89 Octane gas and R80RT

[coyotebmw]

Yesterday I was talking to a mechanic about my '83 R80RT and the type of gas I use. He made an interesting suggestion that I consider using 89 octane gas versus the 92 premium I have been using. Anyone out there have any experience with running the mid-range grade gas in their bikes, versus using the high-grade gas? Since the compression is only 9.5:1 on the R80RT, the mechanic said it should actually run better.

[rmoser]

Generally, engines with compression ratios of 9.3 : 1 or less will safely operate with unleaded 87 octane fuel. But "safely" doesn't necessarily mean "well". I get a lot of spark knock running less than 91 octane in my 85 R80.

The good rule of thumb is to not to run a lower octane than that recommended by the engine manufacturer. It's not only impacted by the compression of the engine but also the shape of the combustion chamber, fwiw.

To be honest, my bike runs absolutely the best, and gets the best gas milage, when I fill it up with 100 octane aviation gas (which has lead in it).

[Rbike]

rmoser is right about octane, raising the octane level simply raises the ignition temperature of the fuel. High compression creates more heat prior to the spark and can create enough heat to pre-ignite the fuel. The combustion chamber design is also a factor because it can cause one spot to be hotter than the rest which can then lead to pre-igniton.

I've got the later model with lower compression and anything I put in runs fine without any knocking. But I thought it was only the '77 and '78 models that had the higher compression...

[James.A]

The owners manual for my R75/5 recommends premium fuel with a "minimum research octane" of 97. I always use Amoco or Shell premium (93 ROM) and the bike runs O.K. It used to love Turbo Blue when I could find it. I understand that lowering compression and/or dual plugging an airhead makes octane less significant. I had to dump a tank of regular un-leaded in it one time. It accelerated like a diesel. I think a rider should always use the best fuel you can get.

[coyotebmw]

I checked the BMW Motorad site for the specs on the R80RT and the compression ratio is listed as 8.2:1, not 9.5:1 as I earlier listed. This means that yes maybe I can use 89 octane gas, it the comment about lower compression engines can use lower octane gas is true?

[James.A]

Why not give it a try? If the bike runs poorly you can always drain off the tank and top it up with premium. We owe it to ourselves and the oil producing countries to use as little gasoline as possible. Given the price of gasoline, a motorcycle getting 40mpg on premium fuel isn't much better than a Honda Civic getting 35 mpg on regular unleaded.

[rmoser]

A conservative estimate (it has to be since the other factor, head design isn't included) would be something like this:

Octane
(R+M)/2 C.R.
85 7.3:1
87 7.6:1
89 8.0:1
91 8.3:1

So probably it would be ok. But again, it's simply best to follow the manufacturer's recommendations in this regard, since they can factor in the head design.

The engine probably cannot run "better" (meaning, develop more horsepower) with lower octane gasoline than high, although it very well could run worse. While higher octane gasoline has more potential energy than lower octane gas, you may not be able to realize all of that potential energy in a lower compression engine. So worst case, you waste a little money running the higher octane gas.

You can test it by consuming at least 75% of your gas from the tank (more is better) and then filling with the lower octane gas. Try driving it, particularly noting hard acceleration. If it knocks or pings, it's simply too low an octane for the engine.

The only case where stepping down octane could plausibly result in marginally better power is if the added octane comes from oxygenating agents added to the fuel. If this is the case and if the engine isn't efficient enough to make use of the higher octane fuel, then perhaps you might see a small increase in power by more closely matching the octane to the engine design. To be honest, I think pinging is a lot more likely going down than gaining power.

Rob

[coyotebmw]

I interested in finding this thing out about using 89 Octane because I just recently had to have the exhaust valves replaced and intake ground because of burnt valves!! That is why the mechanic suggested using the lower octane gas. From the chart it looks like 89 octane may help prevent a reoccurance of the valve problem. Since, 92 octane does burn hotter.

[TheSlashFiveTourer]

Originally posted by rmoser
To be honest, my bike runs absolutely the best, and gets the best gas milage, when I fill it up with 100 octane aviation gas (which has lead in it).
INTERESTING! What set of circumstances is involved in purchasing AvGas 100 for your R80? Do you just drive up to an airport ramp and tell the jockey to "Fill 'er up!" or what?

And would that be like a VIAGRA boost for my 31-year old R75/5?? :idea

[rmoser]

I own a small plane and I keep it at a small airport near my house, so I'm around the airport a fair amount.

Both of the gas pumps can be driven up to (it's certainly not the case at the larger airports which may not even have pumps, just tanker trucks). And the pumps are self serve.

Rob

[rmoser]

I'm not much of an engineer, but I'm not sure that higher octane gas burns any hotter. I think that's an urban myth, to be honest. In any case, if a vehicle can't make optimum use of the higher octane, I think that there would simply be some unburnt products.

Burnt valves are much more likely because (due to pollution controls) the engines are set up to run leaner than optimum. The best power and operation for an engine is somewhat richer than the optimum for pollution control.

