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Altitude Compensation Unit

Published: 23 Apr 2020   updated: 24 Apr 2020

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 a way off yet. Watch this space.....   


Neil Revington