High octane fuel ?

[lucky]

I have a 60 hp merc 2 stroke and i run the highest octane that i can, usally about 91-92. First, am i waisting my money by running that high of octane, and if i have to run the higher stuff can i buy the 87 octane and put an octane booster in with it to make up the differance? and is there a chart avilable for how much to put in. Thanks

[eurolarva]

Higher octane fuels are designed for four stroke motors and you will get little or no value using them in a two stroke. The only advantages of using them would be when fuel gets old it starts loosing some of its octane and the only other one is the only non oxygenated gas you can buy is typically higher octane fuel.

[BobT]

I think it's best to use what is recommended by the manufacturer. With the newer engines and computer controls, the system is designed for a specific octane level. You'll get the best performance using the octane the computer is designed around. Using high octane fuel when the system is designed for 87 may affect engine timing and performance. My truck for example actually runs worse and economy drops using high octane fuel just like the manual suggested it would.

Bob

[Giant_Jackpot]

I agree with BobT. Use what the manual says to use. Also keep in mind that the 87 is going to be "fresher" than the non-oxy.

[fishorgolf]

Giant, why is 87 going to be "fresher" than non-oxy?

[Gissert]

Octane is a measure of resistance to detonation. The higher the octane number, the more resistance to detonation it is. There has been a misconception out there for decades that higher octane means higher power. While it may be technically be true, you must have an engine designed to run the high octane stuff

To run high octane fuels, your engine (2 or 4 stroke) must have enough compression to generate the heat for optimum burn.

Some motors that are designed to run on 87-88 will run lousy and foul plugs on 91-92. They just dont have enough compression to get good combustion, and as a result you can get fouled plugs and carboned up combustion chambers.

I can't vouch for octane booster, never used it.

[boatfixer]

Good info Gissert. The molecular structure is different. The molecules are longer in the non-oxy 91-92 oct fuels, requiring a longer burn. The base stock for the two are different. However one of the more beneficiary results for the non-oxy in small or recreational engines, is moisture resistance over a longer period of time. When in doubt, follow the manufacture recommendation.

[UMDSportsman]

What Gissert said. High octane is made for HIGH COMPRESSION motors. its the high compression that matters, not 2 or 4 stroke.

[Giant_Jackpot]

 Originally Posted By: fishorgolf

Giant, why is 87 going to be "fresher" than non-oxy?

Of course I have no data that can prove it but it does make sense that 87 is being delivered to any given gas station more frequently than the non oxy.

[fishorgolf]

I guess that depends on were you live. Up here we go threw allot of non-oxy for sleds, atv's, boats etc.

[fishorgolf]

Gissert,

What compression ratio would that be when a 4 stroke will not run as well on a 91 octane vs a 87 octane? I run non-oxy 91 octane in everything I own but my truck. I have never had any performance of other fuel problems but am real interested in the to much octane for some engines thought.

[Gissert]

I don't know whathe compression ratio would be. Each engine will behave differently. Some run the 91 stuff with no issues, some do not.

For myself, I have a sled (95XCR) that was made to run non-oxy (90-91)fuel, and I think the ratio is about 10.5 to 1. I also run the stuff on my XC700, and my 90hp 2 stroke outboard runs well on it as well too. While the XCR is speced for 91, I run it in my other small engines because the brand I use has no oxygenate in it (ethanol)

Most of my small engines run fine on 90-91, with the exception of a chainsaw and a 2 stroke tiller.

I have a friend that has a 175 HP merc and it runs lousy on the high octane stuff, but just fine on the 87-88 octane fuel.

You just dont know until you try.

[Whoaru99]

I'd be of the opinion that the difference from 87 to 91 probably isn't enough to cause noticeable problems such as spark plug fouling and poor running. Although, I certainly agree this can be the case with high octane fuels (thinking high 90s and above, like racing gas or some aviation gas).

Higher octane fuel actually burns slower and it's this slower burn that hinders detonation/pinging. It's also this slower burn why it's possible that an engine may actually loose some power and or fuel economy using too high of octane.

