.

Detonation (a.k.a., "Knock" or "Ping") is when the fuel/air mixture ignites too soon and "hammers" the piston on the compression stroke. Often heard when the engine is placed under a load, this condition is usually caused by having the timing too far advanced (spark fires early on the compression stroke) or using a lower octane gasoline in a higher compression engine (heat and compression cause combustion).

As to octane, remember that octane is essentially how we measure gasoline's resistance to ignition. If you have a high-compression engine, you'll want a higher octane fuel since the act of compressing the fuel and air can cause combustion in a hot engine. Also, higher octane fuels often (but not always) will come to complete combustion faster than lower octane fuels. Needless to say, the faster you're spinning that mill, the faster you'll need your fuel/air mix to do its job.

There are quite a few great articles regarding dieseling, detonation, and fuel selection available on the web. Many of these articles go into great detail and are likely to dispell many of the myths about octane, additives, and so forth.

[smokekiki]

72 Custom Camper,the rating on diesel is called it's cetane rating.Usually between 40 and 45 in the USA.The higher the cetane rating,the easier the diesel will ignite under compression,which is a good thing.The higher the octain rating on gas,the more resistance to pre ignition in high compression,which is also a good thing.

[Oborny71]

87 actually makes more power than 93 octane due to the fact that 93 octane less explosive= slightly less power. Is it that noticable? prolly not. but i think a lot of people put in premium (93) octane thinking it makes more power. Now if your talking about racing gas, thats a different story.......

[Cat Fuzz]

Originally posted by 72 CustomCamper
Diesel only has an octane of something like 42...

Chuck

The reason for that is that diesal engine fire by heat and compression only. There is no spark to set it off so it has to combust easily.

[Longhorn Man]

You have a very good explination on the terms here shuttermutt, however, the last part of the first paragraph I have a problem with. Timing CAN EASILY cause dieseling.

Originally posted by shuttermutt
Dieseling (a.k.a., "Run-On") is when the engine continues to ignite the fuel/air mixture after the ignition is turned off. This condition is usually caused by an over-rich idle, an extreme lean condition, or carbon deposits in the combustion chamber. Timing is rarely a significant contributing factor here.

[smitty62]

Higher octane WILL make more horsepower provided the engine is built for it and all other conditions remain the same--ie: driver input. Higher octane has additives to SLOW down combustion, or raise the flash point thereby causing the gasses to expand more slowly, developing thrust or "push" farther down the cylinder wall the piston travels. In laymen's terms, think of lower octane fuel as "hammering" the piston, while the higher octane "shoves" the piston. That's way over simplification, but you get the drift. Putting a higher octane fuel in an engine that does not require it will empty the owner's pocket quicker--that is absolutely ALL it does! If you can run 87 octane without pre-ignition, DON"T make the oil companies richer by using a higher octane. It is ALL refined in the same manner--only additives raise the flash point or octane. Benzine has a much lower octane, but is extremely explosive--get it?
Everything else you hear is usually what their " Daddies" told them and is not based on fact. So is the cliches that "brand "X" runs better in my car than brand "Y". I'm sure this was true in 1920, but not with modern refineries. I run the cheapest and lowest octane I can without pre-igntion. I also build my own engines and NEVER had a problem caused by fuel--Oil change intervals are much more important than fuel.
BTW--Diesel fuel develops more power for several reasons, but primarily because of the inherently higher flashpoint and the compression ignition.