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Lousy mileage?
Why do motorcycles get such lousy mileage compared
to many cars, which are vastly bigger and heavier? 
How much horsepower and/or torque increase does it take
to improve performance?  
 What about RAM AIR?     
Problems that can reduce fuel mileage.
What is the best cruising RPM?
Premium fuels versus mileage and combustion temperatures.

© Copyright, 2014, R. Fleischer; copyright is on MY work only

Part 1:  Overview:

All objects, motorcycles, bicycles .....and you! .....when moving through air .....create friction and drag.   On earth, the pressure of the atmosphere causes a higher pressure at sea level, than it does on a mountain top.  The higher you go with your motorcycle, the faster you should be able to go, theoretically, due to less friction with the air ....OR; conversely, the less power from your engine you would need at the same speed as at sea level; due to the reduced air drag.  While friction and drag are not the same, you can regard them as the same for this particular bit of reasoning, which, other things not considered, is true about the speed, etc. 

As altitude increases the oxygen content of the air is less (by weight), and your engine will have less power as you go up in altitude, which can totally offset performance increases from less air drag/friction.  Usually performance suffers as altitude increases, particularly above 5000 feet.  In some instances, fuel mileage WILL increase with an altitude increase, especially on fuel mixture compensating engines, such as those with fuel-injection.  On some uncompensated carbureted vehicles, mileage may decrease substantially. Sometimes supercharging or turbocharging can improve fuel mileage, particularly with small engines with smaller combustion areas (cylinders and heads).  MANY small factors also enter into fuel mileage.

CARS, especially modern ones, are generally designed to slip through the air with relatively low friction and drag, so as to get the best possible fuel mileage consistent with such as body shape appeal and other factors.  Manufacturers generally do everything reasonably possible to reduce friction and drag within design and function restraints, due to various government-imposed penalties for poor gas mileage, not to mention selling features.    Of course, there are limits to designs that would be acceptable or practical.    The shape of the car, friction of tires, closeness to the ground, irregularities in bodywork and under the car, and literally many hundreds of things, all have plus and minus effects on fuel mileage.

Motorcycles are generally designed for performance, not necessarily fuel mileage; although that is or can be a factor.   There are large differences between various motorcycles depending on what the designers/engineers intended.  Some motorcycles are designed for strictly for performance; particularly regarding acceleration, top speed, and stability at high speeds.  Some may well be quite compact, presenting a small surface area to the oncoming air, which helps fuel mileage.

When an engine is designed for performance, fuel mileage will almost always suffer.   Motorcycles into the late eighties used carburetors having rich mixtures (especially before ~1980), for better power and performance.  Even with fuel injection, mixtures on SOME high performance bike engines tend to be 'somewhat' on the richer side; using more fuel.  This is particularly so for high performance motorcycles.   In particular and quite notable, camshaft timing and other such 'tuning' is such that fuel tends to be wasted during aggressive acceleration.  

In all engines, fuel is used for COOLING the burning mixture, and a slightly more fuel-rich mixture will usually provide more horsepower.   This wasting of fuel is much more pervasive during fast acceleration, than at constant road speeds.   

As speeds rise, the power required greatly increases.  Fuel usage is not on a flat upwards curve with speed ....rather, it is a very steep curve.  This is particularly so on vehicles with high coefficients of drag.  I will get into this in depth later in this article.  The basic idea you should have at this point of discussion is to consider not acceleration, but constant speed ....and the idea here is that for every same increment of speed increase, the fuel required becomes more than for the previous increment.    If you were traveling at a constant 50 mph, and previously had been at a constant 45 mph, you used a certain amount more fuel per mile, for the extra 5 mph of speed.  For 55 mph, you use MORE than the just the extra amount for the 45 increase to 50.  FURTHER, the faster you go, this idea continues, and is on a fast rising curve.  It actually approaches a cube function!  F³

The Coefficient of Drag (Cd), is a huge determining factor on performance as speed increases.   Cd is NOT a straight-line function; which most everyone thinks it is ....from looking at only ONE of the two formulas involved with drag and horsepower needed.  Cd is a complicated thing to measure without a wind tunnel or some fancy computer and graphics work.  It is possible to get a working approximation from a photo and a type of graphing paper, and a lot of pencil work.   Cd is not very usable on a practical basis for anyone but an engineer.  Motorcycles, particularly large touring bikes, have a very high Cd, and hence get lousy gas mileage compared to a car; much worse in comparison, considering the weight and size difference. Motorcycles have been designed and built that get very high fuel mileage, but these are almost always totally impractical as vehicles to carry a rider.  Craig Vetter used to have yearly contests for high mileage modified motorcycles.  >>>>He is, lately, back into it this again!

