I have a 1992 k75. I'm filling my car up with gas today and I see  the option to use ethanol free gas that's available, however the octane rating is 87. My question is if there is some thought of using ethanol free at 87 octane, or go with premium at 92 octane and 10% ethanol in the K bike. Thanks in advance for any insight here.

Rider manual recommends 89 and unless you let it sit for months and months ethanol is OK.

If you ride fairly regularly I'd just go with medium grade with ethanol.

If it's going to sit then get gas w/o ethanol because ethanol absorbs H2O from the atmosphere which can degrade the fuel pump vibration dampener.

What Duck said. 

I run 87 octane 10% ethanol gas in my bikes all summer with no problems.  At the end of the riding season I switch over to the non-ethanol gas for a couple tanks to run out the ethanol gas.  Stabil and Chevron's Techron fuel system cleaner go in the tanks for the winter which around here can last up to four months.

For the first year or two of riding Bricks I would run as often as possible with the non-ethanol premium, which around here runs 91 octane.  I saw no noticeable difference in performance or fuel economy between the Premium and the 10% ethanol, so I decided it wasn't worth the extra $2.50 per tank for the premium, saving me at least $200 each riding season.

Odd how different bikes respond to different fuels. My OZ-based RS sings verses when run on ethanol 98 RON premium versus ethanol low grade 91 or 95 (87 & 89 in North America).

Two Wheels Better wrote:Odd how different bikes respond to different fuels. My OZ-based RS sings verses when run on ethanol 98 RON premium versus ethanol low grade 91 or 95 (87 & 89 in North America).

We're possibly comparing pears to apples here.  Fuel formulas differ from summer to winter, and from state to state.  I have no idea how fuels are formulated in Oz.  I know that when I ride my K75RT and K75S all over the U.S. and Canada using a lot of different fuels, I feel no difference in performance or fuel economy which is fairly predictable within a range of 40 to 48 mpg depending on how heavily loaded the bike is, how strong the headwind is, and my cruising speed.

I have a daily commute of 55 miles each way, mostly at 65 to 75 mph.  One way is daytime, usually in traffic, and the return is in late evening in light traffic.  Regardless of traffic, weather, or fuel, my range to the low fuel light is a consistent 190 to 200 miles(I will admit that I might get a couple extra miles per tank when running 91 octane non-ethanol).  I can discern no difference in starting, idle, acceleration, or high speed running.  I will admit, that while I like to cover lots of miles in the shortest time possible, but at 70 years I am no longer a peg scraper.   Maybe that's where the higher grade fuels are important.

My understanding of octane is that it controls combustion in the cylinder to prevent detonation which can damage pistons and valves.  I also understand that octane has no bearing on the actual energy available per pound of fuel.  The negative effect of ethanol is that it has less energy per pound than gasoline, so there will be a slight loss of miles per gallon when using it.  The loss of energy is just a couple percent and probably amounts to 1-2 mpg on a K bike.

Actually I was merely being a smart alec and looking for a clever way of comparing verses to versus.

The k75 has higher compression so the higher octane is recommended.

Of my five bricks, those that used ethanol had good tanks but bad rubber and ferrous parts, ethanol free were good except the lower points of the tank were leaky. 

I assume when bricks were designed ethanol blend did not exist (l know I'm wrong, I'm generalising, and that even the mighty T model could run on 100% ethanol) therefore the rubber holder was not designed for an ethanol environment.

Alcohol (~ethanol) is distilled (~dehydrated) therefore it absorbs moisture, therefore it also attracts it. 

My bikes live in Australia with our poorer high sulphur oil (granted, it's blended.) Water,  from condensation, plus sulphur equals sulphuric acid.

In both cases the harmful components are diluted if ridden often. 

Therefore:
What I should do; ride often, ethanol free,  clean the  tank once yearly.
What I actually do; ride often, one ethanol blend        tankful per year (more if a trip is a tankful)

Key lesson from this thread:  RIDE OFTEN! 

Interesting old fashioned logic being used here. 
The k75 has higher compression so the higher octane is recommended.
My Ducati has a compression ratio of 12.6 to one and runs on 95 or 98 without any problems. I has no knock sensors either. 

It's logical to me the higher the compression the more liable the fuel is to knock (self combust)

I'm all ears for another reason k75 has a higher recommendation than k100

I'll be even more old fashioned, a Ducati is not a BMW k75

The higher the octane the less liable to knock.

Its not unknown to put K75 pistons in a K100. Some do it.

K75 puts out 75bhp, 100bhp per litre.

K100 puts out 90bhp or 90bhp per litre.

daveyson wrote:It's logical to me the higher the compression the more liable the fuel is to knock (self combust)

I'm all ears for another reason k75 has a higher recommendation than k100

I'll be even more old fashioned, a Ducati is not a BMW k75

That is really funny. 
I suppose that you are saying that BMW and Ducati work out their compression ratios in different ways.
You will be telling me that they use a different kind of volts in the batteries next.

92KK 84WW Olaf wrote:Its not unknown to put K75 pistons in a K100. Some do it.

K75 puts out 75bhp, 100bhp per litre.

K100 puts out 90bhp or 90bhp per litre.

K100 16v is 100bhp per litre

My policy with ethanol fuel is to give it as wide a berth as possible!

Sadly, there are places in the world where that’s not possible, but everywhere I ride it is. 

Not good for anything that has a significant non-use period. 

Bill

I took some outrageous liberties in that post but I didn't think octane rating higher on k75 was one of them.

Cam timing different, swirl pattern different, compression rings different or number of different, included angle different, maybe your high km worn rings,  I don't know. 

I love Ducati, I'm only saying the're different.

I'm open as to why k75 has higher octane recomendation than k100.

