What is the difference between the three generations of WEGO units? 

The first generation WEGO (sold by Daytona Twin Tec) used a Honda/NTK L2H2 wide-band sensor and did not have any built-in data logging capability. The WEGO II series was a second generation product. The WEGO II used the new Bosch LSU 4.2 wide-band sensor and included built-in data logging. The Bosch LSU 4.2 wide-band sensor was available at a much lower cost than the earlier Honda/NTK part. The result was that we were able to offer the WEGO II for substantially less than the price of the original unit.

The third generation WEGO III series is packaged in an improved low profile housing. WEGO III units with display and data logging use an ultra-bright daylight readable blue LED display that is water-proof and include a built-in USB interface. The older WEGO II units had a red LCD display that was prone to moisture intrusion and used an RS-232 serial data interface. 

From an operational standpoint, the WEGO II and WEGO III units are similar. 

What kind of engines can I tune with the WEGO? 

The WEGO system can be used to tune most four stroke gasoline powered internal combustion engines:

• · Motorcycles with carbureted or fuel injected engines. P/N WEGO3-SYS is intended for motorcycle applications. The WEGO is an ideal tuning aid, either for on-road or dyno tuning. Knowing the exact air/fuel ratio greatly simplifies the task of carburetor jetting. 

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• · Automotive applications. P/N WEGO3-SYS-A has an extended length harness. 

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Please note that the WEGO cannot be used with two stroke or marine engines where oil or water vapor in the exhaust would cause serious problems with the sensor. 

Can I use the WEGO in place of an expensive exhaust sniffer when dyno tuning? 

Regardless of what some people may claim, it is impossible to properly tune a fuel injection system or carburetor on a modified engine without some means of exhaust gas analysis. Shops can use the WEGO as a low cost alternative to expensive systems such as those sold by Dynojet. You can easily fabricate your own exhaust sniffer to use with the WEGO, eliminating the need to install weld nuts in the exhaust (refer to our Automotive Exhaust Sniffer Tech Note  and Motorcycle Exhaust Sniffer Tech Note for details). The disadvantage is that you won't have AFR data on the Dynojet chart or be able to directly link to a Power Commander. 

If you are using the Screamin Eagle^® Race Tuner (SERT) or Dynojet Power Commander, check out the features of our Twin Scan II+. The Twin Scan II software analyzes logged data and displays AFR and the required volumetric efficiency (VE) correction (in percent) with the same RPM rows and throttle position sensor (TPS) columns used in SERT and Power Commander tables.    

What AFR values are optimum when tuning an engine? 

Higher AFR values correspond to a leaner (less fuel) condition. The practical operating range for most engines using gasoline fuel is from approximately 11.5 to 14.7 AFR. Combustion of a stoichiometric mixture (exactly enough air to burn all the fuel) results in 14.7 AFR indication. Automotive engines with catalytic converters operate near 14.7 AFR during cruise and idle. Race engines require a richer mixture to limit cylinder head temperature and prevent detonation. The table below lists recommended AFR values for race engines without emission controls.

Operating Mode	Recommended AFR
Cold Start (first 30 sec)	11.5-12.5
Idle	12.8-13.5
Part Throttle Cruise	13.0-14.0
Wide Open Throttle	12.5-12.8 (values down to 11.5 may be used to reduce detonation)

Can the exhaust system affect wide-band AFR readings?

Wide-band systems will give inaccurate  AFR readings in certain situations: 

· Excessive exhaust back pressure. Wide-band sensors are affected by back pressure. Excessive back pressure causes exaggerated AFR indications under rich and lean conditions, but has little effect at 14.7 AFR (stoichiometric). The WEGO is intended to be used with a free flowing performance exhaust. Overly restrictive stock mufflers may cause excessive back pressure under wide open throttle. When used with a turbo system, the sensor must be mounted downstream of the turbo. Motorcycle exhaust systems are relatively free flowing and problems with exhaust back pressure are not likely.