Motorcycles do NOT slip through the air with low friction and drag.  Even motorcycles with well-designed full fairings are 'dirty' as far as friction and drag goes ....compared to a small car and many a larger car too!   Motorcycles have a huge number of exposed parts, each of which presents its own interference disturbance with the desired smooth flow of air.  These air disturbances, many of them small, tend to interfere with each other.  Some parts will affect the air before other ones do.  Thus, there is constant "interferences mixing".    The over-all effect is a lot of friction and drag.  A motorcycle might actually do better at fuel mileage if it was a square ugly vehicle ....a long rectangular box!...     or, at least an ugly non-streamlined car.

Think about this in another way.  Suppose a motorcycle has a streamlined-looking fairing, and it was traveling through air full of multi-colored smoke.  Suppose you were some decent distance away, but able to stay at the same speed, and look sideways towards the motorcycle.  You would see that smoke travel around and about the motorcycle in very strange ways.   It would have all sorts of ripples and strange movements.  JUST talking about a factory stock FAIRING (ONLY!) ....typically, as the smoke-filled air passed the rear edge of the fairing, it would be seen to combine horizontally and vertically (air is coming over the windshield, and the rider...) ....and try to rejoin the air doing the same sort of thing on the other side of the motorcycle.   One fundamental reason for this is that when the oncoming air strikes curved surfaces, it ACCELERATES, creating a lower pressure, that varies from surface to outwards, somewhat.  Thus, the pressure difference alone causes a tendency to re-combine.    Highly streamlined motorcycles, usually fish-shaped, are used in top speed trials/racing, at such as the Bonneville Salt Flats.  The longer the fish shape, in general, the more likely the air disturbances will recombine BEHIND the motorcycle, and, in fact, if this is controlled well enough, that can actually give a forward push on the motorcycle.   If a fairing could be extended to a number of feet behind the motorcycle, and the entire structure made to look somewhat like a fish, then the friction and drag would be GREATLY reduced.   That is not practical for a street-going motorcycle.  Most high mileage motorcycle designs look something like a fish. As you might very accurately think, that effect varies with speed, and only a rather narrow band of speed will let the effect be worth much.   

If saddlebags, or a passenger, or a passenger backrest, or a tour trunk ....are on a  motorcycle ....air currents would strike these things in all sorts of weird patterns .....and cause additional friction and drag.

In the classical way of thinking about friction and drag, one converts by one means or another the ongoing face and structure of the vehicle into a equivalent 'flat plate', of some number of square feet and square inches, and then calculates using commonly known mathematical formulas, the Coefficient of Drag, Cd.

You could do that yourself, by taking a photograph from the front, enlarging it greatly, and then calculating the area of things and of no things, in the photograph.   If the photograph was full size, you could, with some effort, calculate the number of square feet and square inches, of effective frontal area.  The answer would not REALLY be completely accurate, however, as all those air flows around and in-between everything DO interfere with each other, making things WORSE than calculations by area alone.  But, you would get some decent information about Cd least as far as converting to flat plate area.     In the real world of doing this, you could make a huge chart with small identical-sized squares, and then doing area calculations.   For over-all design purposes, Cd work is almost always done by means of a wind-tunnel with full instrumentation; and preliminary work is almost always done by means of 2D and 3D computer software.  Special software programs are used with computers to do the final analysis of the wind tunnel measurements.

Road-going production motorcycles generally have lousy CD, compared to any modern car.  Thus, a motorcycle uses a fair amount of horsepower to maintain any speed, let alone accelerate or get to high speeds.  Using horsepower means using fuel.   Increasing engine efficiency in its burning of fuel has a VERY SMALL effect, compared to Cd.  A large bike getting perhaps 38 to 44 mpg in steady riding at perhaps 60-75 mph, might gain as much as 5 mpg more by going from carburetion to fuel injection, with a few other changes ....and maybe a few more mpg if the rear drive ratio was flatter (allowing the engine to turn more slowly).

All road-going vehicles have TIRE friction with the road that produces heat.  That friction is needed, but it is a user of horsepower.  It could be thought of as power that the engine produces that ends up as heat from the tire friction (from contact, squirming, shape change, etc.) that is transmitted into the road surface ....and into heating the tire itself.   There are other ways of thinking about this sort of thing ....such as the air resistance of the tire/wheel combination to-the-surface, effects of the wheel spokes, and many many other small things ....all of which tend to add up.   These effects also use horsepower in maintaining any given speed. 

