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Altitude compensation for Lucas MU really works?


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Hi there,

last week in a german forum a guy showed me a picture of the altitude compensation for the original MU. I have never seen that before. But I  really love to cross the Alps and drive as many passes as possible. But at altitude from 1800 meters and above the injection systems becomes too fat and passes higher than app. 2100m are not really driveable. So I am wondering if that altitude compensation really solve that problem? And if so, how do I get one? Will it fit on a normal MU or will I need a complete set consisting of MU and compensation?

 

image.png.02288dc5fe4147d8952632dc3d8a34da.png

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They are a rare item to find these days Im afraid. They did work AFIK.

Stuart.

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Slightly more common than hens’ teeth.

Not many were supplied to high altitude markets - many/most of the PI cars were simply adjusted to run leaner than those supplied for sea level.

Manually aspirated cars lose power at altitude which the PIs compound by its failure to compensate for low atmospheric pressure. However many of us have gone over some of the high passes such as Stelivio.

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Last new one I sold was in the 1980's, and that was to a German TR Owner.

Peter W

Edited by BlueTR3A-5EKT
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Contact Neil Revington at RevingtonTR who is looking into a modern adaptation to solve this problem. Given the quality of Neil's work it will work just fine.

Alan G

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This was in Neil Revington's newsletter recently, work in progress for sure. I would recommend that you give him a ring and discuss. The answer to his question at the top was a pic of the later Lucas high altitude unit.

Alan G

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 

Who has ever seen one of these I wonder?

 

This is a genuine Lucas product from the period towards the end of the PI life as the world moved to Bosch mechanical systems then Electronic Fuel Injection in short order and Lucas got out of the game. But not before they produced a few test samples of this baby. So, what is it and how does it work?

The main two units are the Metering section and Distribution section of what we generically know collectively, as the Metering Unit, but below the Metering part there is another unit. This is the Altitude Compensation Unit.  I have only ever seen two. We fitted one years ago to the ex Roy Fidler Triumph 2000 with a factory installed TR5 engine. The owner was doing a lot of mountain driving and, as we all know, or at least those of us with mechanical PI, when you get to 2000m above sea level the fuelling goes so rich it is difficult to drive the car and you pray to be able to get lower down again.

Clearly if you lived at these altitudes the car would be no good to you at all.  This was Triumphs mechanical solution. The capsule on the bottom detects atmospheric pressure and compensates by reacting against the metering spring pressure to lean off the mixture proportional to altitude. Simple but effective. I dare say that if PI had continued, all metering units would have morphed into a version of this.

  That is not the end of the story though. The concept set me thinking, (I do a lot of that but only a bit is useful!) that I could bring the concept up to date.  I have mapped out a plan to use modern electronics, a barometric pressure sensor and a pressure variation device to more accurately control the fuelling proportional to altitude, with the settings being adjustable via a laptop or even a Smartphone. The ultimate goal will be to be able to tune the Metering unit in total via the electronics but that might be away off yet. Watch this space.....   

 

Neil Revington

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thanks a lot Neil for that information. If it ever happened that I get one of these rare parts: will the Altitude Compensation Unit fit on every standard MU or is even the MU itself a special part?

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I see in the 1970 World Cup Rally the Triumph 2.5 saloons had "Lucas petrol injection, incorporating manually operated altitude compensator device." Does anyone know of, has seen, or better have a picture of one of these?

Cheers, Richard

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1 hour ago, Spit_2.5PI said:

I see in the 1970 World Cup Rally the Triumph 2.5 saloons had "Lucas petrol injection, incorporating manually operated altitude compensator device." Does anyone know of, has seen, or better have a picture of one of these?

Cheers, Richard

Im sure someone did post a picture of it and I probably copied it somewhere but it will be on works computer, have you tried a search on here via Google rather than a forum one?

Stuart.

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Thanks Stuart. Yes, there were a few topics from 5, 8 and 15 years ago. You might be thinking of this one TR6 and high alpines passes?  (which I even took part in) but your image has vanished - a victim of Photobucket's new policy no doubt. None of the topics I've found describe how the cable version worked (adds "shims", "negative" adjustment of choke, something else ...) so I thought I'd innocently ask ...

