HP limit of 1973 A body V8 340 manifolds?

[timk225]

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.

 

[junkyardhero]

headers will always make more but how *much* more is the age old question of: is the juice worth the squeeze

it's also highly dependent on the combination. the wilder it be, the more it'll want tubes.

i'll throw a round number out and say 400hp before they become a significant restriction at give or take 20 hp/tq

that being said... there's dudes out there running FAST/pure stock/etx that are in the 10's and 11's with manifolds...
*clears throat*
*side eyes*

 

[toolmanmike]

headers will always make more but how *much* more is the age old question of: is the juice worth the squeeze

it's also highly dependent on the combination. the wilder it be, the more it'll want tubes.

i'll throw a round number out and say 400hp before they become a significant restriction at give or take 20 hp/tq

that being said... there's dudes out there running FAST/pure stock/etx that are in the 10's and 11's with manifolds...
*clears throat*
*side eyes*

Hard to say. 68-70 340 manifolds are good. Long tube headers are better. Use this a guide. Not Mopar but the results are similar.

 

[crackedback]

People waste so much money thinking the 68-70's are superior. Under 400hp, the stock 360 log manifolds are right there with them. That crate motor test had them.
The later SB dakota manifolds are good if iron is your jam.

If you have the correct manifold for an A body and want to run them, go with it.

Headers will always make more HP with a correct tune up.

 

[George Jets]

The 2 1/2" TTI Exhaust System is what you really need to make those stock 340 exhaust manifolds shine.

It will literally pull the spent exhaust gases out of the cylinder heads, as the hot spent exhaust gases keep expanding through the 2 1/2" exhaust system, all the way out to the back of the car.

Yes with Stock Manifolds.


☆☆☆☆☆

 

[jake17201]

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

 

[crackedback]

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……

I think 500 crank could be an achievable goal for us normal weekend guys

If asked the one thing they could do to get the cars to run faster, headers would likely be right at/near the top of the list.

With enough money, time, testing and elbow grease you can get a lot of things to run far outside their initial potential. Fast cars are incredible works of art.

 

[Dart Frog]

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.

 

[jake17201]

Here is some good reading for the original poster

20 plus years ago nearly 450hp was doable on pump gas

I could only imagine with trick flows, air gaps and modern cam tech you can squeeze some more

https://www.hughesengines.com/TechArticles/8408416strokerdynotestingsept2002.php

 

[crackedback]

Hey, it's the 360 logs like in the MM exhaust test on that hughes mill.

 

[toolmanmike]

"Headers will always make more HP with a correct tune up." Well, part of that may be true. I swapped the stock manifolds on my 73 Challenger to a set of Mickey Thompson Try Y's when I was doing a little racing. Without any other changes, I ran the same times at the strip. Like you said, "correct tune up". I wonder how much better it would have ran with a little richer mixture? Before and after the headers would be a fun test.

 

[timk225]

The 2 1/2" TTI Exhaust System is what you really need to make those stock 340 exhaust manifolds shine.

It will literally pull the spent exhaust gases out of the cylinder heads, as the hot spent exhaust gases keep expanding through the 2 1/2" exhaust system, all the way out to the back of the car.

View attachment 1716543445

View attachment 1716543446

Yes with Stock Manifolds.


☆☆☆☆☆

I've seen the TTI exhaust options, I have one 2-1/2" system on my Slant Six in my Duster now. Any benefit to the X pipe over the H pipe?

 

[Illahe]

If your camshaft LSA is matched to your exhaust system, I don't see an honest 400 at the crank being an issue. Street car?

 

[MopaR&D]

The 2 1/2" TTI Exhaust System is what you really need to make those stock 340 exhaust manifolds shine.

It will literally pull the spent exhaust gases out of the cylinder heads, as the hot spent exhaust gases keep expanding through the 2 1/2" exhaust system, all the way out to the back of the car.

View attachment 1716543445

View attachment 1716543446

Yes with Stock Manifolds.


