Quote:
Originally Posted by john_cramer
Hmm. . . 100mm hg =1.93367747 psi. So you're saying that when the pressure drops, the elements inside expand, pushing the pin further out of the cell. This makes sense to me, insofar as if you remove the cell entirely, the engine runs excessively rich. So when you climb in altitude the pressure drops and the pin extends, leaning out the mixture.
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John - Correct - consider the function of both control elements acting on the correction lever - the thermostat and barometric cell. (What i've called the correction lever is not strictly true - the "real" correction lever has the elongated hole and is actuated by the short "H link" - but you get the idea.)
Thermostat - when it's cold (richer mixture required), the lever is at its highest position. When the engine's warm, the thermo discs have expanded and pushed (via the small 90 degree cam) the correction lever down until it bottoms out its end of the lever (leaner)- . The critical setting here is where the compensation bottoms out - i.e. define a "warm" engine. If the compensation did not stop at some point (bottoming out) a hot engine fed leaner mixture gets hotter until some major damage is caused.
Barometric cell - at higher atmospheric pressure eg sea level (more 02 to burn) a richer mixture will be required - so at increased atmospheric pressure,
the pin in the barometric cell retracts 0.58mm / 100mm Hg of barometric pressure increase. Hence the rich mixture when you remove the cell from the unit. (do a web search on "aneroid barometer" or "aneroid cell" to see how it works inside) The critical setting here is for 1 standard atmosphere - achieved by washers under the baro cell.
Several engine builders of repute have replaced the baro cell and/or thermo units with simple set screw devices to meet the requirements of their particular application(check out Henry's 2.8SS motor for example - no baro cell
http://forums.pelicanparts.com/showthread.php?t=239210 , and RSR applications - no thermo compensation)
Pushing the correction lever down at either end will result in a leaner mixture. The various distance ratios (lever rule) determine the total degree and proportion of compensation signal ultimately delivered to the rack. So the thermo and baro compensation base or "standard" settings must be set together during a pump rebuild, to ensure their correct relativity. (refer to dimension "c" in VDT-WJP 711/1 B Suppl. 2 Ed. 2 Figure 1
http://www.pelicanparts.com/techarticles/911_MFI/Bosch%20_More_MFI_Repair_Instructions.pdf)
It shouldn't be too hard to map the actual delivered values for each control signal - just need to put a dial gauge on the end of the rack and replace the control in question with a screw and dial guage and create an XY plot - the "transfer function" of the control element.....(hours of fun for all the family)
A logical extension of this mechanical arrangement could allow for 3 compensation variables (as opposed to 2 used by Bosch in this application), by using a triangular plate with articulated pin in the center - each control acting on the 3 corners of the triangle, and the sum of the inputs delivering compensation at the articulated pin (by superposition) ...but lets learn to walk before we run...and stick with what Bosch gave us !!
Regards
John