If you're familiar with intake systems available for the Type 1, the Type 4 will be really familiar. There are a wide variety of intake systems available for the Type 4, including carbs and fuel injection. These options include single carb(1 2-barrel), dual carb(2 1-barrel / 2-barrel), OEM fuel injection, and aftermarket electronic fuel injection. We'll first look at the stock intake systems and then move on to the aftermarket offerings.
The Type 4 has been unique in the VW lineup in that it has always had either fuel injection or dual carbs. Let's start by looking at the dual carb setup. These motors had two different carbs throughout production. The common model here in the United States was the Solex 34PDSIT, used on the 72-74 Transporters. These carbs were similar in appearance to the 32PDSIT used on the Type 3 series, but are different carbs. Because these carbs were used when the air emissions standards were becoming stricter, numerous circuit/systems are present that clutter the carb. The system used a "central idling system" that allowed idle adjustment from one carb. These carbs are getting to be difficult to find replacement parts for, so there use is limited to original applications. The other stock carb was used on some European spec. VW/Porsche 914 and 411/412. The carb was a Solex 40PDSIT, and it was similar to the Bus carb., only with a larger venturi. These carbs are rare in the U.S., as all 914s shipped here were fitted with fuel injection to meet our stringent smog laws.
These stock carbs can be quite reliable if tuned right and in good condition. This author owned a 1974 Westfalia (1.8) and it was a reliable driver for many years. The performance isn't neck snapping, but for the average Bus, they do work. The key is locating a good set, complete with manifolds, linkage, and aircleaner, and finding someone who knows how to tune them.
Stock Fuel Injection
The stock fuel injection for a Type 4 was essentially the same system, whether it was used in a 914, Transporter or 411/412. These cars came with L-Jetronic or D-Jetronic, but used a lot of common parts. The intake manifolds merged to a box(air distribution box) located on top of the crankcase, to which the throttle body was bolted to. The fuel injectors were mounted in the manifolds(similar to the end castings of a stock dual port Type 1 engine), and the fifth injector(cold start valve) was mounted in the air distribution box.
The stock fuel injection was pretty reliable and powerful if it was properly maintained and not "fiddled with". It has received a bad reputation in the past because most mechanics did not know how to troubleshoot/repair it and the replacement parts are expensive, when they are available. Many Type 4s are still running to this day with these EFI systems and the combination of power and economy is impressive.
First let's look at the single carb conversions available on the market.
Two barrel, progressive opening: The first conversion is the Holley/Weber Progressive 32/36 kit. It comes with a manifold that replaces the air distribution box, carb, air cleaner and misc. hardware. The manifold slides in the stock intake runners with the use of Type 1 dual port intake boots. This conversion will only work with the stock cooling system, so it's use with an upright converted engine is out.
I can't speak from first hand experience of this conversion, but the jury seems split on it's use. A lot of people have converted their Bus from the stock fuel injection to this carb kit and they say it works great. Other individuals have had much difficulty tuning the carb, especially with a modified motor(centrifugal distributor, header with quiet muffler, big bore, etc).
Two barrel, sync. opening: A popular option on Type 1 engines is a single carb, like an IDF or DRLA. The only option I've found is a kit from FAT Performance for installing a single Weber IDF/Dellorto DRLA carb kit. This kit was designed to be used with one of their 911 fan upright conversions. It sits way above the 911 fan, so it's use in a street car is out. This kit will only work in an open engine area car like a desert racer, sandrail, Baja Bug, or dune buggy.
This brings us to the question that a lot of people have asked: Is there a single carb kit that I can install in my street Bug with my upright Type 4? Currently, the answer is no.
You'll get more power, better gas mileage, and a cooler running engine thanks to dual carbs. The fuel/air distribution of a single carb is not anywhere near as efficient as a dual carb installation, so this author recommends running dual carbs or an aftermarket PEFI conversion, which brings us to our next section.
By far the most popular form of carb was the dual carb conversion. The conversions can be broke down into two categories: single barrel(throat or venturi) and dual barrel.
Single barrel dual carb conversions are available from both Weber and Dellorto. The Weber kit offers their 34ICT carb and the Dellorto kit uses the 34FRD carb. Like the progressive 32/36 carb, I've heard yay and nay on these carb sets. Some have had a lot of difficulty in getting them tuned and once they were tuned, they got poor mileage and lackluster performance. Others have said that it was the best thing they've done for their vehicle, and tuning it wasn't a problem. Their dimunitive size means they aren't the way to go for more power, but they are an option to replace the stock carbs or EFI.
The Kadron-Solex carbs are a great setup for the Type 1 engine, and the thousands of VWs running this system prove that. FAT Performace shows a dual Kadron set of manifolds for the Type 4 in their current catalog. The performance would be better than the 34ICT/FRD carbs and probably on par with the Euro. spec. 40PDSIT carbs, seeing as the throat diameter is the same, but the added expense of making a linkage would make it more reasonable to purchase a set of dual 36-40mm two barrel carbs.
