Mopar Slant Six, "Super Six" Two Barrel System
By Reed Speir ("Reed")
Edit & webpage layout by Doug Dutra (Doctor Dodge)




Super Six Two Barrel Intake Set-upThe purpose of this article is to give the slant six owner a realistic look at what is involved in purchasing and installing a factory Super Six two barrel intake manifold and carburetor on your slant six car. While the focus of this text is the Super Six manifold, the techniques discussed herein also apply to installing any slant six manifold, factory or aftermarket. 
(Slant Six manifold installation information)


What is a Super Six


Donor cars to look for,  here is a  complete Super Six set-upThe Super Six was a high performance option offered on slant six powered cars from 1977-1980 and on vans and trucks from 1979-1980. The most common vehicles to find this package on are Aspens and Volares, but all slant six powered Diplomats and LeBarons and even trucks and vans from this era had them as well. The Super Six is made up of the following:



Intake manifold, carb. and linkage partsThe Super Six was not offered in California because of tighter emissions laws. Super Sixing a California vehicle subject to California’s biannual smog checks can be a problem. Super Six vehicles are harder to find in California junkyards because those cars had to be brought in from another state. The following is a list of all the parts necessary to convert your car from the stock one barrel to a two barrel:




Intake Manifolds


The factory installed both cast iron and aluminum intake manifolds, the cast iron unit being far more common. Mopar Performance used to sell a high performance M-1 aluminum Super Six intake manifold. This manifold had slightly larger intake runner ports, but unfortunately this manifold is no longer available from the dealer. However, you may be able to find one at a swap meet or on eBay.


Die-cast and ebeam welded manifolds have a  seamThere is some dispute about the quality of the factory aluminum Super Six manifolds. The aluminum manifolds installed by Chrysler in the 1970s were die-cast as two pieces and then welded together. Some owners have reported that these e-beam welded aluminum intake manifolds suffer from pinhole vacuum leaks due to poor welds and/or porous casting. If you decide to use an aluminum intake manifold but are worried about these leaks, inspect it carefully. Any suspect area(s) can be filled with epoxy and then covered with a good coat of enamel paint or a powder coating. This will seal up any small holes in the weld joint or casting.


Exhaust Manifolds


Over the years there have been a variety of slant six exhaust manifolds produced, some of which are not directly compatible with a Super Six intake manifold. If you have purchased the intake and exhaust manifolds as a unit or know that they came off of the same vehicle, then you do not have to worry. However, if you are piecing together a Super Six conversion from loose parts from different sources, there are several things you can check to ensure that the exhaust manifold will be compatible with the Super Six intake and carb.


1 BBL "closed"  choke  pocketThere are three differences between the generations of slant six exhaust manifolds: the design of the choke coil pocket, the size of the center intake to exhaust manifold bolt (or stud) and the heated air valve counterweight.


Supersix open back wall choke pocketThe biggest difference in slant six exhaust manifolds is the built-in pocket for the choke spring coil. Slant six exhaust manifolds manufactured through the early ‘70s were built with a “closed” choke pocket. Manifolds with this style of choke pocket are not compatible with the factory two barrel carburetor choke coil. However, if you cannot find the “later” exhaust manifold with the “open” style choke pocket, you can cut the back wall out of a “closed” style manifold to make it work with the Super Six choke coil. A manual choke or home made linkage can also be used to adapt the different exhaust manifolds to the Super Six intake and carb.

 Flat stamped counterweight vs. large-round "later "counterwight

The next difference in exhaust manifolds is the heat riser counterweight. Early manifolds use a rectangle-shaped counterweight while later model manifolds use a larger round-style counterweight.


This area must be clean and flat for a good sealThis is a helpful way to identify the unit since manifolds that have a round counterweight usually have an open choke pocket. While we are on the topic of counterweights, always make sure that the counterweight in whatever manifold you are going to use turns freely and the spring returns the flapper to the closed position. Stuck counterweights can lead to drivability problems and are a hassle to repair. Also be sure that the surfaces where the intake manifold contacts the exhaust heat riser area is smooth and flat.


Automatic Transmission Kickdown Linkage


 Kickdown and throttle linkage for the 2bbl Super Six

When converting from 1bbl to 2bbl you must replace the kickdown linkage. The orientation of the carburetors is very different and these parts will not interchange. When purchasing Super Six conversion parts, always get all the special linkage brackets along with the manifold and carb.


Adjustment "block & peg"  slides along the rod, clamps with the boltThe automatic transmission kickdown linkage serves two important functions: it tells the transmission when to shift into the next gear and it helps control the main line pressure inside the transmission. Transmission line pressure regulates the holding power of the clutches and bands as well as the shift points and firmness between the gears.


