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AFR probe question

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Steve B Avatar
Steve B Silver Member Steve Brannan
Weeki Wachee, FL, USA   USA
I spent a happy hour or so checking my plugs and tweaking mixture on my three SU carbs and wishing I had the ability to check the air fuel ratio with an 02 sensor. I have retained the original exhaust header but capped the two intake heating ports with steel plates when I added my Herbold intakes. Has anybody tried putting bungs and sensors in at this location? It would be very easy to do but I am not sure if I would get an accurate reading in this location.....


Any thoughts or expertise to add to the concept?

Steve

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Ken Plumstead Avatar
Smithers, BC, Canada   CAN
1965 MG MGB
1968 MG MGC GT
Steve,

I know that this goes against the prevailing wisdom but when I installed an LC-1 in the exhaust of my supercharged 'B' the instructions from Innovate included the following:

Weld Bung into exhaust.

A bung is easy for any exhaust shop to weld into the exhaust pipe (for ~$10 bucks). For
naturally aspirated cars, weld the bung before the catalytic converters. If your car does
not have catalytic converters, WBO2 Sensor should be at least 2-3 feet from end of
tailpipe exit.
The WBO2 Sensor should be oriented between the 10 o’clock and 2
o’clock position, with 12 o’clock being the best position.


Ken



MG: Transforming gasoline into Fun!!!



Edited 1 time(s). Last edit at 2019-01-10 03:52 PM by Ken Plumstead.

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JMA Avatar
JMA Silver Member John Anderson
Donvale, Victoria, Australia   AUS
Although my car has headers and will have EFI, I placed my O2 sensors after the collectors on each pipe far enough away from the head as not to overheat the sensors. I also chose this position as there's enough space in the engine bay whereas under the cabin is too tight with the torsion bars etc. Personally if I was still using the original cast manifolds I would fit sensors in the same postion on the downpipes.

What the photo doesn't show is that the sensors are pointing slightly "downhill" so that condensation won't collect around the sensor.



Edited 1 time(s). Last edit at 2019-01-11 12:56 AM by JMA.

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Ken Plumstead Avatar
Smithers, BC, Canada   CAN
1965 MG MGB
1968 MG MGC GT
And that's the conventional wisdom I was referring to. Everyone seems to put the sensors up close to the engine and Innovate, one of the AFR manufacturers, wants it 2 to 3 feet from the exhaust exit.

I mounted my sensor in the exhaust by the left battery box about three feet from the end of the exhaust. That also meant I could keep the sensor controller inside the car (under the driver's seat). Sorry, I don't have any pics.

Ken



MG: Transforming gasoline into Fun!!!

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Snowberry C Chris A
Havant, Hampshire, UK   GBR
1969 MG MGC
But.. the guidance quoted above is:

'Sensor should be at least 2-3 feet from end of tailpipe exit'.

I take it 'tailpipe exit' is at the rear of the car, under the rear bumper. Hence no less than 2-3 feet, so can be more. So 6-8-10 feet, whatever, i.e. at the foot of the headers etc, is absolutely in line the above guidance is it not?

Chris

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twentyover Avatar
twentyover Greg Fast
Lives in SoCal, Resides in the Burbs of Detroit MI, USA   USA
I understood Innovate's rationale was that they ddn't want reading corrupted by outside air aspirated into the exhaust, giving a false lean mixture.

When California was tailpipe testing they used a probe about 3' long they shoved up your pipe to keep from measuring outside air contamination

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Ken Plumstead Avatar
Smithers, BC, Canada   CAN
1965 MG MGB
1968 MG MGC GT
That sounds logical. The recommendation is not very specific, is it? One would think they would also include a minimum distance from the exhaust ports on the head to avoid excessive heat or maybe that isn't an issue.

I went with the installation further back so I could mount the sensor vertically from the top of the exhaust pipe to eliminate condensation issues.

Ken



MG: Transforming gasoline into Fun!!!



Edited 1 time(s). Last edit at 2019-01-11 03:27 PM by Ken Plumstead.

