Topic: Does lean running rob MPG?

[manjisann]

I have tried to search for an answer to this question but haven't had any luck. I'm wondering if running lean can cause lower fuel economy?

Thanks,

Brandon

[Pawsoff]

I am not nearly as educated as most on here but I belive running lean would improve gas mileage.
However it does end up being riskier than running a bit rich, as being a bit rich won't kill the engine and will not increase the operating temperature of the motor.

I think the trick the pros have use to get the best out of a motor is to run it as lean as they can without hurting the engine (more power) BUT they are pros and have more knowledge / $$ / resources than most and are trying to race... not have a daily driver.

I believe it is better to run a bit rich to have a kind of insurance in engine protection.

[OldSchool_IsCool]

To achieve optimal MPG, make sure you brakes aren't dragging, that your chain & wheel bearings is well lubed and that you ride PAST the Old Country Buffet instead of stopping in. 

[TwoTired]

Running lean usually improves economy.  However, at some threshold you lose power.  It is possible that the rider response then positions the throttle more open to restore the power needed to achieve their speed goals.  The higher throttle setting then uses more fuel, as you are then operating on another or additional metering orifice in a carb.  But, you would have to know more about the specific carb in question for a better (or more definitive) answer.

Cheers,

[manjisann]

Old School, that's where I get gas 

Pawsoff and TT, you have both answered my question, thanks!

Another kind of along the lines, in order to maximize mpg, would it be best to ride at the lowest rpm for a given speed, as long as it's high enough to charge the battery? Since the engine is rotating as low as possible it will use less fuel right?

Sorry for the seemingly obvious questions, I'm just trying to be sure I've got all the thoughts straight in my squirly brain.

Thanks guys!

Brandon

(in case anyone is super curious, it's a cb650 custom with 750 k4 carbs with pods on it... not exactly stock eh    But the question is pretty universal so I didn't want to confuse it with specifics when I first posted it.)

[OldSchool_IsCool]

Another kind of along the lines, in order to maximize mpg, would it be best to ride at the lowest rpm for a given speed, as long as it's high enough to charge the battery? Since the engine is rotating as low as possible it will use less fuel right?

Think of your engine as an air/fuel pump.  The faster it's spinning, the greater the volume of A/F it is pumping.  so yes, if speed is held constant, and the grade of the road is also constant (insert your favorite external factor here), then a slower spinning motor will, in general, "process" less A/F.  This is the reason why dynamic, or engine braking, while saving your brake components, is wasting gas.

I'm going to go get a nice tall Scotch while the REAL experts correct my statements! 

[TwoTired]

You put carbs on that have a bigger venturi, right?  People do that to help breathing at high RPM.  Larger venturis are less efficient at lower RPM or low air speeds through the carb.

I'll take a whack at an explanation.

A 750 sucks more air than a 650.  The air volume drawn through the venturi determines the suction placed on the fuel jets.
A 650 with less volume and less suction may require larger fuel orifices to get the proper mixture ratio. 
The low volume of air makes full fuel/air atomization more difficult to achieve at low engine RPMs and you get more unburned fuel out the exhaust because the hydrocarbons have a difficult time pairing off with the available oxygen.
You can make more power at or above red line with oversize carbs.   But, most operations in street regimes becomes less efficient.

Back when I was racing Chevy V8s, it was common to put a larger carb on the engine and run it 7000-8000 at the track.  Stock 4 barrels were about 500 to 600 CFM.  The racers were going to 750 or 850 CFM carbs, to get extra power with the engine wound up tight.  This set up was awful on the street, with poor off idle response, (because of low venturi speeds) and very bad fuel economy (which was accepted because high HP engines eat lots of fuel anyway).


Cheers,

[crazypj]

Contrary to 'common sense', running lean destroys gas mileage.
 You end up running lean at every throttle opening, but, your using a lot more throttle for any road speed (plus still running lean)
Best bet is to improve airflow (ported, cam, exhaust etc) jet correctly and see major improvement in gas mileage

PJ

[camelman]

Running lean can lead to tremendous improvements in efficiency.  Most industrial gas turbines and jet engines run as lean as possible without creating excessive NOx (created in large amounts above 1800F - lean burn leads to very high local combustion temps).  The same combustion requirements that define turbine operation are also at play in our engines.
Many newer engines (and even older engines back to the Chrysler products in the 70s) have lean burn engine controls.  The benefit of a lean burn is that the engine is effectively detuned so that it makes less power at wide open throttle.  Since an engine is most efficient at wide open throttle, this allows the engine to run at a more efficient point.  Then, when more power is needed, the fuel management system provides more fuel to increase total power output.  This is EXACTLY how diesel engines work, and is how the newer direct injection engines are being designed.

That being said, we do not have the combustion chamber design, or fuel controls capability, to adequately and safely vary the fuel flow in our engines throughout power/throttle/rpm ranges.  So, we are stuck with one setup for daily operation.  Running slightly lean might not hurt your engine, but the gains in fuel mileage will be negligible compared to the possibility of frying a piston.  I'd suggest getting a fly screen for the headlight to improve fuel economy before trying anything with your carbs.

Camelman

Running lean also creates more smog.

[crazypj]

On an engine designed for lean burn it will increase fuel economy.
 Unfortunately. these engines were designed to run at 12~14:1 F/A ratio (and around 17:1 cruising about 1/2~3/4 throttle)
 In my experience, 550/750 fours drop about 4~10 mpg when running lean.

PJ

[manjisann]

You put carbs on that have a bigger venturi, right?  People do that to help breathing at high RPM.  Larger venturis are less efficient at lower RPM or low air speeds through the carb.

I'll take a whack at an explanation.

A 750 sucks more air than a 650.  The air volume drawn through the venturi determines the suction placed on the fuel jets.
A 650 with less volume and less suction may require larger fuel orifices to get the proper mixture ratio. 
The low volume of air makes full fuel/air atomization more difficult to achieve at low engine RPMs and you get more unburned fuel out the exhaust because the hydrocarbons have a difficult time pairing off with the available oxygen.
You can make more power at or above red line with oversize carbs.   But, most operations in street regimes becomes less efficient.

hmmm, that part with the air volume venturi thing is something I hadn't adequately understood when I did this, but I think I understand it better now. Not perfectly, but slightly better 

On my bike I'm not running lean to try and improve gas mileage, it's because I'm still in the process of working out the jetting on my rather unique setup. I've now got all 4 of the plugs looking pretty close to the same at any given throttle range, which unfortunately is lean. I was just wondering how this was affecting my MPG. Unfortunately I've never ridden a pure stock 650 so I have nothing to compare to my bike as a base line, so I'm just trying to feel my way through this all.

I really appreciate everyones input and willingness to share such great knowledge with me.

Thanks,

Brandon

[TwoTired]

Others in this forum have said their stock CB650s MPG range in the 50 to 55 range. (One said 60.)

Of course, on the freeway in top gear will usually be more economical that never getting out of third in the city.

[manjisann]

Others in this forum have said their stock CB650s MPG range in the 50 to 55 range. (One said 60.)

Of course, on the freeway in top gear will usually be more economical that never getting out of third in the city.

I haven't fully tested it yet, but yesterday I pulled off about 40 mpg, a bit less than 50 to 55, but better than the 23 I was getting. I think a lot of it was jetting issues, brakes issues and romping on the throttle all the time. I think with some better rpm management I can increase my mpg some. I'm going on a long ride today so I'll have to see.

In retrospect, stock was probably the best option, however because of my choices in modification, I understand a lot of things a ton better than i would have otherwise so I guess it's something of a tradeoff..

Brandon.