I am trying to start modifying a stock Skoda Octavia MK1 Engine type APK, but not sure what turbo to fit. I am planing to start the project and been advised to start with the turbo before the airflow system and exhaust system.
Engine specs:
Cylinders: L4
Power: 85 KW @ 5200 RPM, 116 HP @ 5200 RPM, 114 BHP @ 5200 RPM
Torque: 125 lb-ft @ 2400 RPM, 169 Nm @ 2400 RPM
Fuel: Gasoline
Gearbox: 5-manual speed
TL DR: Without knowing your power goals, we can't tell you which turbo is right for you, however, with a little knowledge you can figure it out for yourself.
Before you can even start thinking about which turbo to get, you have to know your power goals. How much power do you want your engine to make? Once you have that number, then you can start looking for a turbocharger which will match that goal. Turbo manufacturers will tell you what the maximum power you can expect any given turbo will make. The maximum output of a turbocharger is based on the amount of air it can move and is true regardless of what type of engine you have (i.e. I4, V6, V8, etc). Once you have the number, you can start looking at which turbocharger to get.
There are three basic approaches to choosing a turbo. The way you need to think about it though is more of a system, rather than just thinking of the turbo itself. When turbocharging the engine, if you understand which parts you are going to be using, you'll waste less time and money in the process of building it out.
The first of the three basic approaches is to research if your manufacturer (Skoda in this case) already has a version of your engine which is turbocharged. If so, see if the amount of power this OEM system provides will meet your power goals. If it will meet your power goals, go to a wrecking yard (or breakers or whatever you want to call it) and see if you can find the system and purchase all the parts for it. This is BY FAR the easiest way to make it happen (and most likely the cheapest). Not only will it fit your engine, but most likely will also fit in your engine bay with little, if any, modifications to make it work. If the OEM system doesn't meet your power goals, you may want to still look to see if by adding a larger aftermarket turbo to the same supporting parts (mainly the exhaust manifold) you could get there. A lot of aftermarket manufacturers make turbos which are a direct bolt on, but will make more power than the stock one. Again, it will be easier going this route than it would by going the other two approaches I'll describe next.
The second approach is researching if a kit exists to add a turbo system. A kit can come in one of two styles. It can be specific for your vehicle, which should have all of the right bits and pieces to make it all work with minimal fuss. While this kind of kit is supposed to be a direct bolt in, the fact of the matter is most kits will still need modification in order to fit correctly. If there isn't a kit specific to your vehicle you can always purchase a universal kit. The universal kit will have most or all of the parts you'll need, but will require heavy modification to make it work.
The third approach is piecing a kit together on your own. This is the most time consuming as well as probably the most expensive. Plus, you'll need to know exactly what you are looking for and what parts you'll need to make the everything work. If you are going this route, look at other manufacturers who produce engines which make the amount of power you want in their turbocharged engines which are the same size as yours. This will give you a turbo source which should suit your needs.
Regardless which approach you take, realize your small I4 engine can only take so much. You cannot expect an engine which is designed to make 85 KW of power to start making 400 KW and stay together. Make sure you set realistic goals as this will make for a better outcome. Also realize, once your engine starts making more power, other things tend to break besides just the engine. Once you get to a certain power level, your stock clutch won't hold it anymore, so will need replaced with a performance one. Then the transmission will only handle so much power before it'll break, then you'll need to source one which will handle more, or have yours rebuilt with performance parts to handle the power. Once that is done, you'll need to ensure your brakes are up to the task. Then there is drive axles, fuel system, and the list goes on.
