Originally Posted by Fuj1wara
Why worry about keeping them in sync? Keep the pid controllers motoring backpressure, outlet PSI, EGTs and shaft speed. Just have them both pump out the same pressure. So long as they are targeting their pressure it shouldnt be an issue. Remember you will have some differences in flow between the two heads as nothing is perfect. Or you could just average the two in your controller and send a single command to both turbos. Either way would work fine. I don't get a lot of the finer points, but I'e gotten away from overthinking all of this stuff.
Imho any differences in flow between one head/port and another will (should) be compensated for in mapping and will remain a “constant”. The issue is that the change in turbo geometry is by definition not a constant, whilst the maf will compensate for changes in airflow, it cannot compensate for changes in VE between the two halves of the engine (at least not the ecu I am using). At present, at start up and shut down, I check that the vane mech position is being reported correctly by cycling through full closed and full open, and referencing to previously stored data. Currently I am thinking that it would be possible to measure the vane movement with a dial gauge (easy on a Holset, not so easy on a Garret) and then ensuring through the software (firmware), that, for a given input, each set of vanes move to the same position throughout their range. I don’t see how it is possible to measure individual turbo pressure, however measuring individual egbp (as you pointed out they will not be identical) and comparing the two could be used as an alarm.