I know that the early 80 R80's have a reputation for burning very lean and additional clean air circuitry that BMW put in the airbox to burn exhaust gases also has a reputation for burning exhaust valves (which is why you see very few r80's with this stuff still in place; people just take it off and plug the holes). If you have this sort of apparatus, I'd consider removing it.

Rob

[donkey doctor]

Hello; I use regular gas all the time. I have a 77 R100/7 that has two spark plugs in each head. I have noticed that using premium gas does make the engine run happier, but regular gas doesn't hurt my engine. On the cost front; here in Canada premium gas is 10 cents a litre more expensive, that's more then 40 cents a gallon! Too expensive! One way to save a little is to buy boat gas, I can find it here for about a cent a litre cheaper then regular, and it is premium gas, the only difference is that a provincial road tax is not on it. I live in a small town and there is no problem with the regulating authorities (what regulating authorities?), Your situation may vary, as they say.

[rmoser]

That's one of the advantages of a dual sparked head; you can usually run a lower grade of gasoline with such a head.

Rob

[flash412]

You should always run the lowest octane (cheapest) gas that your motor will HAPPILY run. If the manufacturer says only run super premium, try a tank of the next grade down. If it doesn't ping or knock on a long uphill, you're good to go. Try a tank of regular. You might be good to go there, too.

As far as only running what the manufacturer says to run, that is a HUGE load of crap. That stuff is written by marketing droids who want to appeal to the moneyed but totally uninformed segment who thinks that if it is expensive to buy and expensive to own, it MUST be good.

BMW says to run only premium in the F650. I run 85 octane in mine. Mine have two spark plugs (as opposed to the "dual spark" that BMW claims to have invented recently). But there are folks with (single plug) F650GS who had problems with stalling that they FIXED by running regular rather than premium.

Then again, as airheads get older, they tend to have increased carbon deposits in the combustion chambers. This raises the compression ratio and may require running a higher grade of gas. Pay attention to what your motor tells you when you put lower grades of gasoline in there and you'll be fine. Pay attention to people who tell you to only run premium and you may well be literally burning your money for no good reason.

[TheSlashFiveTourer]

Originally posted by donkey doctor
Hello...One way to save a little is to buy boat gas...a provincial road tax is not on it. I live in a small town and there is no problem with the regulating authorities (what regulating authorities?)

Donk! SHHH! You're being watched!! :yow

Those people down the road in the Victoria taxation offices are looking for guys like you. Be very careful out there!

[Jeff]

You will have to watch what is added to the fuel, with all the different additives put into fuel to supposedly reduce emissions the airhead may not like some of them. I live in a non atainment part of Wisconsin along Lake Michigan and while my 4 wheel vehicles don't mind the fuel I can notice a difference with my bikes. sometimes the lower octain will run just fine. fill up and try.

[coyotebmw]

It seems I could have found out my answer by checking the Owners manual under the seat. It says on Page 84:

"Fuel and Lubricants: Fuel - Leaded or unleaded, min RON = 91 resp. Minimum Octane Rating. (R + M)/2 = 87"

That would seem that it can run on 89 Octane gas.

[lkchris]

Originally posted by coyotebmw
I checked the BMW Motorad site for the specs on the R80RT and the compression ratio is listed as 8.2:1, not 9.5:1 as I earlier listed. This means that yes maybe I can use 89 octane gas, it the comment about lower compression engines can use lower octane gas is true?

You're starting to get close now.

In the very early 1980s premium fuels were not readily available in the USA and BMW responded by modifying all USA-spec Airheads to run on regular fuels. This is all you need, and running premium (or mid grade) does nothing but cost you more money.

Now, you need a new mechanic.

ALL 1981-1984 Airheads had inappropriate exhaust valve seat material and they all suffer valve recession. This is just a fact and they ALL have to be repaired. Normally the recession is loss of material from the valve rather than the seat, and they both have to be replaced, the seat because it's an incorrect material that doesn't transfer heat as required, and the valve because it's been damaged. None of this had anything to do with the fuel run through the bike to date.

BTW, in the rest of the world BMW retained high compression engines, and for your R80 you can obtain 9.2-1 pistons, easily installed in your Nikasil cylinders assuming you get the right size. The sizes are A, B, C as stamped on your cylinders. Get the pistons with new rings. Do nothing to the Nikasil cylinders. Then, you'll need premium fuel.

[lkchris]

Originally posted by coyotebmw
It seems I could have found out my answer by checking the Owners manual under the seat. It says on Page 84:

"Fuel and Lubricants: Fuel - Leaded or unleaded, min RON = 91 resp. Minimum Octane Rating. (R + M)/2 = 87"

That would seem that it can run on 89 Octane gas.

Nope, run the lowest you can get from a reputable source. The figures in the manual were not computed the same way as we see figures on pumps in 2004.

[coyotebmw]

Well I've been running 89 Octane for a couple of weeks and the bike seems to love the lower octane gas. Actually saw an increase in gas mileage the last tank full!!!
:clap