Moparts Tech Archive
Exh manifolds on a hot small block?tlt
From: Mount Vernon IA
Registered: Aug 2000
posted 07-01-2002 09:30 AM
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.?
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.
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
'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.
I'm not sure How "HOT" you want?
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.
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?
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
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.
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.
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.
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??
as a casual observer i think that would be an interesting series of tests.
all info provided by members listed above.