So I got this Rabbit Turbodiesel-

[4x4_Welder]

I just recently picked up an 83 Rabbit Turbodiesel, and it's turned out to be a bit rougher than I initially hoped.  138k on it, and found out tonight the turbo is pretty well trashed.  It still has the stock gooseneck pipe and stock exhaust system, so I figure on killing three birds with one stone and put on a turbo I have laying around, off a Mercedes OM617 and make up a 2 1/4" or 2 1/2" down pipe and carry that size all the way out the back. 
Before I go nuts on tearing down my car, I do have a couple questions-  First, what pattern is the turbo flange on this engine?  It looks like a modified T3, but am I going to get any surprises in pulling that off there aside from the bolt pattern?
How much boost can these engines reliably hold?  I am not looking at making it "fast", I have a car for fast.  I have also spent way too much making slow cars fast playing with Volvos, I just want efficient.  I live in a fairly mountainous area, so being able to avoid getting run over on a hill is a plus. 

[RunninWild]

should be the flange thats on your manifold if its a t3 turbo, I'm not 100% sure but I think its a vw specific flange. No idea on how that turbo will preform, but the t3 was the largest turbo available on vw idi engines and its quite laggy. The type of power you want will depend on your turbo choice, a k03 is great for almost 0 lag but not very much boost, the t3 or k24 are great for high boost levels but are a bit laggy, the t2 or k14 seem to be a happy medium.

stock headgaskets are supposedly good for around 20-24psi of boost before you need to worry too much. Make sure you install an EGT gauge pre-turbo if you want to experiment with fueling and boost levels.

a better flowing exhaust, some pump mods and a decent boost increase should do quite a bit for more power. I just got a 97 golf aaz and I was pretty surprised how much quicker the car is just from raising the fuel screw about 300rpm,. More boost, fuel and a better exhaust would probably make a pretty substantial difference.

[4x4_Welder]

That is good to hear, thanks.  Looks like I'll have to break out the mill and make an adapter, not a big deal.  Bit of an odd shape, seems closer to a T25 flange but too square. 
This car will see a bit of freeway use, so a laggy turbo isn't a huge deal.  I'll probably only run 10-12psi.
 

[RabbitJockey]

the vw flange is almost the same as a t25, the holes which are further apart are the same as a t25.  if you're only running 10-12psi i'd recommend a k14 or k03. k03 will require a different exhaust manifold.  but i also recommend running about 20psi haha the engine can handle it just fine and it will drive much nicer at that boost level.  whats wrong with the current turbo?

[RunninWild]

agreed, these are such simple engines and are so easy to get free power from. As long as you have an egt gauge and keep temps in check the only real difference between 12psi and 20 psi is a couple turns of the fuel screw and a boost controller.

[4x4_Welder]

The turbo was much more howly than it should be, rather than a crisp whistle, so I popped the inlet off and confirmed it had way too much play.  It also had a fair bit of oil in the inlet, so I'm going to throw one of my proper oil separators on as well.   
The turbo I have is a T3 flange so I can just mill out an adapter and make it all play together. 

[RabbitJockey]

that mercedes turbo will probably spool slowly, but if it does it just means you'll have to do more mods to make it run better haha.  or you could rebuild your vw t3, or build a hybrid from the 2.  use the vw turbine housing and turbine wheel with the mercedes compressor.  and throw a 360* thrust bearing in there.

[RabbitJockey]

actually if ur just worried about highway use that merc turbo will probably be fine, definitely will spool slowly though

[theman53]

For 10-12 psi I would rebuild what you have with the 360 bearing. I would also see about getting yours checked out by a pro, you might not need any work done to it.

[4x4_Welder]

The compressor wheel is contacting the housing, so it needs a CHRA or full out replacement.  I play with turbodiesel trucks for work, so while this is a bit smaller of a scale, the principles are the same. 
It sounds like the exhaust manifold gasket let go, so I may tear it down this weekend and see if hybrid is an option.  I'd really like to upgrade that downpipe, it looks insanely restrictive, but if I can get it back running without breaking out the mill then all the better.

[theman53]

Contacting when you push it with your fingers or when running on the car?

    I have bought several brand new turbos and they will have a lot of up and down play, just 0 in and out. But if it is contacting when running that is an issue, usually the oil isn't keeping the shaft a float. The 360 bearing rebuild will spool as fast as the smaller turbos, especially if you go to a 2.25 or 2.5 inch exhaust. I didn't have luck with the hybrid, but rabbit jockey has, so if you go that way good luck.

[4x4_Welder]

It sounds like the Jetta in this link, not always but that is the sound of the compressor hitting the housing.  I got first hand experience with that on my old 245 turbo, it was howling for a few weeks then let go going up Snoqualmie pass about 30miles into a 700mile round trip.  It was a bit slow after that.

https://youtu.be/lGhmm2RGUEI

[4x4_Welder]

RabbitJockey, can you point me towards the thread on your hybrid turbo?  I did a little searching but only found references to it. 
I didn't have the time to really mess with it today, I just replaced a really bad inner tie rod end and checked the valve lash.  It wasn't terrible, intakes were all between .007" and .009", the exhausts .016" to .019", so it'll get left alone for now.  I did take the opportunity to address the rusty cam cover though-

[RabbitJockey]

wow that looks like a factory turbo diesel car, that is very very rare.

the hybrid is pretty simple, use the mercedes compressor wheel and compressor housing, use the vw parts for everything else,  have a professional build it if you are concerned about balancing.

[4x4_Welder]

The guy I got it from had this one and an n/a with a four speed.  He had sold both, but the guy backed out on this one so I ended up with it.  His loss.  I'm at 3000ft and there is a pass in any direction to get out of here so an n/a isn't a good choice. 

I'd like to use the Mercedes CHRA and compressor housing with the VW exhaust housing if that's possible.  Otherwise I'll just make an adapter and put the Mercedes turbo straight on.  I was hoping to upgrade in stages as I'm not quite ready to build the new downpipe, but fewer steps is cheaper in the long run.