So, what is typical fuel mileage for an Airhead, in fact, what's typical for most motorcycles?
Depends MOSTLY on how you ride ....aggressively; higher speeds; higher rpm; on and off the throttle; ...etc.  Alcohol in fuel lowers fuel mileage, by 4% to 8%.  High ratio rear drive lowers fuel mileage; typically this is another 5%. Saddlebags and tail trunk lower mileage.  Many windscreens lower mileage.  Sitting very tall in the saddle lowers mileage.  Passengers usually cause a lowering. Richer settings in the carburetors beyond the optimum setting will always lower mileage.   Side winds and head winds will reduce mileage, sometimes drastically. Probably 35-48 miles per American-sized gallon is typical for most Airheads, at varying but legal speeds.  A few motorcycles, any brand, will get as much as mid-fifties, and if driven quite moderately, as much as 60; others will be in the high twenties. Airheads can sometimes reach 50 mpg+-.  Your average speed is quite important. As speed increases, mileage goes down on a steepening curve.

Bottom line:  Motorcycles are generally very 'dirty' compared to even pretty decent size cars, as far as Cd is concerned.

Part 2: The basics .....first, the easy to understand things:

Drag varies with shape.  A flat disc, moving through the air, flat surface forward, will have a Cd of approximately 1.1.  As you study the various charts and list in this article, you will see that some vehicles have WORSE drag than a plain flat plate! 

Here are TWO simple charts for easy visualizing. 
NOTE that these TWO charts are a comparison of Drag Coefficients for SHAPES ...and is ONLY, HERE, to compare NOT try to use these TWO charts for absolute measurements of drag AMOUNTS ....that is in a following chart.


The chart just below can be plenty informative.  Note the last item, the common brick, in the first section.

0.0224                                F4 Phantom fighter jet
0.189                                  VW hybrid diesel-electric model VW XL1.  Requires only 8.3 Hp to maintain 62 mph
0.25                                    Toyoto Prius, all models/versions are approximately this value
0.26                                    BMW i8 hybrid car; Mazda 6
0.28                                    Snowbum's Alfa Romeo Giulietta Sprint Specialé; C6 Corvette.
0.303                                  Honda, 1996, RS125GP
0.32                                    2015 Mazda Miata  
0.33                                    Subaru 2014 Forester
0.35                                    Lexus LX570 SUV.  An amazingly good Cd for such a big vehicle.
0.38                                    BMW K1, rider sitting upright.
0.43                                    BMW K100RS, rider sitting upright.
0.495                                  BMW K100RT, rider sitting upright.
0.562                                  Vincent Black Shadow, rider sitting upright.
0.6                                      Approximate for the Suzuki Hayabusa, a very well-designed & slippery street bike.   
2.1                                      Common brick used in construction (no, don't know the orientation, think it was sideways)


Vehicle                           Drag Coefficient
Description            Low       Medium        High
Experimental          0.17         0.21            0.23
Sports                      0.27         0.31            0.38
Performance           0.32         0.34            0.38
60's Muscle cars    0.38         0.44            0.50
Sedan, newer         0.34          0.39           0.50
Motorcycle              0.50          0.90           1.00
Truck                       0.60          0.90           1.00
Tractor-Trailer       0.60          0.77           1.20

The following information was derived from wind-tunnel measurements, and is expressed in different units.  The conclusions you will make are quite valid!

Cd (as drag area)                       VEHICLE
which means

5.54                                    Ferrari 308 GTB, year 1980
5.61                                    Mazda RX-7, year 1993
5.92                                    Porsche 911, year 1994
6.24                                    Toyota Prius, year 2004
6.27                                    Porsche Carrera, year 1986
6.81                                    Subaru Legacy, year 1989
7.34                                    Honda Civic, 2001
7.57                                    Toyota Camry, year 1992
8.71                                    Buick LeSabre, year 1991
16.8                                    Hummer H3
18.06                                  Hummer H1, year 1993
26.3                                    Hummer H2, year 2006


Want to know about other vehicles? 
Want to know Cd, and not CD-A, what about total drag?...try the links below.

That linked article has a very long list of Cd's, for COMPARISON ONLY AS ON THAT LIST.  That is because there are different types of drag measurements in use. The article measurements were not taken and expressed in the same values and methods, so use these tables/charts/lists, for comparison purposes, WITHIN a chart or listing, ONLY!   As drag coefficients get quite small, the differences become less and less.

More places:

NOTE:  Cars generally have the edge over bikes, as speed increases greatly, due to the dynamic characteristics.

