Certainly air pressure changes with altitude which would affect the amount of advance the VSD's "baseline" is, but the bellows ( which sends the variable signal to the ignition module to control timing) is controlled by vacuum pressure from the plenum. Ordered it from Alfaholics as a used item but it truly looks pristine. It is new to me for sure. If you have disabled yours I would consider finding a "new" one. Acceleration is noticeably smoother, idle is much better too. I am waiting the see the fuel mileage comparison.
Good on you!
If your idle improved with a new VSD, the original must have been leaking seriously. Cracks usually present as a quite small leak and if yours progressed to an idle issue, well...
Several years ago I worked with a control engineer who designed and tested a mostly electronic VSD. I used a Chrysler corporation MAP unit for a vacuum sensor and sent him ignition advance data which I laboriously documented with a vacuum gauge. He used those #’s to design a device that converted MAP output to a signal the Ignition CPU could work with. Still operates!
There are posts searchable with my user name.
I did a before and after mpg test and went from about 26 (+/-) to 32 on a flat, 75 mile (+/-) round trip, but economics did not pencil out for production.
Now, I am the only owner of an ‘87 Quad on this side of the Galaxy with a semi-electronic VSD!
Contrary to your results (placebo effect?
) I found no observable difference in acceleration. That was seat-of-the-pants of course, since I was laughed out of all local drag strips
Also, there is a separate barometric sensor under the back shelf to adjust ignition (I think) and injection parameters.
On a related, performance topic, and at the risk of highjacking the thread, I still await Dyno results for w/wo our pioneering variable valve timing device (VVT). I believe it only actuated when a switch on the throttle mechanism closes at WOT, but no matter how carefully I sneak up on it, I’ve never felt any performance difference. I’ve confirmed clicks at VVT and throttle switch, and energizing it directly with a jumper at idle causes roughness.
My understanding is VVT was incorporated into L-Jet to provide a stable idle with improved WOT performance.