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I'd be surprised if it was even that much. I'm running HP manifolds BUT I'm also using 2 1/2" mandrel bent TTI exhaust with low restriction Dynamax Ultraflows......I'd be willing to bet that my setup is less restrictive than a set of cheapy headers mated to the stock 2 1/4" compression bent exhaust pipes with the stock mufflers.

Way too many people think only of the headers vs. manifolds without considering the rest of the system.

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i have the same setup (hp's and tti setup), and i came from bone stock (original exhaust was on my car) and headers (headdman), and i can attest that the difference is night and day, but in the opposite of what the consensus is...
i'll explain: with totally stock (original as my car had it when i got it) and headers, there as a tad more power up top (4500-5000rpms +). this all on a stock 440 with a ch4b eddy intake and 750 afb carb; but when i went to the tti and hp manifolds (which is basically stock but with h-pipe and 2.5" after muffler instead of 2 1/4") i noticed a complete difference in power, but not what you'd expect....
slightly less top end (but very slight), and MUCH more on the bottom end with hp mani's...it was like a different motor when i started it up for the first time...it ran so much better, and i could up the timing to 34-36*degs with no ping, where i used to ping at 32* total...
you mentioned on a stock motor, which basically mine is minus the intake and carb, but the internals are bone stock....
i MUCH prefer the hp's on a STOCK motor over headers, drop in engine bay temps was unreal, my pinging issues and vapor lock issues were GONE...but that's just me...

on the street, i noticed much more bottom end power, and like i mentioned very very slight drop in top end (on the butt dyno); but that increase in bottom end power was a big surprise and is where the car is more often then at 5500rpms which on the street translated in overall more

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I'd be surprised if it was even that much. I'm running HP manifolds BUT I'm also using 2 1/2" mandrel bent TTI exhaust with low restriction Dynamax Ultraflows......I'd be willing to bet that my setup is less restrictive than a set of cheapy headers mated to the stock 2 1/4" compression bent exhaust pipes with the stock mufflers.

Way too many people think only of the headers vs. manifolds without considering the rest of the system.

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i have the same setup (hp's and tti setup), and i came from bone stock (original exhaust was on my car) and headers (headdman), and i can attest that the difference is night and day, but in the opposite of what the consensus is...
i'll explain: with totally stock (original as my car had it when i got it) and headers, there as a tad more power up top (4500-5000rpms +). this all on a stock 440 with a ch4b eddy intake and 750 afb carb; but when i went to the tti and hp manifolds (which is basically stock but with h-pipe and 2.5" after muffler instead of 2 1/4") i noticed a complete difference in power, but not what you'd expect....
slightly less top end (but very slight), and MUCH more on the bottom end with hp mani's...it was like a different motor when i started it up for the first time...it ran so much better, and i could up the timing to 34-36*degs with no ping, where i used to ping at 32* total...
you mentioned on a stock motor, which basically mine is minus the intake and carb, but the internals are bone stock....
i MUCH prefer the hp's on a STOCK motor over headers, drop in engine bay temps was unreal, my pinging issues and vapor lock issues were GONE...but that's just me...

on the street, i noticed much more bottom end power, and like i mentioned very very slight drop in top end (on the butt dyno); but that increase in bottom end power was a big surprise and is where the car is more often then at 5500rpms which on the street translated in overall more

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I think if the engine runs better with the manifolds, it was either running lean, or near detonation? The extra restriction from the manifolds could be causing exhaust to contaminate the intake charge, like EGR, without the valve. That would help lower combustion temps and help prevent detonation.

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My experience with 2 chargers with 440's and headers, is that it's not really worth the aggravation, leaks in the studs, burning the starter out, increased under hood temps, all for 10hp??

440's are prone to detonation with such a big chamber, add to that no back pressure, you loose quench, and result with ping....
the stock hp mani's actually flow quite well, and if the engine is stock, just throwing a set of headers on it will actually decrease performance from 1000-4500rpms or so from my experience...sure you gain 10hp at 5000rpms+ but on the street, i'd much rather have that bottom end power.
all i can say is that going from headers to hp manifolds made a BIG difference to my car. it was jetted properly with the headers, i had to lean it out going to the hp's, using a 1407 eddy carb, leaner main jets, metering rods, and springs...but one big plus was that off-idle bog which i could not for the life of me get rid of completely vanished going to hp manifolds....
the ping i was getting with <32* total i'm sure was from the headers, not enough back-pressure, increased under-hood temps, ect....

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Ok, if you want to know the real answer to the question, send me $500 and I'll chassis dyno the R/T with the exhaust manifolds and then with headers

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69yeehaa, IMO your tuning insights do not compute. A lot of people complain about headers, but I have had a good experience with my el-cheapo summit headers. They fit my car very well, shed a bunch of weight over the lo-po log manifolds that were on there and I never burned out a starter.

Exhaust back pressure has nothing to do with quench. The only possible difference headers could make is the increased exhaust flow can lean you out if you're not jet for it. You say you were pinging at 32* timing and bogging with the headers and that magically went away when you put the hp manifolds on. The only real conclusion one could draw from that is your carb and distributor were not dialed in correctly for your combo. Like already said by another member, the increased exhaust backpressure of the hp manifolds gives your 440 a egr-like effect(reversion) helping to cool and dilute the intake charge to cover your state of tune. There's no such thing as not having enough backpressure. HP manifolds flow decent for what they are and they keep the exhaust gas velocity up at lower rpms, this is the reason they produce good bottom end power, not backpressure. Regular log manifolds act the same way at very low rpms, just they run out of flow potential so low in the rpm range they become a hinderance much earlier than the hp manifolds.

HP manifolds are a decent choice if you have a combo suited to them. For myself, I wouldn't go out and buy a set but if I bought a car already with them I think I would cam to take advantage of them.

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well, i can tell you that weather or not you believe that the difference from headers was night and day with my car, i can tell you it was, and not only on one car but 2.....
i've spent hours and hours jetting, re-curving my distributor (professionally) and still resulted in ping with 32 total, anymore and it would ping. after putting on the hp mani's, i had to jet leaner, and now i'm running 35ish total timing with no ping at all....if you search from my posts years ago, you'll find the trouble i went thru in tuning, and i can honestly say going from crap headers that took me hours to get out of the car, burning my wires, vapor lock, and underhood temps that were crazy, i sure dont' miss those leaky headers. put on the hp mani's and wow...temps dropped, they've never leaked, and the motor just plain old runs like a FI car now.
i think headers have a place, but with my stock motor, they really did not suit my application. if i were to build a hipo motor, sure i'd use them, but stock; i'll take the hp's. and regardless, the low-end power increase was very noticeable....
i'm not sure i understand your theories on back pressure either, so it goes both ways. it's a well known fact that a car with a stock engine, and over-sized exhaust actually will decrease performance; which is exactly what the headers did in my case....
i drive the crap out of my car and sit in traffic on a hot day, and take 100mi cruises weekly, and headers just don't cut it for me....