Topic: Your mileage may vary....

[nickjtc]

We've had discussions about the type/octane of gas/petrol we like to use in our bikes, and we've had discussions about the kind of mileage we get.

So here's a question. Sophie routinely gets 54 mp(imperial)g. My riding does not change much. I seldom rev over 6,000 and usually cruise at around 5,500 which with my gearing = 110kph-ish. I always use the same gas, usually from the same chain of gas stations.

I've just got home from a two day trek to work in Revelstoke, so have put about 460kms on the bike. I filled up on the way home and found that I had averaged 62 mp(imperial)g for the trip. The only thing that is different is that today is stonking hot (currently 35c).

Why would there be a significant increase in mileage/decrease in consumption? I cannot remember there being this kind of variation in hotter weather before.

Let the discussion begin.

[medic09]

One more reason for me to move back to BC.  I can't get that kind of mileage down here, even on my fuel-injected bike.

[Pinhead]

Hmm my '79 CB650 gets 65mpg (US) in town all the time, and 47mpg on the highway (75/80mph).

You may be running slightly lean under normal conditions... And then when it's really hot out the air is thinner and therefore closer to the magical 14.7:1 ratio... Just a guess, though.

[nickjtc]

You may be running slightly lean under normal conditions...

Isn't that status quo for Honda carburation??

[TwoTired]

Hot engines burn fuel more efficiently than cold ones and can convert more of the fuel charge into energy into propulsion.
Cooling fins are fixed in size and transfer heat to passing air in proportion to the temperature differential.  They would transfer no heat if the air temp was the same as the fin temperature.
Your air cooled engine will run hotter in hot weather than in cold weather.

Another factor is piston to cylinder fit.  The aluminum piston expands more rapidly than the steel sleeve.  A hotter engine has a piston to cylinder fit that is tighter and this makes for better compression.  More compression means more power and you can hold the throttle at a slightly lower position during cruise, using less fuel per unit of distance traveled.

Cheers,

[Tom Stark]

Hot engines burn fuel more efficiently than cold ones and can convert more of the fuel charge into energy into propulsion.
Cooling fins are fixed in size and transfer heat to passing air in proportion to the temperature differential.  They would transfer no heat if the air temp was the same as the fin temperature.
Your air cooled engine will run hotter in hot weather than in cold weather.

Another factor is piston to cylinder fit.  The aluminum piston expands more rapidly than the steel sleeve.  A hotter engine has a piston to cylinder fit that is tighter and this makes for better compression.  More compression means more power and you can hold the throttle at a slightly lower position during cruise, using less fuel per unit of distance traveled.

Cheers,

I think I am going to disagree here just a bit.  So far as engine temps go, I can't speak numbers about the bike, as I have not temp gauges on it, but I CAN speak about numbers on my air cooled Corvair.  The Corvair does have a cooling fan, but does NOT have any thermostatic control on it, so it blows all it can all the time.  When I am driving in cold weather, (42 degrees F), the cylinder head temp, (measured from under the spark plug), will get to about 275 in town, and the oil temp will not read on the gauge, (lowest reading 140 F on gauge).  In hot weather (85+ F) the head temp is still about 275 in town, but the oil temp is 225F or so.  Get on the freeway, and oil temp will hit 275 in about 10 minutes at 4500 RPM, back it down to 3000 and it drops back down to 225 or so. 
I would need test gauges on the bike, but it would not surprise me if the head temp was within 10 or 20 degrees F during hot or cold weather. 
So far as piston clearance goes, yes they do expand like you said, but it is the rings that do the sealing not the piston.  On one of the Pro Stock cars, the fastest engine had pistons that were all beat to crap, with the skirts hammered in.  They looked like hell, and were loose, yet that motor was the fastest. 
Why did he get better mileage when it was hot???  Beats me.  Hot air is less dense, and that means less air/fuel in the cylinders, and that means less power.  The engine is going to pump X CFM of air.  The thing is, that engines make power from the POUNDS of air they burn, the CFM does not matter.  On a cold day, you will get more pounds of air in the same CFM so you will make more power.  That is why if you do not qualify on Friday night, you most likely will not be racing Sunday.  Now, I suppose that because the air was hot, so he was burning fewer pounds of air, and was cruising, so he did not need max power, the lower air mass may have helped the fuel mileage. 
Tom

[SOHC steve]

Maybe higher engine/ambient temps=better fuel vaporisation and more complete combustion and hence better economy.Could also be different grade fuels?To be sure to be sure you would need to do another run and check figures.Remember these are 30+ year old machines,they will no doubt all have their own little quirks,but 62MPG sounds like a real good quirk to me...well done These old bikes never stop amazing me,on a totally unrelated issue my K6 has been sitting for over 2 years now without running,it has been undercover awaiting me to get around to rebuilding her (the oil leaks finally got to me).Today I went out,put some power on her,turned the fuel and choke on,kicked it over twice to prime it and then attempted to start it.To my amazement it started not only on the first kick but on the first compression stroke...and run as good as when I last rode it!