Part 3:  Getting into the actual mathematics:

I put two formulas below, and will do a bit of discussion about each.  These are not my made-up formulas, but well-known classic formulas.

Formula #1:
Fd = (1/2) Pv2 ACdV

For the above formula, Fd is the drag force; P is the density of the our example we mean air density; V is the direction of the velocity; and A is the area, usually taken to be an orthographic projection on a plane perpendicular to the direction of motion.  If not 'simple', such as a sphere (your motorcycle certainly is not a sphere!), then one needs to calculate for each and every area.  Finally, v is the speed, relative to the medium.

NOTE!.....did you see that air density, P, is a direct factor?  That is JUST ONE reason that as altitude goes up, your fuel mileage will go up, all other things constant.

For an object with well-defined points (HAH! for a motorcycle!), such as a circular disk plate, perpendicular to the flow, then Cd is a constant for Reynolds numbers over about 3,500.   Cd is a function of the orientation of the flow.  There are many types of drag, one of which is FORM DRAG, which is pressure variations around the object.

There is no need for you to get deeply into the formula details.  The important part is quite simple.  The drag force is proportional to the SQUARE of the SPEED.  If the drag is proportional to the square of the speed, guess what that proportionality means to horsepower required (and gasoline required!)!   >>>>Your guess would LIKELY BE WRONG!!

It is at this point that confusion sometimes comes about, and arguments can start.  That is because fuel usage...and horsepower requirements...are NOT proportional to the square of the speed.  The confusion and arguments probably come from MISUSE of the information in TWO formulas.  You can NOT use just ONE of these formulas & gain an understanding.  They MUST be combined or both used with understanding.  Because of the complexity I have NOT combined the formulas, because I want YOU to easily understand the reasoning why the increased horsepower required for a higher speed is not proportional to speed, nor to the square of the speed....but is proportional to an even higher figure.

Formula #2:
Power required = Av + Bv
2 + Cv3

Let's take a look at that formula:

A is the resistance....which includes the tire rolling resistance...this is a linear function.
B is mostly concerned with internal engine friction components.
C is the aero-forces, including the coefficient of drag, air density, and so on.  NOTE that, as opposed to the previous formula, C is being multiplied by v cubed and added.
v, as before, is the velocity, or speed

As earlier here, there is no need for you to get deeply into the formula.  The important part is actually simple.  The power required is proportional to the CUBE of the VELOCITY (speed, that is) as far as as the external things are concerned.  NOTE that POWER is directly proportional to fuel consumed.

How to reason it all out, if you are still confused by the math:
One formula says that drag force is proportional to square of the speed.
The other formula says that the power required is proportional to the cube of the speed.
Both formulas are effective at the same time for your motorcycle.
Power needs go up MUCH faster than drag force, as speed is increased. 
In order to come to a complete understanding, you must use BOTH formulas, combining them.

No need for you to get into it, unless VERY nerdy.  I will summarize what you need to know:

If the actual power needed is going up at a faster rate than the drag force effect...then the FUEL need is going up at a faster rate too!  Can you can now see that the final effect is someplace between the square and cube functions?   THAT sentence has been highly miss-understood.  Many seem to use what someone says, who does not seem to understand that BOTH formulas must be combined.  That is why many people, ignorant of the FACTS, seem to think that fuel mileage is proportional (to something or other, but not BOTH things).

All the things discussed are the reasons why your motorcycle can have comparatively lousy mileage, compared to a car, which is vastly heavier and larger.  

If you were to put a sidecar on your motorcycle, the fuel mileage will decrease considerably more....due to the greatly increased Cd.    How much of a decrease?  Perhaps 30%-40% at moderate highway speeds!   VERY FEW sidecar rigs that are even capable of maintaining 55 to 70 mph will get over 35 mpg.  If you have a modest sized engine, and do not drive over 40 mph or thereabouts, you might get 40 mpg.  If you have a 1 liter bike, pulling a substantial sized sidecar, in mixed traffic and cruising on backroads and occasional freeways, you can expect ~30 mpg!!

Part 4: Additional information:

There is a LOT of VERY wrong information on the Internet, and even in printed books, about Cd, Cd-A, etc.  MUCH of the wrong information is due to those persons who are writing these things failing to understand that TWO formulas are in play, and must be used TOGETHER. Many do not understand CD, nor the effect on Cd from various object shapes.   Many of these websites are done with lots of mathematics, and look scholarly.  I think many copy from each other, re-arrange things, and think they know what the truth is.  In my discussion of Cd, and Cd-A, above, I have simplified things for your easier understanding.  Here is a link to something written by Tony Foale on aerodynamics. Tony's work you can generally rely on.

