I just did some "experiments" on the coasting issue and didn't get the results I expected. Since the fuel is supposed to be shut off when the car is decelerating, I expected that going down hill in 6th gear would give better mpg than coasting with the clutch in. I used a ScanGaugeII to see the mpg reading because it will display higher numbers than the tach digital readout.

Going down a steep hill at 55 mph in 6th produced 120 mpg. Going down the same hill at the same speed produced just under 300 mpg with the clutch in. Foot completely off the gas pedal in both cases.

I also noticed that going down my very steep driveway in 1st gear gets no better than 24 mpg (going up is 6-8 mpg ). In theory, if the fuel was cut off on deceleration, it should be pegging the highest number the ScanGaugeII could display.

 

More years ago then I want to remember I had a Vauxhall. I would coast with it until I ran into trouble with break problems as that will put a lot of stress on them having to do all the work to slow the car.

Jim

 

Fuel flow isn't directly measured in mosts cars as low-flow fuel transducers are fairly expensive so it is "inferred" from other data. At the levels of mileage you're attempting to measure, I think you're looking at anomalies of the sensors and the algoirthms, not accurate data. On your driveway, the speeds (both engine and road) may be low enough that the engine is using idle fuel.

- Mark

 

That makes sense. But the original post question went like this: 2 Minis start at the top of a long hill and end the contest 30 miles later at the bottom. Mini #1 goes all the way in neutral. Mini #2 goes all the way in 6th. Neither uses the gas pedal. Who uses less gas? Answer: Mini #2. (see pages 1 and 2 for reasons)

 

That would explain why the Car Data mpg from the OBC doesn't always match the values measured at the pump.

 

In the blue 'paperback' that came with it,
ScanGauge details how to 'calibrate' the MPG function of its 'Trip' features by entering actual amount fuel added.

Believe it also has a Gallons-Per-Hour reading, that would give you a good idea of the actual fuel use while idling.

If it would read in "Pounds Per Hour" (or mile) it would be more accurate, we buy it by the gallon, but engines burn it by the pound
and there is a measureable difference between Winter and Summer 'pounds per gallon'.

 

I did an experiment last year when I took a trip to Colorado. I thought the hills would kill my mileage. But I got several mpg better overall by all the coasting downhill. I saved more on the downhills than used on the uphill portions so net gain. Thought it was weird...

I was fairly careful and let momentum build up on downhills, allow momentum to carry me up part of the hill, let the speed drop some as I crested hills.

The whole time I was in Colorado in the hills I got better mileage than the flats.

 

Were you coasting in gear or out of gear?

Jim

 

When it was a decent size hill I'd engine brake, so it was in gear. I'd drop out of gear if my speed dropped and there was too much engine braking.

Also-I'd heard the assertion that higher octane gas has fewer btus available, lower energy, so could lead to lower mileage. I don't know if true.

It all depends on how much the timing is being pulled to keep the motor from knocking, but I'm not sure how one can tell. If the timing isn't being pulled, the motor isn't trying to knock, so you wouldn't lose anything on lower octane.

(I finally found the UK vs US english setting that threw my OBC mileage so far off-imperial vs us gallons lol)

 

That's not true: octane is a measure of how fast the flame front will move after ignition, given a set condition. It has nothing to do with the amount of energy contained in the fuel. Octane is actually an outdated term, the rating is called "anti-knock" these days. Octane comes from the WWII era, when the hydrocarbon "Octane" was added to slow the fuel's flame front speed.

Basically, "octane" dictates how much pressure the fuel can tolerate before it starts buring too quickly (aka detonation, aka knocking) when under pressure in the combustion chamber. Greater tolerable pressure means the ignition can happen earlier, resulting in greater pressure pushing the piston down. That causes more power to be generated by the engine for the same amount of fuel put into it, which is greater efficiency.

I've tested this in my MCS, 3 points of anti-knock rating equals a 3-5 mpg increase and offsets the greater cost per gallon of premium fuel. Not only is less gas being burned, but it also costs less money to make the car go over a fixed distance.

