273 with 340 exhaust manifolds

The 273 had a rated HP of 190-200. The 340 had a rated HP of 290. These HP ratings are pretty much tested and proven. The 273 came from the factory with 273 manifolds while the 340 came with 340 manifolds. The 273 manifold had a 1 5/8" exhaust exit hole while the 340 had a 2 1/4" exit hole.
If there is such a small difference (5 HP) between the 273 and the 340 manifolds how do you account for the 90 HP difference???

Mopar designed and manufactured the 340 manifolds to allow the 340 to produce all the extra HP. "cudfish65" is asking if the 340 manifolds will fit on his 273. I answered his question, yes...but it depends on which 340 manifolds he's talking about. As far as horse power gains, if "cudafish65" has a stock motor then he won't see much gain in HP. That's why I stated earlier "depending on your cam, heads and exhaust pipes the larger 340 manifolds can make quite a difference."
Are you saying that if "cudafish65" has a wild cam, great flowing heads, 340 manifolds and 2 1/2" exhaust pipes he won't see a dramatic increase in HP???

Treblig