Thanks, Jeff. I'm still looking for new, nearly-new, or freshly-rebuilt 0 280 100 037, 043, and 049 MPS's.
The few I've tested so far verify a lot of things that I had suspected, but I need more units to make certain of the results. Here are some preliminary findings:
1. All of the aneroid cells seem to be the same, as they all have expansion rates within a very small window.
2. Calibration differences between the three MPS's used on 914's are significant, but small, and mostly affect full-load operation. That's why people have been able to use different MPS's than spec in their cars and get decent performance.
3. The linear range of operation is smaller than one might suspect. At high manifold vacuum levels (overrun), the armature in the MPS core begins to be pushed up tight against the spring stop at about 20 in. Hg. Transition to full-load operation begins at about 6 in. Hg, so you have about 14 in. Hg of linear range.
4. The transition to full-load operation begins at higher vacuum levels than one might suspect. It starts at about 6 in. Hg and ends at about 2 in. Hg when the diaphragm touches the full-load stop.
Assuming I get some more units, I should have a decent set of calibration curves ready in a couple of weeks.
Brad Anders
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