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MG MGB Technical - Air fuel ratio meter

I have now got an Air/fuel ratio meter, with an oxygen sensor to put into the exhaust manifold. This is the question; will it restrict exhaust gas flow too much, the placement of the oxygen sensor. It is quite large. I do have a blanking plug, so I do not have to leave it there all the time.Although good to see a reading at all times.I am going to use it to 'tune' the needles of the carbs. Mike
J.M. Doust

I considered this a few years ago but eventually got the car running well without it. The main reason I didnt do it was I had a lot of trouble getting the exhaust system to fit together and work well and decided against disturbing it. The TF has a stainless sports system with pre and post cat lamda probes and the gasses fly through that :-). The reason I was considering it on the B was some examples of readings I read where at variuos times the mixture was way out with no obviuos aymptoms, but a big difference in driveabilty once you could see what was going on.
Stan Best

I fitted one to my 1500 midget a few years ago, this has the standard exhaust with a single down pipe from the manifold so it was quite easy. Being mild steel it was quite easy to weld the adaptor into the downpipe. I gave up trying to use it because of the erratic reading I was getting and it was running well anyway. I had thought about trying a similar system on my B put the problem is the twin down pipes. Should I use 2 sensors or put it after the collector box? The problem with using a single sensor after the collector box is I'm not sure the sensor would get up to temperature. I did use a heated sensor last time but I'm not sure that would be enough. I'd appreciate others thoughts on this as I'd like to give this a go.

R.A Davis

Mike I have had one in my supercharged B for several years and find it great. I fitted mine just after the engine pipes join as there is a lot more room there and the Bosch sensor does not like extreme exhaust heat either. I leave it in all the time and its probably done about 30,000miles without a problem. Bob the gauge reads 14.7 for up to about a minute in winter and then comes to life. It is a heated sensor.

Bob I missed this in my last post. Its no advantage to have a sensor in each down pipe as the front carb feeds no 1 & 2 cylinders the rear no 3 & 4 cylinders, where one down pipe services 1 & 4 the other 2 & 3 so you do not have two separate systems. Denis

Bob I missed this in my last post. Its no advantage to have a sensor in each down pipe as the front carb feeds no 1 & 2 cylinders the rear no 3 & 4 cylinders, where one down pipe services 1 & 4 the other by 2 & 3 so you do not have two separate systems. Denis

Ah good, I was looking in the wrong spot for some replies to my post. Good much as I thought and thanks Dennis, as I intend to ultimately supercharge this is probably a good excercise. I shal experiment with the carbs and needles. The SU tuning book says there may be a lot possible just off idle upwards in terms of getting the mixture just right for driveability. the trick will be idetifying exactly where on the needle, ie which stage might be in need of 'opening'. I have probably a standard ? narrow band? I presume this means the range of register? Will not know till I hook it up. Mike
J.M. Doust

Oh yes you will, unless you stole it
Narrow band is cheap, and simple, and some but not a lot of use. It only tells you where the mixture is almost exactly right, and readings out of that "narrow band" are useless. One to four wires from the sensor, two of which are a heater circuit if it has that. The meter is just a simple voltmeter, also cheap. Whole deal might come to $100.
Wide band is dear and complex and tells you lots. I think 5 sensor wires is usual, and the meter end is a computer/control box, so $500-1000, though they used to be that much for the sensor alone. But it tells you exactly what your mixture is doing, over a very "wide band".
Google is your friend!

You have a quite wonderful resource there, in the form of Autospeed Magazine. It is possibly the best source of info for DIY/broke/cheap types available.
Here's a link to the first of many hours of your time, most relevant place to start:

There are about a thousand articles under the DIY Tech link. In looking it up to copy that link, I find that wide band kits are now down to the $200 range - exciting!

It was only a couple of years ago that I figured it would cost me $500 min and that was a kit that required me putting the circuit boards together!

One point not to miss: The sensor must be installed with the tip pointing down, else condensation get in it when the engine is cold, which kills the sensor. About 3-4 feet from the head is good.

FR Millmore

Thanks Frm, FOR YOUR ADVICE. apt aDVANCED pERFORMANCE TECHNOLOGIES, HAS AN a/f METER LISTED IN THEIR CATALOGUE, IDEAL FOR MINIS AND MG'S SO IT MUST WORK, AND IS RELATIVELY CHEAP. The one I have was moderately priced I hope it delivers! (sorry got the caps lock stuck) thanks all Mike
J.M. Doust

You may already know this but it has not been covered so I will chip it in.

When your wide range sensor is fitted to the exhaust it needs to be powered up when ever the engine is running. Otherwise it will collect deposits that will shorten its effective life.

You could also have a look at
David Witham


I understand the issues of using 2 sensors but when I last looked at this a couple of years ago the problem was getting the sensors up to temp and keeping them there. Placing the sensor to far downstream risked them not getting up to temp. I was using a heated sensor but even so it was recommended to keep them in a hot area, I guess things have moved on since then.

R.A Davis

Bob I can assure you the sensor gets hot enough at the the join of the pipes and on my car thats further back than standard, at about the front of the seat or more. Its a Hanns Pederson system. In summer it works in about 30sec in winter a little longer. The make is PLX and uses the Bosch wide band heated sensor. Was not expensive,about $240 and would be a lot cheaper now. Its a read only, and the next model can download to a laptop.
They even do a end of tail pipe adapter for if you dont want a bung or wish to swap from car to car so the internal heater must be enough. It will start working while still cool enough to hold your hand on. As FRM said if you want it for tuning it needs to be wide band as a narrow band will only read rich, correct or lean where you need to know by how much. I leave mine in the car all the time and my normal readings are cruising about 15:1 -16.5:1 light acceleration 14:1 and as rich 11.8:1 on full power. I like it a little over rich on full power with the SC. Denis

Nice tip about pointing the tip down FRM....common sense ....but then again I think I have to learn everything the hard way!!!!

We use two makes of AFR meter the Innovate LM2

and the NGK Powerdex AFR...scroll down the page to view on the link below

The Powerdex uses a really expensive....3 times the price of the Innovate Bosch one....sensor. It works very well in hostile conditions on our rolling road. Sensors do not like rich mixtures which shortens the life.

We try to measure how far the piston has lifted in the dashpot to reference needle position. Some people tell me the simple expedient of cutting notches in the needles can give good reference points.

Unless the exhaust gases are red hot we tend to find part throttle readings not deadly accurate but we do tend to pick up from the tailpipe!


P Burgess

Thanks for all those well documented and thought out comments and for the downward tilt of the sensor FRM. While I have the car on stilts! I will pre-drill this hole in the right place. Cause I know the exhaust man will not!! Mike
J.M. Doust

I am contemplating a sensor for my B. I know I should stop fiddling - the car runs well, it is my daily driver and not used for competition, and over a period of time I seem to have arrived at a reasonable state of tune. But it would be really satisfying to feel that the mix was 'just right' by an interesting technological means.

I live in Australia and was hoping that J.M.Doust (Queensland) or Denis4 (Victoria) might get in touch to advise what kind of sensors they have installed and what is available locally.
J H Crighton

This thread was discussed between 01/08/2011 and 28/09/2011

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