One of the things that always has bothered me is that so many articles will 'explain' that motorcycles have Cd figures of, let's say, .35 to maybe 1.5, yet they use those figures with cars that have those values, do not understand what that value REALLY is, and make erroneous conclusions.  A Cd 'chart' of curves is misleading, if you do not factor-in the rest of the information, particularly the rest of the math.

Charts that display fuel consumption versus weight of the same vehicle are usually thought to be accurate, in showing a doubling of weight as a doubling of fuel consumption.  If only that sort of chart was used, in reverse, you could say that a light weight motorcycle should have monstrously better mileage than any car.  NOT SO, as you have seen in this article you are reading.  Do NOT jump to the conclusion that I am wrong here, after all, some mopeds get 80 mpg or more!   While true, the wrong use of the charts and formulas, as many do, will likely show that a light moped should get MANY HUNDREDS of miles per gallon. Thus, ideas are expressed, without analyzing or thinking.

I am aware that few of you will take the time to really read, think, etc., about the minimal math in this article ....but I HOPE some of you DO, and I hope ALL of you take away the real reasons for lousy motorcycle fuel mileage, and the principles, at least in a broad sense, of why fuel mileage goes down so very fast as speed rises ....and why this happens so quickly with motorcycles.

Below is an interesting chart. 
It does not have a curve for large displacement BIG cruisers, but it is going to be from about 0.48 drag area to about 0.75, with a few even a bit higher.  Notice how quite high speeds separates the drag curves. ESPECIALLY notice the curves SHAPE.  While the curves shape shows an increasing steep drag force with increases in velocity, the effect also increases at slower velocity as the actual velocity increases. 


Part 5:  Equivalents/conversions:

cubic inches x 16.39 = cc
liters x 61.02 = cubic inches
cubic inches ÷ 231 = gallons
Imperial gallon x 1.2 = U.S. gallon
mpg x 0.354 = km/L
km/L x 2.825 = mpg

Km x .621 = miles
one Km is approximately 5/8th of a mile.

Part 6:  Pumping losses, engine friction, efficiency versus load, cruising RPM considerations:

In the mathematics in this article (up to this point) not every possible item and effect is discussed.  The inefficiency, that is, less-perfect, use of carburetion versus fuel injection is barely dealt with.  Engine pumping losses are not discussed, which can be high, and should be considered, especially for carburetor bikes.  Pumping losses are relatively high in any engine, no matter if carbureted or not; particularly at part-throttle. 

The friction losses increase with a change to larger pistons, as piston sides area and rings contacting area increase.  This increase is a square function too, so losses with larger displacement in a cylinder increase fast.  Friction is power robbing.  That means more throttle is required.  Obviously, the more distance the piston travels, the more friction per stroke, thus the stroke length has an effect.    One other item has a BIG effect, and that is camshaft timing.

There are LOTS of other factors,  one not discussed is this following one, barely alluded to in my comment on pumping losses.  Since FEW seem to know about this, I am getting into it a bit here:

A gasoline 4 stroke engine is more fuel efficient at higher loads.  Engine friction from JUST RPM is often said to vary linearly with RPM, which is close to the truth. FRICTION uses up horsepower.

A primary reason that the engine becomes less efficient as you close the throttle is due to pumping losses.   MOST gasoline 4 stroke engines have a throttle plate butterfly valve, perhaps called by that name.  Motorcycles with fuel injection to a manifold, and also slide-only carburetors can both be lumped in with those.   There are several types and situations of pumping losses, but, I am using the term to include them all.  Pumping losses are a MAJOR effect.

There are other variables, such as engine friction changes as pressures in the cylinder change, these effects are smaller, and all others, not mentioned, are also MUCH smaller.

One thing often not thought-of, is that, at its core basics, our engines are HEAT engines.  That is, the combustion process produces heat, and expanding gases.  The force of the expanding gases (rise in pressure) moves the pistons inwards, which produce the power we lust after.  The energy in the combustion process is very IN-efficient, and a LOT of the energy is sent out the exhaust system, and a fair amount is also radiated into the air, for cooling.  It takes a lot of power to just make the engine move AND TO COMPRESS THE FUEL-AIR, BEFORE IT IS IGNITED.  Pumping losses!

The chart below is self-explanatory.  One can expect somewhat of a similar chart for any given engine.  Do not idly take a quick look and pass this chart by.  STUDY THIS CHART!!

