ST1100 Gas Mileage Decreasing

I also have a 55 mile commute and was very pleased when I got 49 mpg recently (I also recorded 52 another time but not on commute). That's about 5 mpg better than I usually get with my 750!

Did a pre-ride of some of the DixieSTOC roads today. 200 miles before gassing up, half commuting and half on the route (2 up), 45.41 mpg. Haven't filled the tank from the rest of the ride yet.

Another update to my ST11's gas mileage...



A few changes since my last update...

• I ran a few months on 93 octane (blue dots) to see if I saw a difference in MPG. No significant difference observed, and none was really expected on a carbureted engine. Was going to run longer on 93 but got tired of wasting money!
• I installed iridium plugs in early June 2015.

Here's the basic stats...


'til next time...

paulcb said:

I ran a few months on 93 octane (blue dots) to see if I saw a difference in MPG. No significant difference observed, and none was really expected on a carbureted engine.

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Octane requirement is a function of engine compression ratio, why would it matter if it were carbureted or fuel injected?

IMHO, there are a lot of good suggestions above.

Does the rear and front wheel spin easily on the center stand and with the front wheel raised ?

Also, you could run some carb cleaner with the next few tanks of gas.

Try some non-ethanol gas and see if that improves your mileage.

dwalby said:

Octane requirement is a function of engine compression ratio, why would it matter if it were carbureted or fuel injected?

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It's also a function of spark timing. On FI engines (with computers), timing can be automatically advanced without knocking using a higher octane, which generally gets you more efficiency at higher RPM's. Carb'd engines generally have a fixed timing range that is independent of octane and knocking.

At least that's how I understand it.

Jim Van said:

IMHO, there are a lot of good suggestions above.

Does the rear and front wheel spin easily on the center stand and with the front wheel raised ?

Also, you could run some carb cleaner with the next few tanks of gas.

Try some non-ethanol gas and see if that improves your mileage.

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The last two places I went to that offered 'pure' gas had those pumps closed off, i.e. they were out. Not sure if this is a trend or just my bad luck.

paulcb said:

It's also a function of spark timing. On FI engines (with computers), timing can be automatically advanced without knocking using a higher octane, which generally gets you more efficiency at higher RPM's. Carb'd engines generally have a fixed timing range that is independent of octane and knocking.

At least that's how I understand it.

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All engines advance/retard timing, but computerized FI systems are more flexible and precise, and have knock sensors, I'll grant you that. Higher compression ratios are more efficient inherently (if you read a theoretical analysis of engine design, compression is the main variable that improves efficiency, in theory). I think what's improved over the years with electronic FI systems, is they can avoid knocking on lower octane fuels than would normally be usable at a given compression ratio. So I think the benefit of modern electronic FI systems goes the opposite way to what you described.

Also, from what I understand the energy content of lower octane fuel is slightly higher, so using higher octane when its not needed to prevent detonation is not going to be more efficient. Since the ST1100 engine is low compression, that's why it does well on 87 octane.

Not claiming to be an expert on the topic either, but that's how I understand it.

I've always assumed that the higher efficiency of advancing the timing to the edge of knock outweighed other variables but that certainly may be wrong. In any case, I'm stickin' with 87! I would be interested in seeing the same type of study on an ST1300 though.

I have to admit, I did a similar study (not nearly as long) on my 2010 F150. I saw a slight increase in mpg with 93 octane but not nearly enough to justify the increased price and since it was a small sample size (only a dozen tanks or so of 93), there may not have been any real difference between 87 and 93.

Good discussion and thanks for the comments.

paulcb said:

I've always assumed that the higher efficiency of advancing the timing to the edge of knock outweighed other variables but that certainly may be wrong.

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I guess I've always assumed that the engine advance curves were already setup to optimize performance for general street use, and that the fuel grade was used as your 'knock sensor' to operate at maximum advance without knocking, but I don't know for sure either. I have no idea if any power gain was available simply due to additional timing advance by being able to operate right at the threshold of knocking, so I wouldn't try to argue that point either way.

I suspect the main power gains have been the result of better airflow through the cylinders, which has allowed higher compression engines to be used with the current pump octanes available. Back in the '60s we needed octanes up to 108 or so to run compression ratios in the 11-12:1 range. I can't remember for sure, but they were well over 100. My 1989 FZR1000 had 12.0:1 compression and ran on 91 octane because it had a very efficient downdraft head (with 3 intake valves) that flowed very well. It wasn't fuel injected with knock sensors, it had plain old carburetors. So if I had to guess I'd say the better airflow has provided a significant amount of the power improvement, and the electronic FI with knock sensing has probably contributed to a lesser extent.

dwalby said:

I guess I've always assumed that the engine advance curves were already setup to optimize performance for general street use, and that the fuel grade was used as your 'knock sensor' to operate at maximum advance without knocking, but I don't know for sure either. I have no idea if any power gain was available simply due to additional timing advance by being able to operate right at the threshold of knocking, so I wouldn't try to argue that point either way.

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Could very well be true... my experience with all this stuff goes back quite a few years, i.e. I may be living a bit in the past.

dwalby said:

I suspect the main power gains have been the result of better airflow through the cylinders, ...

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Agreed 100%. To add to that, I would say airflow improvement overall (in and out) has been a major factor in power gains over the years. Add in FI and computer control of spark and fuel delivery and now we can easily get 100hp out of a 1 liter motor that runs on 87 octane for 200k miles. Pretty impressive when you think back to the 70's & 80's cars compared to what we get today.

One more thing I wanted to expand on from before, is that with knock sensors one of the ways the engine can get by with lower octane fuel is because the electronic FI system can retard the timing a bit if it detects knock. With an old-school vacuum/mechanical advance mechanism it didn't know what octane you were running, so it either knocked or it didn't, it couldn't compensate. So it allows the benefit of being able to run without knocking on lower octane, but not necessarily extracting as much power as would be possible with higher octane.

I drive a Hyundai Genesis Coupe, and the thing runs fine on 87 octane, although it technically puts out just a few more HP running 91 according to the factory. I assume its doing a bit of timing retard at max output using 87 octane (11.5:1 compression ratio), but the factory HP spec is nearly identical with either fuel, its within a few HP. So I assume from that data that a little extra advance/retard adjustment isn't going to change the power output significantly, but I'll also admit that just because its true with one particular engine doesn't mean its true for all engines.