A large enough turbocharger can produce hundreds of horsepower on any engine without necessitating any kind of internal modifications whatsoever--at least up to the point that blows up your motor. Turbochargers are air pumps that use exhaust pressure to spin a compressor blade that forces air into your engine. This additional air allows the engine to burn more fuel to increase horsepower, but it also increases heat and pressure in the engine's combustion chambers. You ignore excess heat and pressure at your peril.
Pistons
If you replace no other components in your engine, it better be the pistons. High heat and cylinder pressures will stress the gasoline molecule until it ignites on its own, resulting in a phenomena called knock, detonation or pre-ignition. Detonation is the number one killer of turbo engines, which is why many boosted motor builders install lower compression pistons. Lowering compression ratio will decrease engine horsepower off boost, but it will allow you to safely increase your boost settings for more top-end power. Many factory turbo cars have a compression ratio of between 8.5:1 and 9:1, but 8:1 is a safer ratio for 20+ psi boost applications.
Head Sealing
Strong pistons won't do you much good if those high pressures blow out the seal between your cylinder head and block. At the very least you should consider installing a turbo-specific, multi-layer steel head gasket with an integral reinforcing ring around the bore holes. Use a set of quality chrome-moly studs instead of standard replacement bolts to mount the head, since they'll better withstand stretching and subsequent head lifting.
Rotating Assembly
A rotating assembly upgrade shouldn't be necessary unless you're boosting the engine to more than double its stock output, but that's not a difficult task these days. Turbochargers are easier on connecting rods than nitrous or revving the engine past its stock limits, but at some point the rods will bend under the pressure. H-beam style steel rods will resist bending far better than your stock rods or competitive I-beams. If you can't find a quality forged steel crankshaft for your engine, consider having your stock crank micropolished and nitrided (nitrogen impregnated) for increased strength and reduced parasitic drag.
Engine Coatings
Engine coatings aren't technically a "component," but using them can vastly increase your motor's efficiency, detonation resistance and durability. Coat your piston tops, combustion chamber roofs and the insides of your intake and exhaust ports with ceramic powder-coat for superior heat control. Coat the bearing surfaces and piston skirts with a dry-film lubricant like Teflon. Cover the underside of the pistons, the connecting rods, crankshaft counterweights, the entire inside surface of your engine block and the valve-train area of your heads with an oil-shedding compound. This will help to keep oil from sticking to components instead of returning to the oil sump or oil cooler where it belongs.
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