Fitting lambda sensor

[mjheathcote]

The thing with production cars with two sensors, is that they are fuel injected so the ECU can control the fuelling on a particular bank.
With the Stag having the inlet manifold split so each carburettor services 2 cylinders per bank, the controlled fuelling is split between banks...hence no advantage to having two sensors, they will see the same mixed gases, you will not have one bank lean, and one bank rich, well you could, swinging from lean to rich if the carburettors are not adjusted equally of course.

[Stagsongas]

'well you could, swinging from lean to rich if the carburettors are not adjusted equally of course.'

Precisely.

And given that the idea of having one sensor fitted is supposed to aid mixture adjustment, the reason for fitting it collapses if it cannot lead you to correct synchronisation of just 2 carb.s.

The lights might be nice at Christmas though.

Regards

Steve

[mjheathcote]

....I've a Holley though so no synchronisation issues, so the fuel mixture will be equally split through the inlet manifold going to both banks, and therefore both banks will have the same exhaust gases, so one sensor monitoring the exhaust gases will give an accurate reading of the fuelling.
i even have symmetrical tubular exhaust manifolds both going forward then back under the car, so the inlet and exhaust paths are symmetrical in my case.

[mjheathcote]

'well you could, swinging from lean to rich if the carburettors are not adjusted equally of course.'

Precisely.

Steve

Same situation with two sensors though, you don't have one sensor monitoring the left carb, and one sensor monitoring the right carb.

[mjb67]

Steve in your opinion is fitting a lambda sensor (or two) not worth the effort then in terms of helping with tuning? Do you personally simply adjust the mixture for highest idle and/or do the thing where you lift the air valve to see how the rpms change? (like here: http://www.triumphspitfire.com/images/carbs/zs1.jpg)

I too was thinking of installing a lambda sensor (or two) somewhere so I could get some hard numbers. I'm not a mechanic (I'm a software engineer) so I can't do it by feeling and experience, and 'reading plugs' and 'colourtune' just sound like voodoo to me.

[Stagsongas]

Steve in your opinion is fitting a lambda sensor (or two) not worth the effort then in terms of helping with tuning? Do you personally simply adjust the mixture for highest idle and/or do the thing where you lift the air valve to see how the rpms change? (like here: http://www.triumphspitfire.com/images/carbs/zs1.jpg)

I too was thinking of installing a lambda sensor (or two) somewhere so I could get some hard numbers. I'm not a mechanic (I'm a software engineer) so I can't do it by feeling and experience, and 'reading plugs' and 'colourtune' just sound like voodoo to me.

To be frank, there is so little (if anything) to be gained in fitting one, two or however many 02 sensors to an engine that does not have an authorative ECU controlling injection times, that I think none of it is worth doing when stood against the time/cost/complexity. That especially goes for a two carb. system like the Stag where the banks are mixed up, just for illustration 1 and 2 mixed with 7 and 8 from one carb. in what we would normally call a 'bank'. When that is taken into account, Stag 'banks' are really made up of 2 cylinders on different sides of the engine, requiring 4 sensors minimum placed in two - four branch manifolds to get clean, reliable information.

I'm not doing that!

It is much easier to tune using a gas anaylser up the tail where the products of combustion are still mixed form all 8 cylinders but you don't have to go to all the trouble of fitting what is in effect a mobile gas analyser (the basis of a gas analyser is an 02 sensor). Problem is balancing up the carbs.

The best way by far to do that, as things are confused by the twns and the bank mix-up is to tune for least O2 on a four gas analyser, which is what I use for the basis of an LPG set-up. If you get least 02. your AFR will be close to the ideal 14.7 - 1 Stoichiometric mix, so the the CO and HC will falli n and be good too, for a begining of a good set-up. But don't go too lean, she's an old girl allowed to pump out more CO and HC than a newer engine and you don't want to spoil those valves, they will only put up with unleaded because they get cooled by a mix richer than we would use today (less than 0.5 CO typical).

An interesting aside is that now and then my conversions get tested incorrectly by MOT testers, they just shove the pipe up and accept/refuse, but they probably didn't reset the gas analyser to the protocol for LPG which has a Stoich. of 15.3 to 1, not 14.7 to 1. No surprise that the figures are skewed.