[KB02]

Another option that has not been discussed yet is this: What is your normal riding? City? Highway? A little bit of both? As a general rule, city driving will lead to few MPG (KPL) than highway driving. If I ride a tank of gas solely to and from work, I will get less mileage per unit of gas than I do If I go out for a ride all afternoon. The best mileage I ever got was on about a 150 stretch of highway. There was no in-traffic riding, or shifting gears up and down, it was just get up into top gear and go riding.

That, combined with everything else, might have something to do with it.

[nickjtc]

Interesting discussion. As I noted in the original post, though, nothing has changed: same gas, same type of riding (highway); the only difference was the heat yesterday but I've ridden in the same conditions before and not noticed this consumption 'blip'.

When I had my VFR I did notice that it got much better gas mileage when I used premium, even though it would run perfectly well on regular. Perhaps when I filled up up the other day the gas quality was much 'better' than normal coming out of the regular pump....

[OldSchool_IsCool]

My $0.02

Heat Dissipation? The only thing about Lloyd's post I disagree with is that cooling is proportional to the SQUARE of the temperature differential.  It's a parabolic curve, not a linear one.  The greater the temp difference, the faster the heat transfer.  I think this makes Lloyd's argument even stronger, however.

Warm air is thinner?  Depends on what you mean by thinner.  Remember, air is mostly (about 70%) Nitrogen.  It's the Oxygen (around 20%) that plays the bigger role in combustion.  I don't know the answer to this, but just maybe, warmer air has a larger oxygen component then cooler air.  Since atmospheric oxygen is actually TWO oxygen atoms, (O2) it may be more likely to "settle out" and concentrate when the lighter components "fly off" with all that Brownian energy heat has given them.

Warm pistons makes for happy engines?  I don't think the ring gap will change much as the piston swells from heating.  It's the sleeves that dictate gap and the sleeves and rings are both steel.   Now, the fit of the rings to the aluminum piston, that I could see tightening up quite a bit.  Taken to an extream, the swelling pistons will JAM the rings into the sleeves causing engine seizure, so be careful walking this fine line.

[crazypj]

Its already been said early in post, I agree with it.
If ambient temp is higher and air is less dense there is a chance that the F/A ratio is closer to correct which will give better mpg.
 In my experience, when its running lean you give it more throttle to maintain cruising speed, engine runs hotter and mpg goes down.
 I first noticed this almost 30 yrs ago when I did big bore kit on 550.
Bigger engine, semi race cam, K&N filters, much larger jets, but, jump from 26~27mpg stock to around 41~42mpg
May not be scientific but gets my vote. 
PJ

 

[medic09]

Looks like I should have gotten popcorn for this  one.  Who knew?

(how do I transfer a popcorn smiley from another site?)

[ieism]

What about octane? My 550 is 10 km/h faster on regular unleaded (95 octane), compared to super unleaded (98 octane). So I suppose it burns fuel more efficiently too.
Will something lower than 95 make it even better?

[Cvillechopper]

All this tech jargon about CFMs and such, what about the tires.  Front tire happend to be a touch low?  Not sure how much of a difference would be made by a pound or 2 but several will absolutely effect the odometer which, I assume, is one of the basic metrics for your computation.  Just wanted to get in on the discussion for the fun of it.  Now you may laugh

[nickjtc]

All this tech jargon about CFMs and such, what about the tires.  Front tire happend to be a touch low?  Not sure how much of a difference would be made by a pound or 2 but several will absolutely effect the odometer which, I assume, is one of the basic metrics for your computation.  Just wanted to get in on the discussion for the fun of it.  Now you may laugh

I'm not laughing. You are right. Correct tyre pressure = appropriate rolling resistance and circumference. Too low = more rolling resistance/changed circumference.

However, I think that there would have to be a significant difference from 'normal' to make a 10%+ difference in mileage.

Also, sadly, tyre pressure is another one of those things I am anal about, and which I check before every ride.

[Cvillechopper]

Also, sadly, tyre pressure is another one of those things I am anal about, and which I check before every ride.

