EfiMini O2 Sniffer Design

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Build Your Own Tailpipe Sniffer

The whole thought behind this sniffer design was to provide the efiHacks community with ideas on how to build one without a complete workshop with welding and machining equipment in the back.

One should be able to build the whole design with simple off the shelf parts from a hardware store, except the threaded sleeve.

The gas sensing chamber basically consists of copper pipe fittings for water installation, a propane torch and some small, thin wall steel pipes. The T shape fitting is the centre piece, enclosed by 2 reducers on each end. The final end pieces do not need to be part of the copper reducer fitting, it might be as well a steel pipe where the sniffer stinger (long small thin wall steel pipe) fits into or the 180 bend for the exhaust hood for the muffler in case of a dyno setup.
On top goes the threaded sleeve for the LSU 4.9 WbO2 sensor.

All the parts can be brazed or soldered with a propane torch and some silver containing low temp brazing rod (silver solder).
Using regular solder might not work since the sensor heater will heat up the whole exhaust gas sensing chamber.

A flexible high temp silicon hose will fit over the tail end.

more details to come.

We need to find a source for the 18 x 1.5 mm threaded sleeve.

There are 2 ways to use this sniffer.

  • to attach it directly to the muffler stinger with some 90 or 180 bend. Attach a 300 - 500 mm long high temp silicon hose at the end of the sensor to prevent oxygen to be sucked back into the sensor. Do not have the sniffer / sensor housing within the exhaust stream.

Be aware, that this way the velocity of the gas through the sensor is partially dependent upon the engine load and will produce different sense delay times.

  • to attach a high temp silicon hose to the muffler (tail pipe) stinger and U bend and have a water separator before the sensor chamber and a small electric pump after it to force a continuous steady airflow (independent of the engine load). This setup allows for a constant sense delay time.

We need to find a water separator and a small inexpensive electric air pump (diaphragm type preferred)

Application Note :

The sniffer or any wide band oxygen sensor will not work correctly if placed after a catalytic converter or if the engine has any kind of mechanism or leak to allow oxygen to enter the exhaust system before the point of sensing the exhaust gas.

more pics to come


EfiMini Sniffer.jpg

Sniffer Pict

Design Philosophy

We agree with the concept of http://wbo2.com/lsu/position.htm to slow down the exhaust gas flow and causing some turbulence to get as much gas to flow through the sensor itself. The small partial blockage right across the sensor head might even cause more turbulence and forces more gas through the sensor. The dia of the chamber should not be smaller than 22 mm (3/4" is too small) and not larger then 25 mm.
A gap of about 7.5 mm between this blocking wall (purple) and the sensor should give the gas a nice swirl.
The purpose of the whole drawing is to illustrate the proportionality of the parts to each other ... sensor ... housing ... gap.

The drawing below shows how the whole sniffer looks inside when cut open.

EfiMini SnifferDwg.png

Sniffer Drawing

Tailpipe Sniffer Parts

The pict below shows all necessary parts and their preparations for the silver brazing (solder assembly)

EfiMini Tailpipe Sniffer Parts.jpg

Tailpipe Sniffer Parts

Parts A are both end-pieces (reducers) before they are cut down to shape B.

Part T is the T-section which is the main part of the O2 sensor tailpipe sniffer chamber.

Part S is the threaded sleeve which holds the actual O2 sensor.

Parts C are the individual pipes where the ID matches the OD of the stinger pipe and the 180 deg U bend.

Part W is the moon shape wall to cause more turbulence within the exhaust stream.

Tailpipe Sniffer Assembly

The pict below shows the assembled O2 sensor tailpipe sniffer without an exhaust gas pump and condense water separator. The U shape bend is necessary in case it is being used on an exhaust hood being put over the tail end of your muffler (i.e. dyno setup).

This 180 deg bend could potentially also be built copper tubing since the outside is being cooled by air. However we have not tried to use a copper pipe for the stinger itself, since most of the tip is inside the muffler or tail pipe and if the exhaust system is short (motorcycle) it's unknown if the pipe would bend or show some other form of mal-forming being exposed to the extremely hot exhaust (possibly still containing burning gas.

EfiMini Tailpipe Sniffer Assembly.jpg

Tailpipe Sniffer Assembly

more pics with the water separator and exhaust gas pump to follow upon selection of parts, sources, etc ...


Note : All published design documentation is Copyright efiLabs.com and its use is without exception FOR NON COMMERCIAL PURPOSE ONLY. For commercial licenses contact efilabs.com ... try us, we're reasonable :)

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