TDI engine to SDI car

I have no idea why you want to do such thing, apart from small hp boost you can get. But yes you can swap them pretty easy all you need is new engine and maybe the new ecu. However such engine swap will cost a lot of money especially if you don't have the tdi engine.

Yes, it's completely possible. The 1.9 SDI and the 1.9 TDI family of engines all use the same engine block, therefore it's possible to bolt the TDI engine in on the SDI engine mounts. It isn't all plain sailing though and indeed it's worth pointing out that the 1.9 TDI is not a single engine. There are numerous variants of both PD and non-PD types. Things like the routing of the turbo pipework and mounting the intercooler may pose some slight issues but you ought to be able to source compatible parts from another Polo, an Seat Ibiza or a Skoda Fabia.

Obviously you'll want enough of the TDI exhaust to bring the pipework to and from the turbocharger. You may find that the Golf downpipe hits the bulkhead on the Polo so you may have to go Polo GTI, Seat Ibiza Cupra or Skoda Fabia VRS. If there is nothing in the VW Group parts bin you may need to have an exhaust fabricated on the car.

Depending on if your car is a 5-speed or 6-speed, you may need to retain the flywheel / clutch assembly from your 1.9 SDI in order to use the new engine with your gearbox. You also may wish to upgrade your brakes and tyres to cope with the additional power.

Engine management wise, you can use your existing fuse box with the engine loom from the TDI. You'll want to get the TDI clocks (instrument cluster), ECU and ignition key as these parts are all electronically paired. Consider how much space you'll have in your dashboard to fit the new clocks. I believe it is possible to have the immobiliser removed from the ECU if you'd rather go a different route.

The PD variants use a fuel cooler in the fuel return line which is bolted to the underside of the floor pan. You'll want to add this because otherwise the heat in the returning fuel can get high enough to hole your plastic fuel tank.

Finally, speak to your insurers. Paperwork wise all you'll have to do is change the engine number of the registration certificate for the vehicle but your insurers may ask for an engineers report.

This is doable - although the first question should be what the goal is.

If it's "just" wanting a TDI-engined Polo then buying one with the engine from the factory is going to be a far more economical option (fitting a replacement engine is not a trivial job - there's quite a bit of ancillary work involved around the exhaust, the ecu, wiring loom etc).

If your goal is to get a one using a more powerful variant of the TDI engine than was offered in the Polo then it would have to be one of the Pumpe-Duse (PD) models used in the higher-end diesel Golfs, Leons etc and that's going to be slightly more complicated unless the Polo is a Mk4 itself (I don't think they go straight on to the Mk3 engine mounts) and while yes you'll get more power than an off-the-shelf TDI Polo but it would be massively expensive for what you get. Replacing the car with a Mk4 Polo GT (which has the 129bhp PD engine) would get you pretty much the same performance at a fraction of the price.

If on the other hand the challenge of doing it is the point of the exercise then that's a different matter, with a complete donor car from the same platform you can essentially swap the lot, lock stock and barrel. If the donor car is based off the Golf platform (rather than a Polo-based car) then there's going to be a great deal of work involved in fabricating a suitable exhaust, loom, and intercooler plumbing (the Golf platform had it's intercooler in a different place IIRC), the donor ECU should work out of the box but it will depend upon the relative ages of the donor and recipient.

If all you've got from the donor is the engine & turbo you'll have to source a suitable loom, ECU and exhaust.

My wallet is having palpitations just thinking about it.

Also if the Polo in question is a Mk3 (only these and the Mk4 run the SDI engines) then that means another factor to bear in mind is that unless the car is a facelifted 2000-onwards model (aka Typ 6N2) then the TDI engine won't fit - the earlier Mk3 cars had a smaller engine bay which won't accommodate it and the engine mounts are different besides.

Yes I have Done It many times, Lost Count, All diesel engines from German made cars at the same age, are all the same, its very strange, they dont tell you about this. For Example, Seat, Skoda, And Golf Are all the same engines,I have put seat engines into golfs, and golf engines into skodas,and seat alhambra engines into Ford Galaxys, I have put Golf Engine into Polos, and golf engines into VW Tourans, All You need to do is take of the old ecu of the engine and put it onto the new engine. very Simple. It Doesnt matter if its a turbo or non turbo, You Can Make a golf sdi into a Tdi, simple, no changes apart from the ecu. But its hard and time consuming, changing a petrol engine to a diesel engine and vice versa.