340 exhaust manifold vs stock 318

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Slantsix64

Slantsix64

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i picked up a 1973 340 driver side manifold along with a 360 passenger side for $40 bucks so i didnt break the bank. i have a 318 with a mild cam and right now its got the stock 1967 273 exhaust manifolds. wanted to swap them out and was thinking would there be at least a 10 horsepower increase ? i have a 318 with a 600 cfm eddy with a lb4d intake 2.25 exhaust dual. any info will be great! thanks
 
Anything will help. 10 hp. might be a stretch. The manifolds you got are a step up from the 67 273's. The next best are the 67-69 manifolds.
 
That's a fair guess. You will get some people to quote huge power gains. So by the time this post has run it's course. You'll be up 100 horsepower. Will have been told to swap the heads intake & carb & go to fender well headers. You got a great deal on your exhaust. Go with what you can afford.
 
Your passenger's side manifold helps zero. It has the same size outlet as a 318. 1 7/8. The driver's side 340 manifold is good. It has a 2 1/4 outlet, like the earlier ones. I would look for a 68-71 passenger's side 340 manifold. More expensive, but worth it.
 
I’ve got the same the carb and intake manifold as you along with a mild cam and 302 heads on my 318 and a 2 1/2” dual exhaust. The biggest difference I found was going from a 2bbl to a 4 bbl. Next biggest difference was going from a crush bent dual exhaust (wasn’t more than 2”) to a 2 1/2” mandrel bent exhaust. The difference with the 340 exhaust manifolds wasn’t huge compared to the other two upgrades.

340 Manifolds on 318
 
RustyRatRod said:
Your passenger's side manifold helps zero. It has the same size outlet as a 318. 1 7/8. The driver's side 340 manifold is good. It has a 2 1/4 outlet, like the earlier ones. I would look for a 68-71 passenger's side 340 manifold. More expensive, but worth it.
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thanks what if i use a Durango passenger manifold would that work.
 
Slantsix64 said:
thanks what if i use a Durango passenger manifold would that work.
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The standard manifolds still have a 1 7/8 outlet. Only the R/T manifolds have the 2 1/8. Get with member @Treblig he can fix you up.
 
'93,94, 95 Jeep with 5.2 manifold should get you the 2 1/8 you need. Or as RRR mentioned, see Treblig
 
You will not feel 10 HP at the crank. The wheel HP gain will be 6-7. Is it worth your time?
The reasons that the 340 ran so strong were:
1) 22 more cubes than the 318.
2) Higher compression than the 318.
3) Different heads with larger valves and ports.
4) Better flowing Exhaust manifolds and dual exhaust.
5) 4 barrel intake manifold and carburetor.
6) More aggressive camshaft with stiffer valve springs.
7) Windage tray.
 
Slantsix64 said:
i picked up a 1973 340 driver side manifold along with a 360 passenger side for $40 bucks so i didnt break the bank. i have a 318 with a mild cam and right now its got the stock 1967 273 exhaust manifolds. wanted to swap them out and was thinking would there be at least a 10 horsepower increase ? i have a 318 with a 600 cfm eddy with a lb4d intake 2.25 exhaust dual. any info will be great! thanks
Click to expand...

The 340 driver's manifold is the best (cast iron) driver's manifold you can use. Stock 318 manifolds had a 1 5/8" exit hole (L & R). The 360 manifolds came in many different sizes, exit holes. The 92/93 360 Magnum manifolds were 2 1/8" exit hole as well as the 96/97/98 Jeep Grand Cherokee (JGC) but you can only use the passenger (JGC) in an A body. Well... you might could use the driver's side magnum manifold if you did some extensive modifications??
You didn't show a pic of the 360 manifold that you have so I don't know what size the exit hole is?? They made magnum engines for many, many years but they only used Large Hole manifolds in certain years.

92/93 360 manifold- 2 1/8" exit hole
96/97/98 Jeep Grand Cherokee 360 manifold - 2 1/8" exit hole
Stock 318 manifold - 1 5/8" exit hole
70s 340 passenger manifold - 1 7/8" exit hole
All 340 driver's manifolds - 2 1/4"
60s 340 passenger - 2 1/4" exit hole

PS - There are also other 360 manifolds that were used on vans and other vehicles that had a large exit hole but the shape of the manifold causes it to pass very close to the passenger's fire wall which will make the floorboard hot, it will work but why bother if you can get a JGC manifold!!

treblig
 
Kern Dog said:
You will not feel 10 HP at the crank. The wheel HP gain will be 6-7. Is it worth your time?
The reasons that the 340 ran so strong were:
1) 22 more cubes than the 318.
2) Higher compression than the 318.
3) Different heads with larger valves and ports.
4) Better flowing Exhaust manifolds and dual exhaust.
5) 4 barrel intake manifold and carburetor.
6) More aggressive camshaft with stiffer valve springs.
7) Windage tray.
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Fully agree. Kinda sad really, no real voodoo here just common knowledge even back then.
Kinda makes the 340 look boring.
But tell that to the guy in the 396 Chevelle having a heck of a time with a little dart holding her tight down the 1/4 mile
 
pishta said:
anyone got pic of the JGC manifolds mounted?
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I got mine from Treblig a couple years ago.:thankyou: Now these are mockup pics, I'm not done with the engine bay yet. Trans filler tube needed to be bent a little at the top to fit. Car is a '75 318 Scamp.

360 passenger manifold.jpg
PASSENGER SIDE DOWNPIPE (1).jpg
Downpipe (3).jpg
 
FYI...if you grind off the casting number and that threaded nub just above the casting number then grind a section off of the casting in the place where the inner fender lives you can get it to fit on an early A. i made a special one for a member with an early A with that area ground away.
 
Treblig said:
FYI...if you grind off the casting number and that threaded nub just above the casting number then grind a section off of the casting in the place where the inner fender lives you can get it to fit on an early A. i made a special one for a member with an early A with that area ground away.
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Could you machine the port flanges to give a little more clearance? The center bolt flanges may get a little thin though. Just thinking out loud.
 
toolmanmike said:
Could you machine the port flanges to give a little more clearance? The center bolt flanges may get a little thin though. Just thinking out loud.
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The 360 magnum manifold actually has very heavy (thick) bolt hole castings. I don't see why you couldn't remove maybe 1/16" (or a little more) without any problem. I would grind the exterior to help clear the inner fender and take a little off the mating surface (best of both worlds). I used the grinding method to save the buyer some money.
 
Ive used early 340 manifolds on my 69 polara C body with PS
then sold them for a challenger restore project'
went with 92-95 5.2 Magnums
then put the 440 in
 
RustyRatRod said:
Ha! A broke clock is right twice a day. LMAO
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So if I'm right once in 24 hours that's like 50 percent of the time and if I'm right 2 times in 24 hours that's like 100 percent of the time. Damn "RRR"....you're pretty smart!!:lol::rofl::poke::thumbsup::wtf:
 
Nice math,I mean it! 1:24=50%, 2x50% =100%, I like it.
If primaries make 250hp. then 2x250= 600 with the secondaries also open.... except on spreadbores then its triple, so 1000 hp . If 1200 is all it can do NA then 150shot should double something or other and the final output is high enough to launch me to the moon. Somebody please give me an A-plus
lol
 
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