340 exhaust manifold vs stock 318

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AJ/FormS said:
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
Click to expand...
"By Jove, I think you've got it!!!'
 
Treblig said:
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
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I've found a 1997 JGC with a 5.2 is this a different manifold than the 5.9? Is this still a step up from a stock 1968 318 exhaust manifold I think my part number reads 2789045. Edit; sorry 2780945

exhast manifold.jpeg
 
Here are pics of the JGC passenger's manifold. They are found on 96/97/98 JGC with V-8. Your pic is an original manifold 1 5/8". Yes the JGC would be a big step up.

DSC03989.JPG
DSC03813.JPG
 
So, after looking at the chart in post #3, is the moral of this story that headers don’t really make much difference on a stock engine for all the trouble & expense?
 
Mike69cuda said:
So, after looking at the chart in post #3, is the moral of this story that headers don’t really make much difference on a stock engine for all the trouble & expense?
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Actually headers make a hell of a difference, but if you want a nice bump without the hassle of adding more engine heat and blowing out header gaskets and the head ache install of them, the 340 and magnum manifolds are for ya.
 
Slantsix64 said:
Actually headers make a hell of a difference, but if you want a nice bump without the hassle of adding more engine heat and blowing out header gaskets and the head ache install of them, the 340 and magnum manifolds are for ya.
Click to expand...

Not trying to be argumentative, but the data from the chart doesn’t seem to show much difference between 340 / 360 manifolds and headers on a 300 hp engine.

At 4500 rpm, the difference looks to be 10-12 hp, about 3-4%. Doesn’t look like much to me.
 
Slantsix64 said:
Actually headers make a hell of a difference, but if you want a nice bump without the hassle of adding more engine heat and blowing out header gaskets and the head ache install of them, the 340 and magnum manifolds are for ya.
Click to expand...
Yes I was looking at some shortys but as the link that forphorty posted suggests that the shortys are only good for maybe an extra 5 hp over 340 exhaust manifolds. I had a 74 challenger back in 1999 and I had headers on it but the very first bump I crushed the bottom of the headers (long tube) due to terrible ground clearance. I am building a 325 to 400HP max, motor. So with that in mind I think some decent manifolds will be much nicer for my application. But who knows the final project always looks a little diferent than original plans.
 
Mike69cuda said:
Not trying to be argumentative, but the data from the chart doesn’t seem to show much difference between 340 / 360 manifolds and headers on a 300 hp engine.

At 4500 rpm, the difference looks to be 10-12 hp, about 3-4%. Doesn’t look like much to me.
Click to expand...
Yup correct 4500 and up if you need to go up there will help, if not don't bother on my v eight cars I run the 340 magnum manifolds install and go! On my slant six I have custom manifold I have made, but will soon be getting some tube headers so it will be easier to reach the moon lol.
 
magnum or JGC manifolds are essentially free
for shorties I like tri Y style with 1 & 7 paired
1.5 is plenty but they step
 
Bronze Barracuda said:
Yes I was looking at some shortys but as the link that forphorty posted suggests that the shortys are only good for maybe an extra 5 hp over 340 exhaust manifolds. I had a 74 challenger back in 1999 and I had headers on it but the very first bump I crushed the bottom of the headers (long tube) due to terrible ground clearance. I am building a 325 to 400HP max, motor. So with that in mind I think some decent manifolds will be much nicer for my application. But who knows the final project always looks a little diferent than original plans.
Click to expand...
i hear ya on that! I am not sure but I think if you have a later abody, with no power steering and is an auto the driver side magnum manifold might work with some grinding.
 
Not mentioned here so far, but the 68-70 340 drivers manifold is also better than the later 340-360. If you look at them side by side the 68-70 is quite a bit taller. They do however, have the same exhaust hole size.
 
Look at the chart in post #35. It specifically states stock '77 360 log manifold (probably 1 5/8")....that is nothing near the 360 magnum JGC manifold and it's certainly not anything like the ones I used to machine out to larger than the stock 2 1/8"!!!
 
So as far as exhaust manifold upgrades over stock manifolds we should be looking for in this particular order. Talking about a 1967 and up a-body (in my particular case a 1968 barracuda with power steering)
It may have been mentioned somewhere about wether or not certain manifolds will or will not work with power steering other than the 340 or 273/318 drivers side exhaust manifold
1: Both driver and passenger side 68-71 340 exhaust manifolds.
2: Drivers side 340 exhaust manifold 68-71, 96-98 JGC passenger side exhaust manifold
3: Stock drivers side 273/318 exhaust manifold, 96-98 JGC passenger side exhaust manifold
4: Always turf the passenger side exhaust manifold and at minimum install 96-98 JGC passenger side exhaust manifold as they are plentiful and cheap.
:lol:
I think someone led to the fact that a 318/360 magnum/JGC driver side exhaust manifold may work without power steering and with a bit of modification/grinding???

