* The specific 350: Different 350s had different factory specs. A 1969 350 is very different from a 1980s small-block. Compression ratio, heads, intake manifold, and other components significantly influence the results.
* Other modifications: Are you changing any other parts? Headers, intake manifold, carburetor, exhaust system, etc., will all affect the final horsepower. The camshaft is just one piece of the puzzle.
* Tuning: Proper tuning of the carburetor or fuel injection system is crucial to realize the cam's potential. A poorly tuned engine will yield far less power.
* Dyno vs. Real-world: Dyno results can vary depending on the dyno itself and the conditions. Real-world horsepower will almost always be less than dyno numbers.
While a cam like the 268H (a popular choice often compared to a 256) *might* yield a peak horsepower increase of 15-30 horsepower on a reasonably well-equipped stock 350, it's not guaranteed. It might even lose a little low-end torque. The gains might be more substantial if other supporting modifications are made.
In short: You can't get a precise number without knowing all the variables and either extensive testing or simulation software. Think of it more as a general range of potential increase, rather than a specific figure. You'll find many forum discussions and anecdotal evidence, but those numbers are only applicable to the specific builds they describe.
