First of all, you're on the vendor marketplace subforum... this isn't the best/right place to post. I'm not even sure how I came across this myself in the first place...
Second thing, I don't know the tolerances of the 1.8T (I'm more knowledgeable on the 2.0T platforms) but I can tell you 2 things:
1) It's not about HP, it's about torque. HP is a basically a theoretical figure that is a function of torque, torque is the real number because torque, physically speaking, is the force that is actually acting. The motor produces a certain amount of torque at the crank and then a certain amount of that power (after accounting for drivetrain losses, etc.) actually goes to the wheels; we call this crank torque or wheel torque. Crank horsepower, also known as brake horsepower, is just mathematically derived from the amount of torque at the crank. HP isn't what bends a rod in your engine if you have too much, that is what torque does. So what you want to be looking for is what the tolerances of your stock motor are in terms of torque not horsepower. That being said...
2) I can 100% guarantee you that you your stock motor will not handle a big turbo and, if I recall correctly, the 1.8T doesn't always fair so well even at K04 power levels (but again the 1.8T isn't my specialty so bear that in mind). A little nuance that people often don't consider is that while the big turbo makes more power at its peak than a K04 for sure, the K04, being a smaller turbo, is very torquey/peaky and has a more instantaneous onset of high torque, whereas big turbos have a smoother onset. So in a way K04s can actually be more dangerous for bending a rod in some cases than a big turbo. Don't take that the wrong way though, that doesn't mean you can safely run a big turbo, it would still definitely produce more power than your stock block could handle; what I'm really saying is that the K04 can be dangerous too even if it is weaker overall. The good news though is that you could get a K04 and then have the car tuned to stay within the safe range of torque/torque delivery of your motor if you get the tune done custom.
While I can't give you the exact numbers, I hope this helps you understand what to look for and what your options are. I assume you also realize that before upgrading your turbo there are more things to consider than just "can my motor handle it", you also need to have sufficient fueling to achieve those power levels. I'm not sure if the 1.8T fuel pump is capable of outputting enough fuel pressure on the rail to support a K04/tune but my guess is probably not. You will probably also need upgraded injectors. You may need even more. Those are my guesses, I don't know the 1.8T limitations for sure so do your own research but don't be surprised if you need to do more than just throw on the K04 and a tune in order to actually take advantage of it.
The stock bottom end is good for about 310tq. I ran a persision 5857 at 20psi for 10k miles with no issues. You want more replace rods. While i.e rods and such are great their expensive. Scat rods are about 350 a set and I made over 600whp on them.(587wtq iirc) as stated a big turbo has lag so hits peak tq later in the power band. Anything over 400 need to replace exhaust valves with fera or supertech. Around to 500 to 600 level look at replacing main studs with arp or comparable. As the bottom will start to flex. Anything over 700 and your looking at a girdle. I should mention at time of rod install arp headstuds are a must. At the moment everything in my motor arp even flywheel bolts
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