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# Gasoline Air/fuel Ratio

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Lean, rich, or just right?

Getting the proper amount of fuel to a motor during any kind of running condition is essential to making it run right. This is why products like the Power Commander, Bazzaz Z-Fi, and others are so popular. Those products give the ability to change the fueling of the bike for different operating conditions. It is also why dyno shops stay pretty busy measuring and tuning bikes.

So what do we use to describe how much a bike is rich or lean? The answer is....air/fuel ratio.

First, a few definitions...

Ratio: a relationship between two quantities, normally expressed as the quotient of one divided by the other. Example: The ratio of 7 to 4 is written as 7:4 or 7/4.

Air/Fuel Ratio (abbreviated "AFR"): the mass ratio of air to fuel in an internal combustion engine. Example: An air/fuel ratio of 13:1 means a mixture of 13 parts of air and 1 part of fuel.

Stoichiometric air/fuel ratio(abbreviated "stoich"): A ratio of air and fuel such that exactly enough air is provided to completely burn all of the fuel. Example: The stoichiometric air/fuel ratio of petrol(gasoline) is 14.7:1.

Oxygen Sensors

Air/fuel ratio is measured with an oxygen sensor(O2 sensor). The oxygen sensor is either fitted to the exhaust pipe and placed directly in the exhaust stream, or has the exhaust pumped to it(like on a dyno).

Most common AFR range for petrol(gas):

10:1(Very rich)-High concentration of fuel in the air/fuel mixture

11:1

12:1

12.8-13:1(Rich)-this region is typically where you will see a bike make the best peak horsepower(100% throttle, high RPM).

14:1

14.7:1(Stoich AFR for gasoline)-this is the AFR that your bike will run when it is using the oxygen sensor(low throttle, low RPM). It is the "cleanest" burn for emissions.

15:1(Lean)

16:1

17:1

18:1(Very lean)-Low concentration of fuel in the air/fuel mixture

Many guys have their own opinion of what they feel is "rich" or "lean", but air/fuel ratio is what puts a number to it.

I'm sure this might stir up some questions. Let them rip!

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Check this out....

Below is a picture of the exhaust from the new Yamaha 990cc MotoGP bike. Those are oxygen sensors installed in the header pipes...one for each cylinder! It gives you an idea of how serious the GP teams are about getting the proper air/fuel ratio on their bikes.

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That is a real trick set up!

I am very happy to see this tuning section - I think it's a great idea and thank you very much for your time, information, and assistance, Timmer!

It comes at a perfect time, since I am ready to start making some power mods, etc. I will post a new thread to discuss.

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That is a real trick set up!

I am very happy to see this tuning section - I think it's a great idea and thank you very much for your time, information, and assistance, Timmer!

It comes at a perfect time, since I am ready to start making some power mods, etc. I will post a new thread to discuss.

Awesome input BLSDJS...and thanks!

Just post up any questions you have about any of this stuff and I will be glad to answer them.

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Why do motorcycle manufacturers only provide a single narrow band O2 Sensor?

From what I understand these only provide AFR's to the ECU within a specific Throttle % / RPM range.

Wouldn't it make sense to have a sensor that reads the entire range?

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Why do motorcycle manufacturers only provide a single narrow band O2 Sensor?

From what I understand these only provide AFR's to the ECU within a specific Throttle % / RPM range.

Wouldn't it make sense to have a sensor that reads the entire range?

Good question tunnelvision748.

The reason why manufacturer's only use a narrowband sensor(and only use it in a certain range of Throttle % and RPM) is because of the emissions standards that they have to conform to.

From what I have heard, emissions standard organizations(CARB for example) are really only interested in the emissions of the bike under "highway cruising" conditions(lower Throttle % and RPM) because that is where street riders spend the majority of their time while riding. They want the "cleanest" burn for this riding condition. The "cleanest" burn that they can get is an air/fuel ratio of 14.7:1, which is exactly the air/fuel ratio that a narrowband sensor measures. Narrowband sensors are also MUCH cheaper than a wideband sensor which can measure the entire air/fuel range.

I wouldn't be surprised to see more wideband sensors on bikes as sensor technology gets better and cheaper.

Hope that answers it for you. Let me know if you have anymore questions.

Best,

Timmer

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