Bolt-On
 

(Hopefully) quick question:

I always hear people talking shit about bolt-on parts. What is the alternative? Unless someone is crafting an entire engine with the upgrades molded into it, wouldn't everything be a bolt-on?

Nope...
Generally "Bolt on" parts are meant to replace existing ones without further modification. Some times they also work... a bit. There are mods that can't be installed without further modification which can't be put inside that category.

Think about short vs long tube headers. The latter need a different midpipe\exhaust system to be installed as well. This would be the case with individual throttle bodies as well or a different intake manifold that would also need a custom intake\airbox to be built around it to work properly (or at all).

A different transmission may require a custom bell housing and driveshaft so that wouldn't fall in the "bolt-on" category either.

There are many examples like these, i'm just trying to give you an idea.

I'm freaking sure a power chip is "bolt on" lol