Kickdown rod at the transmission lever,  Adjustment PointAs the throttle opens, the kickdown linkage rod is pressed downward towards the transmission, moving the kickdown lever on the transmission case backwards and thus changing the line pressure in the transmission. The kickdown linkage should allow this lever to come all the way forward when at engine idle and push the lever all the way back when the carb is at full throttle. (wide open). If the kickdown linkage is not installed on a Torqueflite transmission, the shifts will happen too soon and be soft with slippage under load. The transmission’s clutch packs will wear quickly if the trans. is run without throttle pressure (kickdown) linkage.


Throttle Linkage


Early rod style linkage vs. cable type for 1bbl (AC cars)Owners of ‘66 and earlier A-bodies will have additional linkage problems because of the rod style throttle linkage on the early slant six vehicles. You have three options: track down a 64-66 V-8 car with the cable style throttle linkage and get all the gas peddle assembly parts needed for a direct bolt in swap, use a later A, F, or M body throttle linkage or get creative with a welder and a grinder. The V-8 throttle cable itself is shorter than the later one barrel slant six cable and fits the best with the Super Six, but in the event one cannot be located there are alternatives, including the use of the 1 bbl cable with a “camel hump” to take up the extra length.




BBD on left, next to the Holley 2280The factory offered two different carburetors in the Super Six package, each with their own quirks. The most common carburetor is the Carter BBD. This is a two barrel carburetor which flows about 280 CFM. The other carburetor was the Holley 2280.




The Carter BBD is a venerable carburetor that was used on Chrysler 318 engines from the mid-sixties until the late ‘80s. It is a reliable and simple carb with only one real drawback: the throttle shaft bushings. Over time the area around the throttle shaft wears out and create a large vacuum leak. You can either replace the carb or have the throttle shaft re-bushed.

 BBD Choke flap link with  Super Six eyelet

To adapt the BBD to the slant six, two modifications were made. The first was the choke linkage piece coming off the choke butterfly valve. The position of the linkage “eyelet” was changed to accommodate for the angle difference in thermostatic choke pulloff coil placement between the V-8 and the slant six.


Modified choke lever, different angle for the Super SixThe other modification is open to some debate. 318 BBDs had 1/8 inch hole(s) in one or both of the throttle blades. Factory slant six throttle blades have no holes. The theory behind this difference is that the V8 needs more idle air flow while the slant sixes does not idle well if holes are in the throttle blades. However, there have been numerous documented accounts of slant sixes running throttle blades with holes and having no problems with idle quality. If you cannot find a slant six BBD, a 318 BBD can be made to work. There are no significant internal differences between the 318 and slant six carbs and the only external differences are the throttle blades and the choke linkage eyelet position. The V-8 choke linkage can be made to work by carefully cutting the eyelet for the choke coil pushrod off of the linkage and re-attaching it 90 degrees clockwise from its initial position.


Choke flap eyelet, V-8The throttle blades are interchangeable and can swap out by removing the two screws that hold it in place. These are special brass screws that are staked in place at the factory. The staked ends must be ground off prior to removing and must be re-staked if they are removed and replaced. If they are not firmly reinstalled they can work their way loose and fall into the motor while it is running and most likely cause serious damage.


 Holley 2280


Watch for this linkage interferance pointThe Holley 2280 was introduced as a bolt-on economy replacement for the BBD on 318s. Eventually Chrysler made it the factory carb for the 318 and the slant six. The 2280 has an airflow of around 250 CFM, and is a direct bolt-on for the BBD. One advantage to the 2280 is that it does not have the same throttle bushing wear problems as the BBD. Similar to the BBD, V-8 and slant six 2280s have different linkages. Also similar to the BBD, the choke flap linkage can be cut and reattached to adapt the 318 carb to a slant six. 2280s also require the use of a thicker carb base gasket between the carb and the manifold. If a thin gasket is used, the throttle linkage of the 2280 will hit the intake manifold and not open fully. The intake manifold itself can also be modified to allow the needed throttle linkage clearance.

 2 BBL Choke Coil

Choke Coil


The choke pulloff coil for the one barrel carb will not work with a BBD or 2280 carburetor. The one barrel choke moves left to right while the two barrel chokes move fore to aft (front to back). As of a few years ago, these units were still available new at Napa, but cost about $30. Try hard to find one in a boneyard first.


