Moparts Tech Archive

Exhaust

Exh manifolds on a hot small block?

I have seen the dyno tests in the magazines where headers gain 15-25 hp, Was curious if anybody is using hi-po or standard exhaust manifolds on their hopped-up small blocks, if so, what is you combo and E.T.?
Also, has anybody tested back to back the standard 360 exhaust manifolds vs. 340 manifolds on a dyno or track? Just wondering if the big $$$ the 340 manifolds are going for are really worth it or not.

I have seen some fast small block cars running 340 manifolds and yes I am fully aware the headers will always be faster, but unless you spend $500 for headers, they also hang too low to the ground and make starter access a pain in the butt. I have never owned a small block car with headers that didn't have the drivers side collector smashed almost flat within the first month of driving.
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ph23vo
Moparts Member
Posts: 982
From: RENTON, WASHINGTON
Registered: Dec 1999
posted 07-01-2002 12:30 PM

my 68 340-s cuda fastback ran 12;90,s@105 with its original manifolds, stock converter and 4;56,s, stock 10,5 comp, LD 340 intake 750 holley, 288' 530 solid cam thats it! i say use em! dan
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kwhmopar
Moparts Member
Posts: 273
From: South Lyon,Mich.48178 USA
Registered: Nov 1999
posted 07-01-2002 12:44 PM

'68 Barracuda-'71 360 .040 over, 284/.484 MP cam, 3000 stall, Cheatah valvebody 727, 4:10, 340 manifolds with exhaust out to chome tips. Best ET 12.61@106 although that was with 150 shot of NOS.
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Brian
Moparts Member
Posts: 1138
From: Houston, Texas
Registered: Nov 1999
posted 07-01-2002 04:24 PM

I'm not sure How "HOT" you want?
13.39 @ 103.54Mph with a 4.10?
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340_6Pak_Cuda
Moparts Member
Posts: 2036
From: Maryland
Registered: Nov 2001
posted 07-02-2002 12:27 AM

360 manifolds dont even begin to compare to hipo 340 manifolds. On the other hand, 340 manifolds cant come close to performing like headers will. The only reason I can see to run the 340 manifolds is for restoration purposes or just to avoid the hassles assoiated with headers.
I paid $275 for one set, and $50 a pair for my other two sets. They are all in good condition, only needing a little heat riser work. Average price for a nice set is $300-$400.
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Spamalope
Moparts Member
Posts: 111
From: pleasant hill, CA
Registered: Nov 2001
posted 07-02-2002 03:30 PM

Ok, well i'm gonna be putting a 340 in my '65 Barracuda soon and i'm going to have to run the origanal 273 manifolds and 2in. pipes w/ no crossover and glasspacks, till i can afford a new exuast. the new motor went 12.91@105 through 340 manifolds. Now I know my sorry exuast is going to choke the motor off, bit how much of a loss can i expect?
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Jesse Lackman
Moparts Member
Posts: 383
From: Center, ND 58530
Registered: Jan 2000
posted 07-03-2002 09:32 AM

The difference in hp between cast manifolds and headers is more like 40-60 hp with mild race engines. A recent 340 had a difference of 60 hp at 6000 and 30 at 3000. Other tests have averaged about 50 hp difference
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craig440cuda
Moparts Member
Posts: 321
From: hopkinsville ,Ky
Registered: Mar 2002
posted 07-03-2002 11:22 AM

Mild 318,727,323 gear street car,with 68-70 340 manifolds,ran 10.20-10.30's all day.Switched to headers,now I'm running 9.70-9.80's all day.thats 1/2 a second in the 1/8th mile!!!My headers dont go under the steering,they are an old set of Dougs headrers,never seen another pair like it.Now,if my MILD 318 gains this much from a header,what will a built 340/360 do????That 500 dollars looks to me like a great HP/dollar investment.
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Jesse Lackman
Moparts Member
Posts: 383
From: Center, ND 58530
Registered: Jan 2000
posted 07-04-2002 06:08 PM

BluCuda, on the 340 those were cast center dump. I've never tested the the good 340 I'm too cheap to buy any. I have a restricted 360 on the dyno right now and the right side Magnum is the best so far. If anyone wants to know how the good 340 manifolds compare, send a set to me, when I'm done I'll ship them back and post the results.
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RUNCHARGER
Moparts Member
Posts: 1247
From: Abbotsford B.C.
Registered: Nov 2000
posted 07-05-2002 01:04 AM

Good topic: I have a 71 340 Duster with Hooker headers and 3 inch exhaust, it has a single plane M1 on it and a 284-484 MP cam. I have been looking for an LD 340 and was thinking about a set of manifolds for it as I don't think it pulls hard enough to warrant the header hassle. Well lo and behold today I scored an LD 340 and a set of 68 340 exhaust manifolds all in one swoop! I think I will install the dual plane first to see if that picks this thing up a bit, I know the exhaust will hurt some though, I will be happy with mid 13's, and I see no reason this car shouldn't do it.
Sheldon
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Jesse Lackman
Moparts Member
Posts: 383
From: Center, ND 58530
Registered: Jan 2000
posted 07-05-2002 10:11 AM

Is the top one the good 340?

What I would like to do is test the good 340 manifolds on both sides, same test. That means getting a hold of two of the good ones. I've had a couple offers to lend manifolds so far what do you guys think of this approach?

The lower manifold on the pic above looks very similar to ones I already tested. (IMHO not worth testing again.)

How many different HP 340 manifolds were there?

Here are the results so far;

restricted 360, 4412, cast intake and exhaust.

So do I need to test two of the top ones and two of the bottom ones??

Anyone know the outlet ID on the 340 hp manifolds? I see some are flanged outlet and others are the ball type outlet. All the manifolds I've tested so far have a 1 7/8id outlet. I plan to open up the Magnums to 2" id outlet. The rules say 2" id exhaust pipes.
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Jesse Lackman
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fast68plymouth
Moparts Member
Posts: 2799
From: so burlington,vt usa
Registered: Aug 2000
posted 07-05-2002 01:28 PM

as a casual observer i think that would be an interesting series of tests.
std vs 340 hi-po manifolds vs small tube(1 5/8) economy type headers with both the 2bbl and 4bbl intakes installed.
i think the difference between the two types of manifolds, and then the headers would be greater with the 4bbl as opposed to the 2bbl.
on our dyno, a low compression, 2bbl 355 SBC oval track motor gained about 40hp from a std type exhaust manifold with 2" pipes vs a set of 1 5/8 X 3 headers.
these motors make about 275-280hp with 8.8cr, a mild hyd cam, 1.72 intake valves, an OE 2 bbl intake, a 4412 Holley, and exhaust manifolds.
with just a swap to small tube headers the motor made just over 320hp.
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