Lots of bad info flying around in here. Let me clear some of this up.
Your cheapest hp will be with N20. But not in the long run. You WILL blow your motor unless you also spend extra money on a new and stronger fuel pump, it has to be a wet shot, and you should also retard your timing. So the cheapest horsepower that you will notice is from a turbo. There is no arguement about it, it's all fact.
No, an air filter will get you a whopping 0hp at the block or at the wheels unless you already have in excess of 300hp. And you don't.air filter will get you maybe 3 whp for $40 so that is about $13.33 per hp.
No, no gains. This has been proven. No gains even on turbo cars.You might get 2-3 horsepower if you remove the vtcs butterflys in your intake.
Your cheapest hp will be with N20. But not in the long run. You WILL blow your motor unless you also spend extra money on a new and stronger fuel pump, it has to be a wet shot, and you should also retard your timing. So the cheapest horsepower that you will notice is from a turbo. There is no arguement about it, it's all fact.