Dual two-barrels, progressive opening: Although this is not an option currently available, I felt I had to share this find with everyone. It seems back in the '70s and early '80s, there was a dual progressive carb kit offered for the Type 4 engine. I actually found these manifolds at wrecking yard, but missed the opportunity to buy them. I later found an advertisement in an old isssue of HotVWs for such a conversion. I don't know why they stopped marketing it: it could have been that demand wasn't high enough, carb supply was low, or they could have been difficult to tune. This is all I know about this conversion and rest assure, if I do learn more, I will post it here.
Dual two-barrels, synchronous opening: If you are looking for the ultimate carb setup in terms of tunability, power, and efficiency, twin two-barrels is where you want to be looking. I have seen three different manifolds made for using two barrel carbs on the Type 4: Weber IDF/Dellorto DRLA, Weber IDA, and Weber DCNF.
The Weber DCNF series carb the king of the street carb conversions in the 70s. They provide greater power and a fuel curve that closely matches the modified Type 1. However the DCNF's popularity has fallen with the halted production and the difficulty in finding replacement parts. The 42DCNF, the most common version, is a compact carb and fits well in the tight space of the Bug engine bay. The only DNCF manifold I've ever seen was found at eBay and was going for an exorbitant amount. This manifold was a low profile and would be suitable for use in a Type 4 Variant (station wagen/squareback) or in a Type 3 with a Type 4 engine.
For most (99%) street applications, nothing beats the Weber IDF series and the Dellorto DRLA series carb. They both make for excellent street carbs and turn out some impressive numbers in the hand of a skilled tuner. The Weber is still available new and small parts are available from most authorized Weber distributors.
There are at least three different makers of manifolds for running dual IDF/DRLA carbs on a Type 4 (I should know, I own a set of each model). They vary in height, but they all should work fine in most upright conversions. Make sure that you get a good quality linkage and that the aircleaners clear your decklid.
According to what John Connolly of aircooled.net and Jake Raby of Raby's Air Cooled Technology have both stated at the Type 4rum that the DRLAs seem to be better suited for a street engine that needs a wide RPM range and the IDFs are better suited for a more narrow RPM band engine. The only problem they point out though is that Dellorto has stopped production of the DRLA and getting parts has become extremely difficult. They also state from their experience that a 36DRLA flows as well as a 40IDF, a 40DRLA=44IDF, and 45DRLA=48IDF.
The Weber IDA series carb is king of the horsepower crowd. No carb available puts out as much horsepower, or has the potential that the IDA has. If you are looking to quieting the local Detriot iron guys or the little Japanese pocket-rockets, than this is your carb. The IDA is a LARGE carb and is difficult to work with in a Bug engine bay. Most of the time it is necessary to run them with only velocity stacks, as air cleaners will interfere with the body.
I'd only consider the IDA for your intake if you are running a pretty hot Type 4 with a lot of head work and cam. Many of the tuners have been able to tame the IDA and make it a good "street-able" carb, but the matter of fact, it's purpose in life is racing. New 48IDAs are still available, but the kits are now selling for $1200-1500, at this writing. This price is also the going rate for the new aftermarket PEFI, a better street alternative as PEFI will produce more power and better drivability.
Aftermarket Fuel Injection
In the last few years, aftermarket electronic fuel injection has started gaining a following, especially with the availability of programmable electronic control units (ECU) and throttle bodies that bolt to standard carb. manifolds. A complete kit can be purchased from CB Performance, with everything needed to including dual IDF/DRLA style throttle bodies, manifolds, linkage, ECU, fuel rails (logs), wiring harness, fuel pump, pressure regulator, injectors, linkage, and other misc. parts to complete the system. This system is a dependable system, but isn't as flexible as the parts available from Gene Berg.
Gene Berg Enterprises offers a wide selection of parts to install fuel injection on most vehicles. Using standard carb manifolds(dual IDF/DRLAs or IDAs) and linkage from another supplier, the entire system can be designed with parts available from Berg. The Berg setup actually uses pieces made from various maunfacturer. The ECU/harness comes from Haltech, the throttle bodies from TWM Induction, and injectors/pump/regulators from Bosch.
One advantage of the Haltech ECU over the ECU from CB Performance is that the Haltech requires the use of a computer (MS-DOS) and an oxygen sensor. While the engine is running, the user watches the oxygen sensor reading and adjusts the richness through the PC. This allows one to buy one brain and use it and reprogram it as the motor is further modified. The same brain is used for a stock 1700cc as it would for a fire-breathing 3000cc.
The aftermarket PEFI is definitely the best setup. The kits on the market right now use throttle bodies that bolt directly up a carb manifold and usually the carb linkage. Once dialed in and tuned, the PEFI gives the engine more power (better atomization of the fuel into the system), runs smoother, and runs cooler. Gene Berg also stated that his experience with PEFI has shown that you can run a larger camshaft and still have a smooth idle with PEFI. He also warned that valve float will become more of an issue with PEFI, so make sure your valve springs are up to the job.