Ok, Rob, these are the 'spare' FEt drivers?
http://shop.vems.hu/catalog/product_info.php?cPath=1_7&products_id=59&osCsid=6775c4817555876203e8157ecce3d177Looking at the schematic (looks like an always hard to read, simplified version) As you have pointed out already Jean, the signal for the Peak And Hold board needs to come from Pre_Inj, this is the 5v signal from the processor.
To the practicality of it. Would simply removing the driver chip, soldering a wire onto the appropriate pads of Pre_Inj for that chip, taking these wires to the 'inputs' on the P&H board, maybe then returning another wire from the P&H driver circuit output to the INJG pads of the removed chip, and solder the P&H FETs inplace of the VEMS Injector FETs. This just leaves the reistors R18, R21,R39 and so on, what to do with these? this would keep the FET heat sinks all in the same place along with the original flyback circuit, with only the add on board needing mounting. Seems daft taking the switched current off the board and then bringing it back again, when you could achieve the same on the 5v signal side of things.
One thing I am concerned with is there is no protection on the processor ? Boom and bust
Also assuming that small gauge wire is only required.
Schematic showing the driver chips Labled with 'U'
And the board Layout again showing the driver chip Chips still labled 'U' (lol, nit exactly the best colours to use together
)