HP limit of 1973 A body V8 340 manifolds?

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jake17201 said:
Stock manifold A body cars are very close to breaking in to the 9s at over 130mph in the fast class. I know this is the extreme with big stroke, race fuel and compression, however that’s over 650 crank hp thru iron. Years ago everyone woulda said no way……keep dreaming

I think 500 crank thru iron could be an achievable goal for us normal weekend guys
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I read that in the FAST class the iron exhaust manifolds are "Extrude Honed" nearly to death. For those who do not know Extrude hone is a process where abrasive putty in pumped under high pressure thru a component. Removes material and can leave a shiny finish in every crack and crevice. Porting with de burring bits and stones cant begin to compare. But E Honing is not cheap.
 
I always thought if I owned a car and I didn't want full length headers but I did not own the good manifolds that I would spend the money on high quality shorties such as TTI or Sanderson rather than on high perf used manifolds . My current project is a 95 Dakota Sport mini tubbed. No long tubes exist. Only shorties for the first gen Dakota. I do have the early 92-93 large outlet manifolds run into true duals. But I really like the Gibson shorties available. Even though the Gibsons are not a significant improvement over my manifolds I think I'm going to buy them regardless. I'm disappointed the ID of the outlet on the Gibsons but Ill say they are a want and not a need.

Screenshot_20260423-104959.png
 
More acid dipped. Extrude honing is not really worth it. Theoretically, I wouldn’t say 150 is out of the question. Put a set of them on big small block with the best race intake/head/cam no muffler in a 2500 lb car. Pretty doable, but it would be a serious effort thing.
 
furrystump said:
More acid dipped. Extrude honing is not really worth it. Theoretically, I wouldn’t say 150 is out of the question. Put a set of them on big small block with the best race intake/head/cam no muffler in a 2500 lb car. Pretty doable, but it would be a serious effort thing.
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I stand corrected. But when one points out how incredible FAST cars are and that they use OEM iron manifolds the full disclosure is those manifolds have a lot of $$$ into them and that's what makes them perform so well.
 
junkyardhero said:
well, if you don't believe in science then the conversation is over.
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Now aren't you a party pooper!
We already had one person claim that cooler gasses expand. Why stop there?
:lol:
junkyardhero said:
tell yourself whatever you want, amigo. if it makes you feel good then that's all that matters.
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Good man. I'll join ya rather than continue to poop on the party.
Lets go get a beer.
 
Michael Harrington said:
I always thought if I owned a car and I didn't want full length headers but I did not own the good manifolds that I would spend the money on high quality shorties such as TTI or Sanderson rather than on high perf used manifolds . My current project is a 95 Dakota Sport mini tubbed. No long tubes exist. Only shorties for the first gen Dakota. I do have the early 92-93 large outlet manifolds run into true duals. But I really like the Gibson shorties available. Even though the Gibsons are not a significant improvement over my manifolds I think I'm going to buy them regardless. I'm disappointed the ID of the outlet on the Gibsons but Ill say they are a want and not a need.

View attachment 1716544367
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Go read Elston's "Log manifold experiment" in the advanced section of Speed-talk. As I recall one of the things it supported was the ideat that the most important part of the header is the first 6 to 12" off the head.
 
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Michael Harrington said:
I stand corrected. But when one points out how incredible FAST cars are and that they use OEM iron manifolds the full disclosure is those manifolds have a lot of $$$ into them and that's what makes them perform so well.
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Pay attention. There are two FAST class racers replying in this thread and both have shown pics of their manifolds. Both of their cars are incredible. @furrystump you ain’t slow.
 
furrystump said:
Mine don’t. An hour with a carbide burr, but I’m slow.
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With what guys do to intake manifolds in that arena, surprised the backside of the exh manifold hasn't been section out to get to the outrigger runner internals.
Stitch it back up, good as new. :thumbsup:

Love the art in those FAST cars.
 
timk225 said:
I just picked up a 1971 340 from a friend who had it for years. Bottom end is rebuilt with 40 over pistons. It has X heads, unported as of yet. Not sure yet if I will build it or just sell it.

But with the purchase came a set of 1973 340 exhaust manifolds, casting dates of May 1973, casting numbers 3418623 (right) and 3614368 (left).

Assuming a moderately built 340, what HP level will these manifolds allow before becoming a restriction? And what HP can the '68-'70 high flow 340 manifolds handle?

Apologies if this has been written 50 times before, I've only been in the Slant 6 engine section here for the most part.
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There isnt any firm hp limit. You can crutch the low flow manifolds to some extent with increased cam exhaust duration.

The stock heads dont have great exhaust ports anyway. I think those were the main two reasons for all the 214/224 @.050 street grind cams back in the day.

For a smallish cam street grind those manifolds will be fine. I would spend the money on a good dual exhaust setup and go. If you spend the money to upgrade the heads later then headers might be the choice.