Air Cleaner


Super Six Air Cleaner ClockYour one barrel air cleaner will not fit on a two barrel carb. You have three choices: factory Super Six air cleaner, an “LA” V8 two barrel air cleaner, or an aftermarket assembly. Most any 318 two barrel air cleaner will work but not the real early 273 units. These have a funky clamp-on base that needs a special carb air horn (top) to clamp onto. If you have A/C on your slant you may need to hammer a dimple in the front of the air cleaner base. Also, a 318 air cleaner lid will not have that neat “Super Six” sticker on it. You can buy new stickers from or the Paddock.




Other Considerations

 1 bbl Holley  vs. 2 bbl , note fuel line and choke system

One other consideration is the fuel line. One barrel carbs have the fuel inlet in a different location than two barrels. You will have to either use the metal fuel line from the donor car (between the fuel pump and the carb) or fabricate your own. Compare the difference between a Holley 1920 fuel line and a Holley 2280 fuel line:

If you do end up making your own fuel line it is very important that the end that goes into the fuel inlet on the carb be double flared. Double flaring tools can be rented from tool rental stores or can be purchased at most hardware or automotive supply stores.


Purchasing Parts


Minimum parts you needUse your head here. Buy parts that are in usable condition and will work on your car. Do your research and ask the seller lots of questions. Try to find everything as a “set,” hopefully pulled complete off the donor car with all the extra brackets, small parts, nuts, and bolts included. Don’t pay too much for a bare intake manifold. A big consideration here is if the assembly is already off of the donor car. If someone else has already scrounged around in the filth of the junkyard to locate and yanked the pieces for you, it is worth paying more for the parts. Once you start to remove your old manifolds you will appreciate this fact. Just imagine removing the parts in a cold, wet, dirty junkyard, without all your tools. Super Six assemblies turn up regularly on eBay and go for between $120 and $200, but only pay this much for a complete setup. If you are willing to go hunting in boneyards you may be able to score a complete set up for $30 - $60.


Doing The Research


The best tool you can ever buy for you car is the factory shop manual. If one of these exceeds your budget or you can’t find one, a Haynes or a Chiltons manual is okay. Try to buy both. Usually each has info the other doesn’t have. Another option is to try the local library. Many times libraries receive used shop manuals from dealerships and repair shops when the shops no longer need them. When you are converting your car to a two barrel I recommend that you purchase a shop manual for an Aspen or a Volare. This will give you diagrams and procedures that are specific to the two barrel carb and Super Six parts.


Once you have these books, read them! When I was teaching myself how to work on cars I would read my shop manuals in bed at night to put myself to sleep. Read them once, read them twice, work on the car some, and then read them again. By getting yourself familiar with how your car is put together and what is different between a one and two barrel, it will make it much easier to diagnose any problem which might arise.

Here are scans of most of the important shop manual sections:

BBD Overview & Vacuum Ports * BBD Exploded View * BBD Carb. Adjustments * Kickdown Linkage Adjustment


The Project

 Get the manifold surface clean and flat

When you have assembled your parts and read your books you should first do some preparation work on all the parts. I like to do some basic grinding work to remove any sharp edges, burrs or casting flash from the manifolds. If possible, have the manifolds resurfaced. Once all the grinding is done, completely clean the manifolds, inside and out then repaint. You should either rebuild or replace the carb with something of known quality. Face it, it is unusual for a dried-out used junkyard carb to “come back to life” without problems. Inventory all your parts and make sure you have every piece before getting started on the swap.


So, you say you are ready to actually do the swap? Plan on devoting a full weekend (including Friday night) to getting this job completed. Friday will be spent disassembling and cleaning, Saturday will be installation and initial startup, and Sunday will be spent troubleshooting and tuning.


It always helps to have the parts and the motor as clean and in as good a condition as possible. Working in filth is never fun. I like taking the car to a coin-op car wash and using the high pressure soap and rinse settings to de-gunk my engine and engine compartment.


Be sure you have your car running well before you start the swap. If your motor has problems be sure to isolate the cause before you do the swap. If you are certain that the current 1 bbl you have is shot or that there is a confirmed vacuum leak then “dig-in”. If your car is running bad and you don’t really know why, stop and troubleshoot the cause if the problem before doing the Super Six conversion. If you don’t, it will make it much harder to tell what the problem is and whether you did the Super Six swap correctly.


Swapping The Manifolds

 Manifold  mounting  nuts, studs and washers

When you get home on Friday night (or day for you night workers), let your motor cool a little bit, then go out and use a penetrating lubricant to soak the three bolts that hold the exhaust and intake manifolds together as well as the two bolts that hold the exhaust pipe to the exhaust manifold WD-40 is not a penetrating lubricant. I recommend Sea Foam brand or Knock Er Loose. Next, clean the threads with a wire brush. Before you even touch the first wrench you must do two things: soak the bolts again and read the shop manual instructions on the proper way to remove your manifolds. I have included a scan of the relevant pages.


