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Just that the 'ease of bolting on' of BBTB compared to headers. Not that they make more power. Headers install is a PITA.
FBO usually refers to what a 'typical enthusiast backyard mechanic' can do in his garage with his HF box of tools. No special skills or tools.
All the 'easy' stuff.
Different cars will have different lists.
An example is on my Jeeps 4.0L I6, I would consider the higher ratio 1.7:1 roller rockers a bolt on that can be done by most anyone. Again after air filter, BBTB, intake manifold, headers, cat back exhaust.
Replacing the rear diff with a lower ratio diff assembly from an auto would be FBO but replacing the gears to a different ratio or installing/adding LSD would not be considered FBO as it requires special tools, micrometers, skills.
FBO is what the 'non-professional mechanic' can do at home in his garage with a floor jack, jack stands and basic 'kit/box' tool chests. The relatively 'easy' stuff.
I can do MILVs, I can't change a overhead chain driven camshaft.
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