The higher the compression, the more liable to combust, is all I'm trying to say, it's not too outrageous. Would you grant me that? 

Maybe I'm wrong.

The final pressure in the cylinder at the top of the compression stroke is a function of throttle opening.  Anyone who has done a compression test without a wide open throttle can recognize that.  The need for a high octane rating has a lot to do with how the engine is operated.

Octane rating is more important for riders who are using wide throttle settings under load.  Riders in their twilight years like myself who are less aggressive don't operate their engines in a way that requires higher octane ratings.  Since BMW can't predict how a given rider will use their bike, it only makes sense that they would recommend a fuel that is appropriate under the harshest operating conditions.

As Daveson mentions, ethanol is good at absorbing water.  This is actually a plus in our rather fragile tanks.  The problem comes when water saturated ethanol sits long enough to separate from the rest of the fuel.  This is called phase separation and is the cause of all the corrosion and deterioration issues.  This is such a serious problem that a friend of mine made a rather nice living making and selling a potion that prevented it in fleet vehicles.

Bottom line, if you aren't using your bike hard with hard acceleration over wide rpm ranges, you probably don't need anything above 87 octane.  If you are refueling on a weekly basis and running until your low fuel light comes on, you can safely run fuel with 10%(and possibly higher since the U.S. is legislating 15%) ethanol with little to no problems.  Just be aware that if you plan to leave the bike unused for anything longer than a couple weeks it's a very good idea to replace the ethanol fuel with stabilized non-ethanol.  A bit of fuel system cleaner is also a good idea to prevent any varnishing that may occur during extended periods of storage.

My latest long term storage strategy is to completely empty the tank - using paper towels to get the last drops out of the corners.  Then put a big desiccant pack in the tank just in case.

I also run a dehumidifier in my garage these days because it rains so much here that a dehumidifier is mandatory to keep tools and bike bits from corroding.

My strategy is to fill the tank with a fuel mix that I can use to run the engine every week or two during the winter for 15-20 minutes until it warms up to normal operating temperature and cycles the fan a couple times.  The idea is to circulate the oil, charge the battery and burn off any condensation that may have collected in the engine and exhaust system.  With the transmission in 1st gear I am gently circulating the transmission and final drive oil as well.

I also get to listen to my bikes make Brick music.

All my machines use 10% ethanol fuel and always have. Except for my 1965 Mustang which was built before ethanol was used in fuel.

All are stored from October, generally, until April. The same is true for lawn mowers and snow blowers but with the seasons flipped.

I have never had a problem. I have owned the same lawn mower since I bought it new in 1985 and I have owned my 1965 Mustang since 1991.

Ethanol has an advantage of absorbing water. Before it became common place we used what was called "gas line anti-freeze: because water would assemble in fuel lines and freeze. Gas line anti-freeze was wood alcohol.

If you have a water problem in your tank then using ethanol fuel is a good thing otherwise the water will sink in the tank eventually rotting it out.

About the only legitimate reason for not using ethanol fuel is when the bike is an older bike with a fiberglass tank. Ethanol slowly ruins those tanks. Other than that the problem is not ethanol but poor maintenance. Poor maintenance includes allowing gas to get old whether ethanol fuel or non-ethanol fuel.

btw, the math is alcohol is 80% of the energy content of petroleum fuel. As such a 10% alcohol fuel has a reduced energy content of 2%. Your mileage will be reduced by 2% or roughly 1 mpg based on 50 mpg.

I am forced by the government to run ethanol  contaminated fuel  here in the suburbs of New York City. When I was up at my camp in Northern New York State way out in the country I was able to get non ethanol high octane fuel,My bike not only ran better but my gas mileage rose about 10% . Now I'm running the garbage gas again .I run the 87 octane.The bike runs fine on it but I wouldn't let it sit in the tank for too long.The previous owner stored the bike with that garbage in it and it rusted all the components inside the tank .The sender for the gas gauge literally rusted away to nothing.The ethanol also ruins rubber.In my other bikes with fuel petcocks all the rubber gaskets fail after about a year from it. The stuff is crap and should be outlawed .

Yamaguzzi, it make me crazy that the millions of owners of vehicles and boats damaged by ethanol have no standing to sue the producers of that crap.

In OZ, for the past six years my red RS will sometimes sit for one year and up to two years between month-long uses. I leave it in a dry storage shed with freshly changed oil and filter to reduce the potentially corrosive contaminants found in used oil. I fill the tank to the brim with 98 RON premium (I rarely use less than 95 RON) non-ethanol petrol with a healthy dose of StarTron or Sta-bil added to the tank and run for the few moments it takes until the pressure drops once the fuse is pulled. I remove the battery. The fuel smells subtly like furniture polish when I push her out into the SE Queensland sunshine and pop the cap, but I go for a good burn, topping it off repeatedly when about halfway down. It feels spunky enough but certainly reacts to 'fresh' fuel better when the last remnants of the old, stale fuel are burnt away. So far, so good. Since the '70s I have never started a motor I've winterised once it's in the shed for the duration. I regularly use a battery maintenance charger on bikes closer to home.

I'm not certain if it's changed recently but last time I lived permanently in Australia (prior to mid-2012)  most premium petrol was non-ethanol with the exception of a few minor brand stations, whereas low-grade and mid-grade were mainly ethanol fuels.

It should be noted that Australia's petroleum ethanol is primarily sugar cane-based rather than corn-based. Does anyone in North America have riding mates with newer Beemers who've had the fuel level strip replaced several times? They'll read full when empty, empty when full, or 'freeze' no matter the level. It's gotten so bad last coupla years that BMW have extended the warranty on that component to 10+ years. Interestingly, it's a rare failure Down Under. Good thing we don't also have fuel level strips in our older Ks.