i recently bought a 72 charger with a 318 motor with the factory 2 barrel carb and an automatic transmission, i wanted to change over to the 4 barrel set-up so i bought a 318 4 barrel intake and set that up. With the intake i was given a thermoquad carb and i was wondering what the best way to set the linkage up would be, or if it is even possible with the current 2 barrel linkage. If it is, which conversion kit is best to change the 2 barrel linkage to work with the thermoquad carburetor. Thanks!

 

How to answer this...

The linkage for a 4 barrel carb is different than a 2 barrel carb and will not work without major modification. Who ever sold you the intake manifold probably has the linkage for it.

I should warn you that you will have less power with the 4 barrel than the 2 barrel. The ports on the stock heads are too small the fuel piles up against the head and does nothing. The 4 barrel intake you have was designed for the 340/360 engine heads which have larger intake ports.

Sorry but thought I would warn you about that.

 

jacilynn_s is right.
you would gain more power by installing a less restrictive exhaust system;
HEADER'S & DUAL'S

 

doug -

You're fine with the carb/manifold set up, and that's typically the first stop most guys do, for power and well, visual impact. The TQ might be a bit big for a stock 72 318, but it has relatively small primaries, so it shouldn't be terrible.

The next step would be headers and a full dual exhaust, when and time allow. After that, you're looking at a cam and possibly a head swap, if you plan on keeping the 318. That's the route I took several decades ago, before a jumped to a 440.

Regarding the linkage, if you want "factory" you might be able to find a "new" one, but there are a number or Mopar graveyards that will be able to pull one off a 340/360 for you. Otherwise, there are a number of kits from Lokar, Holley, etc. IIRC, we fabricated a linkage bracket, from the 2v to 4v, but that was a long time ago.

Archer

 

I agree with Archer, the Thermoquad has small primaries and huge secondaries. Nice thing about a carb with air valve secondaries is the carb will only give the engine as much as it can use, that's how they got away with installing an 800 cfm carb on such a small engine. Actually the swap has also been known to improve fuel economy, if you can keep your foot out of it, since the primaries on the TQ are smaller than the 2 barrel venturies. You should be able to find linkage relatively easily as this setup was used well into the 80's on Police package Diplomats and Gran Fury's. I miss the ka-whoomp when the cops used to take off on a call.

 

Unless you get complete "matching" linkage from the throttle bracket all the way to the transmission, you may be in for bad time, unless you are a good fabricator.

There are a couple of outfits, and Lokar is a popular one, who make aftermarket replacement kickdown cable setups. These are very adaptable. I don't use kickdown---I went to an all--manual valve instead Frankly, the way to "wake up" a stock stone dead 318 is to

1--Dual out th exhaust. Even with stock manifolds and stock "sized" piping, just adding the "other side" helps

2--Get a better cam. A 360 cam, IE the "same cam" roughly as the old 340 cam works GREAT in a 318

3--Thermoquad is fine AND THE FACTORY PUT THEM ON a few 318s

 

Thanks a lot for the responses and so quick too. I think i am going to look for a stock linkage since that would probably be the most reliable. The cam will come after as money allows. but thanks alot for the help.

 

with some tuning the thermoquad will work just fine.. The factory used them on a lot of 318's. You just need the correct linkage -- the bracket that bolts to the intake and the kickdown lever

 

You can't put it in if You cant get it out!
put headers and duels on her first...
you will be surprised at the mid range gains!


Come on every one here knows this.

 

IMO, again only my opinion based upon having seen this many times, and tried it myself decades ago when I didn't KNOW any better..
The BIGGEST mistake anybody can make when switching from a factory 2 bbl Engine(318), to a 4 BBL setup..
is forgeting to add a 4 BBL Camshaft ?

The factory 2 Barrel Engine Camshaft is the " program", responsible for how long(duration), and how much(lift), the Valves will stay open.
The factory 2 BBL Cam has NOT enough of either, to even open the 4 BBL's sufficiently to gain ANYTHING.
The stock " 2 BBL" Camshaft is waaaay smaller/different than the "stock" 4 BBL Camshaft.
Long story short
the 2 BBL Engine can NOT utilize any of the extra airflow provided by the 4 BBL. , even with better exhaust.

Analogy to a Computer..
Whats the point in adding another 16Gigs of hard drive to a Computer still plodding away with a Pentium 75 Processor ?

At a minimum, add the stock 340 Camshaft right away.

 

Kickdown is not to difficult, lengthen the carb link (slotted piece attached to carb) so it fits the new travel of the new stud position. Done this plenty of times and no problems.

 

header's and duels will net you more efficient HP than your 4bbl...
Then you can install your cam and intake/carb like Moparbob is advising.
What we are trying to tell you is that your going at it backwards too make power

 

Wow, from a carb swap to an engine swap in 13 easy posts. I must argue, from my own personal experience, that the 4bbl swap is worth the effort. I performed exactly the same swap in 1982 on my first Mopar, a 1972 Charger SE with a 400. I swapped the stock 2 bbl carb and intake for a Thermoquad setup from a junkyard Memphis Police Dodge Polara. Obviously it would have benefitted more from a cam swap but you go with what you have; I was 16 years old and made $3.35/hr sacking groceries, I was damn excited to have a car that would sqeal the tires after the carb swap. Later I had dual 2 1/4" exhaust installed with turbo mufflers- @ $150 two weeks pay back then, that made less seat of the pants difference than the carb swap.

 

Do it in steps !! 4bbl, later do a cam swap, later do a dual exhaust

 

Gents -

Well,

A lot of people don't have the time or funds to do a full rebuild in one shot. Doing it in steps makes it easier financially and, frankly usually more "fun" for the DIY'er.


TV is right, one thing at a time, you'll learn more and see what each step does. I'd just go carb/intake, then exhaust and finally the cam. (Order of difficulty) Just keep a list of what your using, and make sure the parts ultimately match.

Archer

 

Look, we are not talking about a "full rebuild" here, and you don't have to spend all that much on a cam.

The factory 2bbl 318 cam is a real POS. I would not waste time putting a 4bbl on top of a 318 stock cam. Besides, why pull the intake, change the carb/ intake, and then have to pull it back off again to change the cam later????

 

Installing headers and duels on my pos 318 it increased performance then later i added cam intake and carb all in steps as i could afford it. and later after that i installed ported heads each time it increased the hp.

 

440 -

Because for a carb/manifild change, you don't have to disassemble the entire front of the engine, pull the radiator, etc. A cam change on an engine that old will also require a new timing chain, lifters, pushrods, and most likely valve springs. It can snow ball into a rebuild fairly quickly, depending on what's found once you get into it.

Having a lot of knowledge at your disposal is a good thing, knowing how to use it in the real world is even better.

You do have the shop manual, right?

Archer

 

Thanks again for the help, i am going to do it in steps as i can not afford to do everything at once. The point of my car is just to be a fun driver as it is my first car. It already has a dual exhaust set up so that will be changed eventually but just not the first thing on my list.

 

what moparbob said IS THE LAST word on the subject, end of story!!!!

 

when i changed my 2 to a 4 barrel i also changed the cam, the exhaust, timing set and added a Lokar kit. **see my sig and photo album.

 

What do you foresee as power gain and fuel mileage with this option?