Finally, I the main reason I say putting 02's in that are not linked to a controlling ECU is not worthwhile is that it's main fuel and pollution saving virtue cannot be achieved - When on the overrun in 95% of cars with injection that are driven by an ECU, fuel 'cut off with inbjectors' comes into play, and that cannot be done with a carb. system as it stands. Believe it or not, the fuel cut-off during the overrun is the major player in fuel and pollution saving in a modern car, having the thing running in closed loop is a very minor gain in the scheme of things and does not make anywhere near as much difference.

Regards

Steve

[mjb67]

Thanks, that's very interesting. Maybe I'll investigate getting hold of an up the pipe exhaust gas analyser instead then, as looking at my fuel consumption is going to be at the top of the list of jobs at some point :-)

[KOY 23]

An up your pipe analyser will only help you set up idle mixture. Wheras an AFR meter shows you state of combustion when driving, which in my mind is far more use. I could not have found that my weber was running weak without it

[jakesmig]

Finally, I the main reason I say putting 02's in that are not linked to a controlling ECU is not worthwhile is that it's main fuel and pollution saving virtue cannot be achieved - When on the overrun in 95% of cars with injection that are driven by an ECU, fuel 'cut off with inbjectors' comes into play, and that cannot be done with a carb. system as it stands. Believe it or not, the fuel cut-off during the overrun is the major player in fuel and pollution saving in a modern car, having the thing running in closed loop is a very minor gain in the scheme of things and does not make anywhere near as much difference.

Regards

Steve

Back in the early '70s my dad backed a machinist in Johnstone ( a local town) who produced a valve which consisted of a machined cylindrical housing threaded to fit into an inlet manifold. As far as I recall, the diameter of the thread equated to the normal servo take off thread. In the housing was a spring which retained a largeish ball bearing against a rubber washer held to the body by a threaded cap. On the over run when vacume is high the valve would open, closing again when the vacume reduced to tickover pressure ( about 15 - 18 psi). This replaced the gas flow past the throttle closed carburetors, effectively creating an over run fuel cut off.
Unfortunately he didn't pursue patenting and couldn't get anyone interested in major production runs but he did produce enough on a hobyist basis to supply quite a few people in the area, but the valve did appear to work well, I recall routinely getting better than 45mpg out of my 1275 Midget.
I did think of making one for my Stag when it had carburettors but the trick is to match the spring to the required pressures which put me off.
The machinist also had an idea to modify points to have a double cam so that a twin spark could be made. Don't know if that worked or not.
All goes under the title bizare but true.
John.

[Stagsongas]

Back in the early '70s my dad backed a machinist in Johnstone ( a local town) who produced a valve which consisted of a machined cylindrical housing threaded to fit into an inlet manifold. As far as I recall, the diameter of the thread equated to the normal servo take off thread. In the housing was a spring which retained a largeish ball bearing against a rubber washer held to the body by a threaded cap. On the over run when vacume is high the valve would open, closing again when the vacume reduced to tickover pressure ( about 15 - 18 psi). This replaced the gas flow past the throttle closed carburetors, effectively creating an over run fuel cut off.
Unfortunately he didn't pursue patenting and couldn't get anyone interested in major production runs but he did produce enough on a hobyist basis to supply quite a few people in the area, but the valve did appear to work well, I recall routinely getting better than 45mpg out of my 1275 Midget.
I did think of making one for my Stag when it had carburettors but the trick is to match the spring to the required pressures which put me off.
The machinist also had an idea to modify points to have a double cam so that a twin spark could be made. Don't know if that worked or not.
All goes under the title bizare but true.
John.

Actually John, that is a blistering idea, and with today's materials and technology, it is achiveable.....And I know exactly how I would do it. Want to go into business?

Regards

Steve

[Stagsongas]

An up your pipe analyser will only help you set up idle mixture. Wheras an AFR meter shows you state of combustion when driving, which in my mind is far more use. I could not have found that my weber was running weak without it

If you don't want to invest the cost of fitting even one fixed 02 sensor, most MOT stations will run you a check for a few quid, but as you say, this will not tell you what the fuelling is actually like on the road.

If you want to spend about the same, you could have a roilling road check including a fan to replace the often ignored but important relative airflow.

If you want to spend more, buy one of the portable 4 gas analyers on the market for the last 10 years or so, perhaps even a used one. Then you can drive with that up your pipe all day long, and it won't be stuck on one car.

You could even make your own portable GA by using a sensor, a laptop and a 'scope program and perhaps make a few bob by doing checks for others, but it'll have to be a lot smarter to tell you which of the twin carbs is weak, and which is rich.

Regards

Steve