☆☆☆☆☆

If the outlets of the manifolds are smaller than 2 1/2” then that size exhaust will not scavenge well. For that to happen you need pipes the same diameter or smaller for at least a couple feet off the manifolds before it steps up in size. Sharp transitions in size to a larger flow area kill the refraction waves.

Long tubes will always make more power but if you have more compression and less overlap with the cam the engine will be less sensitive to exhaust gas scavenging. If I had to throw a dart to pick an HP number... 350? Tough to say because the effect scales with HP per cube.

 

[Scampin]

Here is the current passenger side manifold on my car. It has been 10.11@133. The outlet is just under 2". If you look at it and don't think it's a restriction even in its current form on a stock motor...

 

[gdonovan]

headers will always make more but how *much* more is the age old question of: is the juice worth the squeeze

it's also highly dependent on the combination. the wilder it be, the more it'll want tubes.

i'll throw a round number out and say 400hp before they become a significant restriction at give or take 20 hp/tq

that being said... there's dudes out there running FAST/pure stock/etx that are in the 10's and 11's with manifolds...
*clears throat*
*side eyes*

1) On my Duster with a 5.9 Magnum, I picked up about 75 hp. That is not a misprint. I came home from the track, threw the manifolds in the trash, installed Dougs and went back to the track. Made a few passes & opened the headers and with minor tuning the car was running a tick over a full second faster and picked up 10 to 11 mph.

2) FAST guys use a lot of tricks to get where they are going including poking and stroking out to extreme displacements. Looking at FAST will never be an apples to oranges comparison, they have to work around running manifolds and mold their package from there.

 

[furrystump]

On my 68 manifolds I have done practically nothing. Took the lumps down a bit on inlet side and opened the exit up to the gasket size. Car weighs 3245 going down the track at 121. My passenger side is much better than the later ones. Oh, removed the flapper valve.

 

[furrystump]

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.

 

[furrystump]

This looks awful silvery!

 

[340six]

I have the early 340 manifolds but saving them for my Challenger.
So had the stock 1973 340 that came on my car new surfaced ( milled flat few pass) and ported incase I change my mind over the Dougs.
The manifolds might end up on there?
Only reason I may not is how bad the manifolds wood sound with cut outs?

 

[ch1ll]

It all depends on the rest of your combination, as stated elsewhere in this thread. Sure, the stock manifolds, even if ported really well will become a restriction at some point limiting high rpm horsepower. But if you plan and build with that in mind up to that point wherever that may be, you'll have a screamer.

Remember, it's never going to be a top fuel funny car or a super stock car for that matter. Just be realistic with your expectations.

Passenger car parts are just that, for passenger cars. We just hot rod them so we can smile proudly.

 

[StreetSleeper]

The 2 1/2" TTI Exhaust System is what you really need to make those stock 340 exhaust manifolds shine.

It will literally pull the spent exhaust gases out of the cylinder heads, as the hot spent exhaust gases keep expanding through the 2 1/2" exhaust system, all the way out to the back of the car.

View attachment 1716543445

View attachment 1716543446

Yes with Stock Manifolds.


☆☆☆☆☆

If I replaced the ‘pictured straight run of exhaust’ with 2 1/2” hole diameter, 47” glass packs, would that impede the flow significantly from straight pipe?

car.

View attachment 1716543445

 

[timk225]

If I replaced the ‘pictured straight run of exhaust’ with 2 1/2” hole diameter, 47” glass packs, would that impede the flow significantly from straight pipe?

car.

View attachment 1716543445

Do not use glass packs, they have that "BBLLLAAAAAAAAA" sound that does not sound good.

 

[furrystump]

I’m running Pypes Race pro’s NOT the violators those sound HORRIBLE! Picked up 1-2 tenths going from Dyno max to the race pros.

 

[66fs]

Do not use glass packs, they have that "BBLLLAAAAAAAAA" sound that does not sound good.

Good glass packs, Cherry Bomb and others, do not do that. They have a nice mellow sound, if too loud run a second as a resonator, like the Commando 273 cars.