This is a bit tricky though on the mileage thing because the "regular" fuel in MN contains ethanol and non-oxy does not. Since ethanol-blended gas contains less BTU per gallon than straight gas, the mileage may be somewhat better with the higher octane fuel, but not because of the octane. OTOH, the added oxygenate in the regular gas is said by some to balance out the somewhat lower BTU by slightly increasing the efficiency of combustion.

IMO, from a performance aspect, if the engine runs fine on 87 octane there isn't really a good reason to put in higher-octane fuel. FWIW, I think my motorcycle actually runs better on 87 than on premium which was a bit surprising to me (high-revving crotch rocket style). Haven't really noticed a difference in my other vehicles.

As previously mentioned, it's probably best to follow mfg. recommendations unless there is reason to believe otherwise (pinging/knocking, certain performance modifications, etc.).

[fishorgolf]

My main reason for using non-oxy is I will not put oxy gas in my motors unless it is something that gets used ALL the time. eg. truck, riding lawn mower etc. I just will not take the chance of having fuel related problems. What I really wish is we have non-oxy reg. like Wisconsin.

Alcohol is for drinking not powering my cylinders!

[Scott K]

I read somewhere that Wisconsin is going to start blending in 09.

[slipperybob]

I don't know why we associate higher octane as slower burning. It doesn't burn any slower. You've only got so much oxygen in a compressed camber. Maybe I'm thinking something different. It's more resistance to burn(detonation) as in requiring a higher flash point before it ignites into a burn. Racing engines typically has much higher compression ratio as well as much higher engin rev. All that high speed part movement generates a lot of heat, that you don't want pre-detonation before your cylinder is in the optimal position. Other things can affect the detonation like the quality of your spark plug as well as the ignition voltage too. Vehicles that run with distributor cap typically will have a lower ignition voltage compared to your fully electronic ignition system. Electronic ignition system has more precise timing for spark.

Oxy's have a much lower flash point. Makes it easier to start a cold engine in the winter time. In effect it's probably igniting soomer than your optimal cylinder stroke position.

Octane boosters don't boost your octane at all. They may have other chemicals that causes your gas mixture to ignite at a higher flash point. Fuel stabilizers are probably a lot more useful as they help prevent to loss of your octane rating when just sitting around for extended time.

[Whoaru99]

 Originally Posted By: slipperybob

I don't know why we associate higher octane as slower burning. It doesn't burn any slower.

Dunno your source, but everything I've read seems to say higher octane = slower burn.

It's the slower burn that controls the knock, IMO. If the fuel burns too quickly, regardless if it's from the normal spark or from pre-ignition, a shockwave/explosion occurs in the cylinder instead of a controlled burn and that's where the knock or ping comes from.

[Macgyver55]

Basically, the "octane" rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. (exactly like a diesel does) When gas is ignited by compression, rather than because of the spark, it causes knocking in the engine. Knocking (pinging) can/will damage an engine, so it is not something you want happening. Lower-octane gasoline can handle the least amount of compression before igniting. The compression ratio of any given engine determines the octane rating of the gas you should use in it. One way to increase the horsepower of an engine of any given size is to increase its compression ratio. So, with a higher compression ratio, it requires higher-octane fuel.

Controlled labratory testing has found that there is little to no benefit to using higher than recommended octane, and can sometimes actually result in less performance and mileage.

[slipperybob]

 Originally Posted By: Whoaru99

 Originally Posted By: slipperybob

I don't know why we associate higher octane as slower burning. It doesn't burn any slower.

Dunno your source, but everything I've read seems to say higher octane = slower burn.

It's the slower burn that controls the knock, IMO. If the fuel burns too quickly, regardless if it's from the normal spark or from pre-ignition, a shockwave/explosion occurs in the cylinder instead of a controlled burn and that's where the knock or ping comes from.

my source: auto-mechanic teacher in high school...LOL's. I didn't question his teaching.