The manifold nuts and bolts have been subjected to extreme heat for about 30 years. As a result, they have almost fused together and will be very hard to turn. This heat has also crystallized the metal, making the bolts and studs very brittle. With patience and finesse, work the bolts out and then back in. If it gets hard to turn the fastener, stop, spray more penetrating lubricant and retighten. Look at the threads, wire brush them, spray more penetrating lubricant, and try removing the bolts again. Sometimes you can get these bolts out without breaking them if you carefully work them back and forth but most of the time you can’t avoid snapping one off. Make sure you have a pair of locking pliers [vise grips] handy for when you break the bolts, and make sure you know the location of a friendly muffler shop that will extract broken-off bolts in your exhaust manifold. It helps to warm the motor up a little bit before you start the removal. This helps get the bolts loose (and keeps you warm). Also, you really should replace those long bolts that hold the intake to the exhaust manifold. Use grade 8 bolts and clean the threads in the manifold as much as you can. If you manage to separate the intake and exhaust manifolds without having these bolts break on you, congratulations! (Direct heat to the bolt threads can help removal). Most of us will not get this small miracle and you end up drilling, re-tapping and replacing the bolts. You should replace them either way. While the manifolds are apart, re-chase the tapped holes. On reassembly, be sure to use anti-seize compound on the threads of these bolts.

Manifold Mounting Instructions * * * Manifold Installation Article

Once you have successfully completed swapping the manifolds and installing the carb, it is now time to start the car and tune the carb. I am assuming that the carb has been rebuilt, or it was in running order, and that all of the internal settings of the carb are correct. If you just yanked the carb off of a junkyard car or bought it at a swap meet without knowing its history, consider rebuilding it. Not only will you know the condition of the carb, but you now know the carb intimately inside and out.


That being said, prepare to start your engine! Before you turn the key you must ballpark the curb idle mixture on the carb. This is done with two screws on the front bottom of the carb. Screw both screws in (clockwise) until the screws lightly bottoms out, then back out (Counter-clockwise) 2 1/2 turns. This will get you a rough estimate of an idle mixture setting where the car will run. You will fine tune it later. You will probably have to crank the motor for a few seconds in order to get gas into the float bowl. You can also use a small squirt bottle filled with gas to prime the float bowl in the carb. This saves some engine cranking. If all was installed properly, it should not be very hard to start the car. (If you do have a lot of trouble, jump to the troubleshooting section of your shop manual and find the problem.) Once the car is started, immediately check for gas leaks and listen/feel for vacuum and exhaust leaks. Exhaust leaks are easiest because you can hear them and when you pass your hand around the manifold you can feel them. Vacuum leaks are harder to detect. I strongly recommend that you purchase a vacuum gauge from a parts store, or at least rent/borrow one. They are very useful and pretty cheap. You should have around 18-20 inches of vacuum, with a steady signal. If the vacuum is low or if the needle jumps around a lot, you probably have a vacuum leak. The most likely places for both vacuum and exhaust leaks are where the cylinder head meets the manifolds and the manifolds bolt together. Another place to look is for extra hose fittings on the carb that may have been left unplugged.


Once you have verified that you have no leaks, let the car warm up to normal operating temperature. Set the curb idle to 700 RPM in neutral, and the high idle to 1600 RPM. The BBD and the Holley 2280 have very similar methods of setting the fast idle and the curb idle. Curb idle is adjusted by a screw that contacts a stop on the driver’s side of the carb and the fast idle is controlled by a screw that contacts the fast idle cam (a plastic or metal tab with little steps on it which flops around on the drivers side of the carb). Always set the fast idle with the screw on the second highest step of the fast idle cam.


Once the car is warmed up and idling at 700 RPM, turn the mixture screws (remember the ones you turned out 2 1/2 turns?) inward (clockwise) 1/4 turn at a time, adjusting both screws the same amount. Turn one screw a quarter turn, then turn the other. Do this until the engine starts to run rough, (clockwise = leaner) then back the screws out 1/2 a turn. Reset the idle speed to 700-750 RPM, and adjust the mixture again.


It is not unusual to have one of the idle mixture screws set at a slightly different position (number of turns) for the best idle quality. Start with both screws in the same position (2 ½ turns off the seat), adjust both to same amount to find the best idle, then try to “fine tune” by moving each screw independently to see if idle quality improves.