Richard

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All very complicated, I have driven the alps and Pyrenees in my TR6 PI many times and never had a problem with altitude so never even thought about it, maybe that's why there isn't many around? 

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Not overly complicated once the components are made, and in combination with Lambda sensor a nice way to fine tune (part of) the mixture during driving.

Nice piece of engineering.

Waldi

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On 5/30/2020 at 4:00 PM, Casar66 said:

 

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Innocent question: how do Webers perform  at high altitude? Are they flexible?

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This will bring up more trouble than it helps!

The MU must meet the engine requirements.

Lucas had three different sets of springs in use what means

there are more than three versions produced.

 

What we will need in the Alps is a full load reduction of fuel what is not done

by the above hand controlled system. That would be nice to meet different  air

temperature and air pressure when cruising with part throttle.

 

Anyway as Waldi said all requires a Lambda sensing unit.

 

Some are affected more from high altitude than others. Reason is simply that PI is quite thirsty when set up

for maximum performance. So I did see several TRs that go as lean as Lambda 0.94 at full load.

WIthout any trouble you can go in the Alps up to Lambda 0.82. So climbing up the performance will rise

for some 100 metres of altitude and than engine gets thirsty but does not stall.

 

Webers too dislike different altitudes and air temps. They simply perform quite well that some missing horses

do not play that role compared to a US twin carb..... Anyway also they suffer more if set at sea level on a cold day

already to Lambda 0.85.....

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Quote

All very complicated, I have driven the alps and Pyrenees in my TR6 PI many times and never had a problem with altitude so never even thought about it, maybe that's why there isn't many around? 

When my PI runs well in my area (altitude somewhat between 50 and 500 meters) it is not possible to do a montain pass above 2000 meters properly. If I adjust the MU leaner it becomes better but the the risk of overheating at full throttle in flat areas is not worth it. I like the idea of the German TR-driver. And I do think that a lambda sensor is not is absolutely necessary if the range of manual adjustment is not too big. It reminds me of the manual ignition adjustment in the pre-war era. The sensor was the driver. So why not here? My aim is not to have a fully automatic control with the finest modulation. I want just a little help to get over the highest passes in the Alps without torturing my TR.

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2 hours ago, DavidBee said:

Innocent question: how do Webers perform  at high altitude? Are they flexible?

Webers dont mind altitude, ask Tom Fremont about them.

Stuart.

 

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6 minutes ago, stuart said:

Webers dont mind altitude, ask Tom Fremont about them.

Stuart.

 

Thanks Stuart! I'll do that.

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16 hours ago, Spit_2.5PI said:

Thanks Stuart. Yes, there were a few topics from 5, 8 and 15 years ago. You might be thinking of this one TR6 and high alpines passes?  (which I even took part in) but your image has vanished - a victim of Photobucket's new policy no doubt. None of the topics I've found describe how the cable version worked (adds "shims", "negative" adjustment of choke, something else ...) so I thought I'd innocently ask ...

Richard

Found the one of it fitted to a saloon.

Stuart.

post-10590-0-60986100-1322348391.jpg

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2 hours ago, stuart said:

Webers dont mind altitude, ask Tom Fremont about them.

Stuart.

 

I had my '250 at 11,150 ft. with no running issues and plugs were clean when I checked at about 7000 ft on the downhill side of Loveland Pass in the Rocky Mountains. I was a little apprehensive because prior consultation with our leading Weber supplier Mike Pierce was inconclusive. They say you lose a point of compression for each 5000 ft. power-wise and it did seem another 1/4" of throttle was needed beyond what a comparable grade in 4th O/D would have warranted. Still inches to spare. Otherwise, w/r/t temperature, they do idle at lower rpm when it's hot outside. 

Who ever heard of altitude compensation for Webers? 

FWIW, I went over the same pass 30 years earlier in my other '250 on emissions Strombergs. Leaded fuel was still in use, and you could always tell how your engine was burning by the tint of the tail pipes; they were beige after crossing the mountains. No running issues at all. So in my experience at least, carbs don't get altitude sickness.

Pic is Eisenhower tunnel https://www.google.com/search?q=eisenhower+tunnel+elevation&rlz=1C1EJFC_enUS874US874&oq=eisenhauer+tunn&aqs=chrome.2.69i57j0l7.11910j0j7&sourceid=chrome&ie=UTF-8

Tom

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