Maybe we're all misunderstanding each other and using the incorrect words to describe things. In a gasoline engine I used detonation is in reference to the air/fuel mixture igniting before the spark plug fires, although it simply is air/fuel igniting outside of the spark ingnition. While the spark plug fires and ignites the air/fuel mixture, I should have referenced that as normal combustionburn.

If we should accept that, then this is why I think higher octane gas still doesn't burn any slower. It's still the reference to it's resistance to detonation. In other words it's the autoignition that the air/fuel mixture spontaneously ignites by temperature. Higher resistance to detonation allows a cylinder to take a higher comression ratio (as compression tends to lead to higher temperature). That means the piston at the bottom of the cylinder can go up to higher position and get more pressure. If the engine was designed to take higher octace fuel, then it will get you more power, by simple mechanics of the piston's position. You still get a spark, the air/fuel mixture still burns as normal combustion burn. When optimum cylinder pressure is achieved from the air/fuel burn, it forces the piston back down. Typically higher octane are also used in short-stroke engines. That means faster reving engines, yet the ignition timing sequence is all revved up even faster. Probably eluding to a requirement of faster burn time, but in itslef is not. Nope, I don't see higher octane gas burning any slower than lower octane gas.

I've always wonder why E85 ethanol is called that, while it has an octane rating of about 105. Maybe it's the molecular bonding of ethanol that contains less power in comparison to gas when those bonds break in combustion. Maybe it's the comparison of how the fuel affects your engine while your operating it under a load. I think I've heard other's said E85 ethanol is weaker and thus needs more fuel to output the same power as gas.

[Jeremy airjer W]

It doesn't matter how you describe it, its harder for an average compression engine to "use" higher octane fuels!

Is higher octane better? Not necessarily, The benefit is that you can still find it with no ethanol (non-oxy premium). The other benefit is that higher octane fuels can have premium additives which can help clean out unwanted build in an engine. For storage its ideal, for everyday use its not necessary unless your engine requires it (99% do not). If you engine does not specifically require it that it is not necessary to use anything more than the lowest octane fuel required. Will it run better or worse, you will likely not notice a difference overall, except the pocket book will be a little lighter.

I have had no problems storing both of my fuel tanks in the boat with non-oxy premium and seafoam. For the walleye opener its nothing but speed trolling and both tanks are empty after the weekend. All summer I use nothing but the cheap stuff (if you can call it that any more). Before I put the boat away next fall I'll fill them with non-oxy premium and seafoam.

There are some exception. Some engines with excessive ping can benefit from using a higher octane fuel especially if the ping is caused by carbon build up in the combustion chamber. Does the vehicle have issues if it is pinging, yes! Short term a higher octane fuel is a quick fix and can possible prevent further damage to the engine until repairs can be made.

E85 = 85% ethanol?

[311Hemi]

I agree with airjer.

To add to other info provided in here, octane is only a measure of detonation resistance.

Also...the compression ratio of the engine is not the important factor, more it's the absolute pressure (compression) in the chamber thats important.

As airjer said, you are not going to notice an increase in performance unless the engine is designed to use the higher compression fuel. Mainly you are spending more money on something that most of the time you will not notice a gain. you are best off running the octane that the manufacturer recommends.

Now, some engines are built to be able to run different octanes. Take my 06' Skidoo 600 SDI engine. I can run this sled on either 87 or 91 octane, as it was designed this way. The sleds mapping in the computer starts out at the 91 octane programming. If the computer senses any knock it will retard the engine to the 87 octane mapping to stop the knocking. It will then stay at the 87 programming until the sled is shut off. The next time it is started it will again start out at the 91 octane.

Some car engines (possibly a lot of newer ones?) are constantly monitoring the knock sensor in the engine and can adjust accordingly. I believe that why some cars may perform better or get better mileage with different octanes.....maybe airjer can chime in on that

I believe that in most gas stations the 87 octane tanks are filled the most, as that is what the majority of people put in their cars. The premium gas probably sits longer as it costs a decent amount more and most cars do not require it.

