Originally Posted by SouthArk370Z

Back in the '60s, when fuel economy wasn't a big deal, car manufacturers were pretty sloppy and one could take a stock engine, slap a bigger carb and manifolds on it, maybe a hotter cam, and you could expect some pretty large gains. Add a basic balance/blue-print/port/polish job (not exactly a bolt-on but you don't have to buy a lot of expensive parts, either) and it was not unheard of to boost HP by 25% or more.

The days of getting big gains with bolt-ons and cheap mods are gone. The manufacturers have already picked the low-hanging fruit.

As others have said: if you want big gains, expect to spend big money.