AFR O2 Sensor Location: Best Practice

[Tartilla]

Building twin marine 454s with B&M 420 blowers.

Looking to get info on where to place the O2 sensors.

Bosch data says:

Use the hottest location.

Angle sensor down 10° on horiz and 15° when vert facing down. Any reason for this? To shed condensation? What about the vertical position wanting 15° offset perpendicular to flow?

Marine guys are saying to place them 6-8"past the header piper merg.

I have Stainless Marine exh manifolds, ( aluminum water jacketed) that have a bung in the collector. These are not long tube, just big open short port. Doesn't have much of an angle, but is perpendicular to flow. Factory welded etc.

[BLSTIC]

Sensor Placement

Sensor operating temperature should be attained in 20 to 30 seconds when the control unit is attached to a 12 Volts (and up to 19.5 volt) supply. A longer warm up phase indicates either a problem with the sensor, the controller, or where the sensor is positioned. Controllers have in-built diagnostic LEDs and documentation for your controller should be carefully read to determine what your controller is saying.

The image shows the range of acceptable mounting positions. A vertical position can get too hot in confined spaces, so we recommend at least 15 degrees from the vertical. The horizontal position can cause condensation to drip onto the sensor, so we recommend at least 10 degrees from the horizontal. In all cases the sensor should be perpendicular to the gas flow, ie. the bung should sit square over the pipe - this ensures adequate but not an excessive amount of gas enters the sensor.

We recommend placing the sensor around 1 m (40") from the closest exhaust valve. Where this cannot be achieved then spacing the sensor away from the direct exhaust by using a longer M18x1.5 bung, or a nut welded over a standard bung, is recommended.

So it looks like the 15 from vertical is about cooling the sensor in low airflow environments. This makes some level of sense because as hot air rises you want it to go away from the sensor, not further up it. 10 from horizontal is supposedly about condensation, but maybe it's because it could be pooling internally and the angle promotes drainage?

I know you CERTAINLY don't want it at the bottom of the pipe, that's just asking for a dribble of water to cool the glowy thing several hundred degrees in 1/10th of a second, and all materials are happy and strong when you do that...

Edit to add: "to prevent the collection of liquid between the sensor housing and the sensor element during cold start the installation angle should be at least 10 degrees from horizontal with the electrical upwards" from HP tuners.

I forgot for a sec that ehaust is a lot of steam, so you don't really want it able to condense and pool if you can help it. And if it must condense somewhere you want it to drain before the sensor gets hot.

[dfarr67]

Spartan_Lambda_Controller_2_User_Manual.pdf

[F-BIRD'88]

If you use a "heated "O2 sensor, It does not need to be in the hottest spot on the pipe. If you want the sensor to read O2 content at idle and off idle part throttle then you need a long extenstion of exhaust pipe AFTER the sensor. If there is water injected in the exhaust stream I'd think that this will effect the output of the O2 sensor, some.

[Tartilla]

If you use a "heated "O2 sensor, It does not need to be in the hottest spot on the pipe. If you want the sensor to read O2 content at idle and off idle part throttle then you need a long extenstion of exhaust pipe AFTER the sensor. If there is water injected in the exhaust stream I'd think that this will effect the output of the O2 sensor, some.

Fortunately I have dry exhaust risers/tails. They extend for about 6 ft @ 3".

So longer tubing required with less flow so the air coming back up the pipe doesn't contaminate the sensor?

The guy that ran these before, had old Edelbrock O2 3 wire sensors...and they never really worked well. But they were over 30yr old teck.

[RW TECH]

Typically you will see 0.3-0.5 AFR difference between sensors in the primaries near the cylinder head and the collector, assuming a full exhaust or turn-down elbows on the end of the collectors. So if sensor in collector reads 13.0:1, you will probably see 12.5:1 close to the exhaust port.

[F-BIRD'88]

You are not really WOT tuning to a specific magic WOT AFR.. You just need to know if the WOT AFR is safe. ( Not too lean). 11.5-12.5:1 AFR ish +/-
Blower engines need a bit more WOT fuel to help cool the combustion chamber . You don't need to know the AFR at idle. At some RPM - throttle running position above idle it will then start working fairly accuratly.
Again there is NO magic AFR to tune too. Just keep it within an aceptable range Not too lean not too rich. 14-15:1 ish+/- is fine at light throttle slow cruise.

[Tartilla]

You are not really WOT tuning to a specific magic WOT AFR.. You just need to know if the WOT AFR is safe. ( Not too lean). 11.5-12.5:1 AFR ish +/-
Blower engines need a bit more WOT fuel to help cool the combustion chamber . You don't need to know the AFR at idle. At some RPM - throttle running position above idle it will then start working fairly accuratly.
Again there is NO magic AFR to tune too. Just keep it within an aceptable range Not too lean not too rich. 14-15:1 ish+/- is fine at light throttle slow cruise.

Appreciate the insight. I was hoping to ensure the readings are reasonably accurate...or as accurate as I can make them IOT have a better sense if the AFR is on the 'edge' of being too lean. Too rich has other indicators like soot on the transom etc.

I also don't want to run it overly rich, and wash down the cyls.

These are mild 454s with a mild cam, and 15% under 420 blowers.

Prob no more than 6psi WOT, and 90% run time @ 3000-3200rpm. Hoping to have 0psi boost for cruise at 3000.

They shouln't be working too hard there.

[F-BIRD'88]

Overly rich wash down cylinders. The does not happen at WOT ( as long as you are not purly dumping raw fuel down th4 carb.

Soot on the transom is mostly from excessive rich mixture during slow cruise speeds at part throttle.
Even @ WOT hard in it under boost. A "rich" mixture (supercharger rich) will not soot up the transom. (11 to 12.5 to 1 AFR at WOT)
This is NOT black smoke rich.. Its just a step ratio richer than what a N/A engine runs @WOT.

Of course under boat crusie (3000 rpm ish +/- You do not need very rich mixture. 13 to 14 :1 AFR+/- is plenty @ Cruise partilay engine load and near no boost on then gauge. If you keep your blower drive ratio reasonable this won't be a problem.

For supercharged boating (VS balls out competition boat racing) you want to keep the supercharger drive ratio reasonable anyway on pump gas.

When the fuel curve is correctly set up on the carbs for supercharging you will NOT get excessive rich fueling when you don;t need it. Like under part throttle No boost cruising (3000 rpm ish)

[F-BIRD'88]

If you are going to go racing and then want to crank up the blower drive ratio, the boost and power output, use high octane racing gas or methanol injection. Its no fun if you don't finish the day. $$$
When set up like this you'd then want to take a look at how much boost on the gauge is being created at part throttle boat "cruise" (3000 rpm ish). Then a main jet change may be needed. The harder you drfve the blower the more likely you will see "Boost" under the part throttle boat "cruise" situation.