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https://www.motortrend.com/how-to/mopp-0307-installing-exhaust-headers-into-300hp-crate-engine/ Dyno shootout of manifolds and headers on a 300 hp 5.9l Horsepower RPM 318 360 340 SHT 1 58 tti 1 34 2500 186 185 187 189 187 189 187 3000 223 223 225 226 228 228 226 3500...

Here's a good video that shows what basic mods do to a stock 2bbl 360 to a warmed over one And mods to a mild 5.9l magnum

I think you need to do some kind of mod to them, plus there's different ones I think the earlier magnums were better, I wouldn't worry about it, your 360 ones will do.

I doubt there much worse than 318 manifolds, probably 5 hp less at worse, I wouldn't over worry about on a mild early A car. There's another shootout that I can't find anymore that included magnum manifold to and was basically same as the 340. The best was one 340 and one magnum I forget which...

Guess a mild cammed 300 ish hp engine don't need much exhaust wise even the 318 manifolds do OK.

Here's a manifold/header shootout on a 300+hp 360 the difference was 1-5hp depending what rpm. https://www.motortrend.com/how-to/mopp-0307-installing-exhaust-headers-into-300hp-crate-engine/ Horsepower RPM 318 360 340 SHT 1 58 tti 1 34 2500 186 185 187 189 187 189 187...

When the time comes Run it and measure the back pressure if it's low then probably good enough if high then a possible upgrade in the future, if you do measure please share the results, also could do open vs the 2" see what difference it makes. Testing/measuring is the best way.

A 600 hp is pushing out about twice the air as your 340 and has only a 13-20 hp difference between 2.5 vs 3. They said 3” was about par with open headers. So how much you lose by going 2” ? Instead of 2.5” probably about the same. Unless you put some power killer mufflers on some are terrible...

I just looked at an engine masters dyno 2.5 vs 3 with mufflers on a 600 hp big block, was a 13 hp and 16 tq different at peak, but did lose a bit more mid high rpm. To me what mufflers you run will have the biggest impact.

back pressure fights the exhaust from leavening the chamber. Say you need a 2.5" pipe for zero back pressure, obviously a 4" pipe would have zero too but with less velocity. Velocity is important but don't let it scare you into decision that strangle your engine that kill power overall.

Back pressure doesn't create exhaust velocity.

More than likely the mufflers will kill more hp than the pipes, ideally you want zero back pressure, if you already have an exhaust you could measure the back pressure and guesstimate from there.

True overlap likes long tube headers , but most wanting to run manifolds are probably not trying to push it over 350hp with fairly mild cams doubt even with something like an xe268h cam the difference between manifolds wouldn't grow much if at all, what I take away from it any 360 manifold is...

Manifold and header shootout Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine

Impressive 1.44lbs-ft per cid

You'd think so :)

Cam is stock and has very little overlap for the headers to work with, is it worth it in this applications depends on the individual, for me I would be adding a cam forsure if installing headers.

Good thing about headers is the gain is everywhere.

Most mods have pro's and con's Does it add power you'll feel yes, is it worth it that's up to the individual.

It's a stock 5.9 magnum including cam with 4bbl.

Yes cause they want everyone to run 360 manifolds instead of 340

If you look at this dyno shootout a 340 manifold was worth 1hp over 360 and 4hp over 318 manifolds at 300 hp range. Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine

The 9hp is on this dyno comparison doesn't mean the same for the OP's engine, depends on cam, overlap needs long tube headers with the right collectors to function properly, both of the dyno's I shown where stock 360/5.9l cam minimum overlap could say probably minimum power gain from headers.

Here's mods to a mild 360 manifold's vs headers starts at 5:15

Engine Masters did a dyno test on headers where they beat the **** out of them with a hammer and had little effect on hp.

What's done to your engine? Which cam ? If stock 15-25 hp, good thing about headers is they usually add hp throughout the rpm range not just at peak. Header manifold shoot out on 300 hp 5.9l Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine

Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine

Don't see the big advantage there's probably a 5-7 hp difference, there's a 4 hp difference between 318 to 340 manifolds don't see 273 being much worse than 318 and magnum being that much better than 340. There is another shoot out which I can't find now that the best combo is one side 340 and...

The engine they test with was a crate engine so it didn’t come with manifolds, guess they didn’t have any laying around to try, but I have seen another shoot out and they about the same as 340, they figured the best combo was one of the 340”s and one of the magnum”s I forget which ones.

Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine Here's a header manifold shoot out, there's not much deference in HP between manifolds normal 360 are the best deal.