Intake manifolds come as part of a dual carburetor kits (in almost all cases). When you order your kit, I strongly recommend you opt for aluminum (as opposed to steel) intake manifolds. Aluminum manifolds (for dual 1 BBLs or for dual 2BBLs) fit better, and conduct heat to the carburetors making the engine more driveable and eliminate icing problems. Effectively, this means that all dual carburetor kits with aluminum intake manifolds do not need intake manifold heat like the centermount kits! I have routinely run my car at well below zero degrees F, with no problems at startup or during operation with dual carbs. Because of this, if you live in a region that hits colder temps, dual carbs are the way to go!
Intake manifold length
Here's a little side note about intake manifold length. Usually, you will not have a choice on manifold length since your linkage clearing the generator/alternator and the space available in the engine bay dictates what you must use. Racecars are the exception! You have a choice on a racecar because of the larger engine bay and/or no engine tin and generator/alternator. In general, short intake manifolds produce more power at high RPMs, and long intake manifolds produce more power at low RPMs. The power peak between 'shorter' and 'longer' is between 300-500 RPM's - not enough to lose sleep over. Long intakes require a shorter air cleaner which is a limitation for air filter area. Short intake manifolds make it difficult to get your plug wires on and off, but they do allow more room for larger air cleaners. So choose your poison.
When choosing your carburetor kit, you also have the type of linkage to consider. Linkage usually falls in one of two categories.
It's important to understand that VW engines get wider as you run your vehicle, due to metal expansion upon engine heating. This usually isn't a consideration until you contemplate the two different styles of linkage. With centerpull, the engine gets wider and the linkage doesn't, so your idle speed can depend on how hot your engine is (the linkage gets tight as the engine warms up)! With the crossbar linkage, throttle position is virtually unaffected by engine temperature and the associated growth/contraction, so it's more consistent in it's operation. I'm a fan of consistent engine operation, so I favor the crossbar linkage style for all applications. Oh! And when purchasing your carb kit, ask whether the linkage is made of aluminum or steel. In my experience, the aluminum linkage can seize or strip, since the down rod threads are so small and aluminum is soft. Steel is King when it comes to linkage! When you are choosing a Carb Kit,
you will be faced with a choice of carburetor brand and model.
Dual 1bbl carburetor systems
Kadrons. These are Brazilian built Solex carbs. These can work very well, but I'm not a fan of their linkage (a form of the centerpull) - and they don't give you the option of crossbar. The linkage ball-joints have been known to break or pop off, and if you pay attention to cars you see with Kadrons, you will note that bread ties, zip-ties, and rubber-bands are frequently resorted to as a quick fix for keeping the linkage together. Only buy Kadrons if you are also willing to keep your glovebox stocked with these Kadron essentials. On the positive side, Kadrons are the largest of the dual 1BBL carb kits, and produce the most power. They are very tunable, and the kits are reasonably priced at under $400, but we don't carry them because of their linkage design faults.
Dellorto FRD's and Weber ICT. These two brands and models are pretty much equivalent. Again, make sure you get crossbar linkage. Parts for these carbs can be more difficult to come by than the more commonly used 2BBL DRLA and IDF. Electric Choke Solex: Solex's work well, but these use center-pull linkage. And there is almost no power increase compared to a stock single carburetor because these carburetors are so small! These are not widely used because chokes are not necessary on VW dual carburetor set-ups, and the inclusion of the electric choke makes this kit expensive. Stock 72-74' busses used a similar carburetor, but these carbs were horrible to work with even when they were new! Only a masochist should consider trying to adapt the bus carbs to some intakes you have lying around.
Dual 2bbl carburetor systems
Weber DCNF. These are fantastic carbs! They are compact and work exceptionally well, but can be tough to find. Gene Berg utilized these carburetors and made them very popular with VW hot-rodders. They are quite small, so fitting them into the VW engine compartment is pretty easy, unlike many other larger carb sets. These are my favorite carb set, but you should not use them for an off road car, even in a centermount application, as they will flood easily (their fuel level control is not very effective). Street and race use only.
Weber IDF. This is the most widely used aircooled VW carburetor. Weber also made the IDA, which was primarily a race carburetor. The IDF's come in 40, 44, and 48mm sizes, and are excellent carbs for street cars. These are also the carb of choice for off road vehicles due to an excellent progression circuit and superior fuel control! These will not flood like the DCNF. IDA's were designed for race applications, but contrary to popular opinion, these do work very well on the street! They do not have much of a progression circuit, so in unmodified form, they are either off (idle) or on (full throttle)! A carb expert (do your homework to find one) can make modifications that will make them more streetable, and these mods are extremely effective. 26-28 MPG on the highway is attainable, even with huge engines, when they are properly set up. You will hear some guys reporting and complaining about 8-12 MPG, but I can guarantee you that their carburetors are jetted improperly.
Dellorto DRLA. is essentially the same carb as the Weber IDF, just made by Dellorto. It comes in 36, 40, 45, and 48 mm sizes. It's important to note that the DRLA carb sizing is 1 step larger than the equivalent Weber. A 36 mm Dellorto is equivalent toa 40 mm Weber; a 45mm Dellorto is equivalent to a 48mm Weber; etc.
Solex 40P11. These are old carbs (they used to use them on old Porsches), and they work very well IF they aren't worn out (and a non worn out one would be a rare find). They are similar to the IDF and DRLA in appearance and function.
There is one golden rule for carburetion systems of ALL types - you must always run air filters! Lack of filtration is a common source of piston ring wear, since the dirt is abrasive and will abrade your rings and cylinder walls. This wear also affects the jets in your carb! It's not uncommon for jet sizes to 'grow' from the wear associated with non-filtered air and fuel. In addition to wear, dirt WILL clog the jets on your carburetors! Carburetors meter both fuel and air, and many of the passages and jets are very small - it doesn't take much to clog one up. When it's clogged, your car will run erratically, and then you have to chase down the problem. Save yourself some grief and run filters, and just avoid these complications altogether. Filters do not limit performance, and don't let anyone tell you otherwise.
Venturi size refers to the narrowest channel in the carburetor throat. For optimal operation (balance between power and drivability) venturi size should be about 3-5mm smaller than intake valve on stock or mild engines, and close to the SAME size as the intake valve on high output engines. You should also note that heavy vehicles must be more conservative with venturi sizing than lighter cars. All 2BBL Dellorto and Weber carbs have changeable venturis, so you DO have some tunability and re-sizing. However, venturis are expensive, so it's best to get close from the outset! Ask what venturi sizes are in the kit you are considering for your car. You want to get in the ballpark from the beginning since re-jetting a set of dual 2BBLs can cost from $100-150, and take a fair amount of time.