Also [and this is a little cloudy in my head], there is a difference between 87/90 octane, and then jumping to 91 octane. I would have to look into it more, but often 87 and 90 octane will be the same price and 91 will cost more. From what I remember the 90 octane is the same as 87 only it has more [alcohol?] added to make it 90 octane. Where the 91 is actually refined to 91 octane.

[slipperybob]

silly me, E85 = 85% ethanol (up to)

should've just google that up...

Your regular engine only has a certain piston position and you simply can't increase the compression ratio except through a turbo intake. Not sure how exactly that works. I've heard of people replacing the cam shaft to increase compression ratio, then again you'll require a different CPU chip to run your engine.

No load and I generally get 18 hwy on 87 fuel on my truck. Maybe a coincidence but when I tow a load, I found that I can still get my 18 hwy with 89/90 octane fuel but only get 15 hwy on 87 fuel. I've tried 91 octane fuel and got the same results as 87 fuel. I didn't see any benefits to 91, but lighter pockets. It's totally inconclusive, but makes me wonder.

[311Hemi]

 Originally Posted By: slipperybob

Your regular engine only has a certain piston position and you simply can't increase the compression ratio except through a turbo intake. Not sure how exactly that works. I've heard of people replacing the cam shaft to increase compression ratio, then again you'll require a different CPU chip to run your engine.

You can change the head and pistons of an engine to change the compression. I was going to put a high compression head with matched pistons in a sled for the mountains and that would require that I run high octane (non-pump gas) at MN elevations. I decided to not do that because I didn't want to buy race fuel to run at MN elevations or change back in forth between heads/pistons. It would have run fine with the high compression head on pump gas in the moutains because of the change in elevation.

[slipperybob]

Ahh, so that's what people mean when they talk about getting racing headers without changing out the cam shaft.

FM is great to learn, stuff!!!!

[BobT]

 Originally Posted By: slipperybob

silly me, E85 = 85% ethanol (up to)

should've just google that up...

Your regular engine only has a certain piston position and you simply can't increase the compression ratio except through a turbo intake. Not sure how exactly that works. I've heard of people replacing the cam shaft to increase compression ratio, then again you'll require a different CPU chip to run your engine.

No load and I generally get 18 hwy on 87 fuel on my truck. Maybe a coincidence but when I tow a load, I found that I can still get my 18 hwy with 89/90 octane fuel but only get 15 hwy on 87 fuel. I've tried 91 octane fuel and got the same results as 87 fuel. I didn't see any benefits to 91, but lighter pockets. It's totally inconclusive, but makes me wonder.

You raise an interesting comment, bob. If your vehicle is designed to use 87 octane, I wonder if it would perform better to use higher octane when pulling heavy loads? Might have to try it once just to find out. With the engine working harder it is probably also working at higher temperature and I could see how this might influence pre-ignition just a bit.

Bob

[slipperybob]

It's that in my old car 94 Toy 4 cylinder, I've always run 89/90 octane in it. It's a manual. Car still runs better than I've expected. Not quite as solid as was brand new but it still runs very close to that condition. Being a manual you kind of have to mash down the throttle and you just don't get the ride of an automatic. I average 27 city max 32 hwy now. It's suppose to only get 22 city and 27 hwy. Probably manner of driving of quick accelleration (load) then maintaining steady cruise speed. Now if I drive with the attempt to save gas by being lighter on my throttle, I get 25 city and 29 hwy.

I surmise that my nozzles are getting clogged or something and I'm getting a more lean fuel now. All inconclusive but I'm not complaining at the increased in fuel efficiency. If I fuel with standard 87 octane for a month I typically find myself getting 23 city and 27 hwy. 91/92 octane provides the same effect as 87 octane.

I'm thinking fuel additives are making the difference. Higher octane grades generally have better fuel additives. I'm leaning towards believing that being the influential factor. If you're engine can't take advantage of that, then use what's most efficient for it. The only other similar factor is both vehicles are Toyota's, which I don't think could be that much influential.