Please someone correct me if I am wrong.
 
Bronze Barracuda said:
So as far as exhaust manifold upgrades over stock manifolds we should be looking for in this particular order. Talking about a 1967 and up a-body (in my particular case a 1968 barracuda with power steering)
It may have been mentioned somewhere about wether or not certain manifolds will or will not work with power steering other than the 340 or 273/318 drivers side exhaust manifold
1: Both driver and passenger side 68-71 340 exhaust manifolds.
2: Drivers side 340 exhaust manifold 68-71, 96-98 JGC passenger side exhaust manifold
3: Stock drivers side 273/318 exhaust manifold, 96-98 JGC passenger side exhaust manifold
4: Always turf the passenger side exhaust manifold and at minimum install 96-98 JGC passenger side exhaust manifold as they are plentiful and cheap.
:lol:
I think someone led to the fact that a 318/360 magnum/JGC driver side exhaust manifold may work without power steering and with a bit of modification/grinding???

Please someone correct me if I am wrong.
Click to expand...

Don’t forget the 72-76 340-360 manifolds. They are not as good as 68-70 340, but are much better than 273-318 due to port size.

71 was an odd year. The pass side was log style like later years. Don’t know about Driver side.
 
Bronze Barracuda said:
So as far as exhaust manifold upgrades over stock manifolds we should be looking for in this particular order. Talking about a 1967 and up a-body (in my particular case a 1968 barracuda with power steering)
It may have been mentioned somewhere about wether or not certain manifolds will or will not work with power steering other than the 340 or 273/318 drivers side exhaust manifold
1: Both driver and passenger side 68-71 340 exhaust manifolds.
2: Drivers side 340 exhaust manifold 68-71, 96-98 JGC passenger side exhaust manifold
3: Stock drivers side 273/318 exhaust manifold, 96-98 JGC passenger side exhaust manifold
4: Always turf the passenger side exhaust manifold and at minimum install 96-98 JGC passenger side exhaust manifold as they are plentiful and cheap.
:lol:
I think someone led to the fact that a 318/360 magnum/JGC driver side exhaust manifold may work without power steering and with a bit of modification/grinding???

Please someone correct me if I am wrong.
Click to expand...
You're mostly correct.....I would say that, in the case of your car, you can use any 340 driver's manifold with the JGC 96-98 passenger. I don't know if I would mix a JGC with a stock 318 driver (too much imbalance) but it would work. The 340 driver/JGC combination can be used on any 67 and up A body with or without P/S. The JGC manifolds are plentiful and cheap and you can find one close to your location here:
http://www.car-part.com/index.htm
 
Treblig said:
You're mostly correct.....I would say that, in the case of your car, you can use any 340 driver's manifold with the JGC 96-98 passenger. I don't know if I would mix a JGC with a stock 318 driver (too much imbalance) but it would work. The 340 driver/JGC combination can be used on any 67 and up A body with or without P/S. The JGC manifolds are plentiful and cheap and you can find one close to your location here:
Car-Part.com--Used Auto Parts Market
Click to expand...
Always willing to help. Thanks Treblig!
 
I've been preaching the low gain with headers on stock type combos for years, but the 318 guys with their 8.0 smoggers all preach me wrong.

I'll say it again;
for headers to work right, it takes the entire combo; most especially the overlap period and the Effective compression ratio.
They are never a "bad" idea; but on a stocker, rarely worth the trouble, compared to a hi-flo Dual Exhaust, which will make all the difference.
Headers may make a difference on the Dyno, I can't say , but 10hp at 4400 is as good as un-noticeable from the driver's seat... when it climbs at a rate of 2 hp per 1000 rpm, peaking at about 40/50 mph with hiway gears..........
The factory 318 LA cam has 20* of overlap on the advertised, which at .050tappet rise is already looong gone. So headers will never get you significant power at rpm, over and above the meager results they otherwise make..... on the stocker.
The Mopar 292/292/108, cam on the otherhand has 76* at advertised, which works out to 32* at .050 so this cam will respond very nicely to headers. Even the mighty 268/276/114, 340 cam has 44* advertised, and by .050 it is down to just about 2*. Yet when you put headers on a factory stock 340, the power increase is likely to be in excess of 20 hp, over and above what the HO manifolds do. So some of that is in the individual pipes, and some is in the overlap. Just keep in mind that the power peak is up near 5000, so forget running hi-way gears.
anyway; rant over.
 