Kickdown Linkage


The kickdown linkage bracket on 67 and up cars is held in place by nuts on the bottom of those long bolts between the intake and exhaust manifold. There is also one bolt clamping on the lever, down at the transmission case. The simplest way to remove the transmission linkage rod is to loosen the lever bolt down on the transmission and slide the kickdown arm and linkage off the mating shaft.Kickdown Linkage on Transmission Lever

Make sure you keep track of the spring that is sandwiched between the linkage and the kickdown arm on the tranny.

Once you have disconnected the linkage from the transmission, you can disconnect it from the carburetor and from the bracket that is bolted to the intake manifold. Installation of the kickdown linkage is the reverse of removal. Make sure you assemble the new two barrel kickdown linkage according to the diagram and that you adjust it properly as well. Note that there are a number of different kickdown lever lengths so it is best to use one from a Super Six setup in order to keep the ratios correct. I have included a scan of the shop manual procedure for adjusting the kickdown linkage.


Exhaust Line


The Super Six will work fine with your stock exhaust system, but remember that the factory Super Six cars had a 2 1/4 inch exhaust pipe as opposed to the 1 1/2 to 1 7/8 sized system that came stock on most 1 bbl slant six cars. The factory did this for a reason: the two barrel carb flows more air into the engine, and thus necessitates the ability to pass more air out of the engine. If you Super Six your car it is highly recommended that you also upgrade your exhaust system.


Dutra Duals, front section with modified factory manifold for the rearWhen it comes to upgrading your exhaust system you have a variety of options: keep the stock exhaust manifold (clean-up porting work recommended) and simply install a larger diameter exhaust header pipe, buy a single outlet header from Clifford Performance or Mopar; buy a set of dual exhaust headers form Clifford performance, or buy a set of Dutra Dual exhaust manifolds from Doug Dutra (or just get the single front section and modify your rear manifold as per the instructions on the Dutra Duals website). Each of these options has advantages and drawbacks, usually having to do with cost and ease of installation.


The absolute cheapest way to go is to leave your stock exhaust system alone and bolt it up to the new manifold. This will cost you nothing, but you won’t get the full benefit of the Super Six. The next cheapest would be to port out the stock exhaust manifold and install a larger diameter exhaust pipe. For single exhaust I recommend 2 1/4 inch pipe all the way back. Headers from Clifford are expensive, hard to install, leaky, and may crack or rust out on you. Headers do offer the most significant boost in performance but they don’t have provisions for the choke pulloff coil and intake manifold heat. Your can buy either single outlet headers that maintain a single exhaust pipe, or go to dual outlet headers which let you have either dual exhaust or a larger single exhaust pipe. If you choose to go to dual exhaust, I recommend 2 inch pipe all the way back. My personal favorite is a set of Dutra Duals cast iron exhaust manifolds. These manifolds offer several advantages over a set of tube steel headers. First, they are cast iron, just like the stock ones, which means they won’t rust out and don’t leak as easy as headers. Second, they are designed and produced by a slant six enthusiast, so you get a product guaranteed to fit, and you help support the slant six community. If you choose to buy only a front manifold and modify your stock manifold, you can keep the choke coil pocket and intake manifold heating system, something to consider if you live in a cold climate. Again, if you go with Dutra Dual exhaust I recommend two inch pipes off the manifolds.


Fine Tuning The Engine


Exhaust leak will leave a black mark of sootAfter setting the curb idle and mixture, you must test drive the car and fine tune the mixture and timing settings. Most slant sixes run best with the timing set at around 10-12 degrees before top dead center static advance. An alternate method to finding the optimal timing setting is to adjust the timing to achieve maximum vacuum at idle.


One of the most common problems encountered after swapping to a Super Six is a noticeable bog on acceleration. A bog can be caused by non carb related problems such as retarded timing, blown vacuum advance pod or late cam timing. If the engine ran without a bog prior to the swap, then the bog is usually from a lean mixture condition caused by a vacuum leak, misadjusted carb metering rods or a faulty accelerator pump. Vacuum leaks are commonly found where the intake manifold connects to the cylinder head,Warped manifolds will have trouble sealing  together

 and exhaust leaks are commonly found where the intake manifold bolts to the exhaust manifold.

See the scans for the proper method of adjusting the carb.


Congratulations, you have now Super Sixed your Slantsix! We hope you will continue to repair and improve your Slant Six car, ensuring that you will be able to enjoy it for years to come.


Copyright © and Reed Speir, 2003.
No reproduction without permission.