im not posotive what ur question is but i think u are asking about the ECU, in cars up to 1995 with the ECU they used a system known as OBDI anything after 1995 they use a system called OBDII. with OBDI systems you can physicly buy a computer chip and put it into the ECU unit (very simple and nice
) with the OBDII system you have to by a system to plug into the ECU that will reprogram your system for the new fuel managment and such from what i hear this is slighty bigger pain in the ass and what not. i hope this answeres your question!
On a side note, what sort of VW's were u buildin'?
p.s. Welcome to the forum!