318 build #2 "cost minded" for the '74 Duster

273 said
True but it still helpful, it's not like all of a sudden there's gonna be 20-30 hp between manifolds...

No argument. But I too often see this test used to say that 340 manifolds are only worth 4 hp and I think that is bunk. The only data there is that with the small cam motor in the test, the HP manifolds aren't worth much. Anything beyond that is a guess.

It's kind of like swapping a big cam under a 2bbl intake and then saying the cam wasn't worth much. The cam stopped being the choke point and its true potential can't be evaluated until the other choke points are removed.

A similar motor to the one the in the test made 50+ more hp with a swap to a small cam, peak to peak. And it made 70+ more hp where the new cam peaked. Hard to say what the HP manifolds would see at that 5200 peak if compared.

The 300hp crate motor doesn't have lobes on the cam and using this test to validate any of the parts really can't go beyond saying "this is better than that". Any use of quantitative data (eg. 4 hp more) is suspect. In my opinion.

273 said
To me the test shows no point spending money on 340 manifolds for power gains any 360/5.9l are good enough. And if you already got a good dual exhaust with 318 manifolds not really worth the swap either.

See above, this is exactly my concern with holding this test up as "proof". I just can't see using the linked test to say 340 manifolds aren't worth something on a motor with a real cam. The data just isn't there to say one way or the other. I understand that they might not be much better, I'm just saying this test can't be used to show it is so.

None of this helps the OP though. That test doesn't say anything about the Magnum manifolds and how they compare so it's not worth polluting this thread with beyond what has already been said.

I can add that Holdener did a test with the small outlet Magnum exhaust manifolds compared to the big outlet ones, and I think the big outlet manifolds were worth like 5 hp at the peak. So, compared to the referenced exhaust test, the big outlet manifolds might be worth about the same as the 340 HP manifolds. But that's just a guess.