Use the octane thats the engine is rated for. My jeep runs great on 87, and runs very poor on 91. The spark plug gaps, in addition to other factors are matched for a certain octane rating.

You wouldn't run a Ferrari on regular...

scooter47,

No matter what you or your mechanic think, running gasoline with a higher octane rating than your engine is designed for is not good! Your engine is optimized to run on 87 octane. Using higher octane gas is at best a waste of money and at worst can actually cause you to lower your fuel mileage. In a car it causes damage to the catalytic converter due to slow burn causing unburned fuel to be exhausted.

This has been tested over and over ad infinitum by manufacturers, oil companies, private research firms, car magazines, etc.

I don't know why the mechanic says the spark is not hot enough to ignite a higher octane fuel. The higher octane fuel ignites just fine but it has a slower burn rate and requires resetting the timing. As higher octane fuel is for a different compression ratio that would seem kind of stupid.

He is probably thinking that if you are stupid enough to put 91 octane in the 87 octane engine he can easily convince you to buy some unecessary parts for the ignition system and reset the timing.

Your loss, his gain.

Jon

He is not trying to sell me anything. What I am trying to do is sort the BS from fact. I do know in older engines that running higher octane can make it run better. Now that could be because it was designed for it . I am not sure

I disagree with both of you. The spark output with a higher octane gas means it will ignite faster which will make the compression sooner. I've noticed putting racing fuel in my lawn mower, motorcycle or car that it starts faster and runs more smoother. Doesn't save any better on mileage. I beleive it is better on your motor. The only reason they did away with the leaded gas from the 70's was because of emissions, it had more lubricating qualities and it was higher octane than we run today. But also they run higher tolerances in all engines today. I do believe that your E85 gas is a different breed that you can't do any switching. But then again, I think the electronics has something to do with that also.

I agree with you that one reason they did away with leaded gas was emissions. Studebaker engines used the lead in TEL in gasoline to seal the cylinders, and they had real problems when we went unleaded. Dartmouth College (NH) ran a test years ago, taking grass cuttings from fields every 25 or 50 feet in from the highway, and measuring the amount of lead in the grass. Serious lead contamination! What did the cows of that era eat??? Do some investigation on how many thousands of tons of TEL we used for gasoline every year. City streets were heavy with lead dust. I remember in WW II the USAAF tried to really spike up the octane in avgas, and they loaded the gas with TEL. A test P-38 flew a circular course using the higher octane gas, and fortunately it wasn't over the channel when the engines started to misfire due to lead buildup in the cylinders. Then we went to oxygenated gasoline additives....

Actually, high octane ignites at a higher temperature than low octane.The octane rating is actually a measure of resistance to preignition due to compression.The higher the number the higher the temperature required to ignite it.Racecars have very high compression, and need super high octane to resist preignition.It also allows them to advance the timing more to gain more power.

The E85 engines have special parts that resist the corrosive effects of ethanol, including hoses and fuel pumps,(ethanol destroys commutators) fuel injector seals, etc.

I do not recommend using E85 fuel in non E85 rated engines for this reason.

And you are how old young?

I cannot believe that THIS topic hasn't YET been put-to-bed once and forever!

Simple search of CR4 shows the topic has gone 'round in various forms, like this, and this, and this, and this.

There is a REASON for different grades of gas fuels. Google/Wiki "fuels" & "gas" and study the REASONS that the differences exist... and...

you will NEVER again allow yourself (or a friend or acquaintance) to open mouth / insert foot , repeating silly old bar-tales such as:

"Yeah, my buddy-an'-I used to sneak into the back gates at the airport an' siphon 5 gallons of jet fuel into a can, to use at the saturday night high school races...

...and we'd blow everyone else away all-uh the time, an' they never figgered it out!"

Some dodos STILL refuse to acknowledge that (typical/standard) jet fuel is closer to kerosene than to gasoline ... (let-alone hi-test gasoline!).

