Suggestions on altitude compensation for mechanical FI system wanted...

[Michael D. Porter]

Okay, I know it's nuts... but I bought, mostly for nostalgia's sake, an
older Lucas PI mechanical fuel injection system, made for the TR6 2.5
liter inline six. After lots of queries on the Triumphs list, and of
people in England, it seems there was some very crude altitude
compensation built into later units, but that it didn't work very well. 

The metering unit uses a baseline spring and cam calibration which is
controlled by manifold vacuum. Higher the vacuum, the less fuel delivery
at the injectors... It's mostly just controlling shuttle movement, I
think. 

Anyone have any interesting ideas on including workable barometric
compensation on such a system? Digitally, it's fairly easy, at least in
closed loop mode. As I recall, the older L-Jetronic Toyotas had a sealed
bellows with a strain gauge inside which altered the closed loop
resistance. 

But, given the vacuum control system of the Lucas mechanical system,
something like that would not be feasible, unless it operated a valve
controlling a vacuum reservoir or some such which fed the metering unit. 

Cheers.