Headers vs. Manifolds

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middleagecrisis

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I'm gathering parts for my 383 swap into my '75 Duster and have a question regarding exhaust. I have an opportunity to buy some '70 B-Body HP exhaust manifolds and am wondering how much performance loss and I can expect over a set of headers. TTI headers are not in my budget and I don't like the idea of creating "clearance" on a brand new set of headers. I'm going to pull apart the 383 that I bought, but some of the specifics I do know are that it has ~10-1 compression, ported HP heads (452 or 906), TM6 Edelbrock single plane, 750 DP DaVinci carb, and a fairly radical (I've heard it run) hydraulic camshaft. I will be running a 727 w/3000 stall and 3.55-4.10 gears. I know this combo has headers written all over it, but the appeal of HP manifolds has me leaning that direction. Should I expect a 20-30 horsepower drop using manifolds and a necked down 3" exhaust with flowmaster mufflers and turndowns?
 
If it's street car, you won't notice the HP loss. Running the manifolds versus headers would be my choice because unless you buy the TTI headers, you will be
"clearancing" them as you mentioned.
 
Or the Schumacher one fit good. I do agree nothing wrong with stock for the street
 
My 318 2bbl '77 van dropped from 11.17 to 10.92 in the 1/8 mile by going from manifolds to headers. And the manifolds had dual exhaust. The more motor, the more the effect in my opinion.
 
Headers on an A body have to be the good ones or else they smash the passenger side flat on the street...
 
With headers I can immediately feel the increased RPM response. The only reason to not run headers is the budget.
 
The HP manifolds look like they would flow fairly well for a stock manifold. The manifolds I'm looking to purchase are priced about the same as a Summit or Jeg's B-Body header.
 
You might make sure that the b-body manifold will clear the steering on the drivers side.
 
The age old question……. What’s the difference in power between manifolds and headers.
And the age old answer is…….. “it depends”.

I look at it this way……. In terms of the typical ohv v8……The more hp/ci your build makes with headers…….. the higher the % of loss you’ll see with manifolds.

20 or so years ago I dyno tested a friends 396BBC that he’d been running in his 68 Camaro.
The local “396 guru” had given him a “400hp” recipe, and it was underwhelming in the car.
He’d been running it for a few years and was looking for more.

It had ex manifolds on it, and he pulled the head pipes out of the car to use with the manifolds on the dyno.
It made right around 290hp STP.
A set of cheap headers was worth 35-40hp on that day.

It got a cam swap & intake swap, then it made 416hp with the headers…….. and headers went on the car during the re-install.
Huge difference in feel from behind the wheel.
 
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Other option that most people won't do today is cut open the inner fenders and buy used fenderwell headers at a less expensive than HP manifolds price. Now if you build it to breathe then choke it you will be unhappy. It's all about the combination of parts to work together & don't forget about Traction.
 
If it's not in the budget run the manifolds, can always do the swap down the road.
 
Ok here is my opinion.
1) The 383 is a big-bore/short-stroke/high-rpm brawler. You already have everything it needs and wants. IMO, it would be a shame to stifle it with the logs.

2) If you can hear that it has a "fairly radical cam" in it, but you stifle it with log manifolds, then IMO you would be better off going down about two or more cam sizes, to get the pressure up, so you can run the 3.55s, and run a lower-stall convertor, and have a strong bottom end.
But when this is done, you gotta plan ahead so that your pressure does not rise high enough, to put the engine into detonation at WOT.

3) If you're taking it apart anyway; It ain't a lot of work to stuff a 440 crank into her.
IIRC that would make her a 425 low-deck before boring. Heckuvathing that would be in the Duster, even with logs and a 2-bbl cam.
 
If your building a hot rod, run headers (if you're NOT building a hot rod, why a big block swap?).
If your building a cruiser, and you don't expect a big block Duster to be fast..... run manifolds.



And yeah, that combo DOES have headers written all over it! I would expect a 30+hp loss with manifolds on your combo.
 
Also kinda depends on how much the manifolds will cost, and if they will fit. You wont know that till you get the engine in the car.
Personally, if I ever do a big block A-body swap (I have a 67 barracuda candidate, and a rb 511) I'll buy cheapo summit or jegs b-body headers, and "adjust" till they fit. I have a custom built set of headers for a bb/a-body, but they are semi fenderwell and too big, even for a 511.
 
70dart340 said:
A set of A body big block HP manifolds must cost a fortune these days.
Click to expand...
The race car I bought that had this combo in it actually has a set of fender well headers on it. I just don’t know that I want to cut up my fenderwells to make them fit. The HP manifold that I’m looking at buying are $400, which is about the cost of the set of summit or jeg’s generic headers last time I looked. I definitely don’t have TTI or Schumacher Shorty headers at $900 a pop in my budget.
 
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I should have Dyno tested manifolds vs my 2 sets of headers. 2 inch hooker f/w - 2-1/8 tti
The tti were a 26 HP improvement over the fw's
 
I don't think B body HP manifolds are going to fit in an A body so the discussion might be moot. However, in a B body you can build a performance engine that doesn't give up much power to headers. You have to be very, very careful with the cam selection but 500 hp isn't a problem with HP manifolds. Here is an article I did on the subject roughly 20 years ago.
Moparts on the Web - Main Index
 
Hagerty did a you tube on adding headers to a 440, not that long ago. Mild Comp XE Cam, factory intake, factory TQ carb. I do not remember the exact amount of the HP increase, but it was substantial, with a big increase at lower rpms, good for the street.
 
Bewy said:
Hagerty did a you tube on adding headers to a 440, not that long ago. Mild Comp XE Cam, factory intake, factory TQ carb. I do not remember the exact amount of the HP increase, but it was substantial, with a big increase at lower rpms, good for the street.
Click to expand...
I can run a dyno test and prove that headers pick up a lot of power when switching from manifolds. I can also run a dyno test showing that headers don't pick up much power from manifolds.
Just because you see a video of something doesn't mean that you aren't being lied to.
The trick is the camshaft. If you want the header advertisers to be happy then you use an engine with a big cam. Then the engine works better with headers and people buy headers and everyone is happy. Nobody buys ads for factory manifolds so the dyno tests aren't designed to make them look good. But, if a person is racing in a class like FAST where manifolds are required, they can be made to work. I've used manifolds on a few builds and it is fairly easy to make 500+ hp with stock HP manifolds. The FAST guys are making way more than that but those engines are crazy expensive. A normal performance build can use HP manifolds if the cam is chosen with care. That is really all it takes.
PRH on this board is the person to talk to if you want a camshaft ground to work with manifolds.
 
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