Knowledge IS the most powerful tool in the universe ... and (don't kid yourself) ... it ALL comes , one way or another , from the one-and-only Almighty.

Good luck 'handling' your repair guy.

He is saying that the plug specified by the manufacturer works with the 87 octane. You would need to change plug where you to use 91. There is no reason to do this. That is all he is trying to say. Hexane and octane are both straight chain hydrocarbons except that one has 8 carbons per molecule and the other has 6. The octane rating is the ratio of these molecules in the mix.

During combusion, the hydrocarbons oxidize. The higher octane burns differently as a result of these reactions. The engine designer assumed 87, but if you want to run 91, you need to redesign the engine to take into account the fact that you are now running a different (though very similar) chemical reaction.

The simple way to make it work would be to change the plug. But there is no reason to do this. You are reinventing the wheel when there is no need to.

The more complicated way to run 91 would be to rebuild the engine changing compression ratio, valve timing, etc.

Your mechanic needs to go back to school and pass the thing this time! It is a waste to run higher octane fuel than the engine is designed for, but the ignition system cannot tell the difference. Many higher grade/octane fuels have more cleaners and additives in them, but if the engines is designed for 87 then 87 will work best.

I use to own a Jeep Comanche pickup with a 2.5L four. I normally ran 87 octane in the truck unless I was going to drive in the mountains (Sierra Nevada). If I did not use at least a mid grade gas, the engine would ping like crazy. On top of that I usually got my best mileage in this truck when I used a higher grade gas and was driving through the mountains. Go figure.

Don't have to go figure! When in the mountains the air is thinner and the mixture would change causing the pinging! Need I say more to educate you?

I find it hard to believe a Suzuki wouldn't love some 91 octane fuel, although I bet it runs great on 87 too. Unless something is wrong with motor...

Franksgarage,

If a 1996 Suzuki 140 hp outboard is designed for optimum performance using 87 octane why would anyone use any other octane?

Jon

If it were mine, I would run the 87 because it's cheaper. I'm saying I bet it really doesn't make a difference.

Franksgarage,

In an Engineering environment it makes a difference. The boat engine characteristics would be carefully measured and recorded then hooked up to special equipment to emulate a load and an exact amount of fuel of different grades run in it to compare performance.

I filled the tank of my car and drove the same route to and from work until the low fuel indicator activated. Then did the same for each grade of fuel. The computer system took care of keeping track of the data and displaying it on my console. The computer also took care of recalibrating the engine timing etc to compensate.

The boat engine had no such magical devices to compensate for the differences.

I also had a 4 cylinder turbocharged car that lost power over 5000 feet elevation so, as recommended, I refueled with higher octane and it got its "mojo" back and could climb mountains like the big guys.

Jon

With there being more oxygen at lower altitudes, I find it hard to believe that anybody's car needs higher octane fuel at higher altitudes and not the other way around. Maybe your turbo car at 5000 feet was always supposed to run high octane fuel and was retarding the timing until you put the correct grade in it.

Franksgarage,

Maybe you should have read my manual for that car where it says 87 octane unless driven above 5000 feet.

That car was an 87 Chrysler product. They put that engine in many of their models including the New Yorker. The method for correcting the high altitude problem was to use the higher octane. End of story.

Did you read this right?

"I also had a 4 cylinder turbocharged car that lost power over 5000 feet elevation so, as recommended, I refueled with higher octane and it got its "mojo" back and could climb mountains like the big guys."

If it wasn't a turbo I would have been down-shifting a lot on the big hills.

Jon

The original question was about spark output vs. octane. The reason I ran 91 oct. was because it( Engine) had sat for awhile and was having a little trouble running right. I was trying different things to see if I could correct the problem. I know that modern ecm's can adjust for some differences but it is not applicable to this question.

Scooter47,

The original question was about spark output vs. octane.

