Yes, in hilly terrain, you might be able to come up with a test case where the benefit of speed gain with the transmission in neutral going downhill would offset the idle fuel used. I suspect it would have to be extremely hilly though.

One technique is to always select the very highest gear you can going down a hill. This minimizes the pumping drag of the engine and allows the car to attain the best speed it can while using no fuel going down the hill. Of course, if this speed ends up being too fast, then you have to use your brakes which is another loss. Complex problem.

I recall reading somewhere that the prototype "cars" (not street cars) they use to set mileage records (I think the current record is something ridiculous like over 1000 mpg) alternate between accelerating up to speed and coasting with the engine off. This allows the engine to either be off or running at full throttle where it is most efficient. So there are a lot of weird techiques in the corners of the envelope.

- Mark

 

I'm not sure where I would find anything below 87.

Jim

 

Regular grade gasoline in the entire state of Colorado is 86 "octane".

 

Don't know what it is now but when I lived in Idaho in the 80's low grade was 85 octane.

 

That is interesting. When I lived in CO back in 1969 I really didn't know what the rating was as it wasn't posted like it is today. Here on the East Coast I haven't seen anything below 87, but if I do I'll stay away from it.

If regular is lower then 87 then what is mid grade?

Jim

 

jbewley,

I don't remember...I moved from there 7 months ago and didn't use mid-grade. Also, "octane" doesn't really measure octane levels any more, it measures combustibility of gasoline. At higher elevations (e.g. Colorado), the gasoline needs to combust at lower pressures, hence the lower octane. The higher the octane rating, the MORE resistant to ignition the gasoline is...it can be compressed to higher levels without pre-igniting.

(P.S. Now that I've probably messed up some of the details, we'll get a few people who really know what they're talking about to polish up my discussion!)

 

Just remember that TurboCharger's (or SuperCharger's) negate the effect of higher elevations.
__________________________________________________ ____________

In Florida; it's 87/91/93 (usually 10¢ apart) or just 87/93 at some stations.

 

Do you mean that turbochargers are able to completely offset the effect of lower atmospheric pressure (at higher elevations) by adding more boost at higher elevations than they do at sea level? That would suggest that turbos are not operating at their maximum capacity at sea level.

Or by "negate" do you mean that they reduce the impact somewhat?

 

I could be wrong about the Mini, but most turbo cars today have a fixed wastegate and deliver a fixed amount of boost above ambient. I'd be surprised if the Mini is different. So the engine power decreases with altitude just like like a normally-aspirated engine

There have been a few cars over the years (Mitsubishi's mostly as I recall) that did have an absolute pressure controller for the wastegate and would raise turbo boost with altitude and maintain sea-level pressure up to some altitude where the wastegate was totally closed and then the power would stop dropping.

The problem with absolute pressure controllers is mostly expense, but there is also the issue of cooling. As you gain in altitude, the cooling capacity of the radiator and intercooler drops along with the air density. If the engine power drops at the same rate than everything stays balanced, but when you start increasing the boost at altitude, then you are keeping the power (and cooling requirements) fixed, while the cooling system is decreasing in efficiency. So unless you signficantly overdesign the coolling system, you start running the risk of overheating at altitude.

- Mark

 

Your TurboCharger is capable of producing far more boost than your engine could survive.

The ECU uses a Waste-Gate to by-pass enough exhaust gas to control the boost pressure.

Yes, at higher elevation, your boost would be greater as your ECU will maintain the same manifold pressure.

There may still be some loss of power due to the greater compressive heat that the intercooler will have to deal with.

With an aircraft, when the waste-gate is completely closed, you have reached a 'critical-altitude',
above which maximum power is reduced (perhaps 20,000 feet or so).

Had not heard of the above, but would recommend a ScanGauge that would not only give you Manifold Pressure (boost) but also read your inlet temperatures (and a dozen other parameters).