ONE thing even a brief look at this chart will tell you, is that the gearing ratios from crankshaft to rear wheel is important.   For the Airheads, assuming 4th gear (4 speed transmissions) or 5th gear (5 speed transmissions), where the transmission ratio is identical (1.5:1), the rear drive ratio has a noticeable effect on fuel mileage, both for over-all real world driving, AND constant speed driving.  Read the next sentence carefully:
It is obvious from the chart that using a fair amount of throttle at lower rpm, roughly 3,000 here, gives the best mileage.

For the engine for which this chart applies, you can see that around 4000 rpm is a good rpm for typical cruising on the open road, but if at much lower speeds, cruising at 3300 rpm will give good fuel mileage.   NOTE that the chart below is not that of an Airhead engine, but the idea, and curves shapes, are typical.   In real terms, if you increase the rpm listings of rpm at the bottom of the chart, from 1000-5000 rpm as shown, to be re-marked to 2300 to 5800 or so, that is REAL least from what I remember when having MY R75/5 on Axtell's dynamometer (I also did extensive work on the dyno with my Nor-Vin).


As you can see, the fuel consumption varies with both load and rpm.  You could consider this to mean that at a constant speed, on a flat or constant rising road, there is an optimum throttle amount.  It is generally bad for an engine to 'cruise' at very high throttle settings and very low RPM.  That is called "lugging".   For the BMW Airhead engines, due to the design, including primary balancing rpm area, and many other factors, the engines will, OVERALL, prefer to be at 3800-4500 rpm, under mild or relatively flat road conditions.  That is, the throttle will be at, perhaps, only about 35%-60%.   There is nothing all that strict about this.

HOWEVER, as you increase the throttle (but keeping the rpm in that above 'sweet spot'), perhaps doing this by shifting to an appropriate transmission gear, mileage goes up.  Don't overdo this.   There are many rear end ratios used on the BMW Airheads, so I cannot give hard and fast rules, except what I said above, ~35-60% throttle, and ~3800-4500 rpm.  The engines are certainly capable of cruising vast distances at much higher throttle and/or rpm, but fuel mileage suffers more.  Note: if you are in any gear, with a constant throttle setting,...... & opening the throttle some additional amount does NOT accelerate the motorcycle, then you ARE in the LUGGING AREA, so SHIFT DOWNWARDS as needed!!!  Lugging increases cylinder pressures, and heat, and, if excessive, WILL CAUSE SERIOUS ENGINE DAMAGE!

Here is another fuel-robbing situation.  Did you ever think that an alternator uses engine power?  If you are using even near full output of the 280 watt stock alternator, the alternator, being less than 85% efficient, will pull a considerable amount of engine power to produce this electricity.  The effect on mileage is WORSE at lower speeds.

There are other things, such as rear drive ratios.  The higher the numerical ratio, the more RPM for a given speed, the more friction losses in the engine, and the more throttle is used. 

Things ADD UP!

PART 7:  Fuels, jetting, poor mileage due to problems, ETC.   There can be many reasons for poor mileage; here are SOME of them:

(1) Alcohol-laced fuels (Gasohol) has already been mentioned.
(2) Winter fuels, with their higher volatility less power-producing ingredients.
(3) Problems with the emissions control items, such as from the electric solenoid valves and the one-way valve going into the crankcase.  These items are found on late Airhead California models and may be found in total or in part, on other State and Country models.
(4) Any engine with wrongly jetted carburetors; perhaps an R80 engine with wrong jetting right from the factory.  Article is on this website:
(5) Defective carburetor floats
(6) Extremely dirty air filter.  Check for rodent problems in that area too.
(7) Use of quite high speeds.
(8) Use of higher ratio rear end ratio gears.
(9) Defective ignition spark coil, wires, caps.
(10) Use of large alternator output.

Part 8:  RAM AIR:

Sometimes folks ask questions about RAM air possibilities for "no-cost" supercharged horsepower.  Dynamic air pressure by or on a moving object are treated by engineers as a certain size of flat plate moving through the air at some speed.   As you read earlier in this article, drag is proportional to the square of speed.  Drag can be said to be dynamic air pressure.  As we have seen earlier in this article, Dynamic air pressure is proportional to the SQUARE of speed, but the horsepower to attain that speed goes up as the CUBE of that speed change.   Think about that!  This means that for oncoming air, just the SPEED change, is proportional to the cube of gasoline usage.

