At nauseum..318 intake manifold to 340 or 360.

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Spin'em75

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Hey, everyone! New to this forum. Got my first Plymouth Duster a few days ago ('75) with a rebuilt 318. Dual flowmasters were recently installed (not sure on model) and was told the 318 already has 340 heads on it.

I know this question or topic has been probably asked repeatedly, but I am wanting to upgrade the 2bbl carb and intake manifold on it to convert to either a 340 or 360 4bbl. Forgive my freshness, but I am new to this. I am used to your regular maintenance and items that need to be replaced or repaired on vehicles, but none to this degree (which is probably intermediate level for you all lol.)
So because of this, I'd hate to buy wrong parts. I was wondering if anyone has the patience to link certain main products I will need to do this conversion (intake, specifically. ) I know the carb must be 650cfm Carburetor.

If you dive too much into the science behind it all, you will probably lose me. I likely was dropped on the head as a kid. So explain to me like I'm 12yrs old. Lol. Thanks everyone. Just a man trying to dive into this hobby. Been a dream of mine since I was a kid.
 
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I find it unlikely that a two barrel intake would be installed on 340 heads. You may want to take a valve cover off and look at the part number on the head and post back the number so we can tell you what kind of head you have on your 318. The 318 intake manifolds have smaller ports than the 340 intakes. It’s important to put the right intake on the right head or you’ll get a port mismatch and the motor won’t run as good as it should.
 
you can run anything from a 500cfm to 750, it's not gonna matter much. it'll probably be happiest with 600~650

intake isn't gonna matter. the factory ran mismatched heads and intakes. that being said i'd try and find an intake that *does* have the larger 340/360 ports. but again, not critical.

what is absolutely critical is your throttle linkage for a 4bbl is different from a 2bbl as is your transmission kickdown. this must be addressed, because having the kickdown incorrect or not properly operating will smoke your transmission. and having janky throttle linkage is no way to roll.

some of this can be adapted and modified with what you have, or you can pick it up fairly easily aftermarket, reproduction or here in the classifieds.
 
Look by the spark plugs (in between them). Is there a "J" or "X" ?? Or, as mentioned, pull one valve cover and get the numbers. But you can throw a 4bbl on the 318 even if it has 318 heads as well !
 
Regular maintenance? Intermediate, lol
On this site putting together a 500HP 440 build in your garage is intermediate.
I recently did this swap for the 20th or 30th time. It costs about $1000. buying everything new.
Intake if no AC-Assault eddy air gap clone, the port mismatch is the boogy man. It doesn't make any noticeable difference on a mild 318 under 320HP.
if you have AC I'd go with the Edelbrock performer with the ac boss to mount the compressor built in.
Kickdown & throttle cables, Lokar style with brackets included. I bought 2 ,used one cable for kickdown and one cable for throttle and sold the extra bracket on ebay.
Carb-Edelbrock 1406 elec choke with a throttle adapter to make sure throttle and kickdown throw are 1:1
Don't start out trying to setup a holley double pumper, start with a Carter style Edelbrock carb it's easier to learn how to tune it and how it works. Plus it's better for a mostly stock cruiser.
Before you throw away or sell your old carb, take it apart, clean it, possibly rebuild and learn how it works in the process. Get some hands on with it.
When you go to set the kickdown cable err on the tighter side, too loose burns up bands in a hurry. Just one trip around the block with the kickdown too loose can smoke the band, I've seen it happen.
Luckily they are cheap and pretty easy to replace.
If the lean burn smog system is still on it your going to want to change out the ignition box. and possibly the distributor because some of the smog dizzys have too much total spark advance built in and the curve is terrible. It's easier to get this conversion kit.
 
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Maybe it does have 340 heads, maybe it doesn’t. It’s an easy tagline the seller can toss out w/o really needing to prove if the potential buyer doesn’t ask to pull the valve covers before buying it.

Regardless what heads your 318 has, upgrading to a 4bbl will make a difference. Also consider adding a dual exhaust and recurving the distributor as it’s likely set up for a smog advance.

I replaced the 2bbl on my ‘75 318 with a 4bbl and did a summary here: LD4B installation on a 75 318


Oh, and post pics of your Duster!
 
H22A4 said:
Also consider adding a dual exhaust and recurving the distributor as it’s likely set up for a smog advance.
Click to expand...

Did you even read what the OP wrote in his first post #1 ???

Spin'em75 said:
Hey, everyone! New to this forum. Got my first Plymouth Duster a few days ago ('75) with a rebuilt 318. Dual flowmasters were recently installed (not sure on model)
Click to expand...

He's already got that covered.

Tom
 
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intake-head mismatch on a street beater is a huge red herring.

Run whatever intake you have on whatever head you have. Lots of alum intakes recommended for a 340/360 have 318 size ports.
Ede 600-650 and make sure the kickdown is set up properly as @junkyardhero mentioned. REALLY important with an auto car.
 