Actually your thread was not in the form of a question and there was not adequate detail as to what was done or not done so all we could do is speculate or keep asking for more info. Perhaps the real good answers were not forthcoming because those who have them did not want to deal with that kind of interrogation process.

Spark output VS octane: The spark is the same in that engine no matter what octane is used.

If your engine was a high compression type it would most likely have a beefier ignition and use a higher octane fuel.

Changing fuel octane doesn't change the compression ratio.

You didn't mention that you tried a fuel system cleaner with 87 octane or a new set of spark plugs or replaced the HV distribution system that may have gathered moisture during storage.

You also didn't mention if the gas in the tank had a Fuel Stabilizer added to it before storage or if it was drained and let dry. You also didn't mention how it was stored. Like, attached to the stern of a sunken boat?

STA-BIL Fuel Stabilizer eliminates the need to drain fuel during storage, and keeps fuel fresh for up to 12 months or more. STA-BIL now includes more corrosion protection than ever before to help prevent and protect against problems caused by today's Ethanol-blended fuels. For Marine Engines, use Marine Formula STA-BIL Ethanol Treatment– which contains DOUBLE the corrosion protection, and FOUR TIMES the fuel system cleaners to protect the fuel tank, fuel lines and complete fuel system from the damaging effects of Ethanol in the marine environment.

Jon

yes I used stabil,new correct plugs, new fuel filter ,new gas tank. The carbs were rebuilt,. new water pump. Let me rephrase it for you. Does a engine rated 91 octane fuel have a higher spark out put than one rated for 87. Can I put it any simpler?

no

Octane rating is resistance to preignition or detonation, not how easy it is to light off with a spark plug.

Scooter,

That's my point. You could have put it simpler.

They are the same. The difference would be in the timing for the specific engine. You didn't mention if it was a model DT (2 stroke) or DF (4 stroke). Or injected or carburated.

You could have left out the part about the modern car not being hurt by using the wrong fuel. If you had a modern car warranty you would know that certain items are voided if you use the wrong fuel. The catalytic converter is one.

Does a engine rated 91 octane fuel have a higher spark out put than one rated for 87. Can I put it any simpler?

You could have said that in the beginning and avoided looking like you didn't know what you were talking about.

Before you swap out the ignition system check the carbs. 1500 rpms is just above idle so the fuel/air mixture from the carburetor is transferring from the idle circuit to the main jet circuit thus causing the engine not to run as well.

Otherwise, It looks like you could try what I suggested earlier and replace your high voltage ignition system.

Jon

Average flame speed that is one of prime factors of power output and efficiency has no direct relation with octane rating that I know of. It is more related to fuel concistency itself, mixture ratio and other factors that affect final (before ignition) mixture temp like per cycle compression (not same as compression ratio), combustion chamber wall temp, ignition advance time etc . Using higher octane rating fuel has not definite positive or negative effect on a given engine. Result depends on engine specifics eg auto advance adjustment with knock feedback or manual readjustment of it, application of "hotter" or "colder" plugs, chamber geometry etc. Of cource any mods are not adviced unless one knows what he is doing, usally manufacturer has better overview of the compromizes that have to be made for overall acceptable performance and reliability, and better ways to test 'em than you and me

Most (but not all) modern cars have a spark knock detector to tell the ecm to advance or retard the timing.A low octane fuel will spark knock at a lower timing advance than a high octane fuel.The computer is designed to keep the timing as far advanced as possible but avoid spark knock.If you run high octane in these cars, the timing will gradually advance,and you will get a little more power, but not nescessarily more milage.

As far as your boat engine is concerned, it may or may not have this feature.If not, then it will not hurt the engine, but it won't perform any better,or stay cleaner, or last longer,etc.So why pay more for no benefit at all?Doesn't make sense to me.

With the gas prices that go up in proportion to octane in the gas unless you are trying to beat a boat speed record why use the higher octane. It is more money that you are burning up. There are platinum spark plugs that might work better but expensive.