Lexus LFA Engine

V10 masterpiece, hide Mona Lisa.

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Engine swapping

You tuned up your engine, still not enough power.
For example you tuned your b16a2 engine in Honda EG6, it has 200+ horsepower, it's faster than your friend with his stock K20 in a Type R. But K20 has a lot more potential for tuning, it has a 200hp and more torque in a start and a bigger engine. Your friend will only change exhaust system and tune his ECU and he will again be faster than you.
Engine swap tuners economic problem is that it costs a lot of money right in a start, when originally mounted engine can be tuned step by step, not much money at once.
Engine swap isn't just an engine, it's usually also swap of transmission, ECU, engine mounts accessories, drivetrain and much more. Plus money for mechanic to mount an engine and rest of stuff in your car.
Once you're there, all the hard work is forgotten and nothing can compare with happiness when reving up your monster and feel the power of your completed work.
K20 swap in an EG6:

NA tuning - engine modifications

Now that we've changed complete exhaust and intake system and we still don't have enough power, now it's engine's inside turn.
Increasing compression - change pistons with high-compression ones or replace oem head gasket with a thiner one.
Cams - OEM cams don't have the optimum aggression. They are classified by stages of aggression: stage 1, stage 2... Be sure you choose the right ones for your goals. Some of them are better for mid-end range, some of them give more power, some more torque...
ECU optimization- it's necessary for tuned cars. Cam timing, ignition, how much fuel goes to the engine, if there's too less fuel going in, power of car won't be optimized and you can hurt your engine for that. If power is too big for OEM injectors and fuel pump, replacing them with bigger ones is a must. ECU tuning should only be done by professional tuner on a dyno. If you don't know how to do it, don't try to do it yourself, you can make more damage than good stuff.
Valves + valve springs: Put in some sport aftermarket valves and springs, especially when you increase redline of rpms on ECU tuning. OEM valve springs probably won't last long when rpm redline is changed to 1000revs higher than stock and a big damage can be done to an engine.
Engine head - Port polish a cylinder head increases compression, it optimizes power and turns production engine into a racing one.
That's about it for NA tuning, don't forget to optimize transmission: lighter sport flywheel, stronger clutch and LSD in your gearbox for optimized performance.
Don't forget: This are just some basics to imagine how things go in tuning, don't tune if you aren't sure how things work and what will your modification exactly does to your car;)