One of the greatest drawbacks of most high-performance Volkswagens I have seen is the excessive amount of noise they emit. The exhaust is the obvious first source of this audible annoyance, but unless your ultra-high-performance VW is turbocharged, which would make it quite livable with noise-wise, it probably has twin two-barrel carburettors, which, to my ear, are unbearable when you really put the boot into them
Of course, this suits a lot of people who want a "sporty" note to their machine, and who like to hear everything the motor is doing. The trouble is, everyone else does as well, so it is definitely an anti-social, if not an illegal act, to drive hard a highly modified Beetle around the streets. There is also the point that very loud VWs give the marque a bad name. Most people already have an impression of air-cooled VWs as loud and uncultured, and believe it or not, the majority of normal people (that is, non-VW fanatics) prefer motors which are refined, but which make their presence felt in a restrained sort of way.
The type of twin carburettors which are generally used on modified VWs, such as the Weber IDF and IDA, have in most cases been designed for competition. In racing applications, the priorities are high power and good reliability under rough conditions, so elaborate anti-noise components are generally out. When you buy a twin-carburettor kit, the only filters available off the shelf are also specified for competition. They are very simple., consisting of two pressed-metal sheets sandwiching a paper or foam filter, but. this doesn't prevent them from being damned expensive. In the absence of anything else, people also use "wet socks", which are sock-shaped pieces of oiled urethane foam strapped to the intakes of the carburettors. They are cheap and simple, but even louder than the paper filters. Any filter arrangement which provides effective noise reduction for a street car obviously requires a bit of owner fabrication.
Why, then, are these filters, or no filters at all, so noisy? The noise level can approach that of an open exhaust, but is usually more offensive, since the creators of the offending decibels are more in line with the ears of the cars occupants. There are a number of reasons for this high level of noise. Firstly, there is more than one source of noise with multiple carburettor throats. Each cylinder does its own thing on the induction side, so you have what sounds like four single-cylinder motors. Secondly, air going through individual venturi carburettors pulsates very strongly, especially at full throttle. When you join together the four intakes, as with a conventional single carb motor, you kill two birds with one stone. You reduce the sources of noise to one only, and because you have one column of air shared by four cylinders, the violent pulsation’s are damped out. The frequency of the pulsation’s going through the filter is four times that of individual filters and the pressure swings are nowhere near as high. A well designed filter enclosure can further reduce noise emission by certain scientific means, but I wont go into those.
You don’t need to pass all that air through a carburettor to achieve noise reduction this would defeat the purpose of a high-performance car the air inlets of the multiple carburettors can be connected together and routed to a single filter.
Where racing-type carbs have been fitted to street cars by manufacturers, you will notice well-engineered ducting to a central air filter. The Alfa 33, another car with a horizontally-opposed four-cylinder, uses what looks like Weber IDFs and has beautifully cast channels which bolt to the carbs and curve around to a central filter chamber. It's unfortunate that the Alfa has the opposite cylinder offset to VWs, otherwise their filter system might have been a goer for our application. They’re certainly quiet enough to be considered palatable for the general public.
Fortunately, such a system for a Beetle can be built up. Ill give the credit for thinking of the idea first to some German tuning companies. Riechert made their own filter to fit onto a set of Solex ' PII-4s, and Oettinger used a stock late-model Beetle filter turned around for use with Solex 40 PDSITs. (Isn't it great when you can re-use the bits that you bought with the car instead of throwing them away ?) Powertune Engineering also used to market a very neat cast aluminium filter housing for their Weber 46 IDA's. On my Beetle I have modified Oettingers design and used a 1974 Beetle plastic filter housing. It was necessary to saw off the long intake snout and turn the housing end-to-end and around, so that the top then faced to the rear of the car. I cut off the upper part of the filter base, the bit which used to clamp onto the standard Solex, and riveted it to a new filter base which I folded up out of sheet steel and welded up. This new base sits next to the fan housing and has a circular fitting on each side onto which flexible hoses attach. These hoses in turn lead to the carburettor inlets. I have Weber DCNs and was very lucky in that mine came with some nice cast aluminium pieces which accept a piece of hose. If you don’t have any such pieces, it is possible to modify existing individual filters to accept a hose fitting. Obviously the filter elements will have to be replaced with block-off panels to seal everything up. The hardest part for me was making the sheet metal base accurate enough so that it sealed well to the plastic parting face of the old filter. I ended up using some sealant in some of the cracks. I have had a Uni Filter foam element made up to keep flow restriction a s low as possible, and to save on costly replacement filter elements.
There is an inevitable reduction in flow whenever extra plumbing is added to the inlet system. Just how much with this set-up can’t tell you, but the car seems no faster when the flexible hoses are off. What I can report, on the other hand, is that the noise is immensely reduced when they're on, wore than you would believe possible.
There is another great advantage to using Wolfsburg engineering. The stock VW filter comes with a vacuum diaphragm actuated flap which mixes hot and cold air. The purpose of this system is something which is not well understood by many people, judging by the number of them I’ve seen disconnected. Ignorant people think it looks like an emission control gismo, so Ill rip it off but the effects of this system are all good. A little thermo-vacuum valve in the filter housing feeds vacuum to the diaphragm at the filter entry, so that the temperature in the housing is maintained at a set level. When the engine is cold, hot air only is introduced, so a quick warm-up is achieved. As soon as the correct temperature threshold is reached, more and more cold air is bled in. Under full throttle, depending on the type of valve fitted (there are two different types), you get pure cold air, so that peak power is unaffected. The net result is that the carburettor under normal running conditions only has to cope with air at one temperature, so it can be more closely tuned to that temperature. You see, the hotter the air that a motor breathes, the thinner it is and the less oxygen it contains. Therefore less fuel should be metered to hot air. The carburettor cant sense any temperature difference fuel injection can, by the way), so it-s best to keep the inlet air temperature constant. The advantages in drivability when this system is maintained are most noticeable while the engine is still cold, and the engine idle quality is far less dependent on temperature. Fuel economy and emissions are both improved also. You should be taking advantage of this excellent engineering ! Your Beetle deserves to have refined driving manners as well as a refined sound.
The icing on the cake is that the whole system looks as though the factory designed it that way. Undoubtedly the factory would have designed it that way if ever there had been an ultra-high-performance Beetle made in Wolfsburg!
Rod Young
P.S. I hope this advice doesn’t fall on deaf ears.