But, the question here is the use of RAM AIR for supercharging purposes; so, I will give an example:

Consider your Airhead or any other motorcycle, moving at 68 statute miles per hour.  That is easily converted to feet per second, and the value is 99.7, let us just say 100 feet per second.   There is about 12 pounds of pressure per square foot of surface area (over and above atmospheric static pressure).   At 200 feet per second (about 136 miles per hour) the added pressure is 48 pounds per square foot.  (see, I did the calculations for you!!).  Thus, the drag is 4 times higher for a doubling of speed (but, you already knew that from earlier information in this article). 

If one was to divide by 144, you would have the pressure per square inch, in this case 0.33.  We commonly measure pressure in the USA in pounds per square inch (PSI).   This means that the supercharging effect, nothing else considered (such as HEAT rise, which lessens the usable effect), at 136 mph, is only 0.33 psi.

So, 0.33 psi is quite small compared to atmospheric pressure forcing itself into the cylinders (about 15.0 psi at sea level).  Thus any ram-air-supercharging effect is very small .....until speed gets VERY high. However, once the speed is high enough, the pressure rise is more and more usable ....because of that SQUARED function I mentioned earlier.  Speed starts to make some reasonably usable difference around 150 mph, all things considered.  Thus, ram air pressure does NOT help at ordinary road speeds.   AND, no, 'big scoops' don't increase that pressure and performance due to the mathematics and area involved.

Part 9: Horsepower, Torque, miscellaneous:

1. Several methods of 'proving an increase in performance' come to mind.  The best testing is on a dyno (dynamometer)....with the temperature and humidity of the incoming combustion air being measured and written down and accounted-for; and having fuel flow instrumentation. Dyno's cost $3K upwards, and are expensive to rent.  But, they are really great, especially  if you use the same dyno for all testing.  That is because a decent dyno is good enough to compare between readings on the SAME dyno; but not all that great for absolute measurement values.  Very fancy dyno's can cost over $100,000.00.  

2. The second best method is often a top speed run, same loading/equipment and rider position, clothing, etc. You can compare, before & after modifications. Use the same road area, same temperatures, same atmospheric pressure, same wind (if any), same direction.    Calculations could be done based on actual measured increase (or decrease) in top speed, to get an idea of the real Cd.  Runs in two directions within minutes tends to average out any wind.  Yes, if you are a bit clever, you can calculate Cd from performance increases from the basic Cd formulas.

3. A speed run up a slope can tell you a lot, and often can be done at legal speeds. Typically this is done on a slope that allows, let us say, third gear and 6000+ rpm or so (Airheads)...and the slope is such that it won't allow any more speed in a particular gear, or, only some.   This is one of my preferred methods.   I also often use this sort of test over perhaps half a mile, to determine mixture, reading spark plugs or exhaust gases.....etc.  I have one particular stretch of road where I have been doing this up-slope testing for decades now.

4. Acceleration testing can be done with a stop watch, over different speed ranges, and sometimes offers some decent measurements. Unfortunately, for JUST the drag-race start method, there are TOO MANY variables to consider. It generally is a poor method, unless you can have tight control over things.   Where it CAN work out well, is a roll-on test, from a given rpm & speed in a given gear.

5.  In the United States, horsepower means "Imperial" horsepower. The original description is the power a horse exerts in moving 550 pounds of something, one foot in distance, in one second.  It can include just the horse and its own weight.  Horsepower can be defined many ways. Just one such is that ~746 watts is one horsepower.

The term brake horsepower (BHP) came from the measuring of horsepower by a device called a water brake.  A 'brake' loads the engine ...and the twisting force, called torque, is then measured. It can be measured directly, with such as strain gauges, or indirectly. BHP is crankshaft measured, unless specified at some other point, such as transmission output or rear wheel output.  Today, hydraulic brakes, water brakes, electrical brakes ...etc. ...are all in use in measuring BHP. Typically, however, we have some sort of turbine ...and a means to adjust characteristics, such as amount of loading.

Shaft horsepower is NOT the same thing as BHP. It is supposed to mean BHP, less certain losses.   In practice ....especially decades ago with the puffed-up advertising inflated horsepower claims of car manufacturers ....horsepower was measured at the crankshaft, often without alternators/generators, water pumps, etc. .....all these power draining devices were eliminated ...and sometimes some real cheating was done measuring power at unlikely cold and very dry temperatures, ....even using different air pressures, no exhaust back pressure or optimizing the exhaust system for best extraction and input characteristics at a very specific rpm; ....and changing the coolant temperature to non-real-world temperatures.

6. Formulas to calculate BHP: 
BHP = 2 times pi times torque times revolutions; all this divided by 550.
Pi is 3.1416 and torque is in pounds-feet, and revolutions are revs per second NOT MINUTES!   Obviously, we can change the revs to revs per minute, easily!

