Things noobs need to know before they start build threads on vortex lol:
A) 12v VR for mk3 has 2 main strains... OBD1/OBD2. OBD1 is older as you may guess. main differences are harnesses, # o2 sensors, EGR system (1 has, 2 not), head variations, chains, etc. OBD1 engines almost always have the aluminum valve cover (i think).
....a) early early early OBD1 can have a distributor - basically not much aftermarket options for big turbo power. some of these heads have no SAI port, and some have a sealed side-head where all the rest are open (around
....b) later OBD1 have single coil packs like an OBD2, some have SAI ports - some don't.
....c) OBD2 has different throttle body, front AND rear O2 sensors, different MAF, different valve cover plastics (3 versions - i have them all in my garage, lol), no ugly-ass EGR system on between manifolds, can actually get good info from ECU via OBD port. OBD1 and OBD2 engines tend to sound different to a well honed ear - probably throttle body and intake noises, etc.
....d) many many many of the OBD1 VR's i've worked on or chopped have 4.0bar FPR's in them - not sure if that was a standard or not... many passat OBD2's also have these. not sure why.
....e) late OBD2 engines have the mk4 passenger side mount bolt threads in them - the block has a widened boss area for these - cannot (easily) retrofit an early mk3 block to hold mk4 mounts).
B) 12v VR6 from mk4 are all the same. share same chain parts as mk3 OBD2, moved to plastic intake manifolds which have different runner design, and therefore a mk4 cam set IS DIFFERENT than a mk3 one. both will interchange, but not properly (many threads on here... gonna name drop "FOFFA" on this one - search that).
....a) mk4 downpipes moved to a solid 1 piece from the mk3 style of seperateable cat-verter. mk4 shares same cat as 1.8t engines, just different on up-pipe section from cat to manifolds
....b) mk4 has a metal headgasket, mk3 had a fibre type. result is slight compression increase for mk4 - both are form-fit-function compatible, a common upgrade for mk3 12v when doing chains
....c) allllllll 12v water pumps, oil coolers/filters, etc are same. coolant system plastics on mk4 are different than mk3 around the thermostat. mk4 VRT users tend to use the mk3 setup as it gets the hoses out of the way of the SRI throttle body position.
....d) mk4 can use a mk3 valve cover if one wants the baller aluminum status and doesn't mind some hose changes for venting.
....e) mk4 12v has intake temp sensor in the MAF and not the side of the intake manifold like a mk3
....f) mk4 blocks still have legacy rear mount bolt holes to use in mk3 applications easily
....g) early mk4 12v VR's (ie - 99.5) were a different motronic ECU engine management and had throttle cable TB's --- different harnesses and ecu, etc etc than 2000+ 12v's. raaare gem. good buddy of mine is putting one into an early beetle right now... he's bored with his mk4 VRT and his mk2 VRT i guess... lol
....h) mk4 12v uses an aluminum oil pan which has a few extra spots to bolt your O2j tranny to. also holds a bit more oil, which is handy for a VRT application to keep cool.
important things to know about mixing different longblocks into different applications:
1. keep your cams and intake manifold sets paired together (ie - don't mix mk3 and mk4... FOFFA will kill you!)
2. keep your timing chain cover sets together as they are different for mk3/mk4 and for mk3 single chain, double chain... don't even get started on "will my dizzy chain covers work with my..." lol.
3. if you use a mk4 block in a mk3/2/etc - ensure you swap to the correct knock sensors
4. mk3/mk4 injectors are different. most die due to dirt/grime getting into the breather hose which feeds the side of the mk4 plastic manifold... it actually sucks in air to improve the spray pattern... don't replace your injectors until you've cleaned out this hose/line and the tiny little screens in the end of the injector body which plugs into the manifold. fixes a failed injector 95% of the time, 35% of the time.
5. before you buy a VR6 engine, check the mounting holes for the application that you will be using. these blocks rott out in the holes that are not in use (at least in canada they do).
other than all that useless info... for the most part, the bottom ends are damn near identical aside from slight compression different in mk4 pistons (excluding euro 2.9 engines, etc). heads are damn near the same with the exception of the SAI port option and the dizzy heads with the sealed chain end in the casting. yes there are other differences over the years, but most seem to be machining patterns, etc.
oh, and ..... no, its not financially feasible to turn the 12v VR into a diesel, even if your buddies told you so.
oh, and lastly.... with so many 1.8t's rotting in the ditches by now, if you want to get a faster mk4 (assuming you have a 2.0L), don't bother with a 12v unless you are also turbo'n the guts out of it. drop in a 1.8T and take advantage of cheap engine parts, and plentiful spare engines/transmissions at junk yards.
for the record, i love VR's and have had oh so many. mk3 gti, mk4 12v, b4 passat vr, mk4 R32, 4-door mk4 12v (was a tdi, lol). i like them because they sound great, but they are slow and terrible on gas. the R was different. that was forbidden fruit to me being a canadian dubber, but my k04-2x GLI ate it for breakfast and i bored of it... so there you have it... R32 > 1.8t > 12v VR6 > 12v VR6-dizzy VR6 > 2.0L > honda
also... comically - in my area of canada where mk3's have rotted out and you almost can't find them anymore... there are currently far more mk2 VR6's on the road than mk3 VR6's. mk2's love them, but get very nose heavy compared to an equivalent 4-banger setup... but they're great.