 

On the subject of octane levels, why are a few on this board concerned about finding 91 as opposed to 93? It's tough finding 91 in Texas so I pump 93. Are they just trying to save some money without hurting their fuel economy at the same time? Is 91 going to work better than 93 mileage-wise?

 

Yep, octane in this day and age is a measure of the speed of the fuel's "flame front" which is the burn rate of the fuel after ignition. The burn rate is dependant on pressure; the higher the pressure, the faster the oxygen in the air can combine with the fuel because of the greater amount of oxygen per unit volumn of air.

The pressure is controlled by spark timing; an early spark will ignite the fuel as the piston is still rising and the aim is to have complete combustion and peak pressure just as the piston starts moving down on the power stroke. If the fuel detonates, peak pressure is reached while the piston is still moving up, and which wastes a lot of power and can damage the engine. If the fuel can burn correctly at a higher pressure, then the Mini's engine will sense that, allowing higher pressures inside the cylinder and the engine will become more efficient, since more pressure equals more power.

The Mini should be very happy with 93 octane, in fact with accurate enough measurements, you should see 1 or 2 mpg improvement with 93 over 91 octane.

One item that PilotArt and Mark forgot: turbochargers work better at higher altitudes than low. The speed of the turbine is dependant on the pressure differential between the exhaust manifold and ambient, therefore high altitudes actually allow the turbine to turn faster. This is usually only seen in airplanes flying above 8,000 to 10,000 feet, however, the effect might be seen as low as 5,000 feet.

 

Ken, very interesting information! I learned something! Makes me wonder if there's any limit to how high one can go. Has anyone tried Sunoco 93.5? Is there any benefit? I don't understand why they say that instead of 93? Does 0.5 octane make that much of a difference?

 

As I recall, all of Colorado is above 5,000 feet. We lived on relatively flat ground in a valley in SW Colorado at 7,000 feet elevation. Hence the lower "octane" numbers for the state. I didn't own the MINI then. The 4" ground clearance wouldn't have worked there anyway (had to replace the oil pan on my wife's VW New Beetle for that reason; also 4" clearance), since most of the roads are gravel once you leave the cities.

 

Sunoco might be playing with a bit of advertising with that 93.5 octane. It might also be a real 93.5, too, I don't know. Best way to test it is run a couple of tanks of the stuff and see if it makes a mileage difference.

High octane fuel is a chemistry game. Until the EPA enacted a ban on leaded fuels, the highest octanes were aviation gasolines that were rated at 100 and 115 octane. They were used in the really big piston engines during and after WWII and both of these fuels are no longer produced.

 

In California 91 is the highest (at least that I have seen) and that is what my manual calls for. we have 87, 89 and 91 as our 3 grades of fuel.

 

You can't compare octane numbers across different uses - aviation fuels use a completely different octane test. (And some aviation fuels still contain lead.) And most countries use a different octane test than the US.

- Mark

 

Important point would be to NEVER put Aviation Gasoline in a street-driven MINI; the Lead will quickly destroy your CAT.

We are still awaiting a 'lead-free' Aviation Gasoline, but it has not been introduced yet.

For some 80 octane aviation engines, there is an approval to operate on 87 octane (Lead-Free) Automotive Fuel w/o alcohol
and they do better on that, than on the 100 Octane "Low-Lead" that is almost universal now inside the USA.

 

Sorry, I seem to be causing a significant topic drift. I thought it went without saying that avgas is unusable in a car. I was using avgas as an illustration of how wide octane rating can be, although the ratings systems between auto and aviation isn't as different as you might think. I got to play around with this while in college.

I've heard rumor that some of the new biofuels are testing with huge "octane" ratings, because of their extremely slow flame fronts.

 

So, let me suggest a scenario. I'm driving through Denver from another state. I need to stop by a gas station. Am I better off using regular instead of premium even if gas mileage DROPS a little?