Here's another version:
P = 2 time pi times torque times rpm times 1.34 times 10 to minus third power.  Divide the results by 60.  For this formula P is in horsepower and torque is in Nm.

7. Today,  SAE (Society for Automotive Engineering) testing methods and specifications do not allow the large inflated values from the 60's and 70's. SAE standard conditions are 29.61 inches of pressure of mercury atmospheric; 81 degrees Fahrenheit incoming air temperature; and 60% relative humidity.  These are real world numbers.   Just what is what on exhaust, etc....well, yes, exaggerated values still exist, just not quite so large.   For your Airhead, say an older R100 with stock higher compression ratio, rated at, say, 70hp, you will likely, on an accurate dyno, get around 51-54 hp.  AT THE REAR WHEEL.

There are other types of measuring standards.   You may have heard of the ISO international standard. ..often seen on threaded fastener spec's.  ISO for horsepower is similar, but not measured exactly the same way as SAE, but it is only a couple of percent different (higher).

8. Note that both Imperial and metric power can be expressed, and often is (see BMW spec sheets) as Kw (kilowatts). Metric horsepower is now larger than Imperial ...since it is measured in the real, supposedly on-road world ....but not always ....some fudge factors exist ....I am speaking theoretically here about metric hp being larger than Imperial hp. You can usually assume that metric power times 0.986 gives Imperial. You can calculate KW into Imperial, by dividing by 0.746.
9.  I won't go into it deeply here ....many folks HATE math and formulas ...but you probably have noticed that you CANNOT separate a torque effect from a horsepower effect, and vice versa, due to that formula in #6. above. Because of the formula, and some other things, there is a place on every torque and horsepower curves chart [where both curves are shown], where they CROSS-OVER.   It is ALWAYS at the same RPM. You cannot separate the interrelationship: if you change one item, at least one other is changed ... torque times rpm is horsepower.

10.  Puzzler: 
What happens if there is NO rpm ...the piston has not moved ...does this mean torque and power are now equalities within the equation? Just what DOES it mean?  I just thought I'd drop that in here ...and let some old time steam engine enthusiasts have a tad of fun.

Premium fuels versus mileage and combustion temperatures:
There is an old controversy over possible increases in combustion chamber temperatures, ETC., when using premium gasoline's in lower compression BMW airhead engines, where higher octane is supposedly not needed. 

Gasoline burns at about the same rate under normal, that is, not detonating, etc., conditions. The output (BTU) per gallon of Premium gasoline is potentially ....or even likely be a small amount LOWER than for Regular.  I think it is likely that SOME premium gasoline's WILL give LOWER gas mileage than a regular gasoline will .....assuming here that the engine will run properly on Regular grade gasoline in the first place.  

Temper this with the fact that gasoline formulas change, and also change between summer and winter grades of gasoline. Winter gasoline contains rather volatile things like butane or propane.   In the West, California in particular, oxygenates were added to most fuels for decades.   These additions GENERALLY cause ~5-10% POORER gas mileage.  They are NOT good for your engine, carburetors, hoses, etc. Nowadays, ethanol is added to nearly all road fuels.  Without jetting changes, they make an engine run leaner.  The fuel mileage decreases from just the ethanol content ....AND! ... decreases even more, perhaps another several %, when you rejet for performance with such fuels.


05/25/2009:  initial release.
01/03/2010:  slight updating
06/01/2011:  cleanup
09/28/2012:  Add QR code; add language button; update Google Ad-sense code, add Additional Information section
10/04/2012:  Merge information from hp-drag.htm article, so that one can be eliminated.  Go over the whole article.
12/09/2012:  Add shapes chart
03/05/2013:  Add comments "Some Special Details" and the fuel-rpm-load chart
09/02/2013:  Clean up article.  NO important details changed. Language button removed sometime in 2013.
01/18/2014:  Clarify some details.
03/05/2014:  Updated in several areas, add more vehicles, charts, conclusions
12/04/2014:  Expand article more regarding Cd, with more charts and comments
03/04/2016:  Meta-codes; left justification; metatags, etc.  Still needs narrowing and final meta's, be done later this year.
06/28/2016:  Update metacodes, scripts checked, improved descriptions, enlarged font sizes, full justify left, full length lines, etc.
08/02/2016:  Add section, prev. in InExTuning article:  Premium fuels versus mileage and combustion temperatures.
01/23/2017:  Fix totally wrong hyperlink in Part 7 (4).   Also clarified some things in Part 6.
03/27/2017:  Fix typos, improve clarity.

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Last check/edit: Monday, March 27, 2017