If you want more documentation about this, look around for aircraft engine texts. There are also engine manuals printed by Pratt & Whitney Aircraft Engines in the 1950's that are easy to read and go into a great detail about piston engine combustion and how to control it.

 

I know this has nothing to do with MINIs, but so would an engine with a compression ratio of 10.5:1 need premium fuel? Or does it depend on the engine?

 

I don't know it as fact, but it's something I've heard. It may depend on how the octane is raised. Isn't it true that adding ethanol raises octane? Ethanol has fewer btus available for the same volume. It may not be true that premium and regular are exactly the same except for octane rating. Like I said, it's only something I've heard, but it might be interesting enough to look in to a bit...

I get better mileage with premium. But I wonder if there might be cases where this isn't true. With our motors, they change the timing and play with other parameters. But that may not be true for other motors.

 

There is some minor variation in energy content with octane in conventional non-ethanol fuel, but it can go either way (the high-octane fuel may have slightly higher or lower energy content), varies with the fuel brand, and it's generally not sigificant. (Gasoline is naturally-occuring substance and will be subject to natural variations in things like energy content.)

Ethanol is a whole different story as it has much less energy content than gasoline. Everything else being equal, you should be 3% lower gas mileage with E10 vs. pure gasoline.

Sorry, but on a stock engine running properly, there is simply no technical rationale that can explain 3-5 mpg increases in fuel mileage simply with a change in the octane rating of gasoline. Likely other factors are interferring. Because driving conditions vary so drastically and because of fill-up measurement error, you really have to average your mileage over a large number of tanks before you really can say much about anything.

- Mark

 

It depends on the engine. CR is only one factor that increases octane appetite in an engine. There are 13:1 engines that specify regular and 8:1 engines that specify premium. Having said this, if everything about an engine is kept the same, raising CR will increase the octane requirement.

- Mark

 

As long as an engine doesn't change it's ignition timing, anti-knock rating will have no effect on mileage. However, the Mini's engine changes it's ignition timing based on input from the knock sensor. Logically, if the ECU responds to the knock sensor, what the ECU is doing is advancing the ignition timing until detonation starts, then retarding the ignition until it stops.

I've seen plenty of engine analyzer data here at NAM that shows the ECU changing the ignition timing based on sensor data. If the ECU is keeping the ignition advanced as far as possible based on performance of the fuel, then the peak combustion pressure is determined by the fuel's anti-knock rating. Peak pressure determines power output, so the engine will generate more power at a given throttle setting due to higher combustion pressures allowed by premium fuel. This means it will take less throttle to generate the same horsepower when the engine is operated as less than full throttle. Less throttle means better gas milage, all else being equal.

You're correct in pointing out the large margin of error in any sort of mileage test. I tested this over 4 tanks of each fuel, but I'll have to re-do the test to see if I get repeatable results.

 

I've got about 34K miles, and mine probably averages 2-3mpg better with premium. I don't run regular all the time, but I consistently see lower mileage when I do.

 

I suspect that's the reason I always got such great mileage with my Sonett on the Missouri hills. It freewheeled (coasted out of gear) UNLESS you locked it out and drove like a "regular" car. That was actually legally required, in order to meet the emission requirements that were new at the time and driving everyone else nuts. I never locked it out except on snow and ice, so most of the year I coasted.

It's been 3 decades, so don't quote me on this, but I think I routinely got about 36 mpg. That's without anyone (manufacturer or driver) having the slightest interest in fuel conservation. Gas was about 30 cents a gallon at the time.

 

True IF the engine is "octane adaptive" which means the engine is continually adjusting its timing and engine tuning to stay right on the edge of engine knock. Few engines are and the Mini engines are not. Instead, they use fixed engine timing which is backed off in the event the knock sensors fire which happens only under demanding conditions even when running on regular fuel. Timing is fixed at a value which should not cause knock under most conditions with premium and as long as the engine doesn't knock, nothing changes with regular. Thus, the degradation in fuel efficiency is fairly small - I'd guess a few percent worst case. It would take a tremendous change in engine tuning to be able to cause 15% changes in fuel mileage with simply a change in fuel octane (and a minor one at that - 87 vs. 91).