· Exhaust reversion. Reversion is the term for a negative pressure wave that can suck ambient air back into the exhaust and cause an erroneous lean AFR indication. Exhausts without mufflers, such as open headers or "drag pipes" on motorcycles, usually suffer from reversion effects and may not be suitable for use with the WEGO. Reversion effects will also occur with certain exhausts used on "bagger" style motorcycles, where two pipes split off near the rear cylinder. At part throttle, air is actually sucked into the left tailpipe. Reversion effects will be most noticeable at idle, part throttle low RPM cruise, and decel. 

WARNING: If you can insert a broomstick through the mufflers, you have the equivalent of open drag pipes and the WEGO sensors will not read accurate AFR values.

You can reduce reversion effects in open drag pipes and mufflers without restrictive baffles with the modification shown below.

Exhaust Mod to Reduce Reversion

Use washers with an OD that is 2/3 to 3/4 the ID of the pipe (for example, 1-1/2” OD washers are suitable for pipes with an ID of 2” to 2.25”). Weld ¼-20 socket head cap screws to the washers as shown. Drill holes at the bottom of the pipes about 2” from the end and use decorative acorn nuts to secure the washer assemblies. We suggest that you use stainless steel hardware.  

The washers will reflect positive pressure waves that will cancel out the negative pressure waves reflecting from the end of the pipes. You can turn the washers just like throttle blades to provide more or less restriction. Dyno tests will show a significant increase in midrange torque and a small drop in top end horsepower as the restriction is increased.  

· Excessive scavenging. Turbo systems or tuned exhausts in combination with a high overlap camshaft profile can force unburned air and fuel mixture through the cylinder into the exhaust and cause an erroneous rich AFR indication. For motorcycles, some  aftermarket 2-into-1 systems such as the Thunderheader appear to suffer from this problem, whereas others such as the Vance & Hines Pro Pipe and White Brothers E-series seem less affected.

· Misfiring. If the AFR is so rich that the engine misfires, high levels of oxygen will remain in the exhaust gas and result in an erroneous lean indication. 

What is the difference between a wide-band and conventional exhaust gas oxygen sensor?

Conventional (narrow-band) exhaust gas oxygen sensors have been widely used in automotive applications since 1981. Conventional sensors have one to four wires and can only sense air/fuel ratio over a relatively narrow 14.5 to 15.0 range. They are intended to be used with 3-way catalytic converters that require operation near the stoichiometric point (14.7 air/fuel ratio). The range of narrow-band sensors is inadequate for performance tuning. While originally developed for lab and specialized automotive applications, wide-band sensors are ideal for tuning. The 5-wire Bosch LSU 4.2 sensor used with the WEGO operates over a range of 10.3 to infinite air/fuel ratio. 

How does the wide-band sensor work?

The Bosch LSU 4 wide-band sensor integrates a conventional (narrow-band) heated oxygen sensing Nernst cell with an oxygen pump cell. With an appropriate electronic interface such as the WEGO, this integrated sensor element is capable of measuring the air/fuel ratio (AFR) of hydrocarbon fuels over a very wide range. In our case, we are primarily interested in gasoline fuel. 

It is important to understand that the wide-band sensor is measuring the apparent AFR based on the composition of the exhaust gas. The actual AFR is the mass of air divided by the mass of fuel inducted into the engine. For any given AFR, the concentrations of the various exhaust gas constituents can be measured under experimental conditions or calculated using computer programs based on chemical kinetics. Under rich conditions, excess hydrocarbons and carbon monoxide remain  in the exhaust gas. Under lean conditions, excess oxygen remains in the exhaust gas. Earlier generations of exhaust gas analyzers were based on measuring the carbon monoxide level using infrared absorption techniques.  

 

Bosch LSU 4 Sensor Element

(Protective Shroud Removed)

 

 

Diagram of Bosch LSU 4 Sensor and WEGO Circuitry

 

The Bosch LSU 4 wide-band sensor element consists of a heater cell, conventional (narrow-band) oxygen sensing Nernst cell (with associated reference cell exposed to ambient air), and an oxygen pump cell. The three bottom cells (heater, reference, and Nernst) are identical to a conventional heated narrow-band oxygen sensor (4-wire type) widely used in automotive applications since the 1980s. As shown in the graph below, the VSENSE output of the Nernst cell is exactly 0.45V at the stoichiometric AFR (14.67 for gasoline). In the Bosch LSU 4, the Nernst cell compares the partial pressure of oxygen within the pump cell cavity to ambient air (outside the sensor). The sensing range of the Nernst cell is relatively narrow - the output is linear from about 14.5-14.9 AFR.