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Steve B Avatar
Steve B Silver Member Steve Brannan
Weeki Wachee, FL, USA   USA
Thanks for all of the input. As I have the standard headers, installing one sensor to read all 6 cylinders probably wont get me any more accurate tuning than reading the individual plugs. It would be good to know what AFR readings are best for this engine as the conventional wisdom seems to be that they like to run rich. If you do have AFR readings, can you volunteer your readings?
Steve

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kirks-auto Platinum Member Robert Kirk
Davenport, IA, USA   USA
I don't wish to be picky, but to clarify, a "standard exhaust pipe" is not a header. A header is designed to scavenge exhaust, pull it via engineered gas flow. An exhaust pipe simply attaches to a port to allow the flow of burned residue away to the muffler/silencer and out the tailpipe.



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PETE W Sussex Peter H
Ansty, West Sussex, UK   GBR
In reply to # 3872055 by Ken Plumstead That sounds logical. The recommendation is not very specific, is it? One would think they would also include a minimum distance from the exhaust ports on the head to avoid excessive heat or maybe that isn't an issue.

I went with the installation further back so I could mount the sensor vertically from the top of the exhaust pipe to eliminate condensation issues.

Ken

We fitted an LC2 and wide band lambda sensor to our MGTF track car which is normally aspirated and delivers 230bhp on the rollers. We had a lot of trouble with burning out lambda sensors initially. In the MGTF with the mid/rear engine layout and the exhaust manifold we have it was a bit of a challenge to find a place to put the sensor and it turned out that our first attempt was too close to the head - we ended up moving it a foot further down the manifold with an extension bung and now it is happy. Based on this experience I would say too close the head is not good.......but don’t ask me why the standard car has the lambda about 12 inches or so from the head.

As to the best AFR for the C I don’t know. I would get it on a rolling road with someone who knows what they are doing as timing will come into it but instinctively I would go richer rather than leaner, if for no other reason than safety. We have the TF set up to be running at 11/12 flat out I think but that is no benchmark.

Attached below is the instructions for the LC2 which clearly talks about positioning distance from the exhaust ports

https://www.innovatemotorsports.com/support/manual/LC-2_Manual.pdf

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JMA Avatar
JMA Silver Member John Anderson
Donvale, Victoria, Australia   AUS
Postioning a sensor too close to the head has been known to burn out the sensor, but too far away the sensor may not heat up enough and give false readings which is why some have a heating circuit inside the sensor. But this is usually used only on road cars with the latest type of engine management systems. I would suggest the position mentioned by Pete on his track car would be because it was intended as a road car, race cars have everything running hotter especially the exhaust. More power equals more heat.

The only AFR gauge I have used was on an engine trolley that my brother built so we could get the mixtures right before taking the engines to a dyno, on that the sensor was fitted just after the manifold flange. If I was going to do any competition work I would have fitted it further down the pipe due to the heat. I have seen sniffers fitted to dyno headers, some plumbed into the header others fitted to a sandwich plate for tuning purposes, sorry no photo.

I'm going to be the devils advocate on the subject of the recommended AFR for the C, my reasoning is that I suspect that the inlet manifold may be a factor. On my car with the UK log manifold certainly benefitted with an overly rich mixture but this was due to the inner cylinders robbing the outer ones of the mixture. The US manifold I suspect may not need as rich mixture due to the better design(?), and a triple carb manifold should get away with a leaner mixture again. The photo below was of the spark plugs that came out of my car when I took it off the road. Cylinders 1 to 6 are left to right.


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PETE W Sussex Peter H
Ansty, West Sussex, UK   GBR
Ah, you remind me of the other problem with the lambda John! Yes, ours has the heater in it and initially I had wired it wrong. I had it wired to switch on with the ignition and not the starter button so if there was any delay between ignition and start the lambda had heated up - when you hit the starter the first blast of cold air down the exhaust manifold killed the lambda. It of course needed to wired to switch on with the starter. We learned a lot from building the TF!!

I think you are right on the distance from the exhaust ports, that was the only conclusion we could come to. And it does produce a lot more heat than a standard engine - we worried a lot about cooling but seem to have that cracked with completely redesigned plumbing but I still think the oil is running too hot. I get jumpy at anything much over 100c even though the fully synthetic oil is said to be good to 150c.

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chormy Avatar
chormy Gold Member Shaun Holmes
Norwich, Norfolk, UK   GBR
1963 MG MGB MkI "3330 PE"
1964 MG MGB MkI
1967 MG MGB GT "BABE"
1967 MG MGC    & more
This was used on Dyno , impractical for normal use. I will probably do another for trackside use.


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