Deltatech said:
Regular maintenance? Intermediate, lol
On this site putting together a 500HP 440 build in your garage is intermediate.
I recently did this swap for the 20th or 30th time. It costs about $1000. buying everything new.
Intake if no AC-Assault eddy air gap clone, the port mismatch is the boogy man. It doesn't make any noticeable difference on a mild 318 under 320HP.
if you have AC I'd go with the Edelbrock performer with the ac boss to mount the compressor built in.
Kickdown & throttle cables, Lokar style with brackets included. I bought 2 ,used one cable for kickdown and one cable for throttle and sold the extra bracket on ebay.
Carb-Edelbrock 1406 elec choke with a throttle adapter to make sure throttle and kickdown throw are 1:1
Don't start out trying to setup a holley double pumper, start with a Carter style Edelbrock carb it's easier to learn how to tune it and how it works. Plus it's better for a mostly stock cruiser.
Before you throw away or sell your old carb, take it apart, clean it, possibly rebuild and learn how it works in the process. Get some hands on with it.
When you go to set the kickdown cable err on the tighter side, too loose burns up bands in a hurry. Just one trip around the block with the kickdown too loose can smoke the band, I've seen it happen.
Luckily they are cheap and pretty easy to replace.
If the lean burn smog system is still on it your going to want to change out the ignition box. and possibly the distributor because some of the smog dizzys have too much total spark advance built in and the curve is terrible. It's easier to get this conversion kit.
Click to expand...
This is really helpful, thanks for the links. Think I can use all this info with a few YouTube how-to's and get it done. I'm assuming I'll have to purchase the intake gaskets separately?
 
Yup, there are of course two different gaskets, small port 318 and large port 360.
If you're heads are 340 then you need the large port 360 intake gaskets and the Assault intake port will be the best fit, The Edelbrock that has the boss built in for the ac compressor that I linked has the small 318 size intake ports.
I figure you probably do have 340 heads, he just took his Aluminum intake and four barrel off and put the stock 2 barrel back on the sell it as running.
Dual exhaust is another sign, If it has headers I put $1000 on it has 340 heads.
 
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H22A4 said:
Maybe it does have 340 heads, maybe it doesn’t. It’s an easy tagline the seller can toss out w/o really needing to prove if the potential buyer doesn’t ask to pull the valve covers before buying it.

Regardless what heads your 318 has, upgrading to a 4bbl will make a difference. Also consider adding a dual exhaust and recurving the distributor as it’s likely set up for a smog advance.

I replaced the 2bbl on my ‘75 318 with a 4bbl and did a summary here: LD4B installation on a 75 318


Oh, and post pics of your Duster!
Click to expand...

H22A4 said:
Maybe it does have 340 heads, maybe it doesn’t. It’s an easy tagline the seller can toss out w/o really needing to prove if the potential buyer doesn’t ask to pull the valve covers before buying it.

Regardless what heads your 318 has, upgrading to a 4bbl will make a difference. Also consider adding a dual exhaust and recurving the distributor as it’s likely set up for a smog advance.

I replaced the 2bbl on my ‘75 318 with a 4bbl and did a summary here: LD4B installation on a 75 318


Oh, and post pics of your Duster!
Click to expand...

IMG_20250418_184406.jpg


20250420_105421.jpg
 
Congratulations on the Duster! Being from Seguin, I saw that car advertised on Facebook marketplace. I thought it was a good looking car, and it sounded good in the video. PM me if you want to find out about local Mopar stuff in San Antonio. Where do you live? I’d be happy to help you when you do the intake swap.
 
I also have an Edelbrock 318/360 performer intake that has a very small crack in it around one of the bolt holes. I’ll give it to you if you want to take it to a machine shop to get it weld repaired.
 
middleagecrisis said:
Congratulations on the Duster! Being from Seguin, I saw that car advertised on Facebook marketplace. I thought it was a good looking car, and it sounded good in the video. PM me if you want to find out about local Mopar stuff in San Antonio. Where do you live? I’d be happy to help you when you do the intake swap.
Click to expand...
Thanks man! I'm located in South Texas, near McAllen. Nice to see a fellow Texan here. I'll definitely send over a PM to find out more stuff. This Duster needs some interior work like new carpet, panels, etc., but I'm determined to give it the refresher it needs.
 
middleagecrisis said:
I also have an Edelbrock 318/360 performer intake that has a very small crack in it around one of the bolt holes. I’ll give it to you if you want to take it to a machine shop to get it weld repaired.
Click to expand...
I think I saw this on Marketplace, could be you. Lol. I don't mind taking it off your hands. Maybe if I at least cover shipping? Don't know when I'll have a trip up there as life and my job have me pretty grounded at the moment.
 

Spin'em75 said:
I think I saw this on Marketplace, could be you. Lol. I don't mind taking it off your hands. Maybe if I at least cover shipping? Don't know when I'll have a trip up there as life and my job have me pretty grounded at the moment.
Click to expand...
I never advertised this one, since it's got the hairline crack. You can either pay the shipping and I'll send it your way, or it can set in the corner of my shop until you find yourself this way. McAllen is a long way off, so I'll pull back the offer about coming over to your place and helping with the intake swap. For you non-Texans, that's about a 5-hour trip, one-way!
 
Spin'em75 said:
I think I saw this on Marketplace, could be you. Lol. I don't mind taking it off your hands. Maybe if I at least cover shipping? Don't know when I'll have a trip up there as life and my job have me pretty grounded at the moment.
Click to expand...
Lol understood. Let me know for shipping costs in a PM for area code 78541
 
I would have no real issue running a “small” port intake on a “big” port head. Ideal? No. The other way around is going to hurt you pretty significantly. Like a dirt bike going over a jump. One way is fine, going the other way really hurts! :) To the OP, no fear, it is always better to trust but verify. You need to check out what you are told and pictures are truly worth 1000 words. We might see something totally unrelated that might need attention.
 
The Holley Street Dominator intake has the four-barrel carb in the correct location so your linkages will work without modification if I remember correctly. I used one on my daughter's Barracuda with a 1969 318 with the original heads and it worked great.
 
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