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REPEAT POST:

"Look at the chart in post #35. It specifically states stock '77 360 log manifold (probably 1 5/8")....that is nothing near the 360 magnum JGC manifold and it's certainly not anything like the ones I used to machine out to larger than the stock 2 1/8"!!!"

What would have been the HP reading with a 340 driver and one of my machined manifolds with both manifolds having a 2 1/4"+ exits?????
 
What would have been the HP reading with a 340 driver and one of my machined manifolds with both manifolds having a 2 1/4"+ exits?????

That’s a great question right there.
I have a set of trebligs manifolds I’ve been dying to install. A 73 340 driver side and a jgc passenger side milled out to nearly 2.5”. I just finished the k frame and suspension upgrades and install. I cut out the old rusty, caved in headers and have just now installed the manifolds. I expect the 5.9 magnum with a bit larger cam and airgap will perform just as before as a streeter with the occasional 1/4 mile run. I love the manifold look and the tremendous amount of room that’s been freed up by removing the headers. I’ll get some pics later today and post them up.

Thank you Gil.
 
67dartgtgo said:
What would have been the HP reading with a 340 driver and one of my machined manifolds with both manifolds having a 2 1/4"+ exits?????

That’s a great question right there.
I have a set of trebligs manifolds I’ve been dying to install. A 73 340 driver side and a jgc passenger side milled out to nearly 2.5”. I just finished the k frame and suspension upgrades and install. I cut out the old rusty, caved in headers and have just now installed the manifolds. I expect the 5.9 magnum with a bit larger cam and airgap will perform just as before as a streeter with the occasional 1/4 mile run. I love the manifold look and the tremendous amount of room that’s been freed up by removing the headers. I’ll get some pics later today and post them up.

Thank you Gil.
Click to expand...
YES!!! Pics please!! Wonder if my manifolds will be collector's items someday.....:thumbsup:
 
Treblig said:
You're mostly correct.....I would say that, in the case of your car, you can use any 340 driver's manifold with the JGC 96-98 passenger. I don't know if I would mix a JGC with a stock 318 driver (too much imbalance) but it would work. The 340 driver/JGC combination can be used on any 67 and up A body with or without P/S. The JGC manifolds are plentiful and cheap and you can find one close to your location here:
Car-Part.com--Used Auto Parts Market
Click to expand...
ok :thumbsup: thank you for the clarification. I wasn't sure about the driver side exhaust manifold on the JGC. and love that auto wrecker parts finder.
Alright just realized, "340 driver/JGC combination" is combination. Late night with friends and still on first cup of coffee.
 
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67dartgtgo said:
What would have been the HP reading with a 340 driver and one of my machined manifolds with both manifolds having a 2 1/4"+ exits?????

That’s a great question right there.
I have a set of trebligs manifolds I’ve been dying to install. A 73 340 driver side and a jgc passenger side milled out to nearly 2.5”. I just finished the k frame and suspension upgrades and install. I cut out the old rusty, caved in headers and have just now installed the manifolds. I expect the 5.9 magnum with a bit larger cam and airgap will perform just as before as a streeter with the occasional 1/4 mile run. I love the manifold look and the tremendous amount of room that’s been freed up by removing the headers. I’ll get some pics later today and post them up.

Thank you Gil.
Click to expand...
Oh the fun I would have if I had my own dyno meter.
 
AJ/FormS said:
I've been preaching the low gain with headers on stock type combos for years, but the 318 guys with their 8.0 smoggers all preach me wrong.

I'll say it again;
for headers to work right, it takes the entire combo; most especially the overlap period and the Effective compression ratio.
They are never a "bad" idea; but on a stocker, rarely worth the trouble, compared to a hi-flo Dual Exhaust, which will make all the difference.
Headers may make a difference on the Dyno, I can't say , but 10hp at 4400 is as good as un-noticeable from the driver's seat... when it climbs at a rate of 2 hp per 1000 rpm, peaking at about 40/50 mph with hiway gears..........
The factory 318 LA cam has 20* of overlap on the advertised, which at .050tappet rise is already looong gone. So headers will never get you significant power at rpm, over and above the meager results they otherwise make..... on the stocker.
The Mopar 292/292/108, cam on the otherhand has 76* at advertised, which works out to 32* at .050 so this cam will respond very nicely to headers. Even the mighty 268/276/114, 340 cam has 44* advertised, and by .050 it is down to just about 2*. Yet when you put headers on a factory stock 340, the power increase is likely to be in excess of 20 hp, over and above what the HO manifolds do. So some of that is in the individual pipes, and some is in the overlap. Just keep in mind that the power peak is up near 5000, so forget running hi-way gears.
anyway; rant over.
Click to expand...
I put a set of Mickey Thompson TriY's on my 73 Challenger back in the day. Without any other changes it went a few hundredths slower. :BangHead:
 
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