Yeh, you've said that somewhere before so I didn't think it was really a factor, but there seemed to be more theory here than anything else.  I wanted to ask about wind direction as well.  I've had a slight tail-wind give me a bump of better than 5mpg over a 200 mile haul. 

Another question.... I posted a very specific routine I follow when filling up my bikes on another forum that standardizes bike position and the level the gas is filled to.  Were you on any type of hill at the gas station?  Do you use the center stand or side stand? Do you have a specific spot on the filler neck you fill to?  Little differences like these get blown way up when the resulting number is used in a calculation. 
Just trying to eliminate the simple stuff.  That's why I'm so anal about how I fill up the tank now including a spreadsheet with every stop entered, mileage, fuel, date etc.  I can see patterns developing early and hopefully keep her on top of the game.  Planning on starting one for the 550 once she's back up and running.

I think there's a medication I can take, isn't there?

[nickjtc]

I think there's a medication I can take, isn't there?

The medication is more riding. Repeat daily; twice a day as necessary.

Again, sorry...I fill on the centre stand, to the bottom of the filler neck inside the tank (which allows for expansion when the temp gets up to mid/high 30's c).

And the mileage was the same in both directions, with ambient wind being from the same direction each way.

[dusterdude]

james,anal aint the word for it.

[Tom Stark]

My $0.02
...
Warm air is thinner?  Depends on what you mean by thinner.  Remember, air is mostly (about 70%) Nitrogen.  It's the Oxygen (around 20%) that plays the bigger role in combustion.  I don't know the answer to this, but just maybe, warmer air has a larger oxygen component then cooler air.  Since atmospheric oxygen is actually TWO oxygen atoms, (O2) it may be more likely to "settle out" and concentrate when the lighter components "fly off" with all that Brownian energy heat has given them.
...

Warm air will not make the power that cold air will, period.  no question or doubt about it.  Friday night qualifying session is usually the coolest temp wise, and if you do not make it into the field Friday night, you will most likely not make it in at all.  The Dart car was not ready one Friday night, and did not run well that night.  Then during the heat of the day Saturday, they were the 3rd quickest of all the cars, (around 38 cars), only 16 cars get to race, they wound up being #18 or 20 because everyone went so much faster on the cool Friday session.  Lower air temps = more horse power.  General rule is about 1% increase for every 18 degree drop in temperature.  And let's not forget barometric pressure.  The more there is, the more power you can make. 
Tom

[OldSchool_IsCool]

My $0.02
...
Warm air is thinner?  Depends on what you mean by thinner.  Remember, air is mostly (about 70%) Nitrogen.  It's the Oxygen (around 20%) that plays the bigger role in combustion.  I don't know the answer to this, but just maybe, warmer air has a larger oxygen component then cooler air.  Since atmospheric oxygen is actually TWO oxygen atoms, (O2) it may be more likely to "settle out" and concentrate when the lighter components "fly off" with all that Brownian energy heat has given them.
...

Warm air will not make the power that cold air will, period.  no question or doubt about it.  Friday night qualifying session is usually the coolest temp wise, and if you do not make it into the field Friday night, you will most likely not make it in at all.  The Dart car was not ready one Friday night, and did not run well that night.  Then during the heat of the day Saturday, they were the 3rd quickest of all the cars, (around 38 cars), only 16 cars get to race, they wound up being #18 or 20 because everyone went so much faster on the cool Friday session.  Lower air temps = more horse power.  General rule is about 1% increase for every 18 degree drop in temperature.  And let's not forget barometric pressure.  The more there is, the more power you can make. 
Tom

OK, guess that answers that.  Warm air would have a negative effect on power/speed.  Does that also mean a change in millage?  Just asking, I don't know how they would relate.

[Tom Stark]

Really good question.  One would "think" that warm air would hurt the mileage, but there are lots of variables and I have seen things proven to be the opposite of what was expected, so who knows.  When I was project manager for North American Propane Fuel Company, (dual fuel vehicles), I can recall that a lot of independent guys were working on getting more MPG from cars, and heating the air/fuel was a big thing with them.  None of their stuff panned out, but Smokey Yunik (sp?) took the idea a step further using a turbo as a "one way valve" so that while the air/fuel was hot, it did not lose density because it was heated downstream of the turbo.  Who knows, perhaps a little heat can help.  I know that a little lean will help milage, but then when you go too lean, it hurts it.  I can say that with as big a jump in mileage as was seen, the possibility of error in calculation definately exists.  Not say the guy does not know how to calculate MPG, but could be that the gas pump actually pumped more gas than it recorded, so something like that.  Who knows!  Right now I know, with only a couple hundred miles on my bike, that I am getting between 46 and 50+ depending.  (all US measurements).
Tom

[ofreen]

So here's a question. Sophie routinely gets 54 mp(imperial)g. My riding does not change much. I seldom rev over 6,000 and usually cruise at around 5,500 which with my gearing = 110kph-ish. I always use the same gas, usually from the same chain of gas stations.
I've just got home from a two day trek to work in Revelstoke, so have put about 460kms on the bike. I filled up on the way home and found that I had averaged 62 mp(imperial)g for the trip. The only thing that is different is that today is stonking hot (currently 35c).
Why would there be a significant increase in mileage/decrease in consumption? I cannot remember there being this kind of variation in hotter weather before.