 

No, you want to stick with the highest rating you can get. If CO indeed gets a premium gasoline that's lower in "octane" rating that other states, your engine will run more efficiantly on the higher rated fuel than the lower rated fuel, regardless what the absolute numbers are.

Remember that "octane" rating is a measure of the speed of the fuel's flame front under pressure, not how resistant a fuel is to being ignited.

 

I doubt this. I don't think the Mini engine is "octane adaptive" and can automatically take advantage of octane ratings beyond the recommended octane. There are a few engines that do - they automatically adjust engine parameters to stay right on the ragged edge of engine knock. But most are not - they have knock sensors that detune the engine if it knocks, but their engine turning is designed for their rated octane and octane which exceeds this is of no consequence.

C&D also had a big article about octane and -- surprise -- their basic advice was to use what the engine mfg recommends. For a modern car with knock sensors, you can burn lesser octane than recommended without harming the engine, but you probably are going to lose max engine performance about in proportion to the savings in gas (e.g., 10% or so).

- Mark

 

Hmmm.

I've run a couple of full tanks with shifting into neutral and coasting downhill and a couple of full tanks with staying in gear on downhills.

I think I get better mileage coasting. The difference may be as much as 2 mpg (almost 5% for me).

Seat of the pants, this makes sense to me because my car maintains speed better in neutral than when it is in gear but my foot off the gas. That is, I have to maintain pressure on the accelerator in gear to maintain the same speed as in neutral. Yes, the throttle is a drive by wire linkage so this in itself may not be indicative of more fuel injected into the cylinders, but it does feel like more gas is needed to maintain rpm (in gear). You can feel this when feathering the clutch.

Also, my computer mileage indicator (about 1.5 to 2 mpg optimistic, incidentally) ticks up a tenth or two on each of my three long freeway downhills to and from work, but only when I coast (more after a recent mileage reset, less after more miles on the reset).

But it probably would take many more tanks with more careful data recording to get a statistically significant result. There are just too many other variables -- weather, traffic speed, defrost (AC) on or off, headlights on or off, stereo volume, seat heat, etc. So I can't definitively negate the OP's point about fuel mileage while coasting. But from personal experience, I believe it to be incorrect.

To expand on my first sentence, I've also done half tanks in gear followed by half tanks coasting. The effect of the downhill mpg gauge uptick is cumulative, with the next morning's uptick adding to a slightly higher ending mileage the night before. Over two or three days of commuting with coasting, I can make the indicated mileage increase by single digit mpg. Of course, this could also be a problem with the mileage calculation algorithm or sensor calibration, so this is still not definitive.

Note, if you try coasting, be very careful on the next uphill! E.g., I keep my right hand formed in an American sign language "N" when I'm in neutral so I remember not to press the accelerator. This is especially important if you listen to music at high volume. Also, I wouldn't do it in poor traction conditions or in heavy traffic.

$0.02

 

That's pretty convincing stuff. Nothing wrong with your reasoning, methodology, or conclusions.

I can only add that another case where being in neutral would be dangerous is during long, steep downhills, when the car accelerates while in neutral. You'll get better overall mileage in neutral in this case, but you'll also loose the engine braking effect. This requires the brakes to absorb more energy, which would make them become hotter and increase pad wear. In extreme cases, the brakes could overheat and lose effectiveness, which usually happens right at the steepest part of the hill.

Coasting in neutral aside, I've found that large mileage gains can be found by anticipating the traffic and using the brakes as little as possible. Brakes are huge energy wasters (since that's what they're designed to do). It's better to let off the throttle early and coast in gear to a stop, than it is to stay on the throttle and brake hard. This is where cars with regenerative braking get most of their increased gas mileage from; instead of dumping the energy of braking overboard in the form of waste heat, the car stores that energy.

 

I hve been getting 42 to 45 mpg, by not tacking over 3K or gearing down under 2K. I realize this is not the type driving most want to do, but the mpg has remained the same. Doing this I can get into 5th and 6th gear regularly and still get great mpg's.

Jim

Jim