 

Bosch LSU 4 Nernst Cell Output Versus AFR

 

Exhaust gas continually diffuses into the pump cell cavity through a small diffusion gap. The pump cell can also pump oxygen into or out of the cavity depending on the direction of current for the IPUMP terminal (the fifth wire for a 5-wire wide-band sensor). When IPUMP is negative, oxygen is pumped into the cavity. When IPUMP is positive, oxygen is pumped out of the cavity. The pump control loop (shown as summing junction and operational amplifier) maintains the pump cell cavity at stoichiometric conditions (VSENSE=0.45V). 

If the pump cell cavity becomes slightly rich, VSENSE increases and the pump control loop makes IPUMP negative to pump oxygen in. Under rich conditions, this oxygen is generated by electrochemical decomposition of water and carbon monoxide in the exhaust gas at the surface of the pump cell. Chemical reactions between the excess hydrocarbons, carbon monoxide, and pumped oxygen then restore stoichiometric conditions within the cavity.  

If the pump cell cavity becomes slightly lean, VSENSE decreases and the pump control loop makes IPUMP positive to pump excess oxygen out. The pump control loop is a feedback and control system that maintains stoichiometric conditions in the pump cell cavity as the exhaust gas AFR changes. The relationship between pump current and exhaust AFR is shown in the graph below. If the exhaust gas is already at stoichiometric AFR, no oxygen pumping is required to maintain the cavity at the stoichiometric point and IPUMP=0. 

 

Bosch LSU 4 Oxygen Pump Current Versus AFR 

 

The digital signal processing (DSP) block changes the non-linear relationship between oxygen pump current and AFR into a linear 0-5V output as shown in the graph below. The DSP block also filters the oxygen pump current signal to remove noise. The WEGO implements both the pump control loop and DSP functions in firmware that runs on an Atmel microcontroller. 

The DSP block also includes a control loop that maintains the heater cell at 750 deg C. Pulse width modulation (PWM) turns the heater current on and off at a 30 Hz rate. The PWM duty cycle (percent of time that current is on) determines the average heater current. The resistance of the Nernst cell is inversely proportional to temperature. Additional circuitry (not shown) measures the Nernst cell resistance. The resistance value is used as feedback for the heater temperature control loop. 

 

WEGO Output Versus AFR

 

The Bosch LSU 4 wide-band sensor is affected by exhaust pressure as shown on the graph below. The error (%) applies to the oxygen pump cell current. Note that 1 bar corresponds to normal sea level atmospheric pressure. For most performance applications, excessive exhaust back pressure is not a concern and the resulting small error can be disregarded. At high elevations, the error is also relatively small. At 10,000 feet elevation (about .68 bar), AFR values near 13.0 will be shifted up approximately +0.15 AFR. 

 

Bosch LSU 4 Pressure Dependency

What are the limitations of the wide-band sensor?

The sensor will be quickly degraded if leaded racing gasoline is used. Under these conditions, expected sensor life will be less than 10 hours. As the sensor degrades, free air calibration will become impossible. 

Oil or other hydrocarbon residues in the exhaust will affect the sensor readings. Likewise, gasoline containing ethanol will result is slight air/fuel reading errors. 

The sensor responds to the partial pressure of oxygen. Excessive exhaust back pressure will affect sensor readings. This should not be a problem with any performance exhaust system. When used with a turbo, make sure the sensor is located downstream of the turbo. 

Make sure that power is on to the WEGO whenever the engine is run. Without power to the internal heating element, the sensor will clog with hydrocarbon residues and may be permanently degraded. If you want to remove the sensor, we sell an 18 x 1.5mm hex plug.