I've had this experience many times over the years.  It happened again a couple of weeks ago with the 750.  With no changes in the commuting routine, or radical changes in the weather, the bike went 20 miles farther than usual before having to be switched to reserve.  I've had this happen at one time or another on all my bikes.  When it happens with my BMW R100GS-PD with its 9 gallon tank, it is very noticeable because the bike will go 40 or 50 miles farther to reserve than usual.  The jump in mileage always coincides with a tank fill-up, then goes back to 'normal' when that gas is gone.  It has happened so many times that I'm convinced that it is something about the gas that causes it.  What that something is, I don't know.  I doubt it has anything to do with octane, because the 750 gets the same mileage on 87 regular as it does on 92 premium. 

I'll bet somebody in the fuel industry knows the answer to this, but they ain't talking.

[SOHC steve]

Ethanol blends or ratio? I know here in Australia there have been some rogue fuel suppliers (gee fancy that ) selling ethanol blends as normal unleaded.I don't know if you guys in the USA have blends as standard or not,but it would definitely make a difference.On a side note,I run my Mopar daily driver on LPG (or propane as it is known in the USA).It is blended with a variety of other gases over here (apparently this is why they don't call it propane here) including butane and economy and performance differences are noticeable.
    Also,did your ride take you to a different elevation to your normal riding? To really prove your results you would need to do another run,otherwise its pretty hard to really work out a definite answer I thinks

[nickjtc]

Also,did your ride take you to a different elevation to your normal riding? To really prove your results you would need to do another run,otherwise its pretty hard to really work out a definite answer I thinks

Yes, I ended up at a slightly higher elevation. But then returned back home and got the same mpg in both directions.

[nickjtc]

Just had a thought on the commute in to work. The only significant difference in the status quo was that on the return trip I had a pair of Joe Rocket throw-over saddlebags on the bike. The friend I visited whilst in Revelstoke actually gave them to me.

So, are saddlebags more aerodynamic to the rear of the bike??

[Ibsen]

Nick, maybe the wheel bearings spins more freely in hot weather.

I agree with OldSchool. For us who lives in a part of the world where the temperatures can get pretty high in day time, and chill down in the evening, we don't need any scientific proof to tell that the bike will produce more power when the air chills down. The difference between riding in the middle of the day and in the evening is very noticable.

Hot weather requires a leaner mixture setting; cold weather requires a richer setting. It is wrong to assume that colder weather requires a leaner setting to keep heat in the engine and vice versa. Cold air is denser than hot air. The denser, colder air packs more oxygen into the engine, so going from hot weather to cold needs a commensurate increase of fuel to balance ratio of fuel-burning oxygen and the fuel itself. The opposite is true in hotter weather. Going from cold to hot weather requires a leaner mixture setting.

Humidity. Humidity is the amount of moisture (water vapor) in the air. Moisture in the air takes up volume that would otherwise be occupied by fuel-burning oxygen. Less oxygen means less fuel is required to maintain a proper ratio of air and fuel. High humidity requires a leaner mixture setting than dry conditions.

Barometric pressure. A barometer measures the atmospheric pressure (generally listed in the local newspaper or on the local weather forecast on TV). Higher barometric pressure readings mean more air is getting into the engine, requiring a richer mixture setting to balance the air/fuel ratio.

Altitude. Altitude is an important factor that most of us ignore, yet it affects the engine’s performance possibly more than any other element. The general formula for power loss with increases in altitude is 3 percent for every 1,000 feet above sea level. If you race in Colorado at 5,000 feet instead of in California at sea level, you can expect to lose about 15 percent of the engine’s potential power output, if the engine is tuned properly.
Air is thinner at higher altitudes, which means there’s less fuel-burning oxygen than at sea level. You might sense a common theme here: less air (oxygen) means less fuel to maintain the proper air/fuel ratio. So, running at higher altitudes requires a leaner mixture setting than running at sea level.