Sled & UTV Tuning


By Joey Strub

Air Fuel Ratio (AFR) is the ratio of Air (Oxygen) to Fuel (Gasoline). 14.7 units of Oxygen to 1 unit of Gasoline is whats called the Gasoline Stoichiometric ratio. Often AFR is displayed as a digital number, for instance 12.6 on a digit display would refer to only the units of Air. So 12.6 really means 12.6 to 1. Now that you understand what AFR is now lets go over what the different ...values of AFR really mean. At 14.7 AFR all fuel has chemically reacted with all oxygen that was available. At 14.7 there no fuel or oxygen remaining. This is an important number because if the number is higher than 14.7 (Lean) there is too much air and if it is high enough it will not fire at all because the flash from each fuel molecule will not flash far enough to contact the next fuel molecule so energy will be unable to pass from one to the next and the chain reaction can not occur, this is called a Lean Misfire. Another concern for AFR values 14.7 and over is that there is no unburned fuel to carry heat out of the cylinder. Heat can be very dangerous for engine life. In contrary when values are lower than 14.7 (Rich) there is fuel remaining after flash has happened. This extra fuel has 2 main effects. The first is the engine will run cooler the richer it is because the unburned fuel will absorb heat inside the cylinder and carry out into the exhaust. The second is that the flash will occur at a slower rate because the extra fuel molecules mixed in with the flashable fuel molecules acts like an obstacle that that flash must travel around to get to the next flashable fuel molecule. The Lower the AFR value (Richer) the slower the flash will occur. It is possible to have too many unflashable fuel molecules to for the flash to reach the next molecule, this creates a Rich Misfire. Like anything there is optimal value for AFR depending on application. Major factors that affect optimal AFR is engine type, compression, operating temperature, Octane, Ignition Timing and fuel delivery method.

Exhaust Gas Temperature (EGT) is a measurement of the temperature of the exhaust gas as its entering the exhaust system usually 6-8 inches down stream from the piston skirt if possible. Often EGT is displayed as temperature either in Fahrenheit or Celsius. Exhaust temperature is an excellent bit of information for the serious 2 Stroke tuner since you have a digital number to reference how your engine is running. In most cases the lower the EGT value is the richer/cooler the engine is running and visa versa. So you can adjust fueling to hit your desired EGT at Wide Open Throttle (WOT). Note it is quite normal have a higher safe value for Partial Throttle (sometimes 75 to 150 degrees F) since you do not have the volume of heat needed to cause damage where as that same value may be unacceptable at WOT.

Typical Wide Open Throttle (WOT) AFR Values for optimum power-
*** Some engines safe values are outside of typical limits***

2 Stroke naturally aspirated Target AFR/EGT
AFR = 12:0-12.6:1
EGT = 1100-1200F

4 Stroke naturally aspirated target AFR
AFR = 12.8-13.4:1

4 Stroke Boosted target AFR
The more Cylinder Pressure the lower the number!
The lower the Octane the Lower the number!
AFR = 10.9-12.5:1

Where to install EGT bungs 2 stroke exhaust-
EGT Probe should be 6-8 inches from the piston skirt down exhaust stream, If a Y-Pipe joins the exhaust stream in this range install the EGT Probe at the closest point where you could install a hose clamp around each side as close to where the 2 join into 1.

Where to install AFR bungs 2 stroke exhaust-
For best reading without disturbing sound waves AFR bung should be installed as close to towards the engine while still in the belly portion of the pipe, this is the section where the pipe is at its largest diameter. You want to place the bung on the upper half so the sensor will not be upside down when installed (this would cause premature sensor failure).

Where to install AFR bungs 4 stroke exhaust-
Pre-muffler so it will not get inaccurate readings from reversion air pulses, If running a straight pipe 12 inches upstream of outlet should suffice. You want to place the bung on the upper half so the sensor will not be upside down when installed (this would cause premature sensor failure).

Written by Joey Strub
Bikeman Performance Plus

I wrote this to help with a basic understanding of the hows and whys. This is only scratching the surface and I claim no responsibility for any damage that results from inexperience.

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