I'm not recommending using regular fuel. To get full power you need premium. But just because premium is needed to make full power does not mean that premium burns any more efficiently at lower power settings, and unless you're on the autobahn or the track, the engine spends most of its time at very low power settings, seldom going over 25% power in typical steady-state driving. At these low power settings, the engine doesn't care a whit whether it is burning regular or premium.

- Mark

 

When you're "at altitude", the fuel injection system will sense the lower atmospheric pressure (lower oxygen) and feed less fuel so the air/fuel ratio remains near-ideal (stoichiometric). The result: less power and increased fuel mileage. My 50 mi/gal BMW motorcycle got 60 mi/gal while over 7,000 ft. in Colorado, but required a lower gear than normal (due to less power) when passing. I don't think uphill/downhill was affecting your mileage - overall altitude was the culprit.

- Bob

 

Just for the record, "altitude" is a term used to describe position in the atmosphere (above terrain or above sea level). "Elevation" is a term to describe your position on the ground, or the terrain itself. In Colorado, we lived at 7,000-foot elevation (above sea level), not altitude.

If your MINI is at 7,000' altitude, please post photos!

 

Yes, Mark, if the engine is using a fixed ignition timing, octane doesn't have an effect at partial throttle settings. I was under the impression that the Mini adjusts it's ignition timing more than it does, but I reviewed some of the engine data posts and I think you're right, this engine only retards the spark.

What gets me the worst is how little real technical data has been released for this engine. I'd love to know what all is going on with the variable valves, direct fuel injection and the such, but BMW-Mini is being really tight with that information. There's not even an aftermarket manual for the Gen 2's yet.

 

True octane adapative engines aren't very common. It makes the engine software much more complicated and often results in odd surging and other problems. And at light loads that a car engine typically runs, the increase in fuel efficiency with adapative software isn't much if any.

This may change as we get better computer control of more engine parameters, valve timing, and other engine parameters. Variable displacement may also encourage more aggressive octane adaptive strategies as engines which shut down cylinders tend to run the remaining ones at higher loads.

BTW, I'm not discounting that you're measuring better mileage when you use higher-octane fuel - just saying that some other factors are probably at play. Without extensive testing with a dyno, it's just damn hard to separate the factors affecting fuel mileage. Which plays into a lot of people able to sell some real snake oil.

- Mark

 

For aviation performance computation "density altitude" is used (a correction is made for non-standard temperature and barometric pressure)
and temperature has the greatest effect (increases in humidity also reduces 'density', or amount of available oxygen).

At sea level, on a 95f day, your engine would perform as if it were at a 2300' elevation on a standard day
(standard temperature at sea level is 59f and at 2300' it would be 51f).

So, at a 7,000' elevation on a 95f hot day, (60f warmer than standard) you would compute a 'performance altitude' of 10,700'.

Since the actual temperature is more often higher than "standard" in most high elevation locations,
your MINI's power and fuel economy will usually act as if it were at a higher elevation
than your true elevation above sea level. (Turbo/Supercharging reduces performance 'hit' at higher elevations.)
__________________________________________________ _____________

In the days of timing lights and dwell meters, a little occasional spark knock (pinging) was acceptable unless pulling heavy loads or hills
and we knew that better fuel, (or a one degree reduction in spark advance would stop the ping and reduce performance).

Following the 'IKE' created fuel shortage, I was forced to use a tank of regular grade gas in my non-turbo Cooper,
I could detect no change in performance in normal driving.

I have the ScanGauge II installed and watching its gauge for ignition timing shows the spark advance to vary often and a lot,
but could never see any reduction due to detonation prevention.

 

Just a clarification - virtually all engines vary their engine timing, mostly with load and RPM, but very few vary it continuously to keep the engine right on the edge of knocking based on the knock sensors.

- Mark

 

disregard