modern gas

Went for a 100 mile ride today since the weather was perfect and my Commando needed to stretch her legs. Pretty much since I bought the bike 4 years ago, I have been using 92 octane fuel. Lately I was thinking that with a compression ratio of 8.5 to 1, I shouldn't need it and that I was wasting my money out the tailpipe. So the last fillup was on 89 octane. During the ride today I was sure that the bike had less power than I was used to and was not as eager to rev. When I returned home and filled up the gas tank, I was very surprised to see that I only made it 106 miles on 2.4 US gallons. That's 44 mpg with a 36 mm Mikuni carb. Pretty crummy mileage for a Commando methinks. Previous mileage figures that I have recorded under similar riding conditions have been in the range of 50-55 mpg.

So my question to all you wise ladies and men is this: Have any of you been experiencing this same phenomenon with different grades of gas lately? Is it possible that someone upped the ethanol content of this batch of gas and would this explain the 10 mpg reduction and apparent lack of power? Do different grades of gas degrade faster than others? The batch I was running on today was about 3 weeks old with red Stabil added. Are any brands of gas more consistent than others? This latest batch was from Texaco. Any insights greatly appreciated.

Tobin

From my experience with a 450 Ducati single, the engine would ping (pink? pre detonate) using standard 95 octain. If 98 was used it ran fine and had better fuel economy. I was told that lower octain fuel burnt faster than higher rated fuels and the lower octain would actually burn too quickly. (burn before the piston was at TDC, without adjusting the timing of course)
This also can make the engine run hotter than it should. Look at older air cooled race bikes that use alcohol and many have their finning reduced. (and main jets big enough to put your finger through)
I don't know if retarding the timing can let your motor run better on lower grade fuels? I would rather pay a little more and have the thing run nicely. Maybe your engine is telling you something?
This is fuel in Australia though, the lowest octain I've seen here is about 92 or 91 and that's the 10% ethenol mix.
Over here my old 450 likes BP and Mobil premium, any thing else and it complains. And I've got to kick start it, so I don't argue, just give it what it wants.
Graeme.

Some folks on other forums have commented that the change to "summer" fuel formulas has been quite noticable this year, especially in California.

I did not know 89 octane fuel was available....in the UK ordinary fuel is 95 octane, premium is 97....Both The Commando and the Triumph are happier on 97....The Triumph especially so, they pink below this.
Are you sure about the octane numbers? You havn't put diesel in, have you? :shock:
Stu

bigstu said:

I did not know 89 octane fuel was available....in the UK ordinary fuel is 95 octane, premium is 97....Both The Commando and the Triumph are happier on 97....The Triumph especially so, they pink below this.
Are you sure about the octane numbers? You havn't put diesel in, have you? :shock:
Stu

Click to expand...

I think US octane calculations are different from the UK but am not sure of the conversion. Our fuels typically come in 87, 89 and 92 octane. I haven't noticed any pinking with the bike and have my Boyer timing dialed in at 31 BTDC at 5000 rpm as per recommendations so am pretty sure the timing is OK.

Found some neat info on "howstuffworks":

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

And of course the always reliable wikipedia :roll: : http://en.wikipedia.org/wiki/Octane_rating
(scroll down to regional variations)

In the US and Canada, the Octane rating is different than Europe and Australia. US Octane is (R + M) /2, or the average of the Research Octane Number and Motor Octane Number, and is typically 4 or 5 points lower than EU/AU numbers. In other words, your 98 is our 93.

Here is the US, they've been adding 10% ethanol for over a year across the board. This is supposed to reduce our dependency on imported oil, but it is mainly a cheap way to boost the octane rating of lower quality fuels. adding 10% ethanol to 86 octane gas boosts the octane to about 91 (equivalent to 96 in OZ).

This cuts costs for refiners, as they can get more gas out of a barrel, cuts the price of gas slightly (ethanol runs a little less than half of gasoline, so theoretically reduces the price of gas about 5%), but also cuts gas mileage (ethanol only contains about 2/3 the BTUs of gas, so cuts gas mileage about 3-4%)

In my experience, ethanol-containing gas gives me almost 10% less fuel mileage. I think it may be because ethanol mixes with water, allowing some water to remain in mixed with gas, instead of all settling to the bottom of the tank at the filling station.

Ethanol is also a great solvent, and will dissolve old GRP gas tanks

There has been some discussion lately on the list at Britiron about the gas that's available these days. Nothing would surprise me, we're talking about the industry that brought us MTBE. The thing that strikes me is that modern motors with fuel injection, antiknock sensors, and liquid cooling really would have to be fed swill to show any problems. But an air cooled, carbureted, and slightly antiquated engine would have fits on the same stuff. We are the canaries in the mine. Lucky us.

I'm running a panoply of bikes on modern gas (what else is there?). Mileage suffers in winter when they pump up the ethanol content, fiberglass tanks don't do so well, but the engines soldier through. Christ, the Commandos (ex-Combat) were sub-9 compression engines. Mine takes 87 (R+M) octane and is happy. What's the issue?