A/F ratio
Road Master
Joined: Jan 2011
Posts: 893
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From: Knob Noster, MO
Stoich for n/a is 14.7, but you wouldn't want to tune a motor to a 14.7 because is leaves little protection for things like detonation, etc. Normally a good tune a/f for an n/a motor would be high 13's to low 14's on pump gas. Boosted motors are slightly differant in pump and mid to high 11's for a/f. But just because you tune for a safe a/f doesn't mean your motor isn't pinging from bad fuel, too aggressive of timing and there are many other factors. So just going off an a/f reading for a tune doesn't cut it.
Road Warrior
Joined: Apr 2010
Posts: 1,739
Likes: 10
From: Suffolk, England
For a stock bike it is so restricted by the A/C and pipes that throwing more fuel at it isn't going to add much. Though some Xieds to bring the closed-loop AFR down a bit would be a nice cheap way to better engine temps.
As for stage-1...There's more air flowing and you need to stick some more fuel in with it. When running in closed-loop (cruising speeds and low throttle openings) you want an AFR of around 14.0 for a balance between economy and engine temp. When accelerating and at high speed you want AFRs betweens 12.5 and 13.5 depending on the throttle opening and RPMs.
Ultimate HDF Member
Joined: May 2008
Posts: 6,642
Likes: 48
From: Ogden, UT
AFR changes depending on the type of fuel. Ethanol has a lower ratio than gasoline for instance, Ethanol is 9:1 where gasoline is 14.7:1
Bio Ethanol (22 % mix) has an air fuel ratio of 12.7 : 1 as it is an oxygenated fuel as compared to Gasoline, and E85 has an air fuel ratio of 9.765.
So trying to use one number is not quite the right way to go, since in most places straight gas is hard to find.
The power band is also not right at 14.7:1
This is from:
http://www.procyclesinc.com/dyno_tuning!.htm
Bio Ethanol (22 % mix) has an air fuel ratio of 12.7 : 1 as it is an oxygenated fuel as compared to Gasoline, and E85 has an air fuel ratio of 9.765.
So trying to use one number is not quite the right way to go, since in most places straight gas is hard to find.
The power band is also not right at 14.7:1
This is from:
http://www.procyclesinc.com/dyno_tuning!.htm
How does the air fuel ratio relate to making HP? One of the most important factors in making more and better power is controlling the air fuel ratio. When we measure samples from the exhaust what we are actually measuring is the oxygen content in that exhaust sample. The amount of O2 in the exhaust is relative to the amount of 02 entering the engine. What the 02 analyzer does is compare the amount of oxygen present in the exhaust to the fixed amount of oxygen in the atmosphere (this never changes) When the air fuel ratio is considered rich, there is little O2 available in the exhaust stream and when the air fuel ratio is lean there is available 02 in the exhaust stream.
It is well documented and known that in general terms the optimum air fuel ratio for making the most power is about 12.2:1. Understand that this is not an absolute figure and can change significantly from engine to engine. Understanding that each engine has unique characteristics and flow dynamics adds to the challenge of improving the output. What we do know is that engines that run too lean or too rich will not make optimum HP. Being able to see what the A/F ratio is at each and every RPM along the axis of the graph gives us a wonderful tool for adjusting that A/F ratio.
What is most confusing is that there is a lot of information floating around out there about A/F ratios. Internal combustion, gasoline burning, naturally aspirated engines will conform to this nominal ideal A/F ratio of 12:1. Deviation from this ideal is normal and expected. However, we can find that ideal number by performing runs and "learning" from these runs. The real confusion exists with what people hear and read. Many times the A/F ratio of 14.7:1 is tossed around in discussion and is published quite ubiquitously. This stoichiometric A/F ratio is generally ideal for catalytic converter efficiency and not necessarily for making maximum power. This stoichiometric A/F ratio is necessary on modern, catalyst equipped vehicles for the reduction of Carbon Monoxide, Hydrocarbons and Oxides of Nitrogen but not for maximum HP. What's interesting about this often misunderstood A/F ratio is that it is the same value as atmospheric pressure at sea level but has no relationship to it at all. It's simply a scientific coincidence. What's more interesting is that we find the ideal A/F ratios of modern, properly designed, engines and combustion chambers getting towards the leaner side of the accepted standard. That is, they start creeping up from around 12:1 and make the most power at leaner A/F ratios than older engines. This is a result of good combustion, excellent ignition systems and precise fuel control that allows for better fuel atomization.
Keep in mind that internal combustion engines are not very efficient at converting gasoline from potential energy in to kinetic energy. This is what we are constantly trying to improve when we modify an engine. By allowing us to "look" at the A/F ratio of any particular engine we can customize that A/F ratio in to making the most power for any given engine.