Just my two cents. There is definitely more than one way to skin a cat. Doesnt mean my way is right.
 
furrystump said:
More lobe separation helps with manifolds. My car is on the edge of streetablity. It runs cool at 60 mph on the highway. The only thing that hurts it is 12.2 to 1. If you can afford the fuel, it will go anywhere.
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Thanks for sharing.

When you say on the edge of streetability, is that because of the compression/fuel, or because of something else?, or a combination of things?

If you wanted to get to next level in horsepower by only changing one thing engine-wise, and in conformance with the FAST rules, what would be your first choice, and what would be your second?

Thanks
 
Mattax said:
Go read Elston's "Log manifold experiment" in the advanced section of Speed-talk. As I recall one of the things it supported was the ideat that the most important part of the header is the first 6 to 12" off the head.
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I skimmed the numerous pages but they delved pretty deep referencing F1 cars , wave guide terminations and Slot feed dual plenums on Audis.. In a nutshell I assume the Log manifold be built performed better than a name brand expensive shorty header?
 
BSB67 said:
Thanks for sharing.

When you say on the edge of streetability, is that because of the compression/fuel, or because of something else?, or a combination of things?

If you wanted to get to next level in horsepower by only changing one thing engine-wise, and in conformance with the FAST rules, what would be your first choice, and what would be your second?

Thanks
Click to expand...
Yeah 12.2 to 1
 
Michael Harrington said:
I skimmed the numerous pages but they delved pretty deep referencing F1 cars , wave guide terminations and Slot feed dual plenums on Audis.. In a nutshell I assume the Log manifold be built performed better than a name brand expensive shorty header?
Click to expand...
No.
It was a test for a Cup race team (nascar). Bottom of page 1. The power curve shape was similar to the full header but down around 1.5%
Getting the exhaust 'slug' away from the port is most important part of the system.

Therefore we don't want big passages and sharp or cheated bends right off the port. The design goal should be to create high velocity flow through the rpm band without impeding the top end flow.
 
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Dart Frog said:
Your question was what horsepower level before the manifold becomes restrictive. As people have stated these manifolds can be modified ported honed and basically blueprinted to achieve some pretty high numbers. That's not the kind of results you will get with an HP manifold as cast. It has been my experience that even a 300 horsepower 340 will respond well to a long tube header in comparison to a as cast HP manifold.
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That’s true. Kim
 
BSB67 said:
Thanks for sharing.

When you say on the edge of streetability, is that because of the compression/fuel, or because of something else?, or a combination of things?

If you wanted to get to next level in horsepower by only changing one thing engine-wise, and in conformance with the FAST rules, what would be your first choice, and what would be your second?

Thanks
Click to expand...
I would probably bump compression by a point. Then an aluminum two piece converter sounds interesting, but because it would need to be pretty tight, no one is jumping at the chance to make one. At least with any real confidence. Plus that would be $$$$$$$$$. Duster boy has asked around a bit. Weight out of the car is always a good thing.
 
furrystump said:
I would probably bump compression by a point. Then an aluminum two piece converter sounds interesting, but because it would need to be pretty tight, no one is jumping at the chance to make one. At least with any real confidence. Plus that would be $$$$$$$$$. Duster boy has asked around a bit. Weight out of the car is always a good thing.
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Thank you.

You kinda confirmed my opinion. Not wanting to put words in your mouth, but what you didn’t say was you would put a pile of time and money into the manifolds.

Reviewing this thread, I came across your statement below. I like your last sentence. I used to say the same thing all the time, i.e: if you’re building a motor, you don’t really need headers unless you’re going to need a roll bar too.

furrystump said:
….A better question might be, can you get to your performance goals with a manifolds. You can quite easily get to needing to run a bar with manifolds.
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TT5.9mag said:
Nope. In an all out performance situation one only needs to look at what top fuel does. A single pipe per cylinder aka zoomies. Thats the best an exhaust system can get for all out performance.
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Supercharging makes scavenging irrelevant, You look at Pro Stock & Pro Mod,.....not Top Fuel anything...
 
I remember a process that was popular many years ago called extrusion honing. It was an abrasive slurry that was run through something like an exhaust manifold. The abrasive properties of the thick, liquid slurry would remove material from the inside and smooth the surface. I checked, and it is still being done. I would think a pair of 68-70 manifolds could be greatly improved.
Extrude Honing Frequently Asked Questions — Automotive Edition - Extrude Hone AFM
 
harrisonm said:
I remember a process that was popular many years ago called extrusion honing. It was an abrasive slurry that was run through something like an exhaust manifold. The abrasive properties of the thick, liquid slurry would remove material from the inside and smooth the surface. I checked, and it is still being done. I would think a pair of 68-70 manifolds could be greatly improved.
Extrude Honing Frequently Asked Questions — Automotive Edition - Extrude Hone AFM
Click to expand...
7 hp its been done
furystump most likey got 7 hp with a 1 hr cleanup
manifolds run 10s
 
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