The gas analyzer used on the dyno is a "wide-band" O2 sensor. A wide-band sensor is an 02 sensor that has a working parameter of about 0 to 5 volts. When A/F ratios are very lean they generate a very small voltage at or near zero. When the A/F ratio becomes rich they will generate almost 5 volts. This varying voltage is then sent to the A/D converter on the dyno stack and then converted in to a readable A/F in numerical terms. Narrow band 02 sensors, such as those used on automobiles and some motorcycles in their fuel injection systems, generate a useable voltage of about 0 to 1 volt. These sensors have less sensitivity and their working parameter is not as useful for interpreting A/F ratios.
It is also necessary for 02 sensors to be at or above about 600 degrees F in order to generate reliable voltage readings thus explaining the need for the heater in the sensor. This heater warms the sensor very quickly so that it can be useful as soon as possible. This also explains why modern vehicles have heated 02 sensors. The engine management system needs this information as soon as possible in order to begin controlling A/F ratios and achieve a reduction in tail pipe emissions as soon as possible. Understand that disconnecting 02 sensors reduces or eliminates the ability for your engine management system to operate properly. 02 sensors are as prevalent today as spark plugs and understanding their role in modern engine management systems is vital in achieving contemporary goals of reduced emissions and maximum power.
It is well documented and known that in general terms the optimum air fuel ratio for making the most power is about 12.2:1. Understand that this is not an absolute figure and can change significantly from engine to engine. Understanding that each engine has unique characteristics and flow dynamics adds to the challenge of improving the output. What we do know is that engines that run too lean or too rich will not make optimum HP. Being able to see what the A/F ratio is at each and every RPM along the axis of the graph gives us a wonderful tool for adjusting that A/F ratio.
What is most confusing is that there is a lot of information floating around out there about A/F ratios. Internal combustion, gasoline burning, naturally aspirated engines will conform to this nominal ideal A/F ratio of 12:1. Deviation from this ideal is normal and expected. However, we can find that ideal number by performing runs and "learning" from these runs. The real confusion exists with what people hear and read. Many times the A/F ratio of 14.7:1 is tossed around in discussion and is published quite ubiquitously. This stoichiometric A/F ratio is generally ideal for catalytic converter efficiency and not necessarily for making maximum power. This stoichiometric A/F ratio is necessary on modern, catalyst equipped vehicles for the reduction of Carbon Monoxide, Hydrocarbons and Oxides of Nitrogen but not for maximum HP. What's interesting about this often misunderstood A/F ratio is that it is the same value as atmospheric pressure at sea level but has no relationship to it at all. It's simply a scientific coincidence. What's more interesting is that we find the ideal A/F ratios of modern, properly designed, engines and combustion chambers getting towards the leaner side of the accepted standard. That is, they start creeping up from around 12:1 and make the most power at leaner A/F ratios than older engines. This is a result of good combustion, excellent ignition systems and precise fuel control that allows for better fuel atomization.
Keep in mind that internal combustion engines are not very efficient at converting gasoline from potential energy in to kinetic energy. This is what we are constantly trying to improve when we modify an engine. By allowing us to "look" at the A/F ratio of any particular engine we can customize that A/F ratio in to making the most power for any given engine.
The gas analyzer used on the dyno is a "wide-band" O2 sensor. A wide-band sensor is an 02 sensor that has a working parameter of about 0 to 5 volts. When A/F ratios are very lean they generate a very small voltage at or near zero. When the A/F ratio becomes rich they will generate almost 5 volts. This varying voltage is then sent to the A/D converter on the dyno stack and then converted in to a readable A/F in numerical terms. Narrow band 02 sensors, such as those used on automobiles and some motorcycles in their fuel injection systems, generate a useable voltage of about 0 to 1 volt. These sensors have less sensitivity and their working parameter is not as useful for interpreting A/F ratios.
It is also necessary for 02 sensors to be at or above about 600 degrees F in order to generate reliable voltage readings thus explaining the need for the heater in the sensor. This heater warms the sensor very quickly so that it can be useful as soon as possible. This also explains why modern vehicles have heated 02 sensors. The engine management system needs this information as soon as possible in order to begin controlling A/F ratios and achieve a reduction in tail pipe emissions as soon as possible. Understand that disconnecting 02 sensors reduces or eliminates the ability for your engine management system to operate properly. 02 sensors are as prevalent today as spark plugs and understanding their role in modern engine management systems is vital in achieving contemporary goals of reduced emissions and maximum power.
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