Weber Ram Tubes.

The Weber DCOE 40 – 45 carburettor, is known to provide superior performance when fitted with proper ram tubes. Some people don’t realise the importance that the humble ram tube plays in promoting best performance from the Weber side draft. (Or any carburettor for that matter).
If normal air filters are fixed onto the induction end of the Weber minus the ram tubes, there will be a considerable loss of performance (reduced ram effect) from the Weber carburettor.

In general, a very short ram tube length will provide more power at the top end, while a longer ram tube will produce more low down torque, which is what most people are looking for on a road tuned engine.

Ram tubes can also be purchased as a tuned length. The aim with this page is to produce custom made Weber Ram tubes that equal or better the original items… and this is what the writer did… more than three decades ago.

The addition of a well designed cold air box will also make a difference to the performance of the Weber side draft, (Another item that some people consider to be unnecessary)

An old engine tuner made the comment to the writer many decades ago; to always use ram tubes in conjunction with a properly designed large capacity cold air box attached to the correct backing plate, inlet trunking should be 100mm (4″) Dia, and the distance between the ram tube outer rim to air box cover (Inside face) should be about 30mm, that was sage advice indeed, because it really does work, study the photo below for details.

Even with a 1600 or 1700 CC capacity twin cam engine, there needs to be plenty of breathing space at the entry and inside of the air box for good airflow into the ram tubes. If the outer end of the ram tubes are set too close to the inner air box cover surface, there will be insufficient clearance which will tend to restrict the inflow of air, in a worse case scenario, it could starve the engine of air at the higher end of the rev range.

In the writers view, the standard elan Weber air box is inadequate for the purpose, that view is also shared by many other people.

So Lets Discuss The Building of A Set of Custom Made Ram Tubes.

This project will require certain metal working skills, the most essential being old school metal spinning, so it will be essential to have access to a lathe, you will need to know how to silver solder, or how to bronze braze of various components together using an Oxy Acetylene welding outfit. and of course, the requirement to machine various metal parts in the lathe.

There is much joy and satisfaction to be had from this type of light engineering work, and it will be a shame to hide the shiny new ram tubes inside of the air box.

The photo above, illustrates, the DIY ram tubes (Often called air horns and velocity stacks) they all the same thing.

Things that should be noted; The cold air box above was also folded, hammer formed, and fabricated in the home workshop, including the small metal ram tube retainer tabs under the nyloc nuts, were also DIY made.

Note also the extended, threaded posts with diagonal support tabs for the cold air box retainer bolts (1/4″ UNF).

Before we get into more ram tube detail, the items depicted in this article were made by the writer more than thirty four years ago, and they still look as good as the day they were made. (See picture above)

The tooling is quite simple in order to manufacture ram tubes. The method described here is different to what you might find elsewhere.

The large diameter metal die to the right in the picture above is the main ram tube forming tool, the tall press die in the background is pressed into the tubes to form a gradual taper, the taper in both die tools must match, otherwise this method wont work.

There are few photos of this project, and even fewer written notes, if any, about how the writer accomplished this project, after all it was a little over three decades ago, (Written on the back of one of the B&W photos is “6th October 1982)” but I will do my best to share and describe the process.

The photo above shows the completed ram tubes ready for chrome plating. The tail and the ram tube are silver soldered together, which should be hardly noticeable, and if it does show, it can be cleaned up in the lathe with a file, and then polished with fine emery while spinning at high speed in the lathe.

Although Weber do provide measurements for ram tube length, there could be times when a custom length is required. The twin cam that these ram tubes we on, delivered amazing low to mid range torque, in fact, some passengers asked if the engine was a six cylinder, not a four cylinder, such was its performance, it was a really nice tractable road engine.

We are hoping that the new 1700cc Lotus twin cam which is nearing completion, will be equally as good, if not a helluva lot better, but only time will tell.

The ram tubes, looking awesome in polished chrome plating, they truly complete the look and provide the best performance possible from a Weber DCOE carburettor.

Now for a description of the ram tubes in the making.

The steel is nothing more than 1.25″ Dia ERW or (Seamless drawn) mild steel exhaust tubing, Cut off a long piece, enough to cut four ram tubes. Then the entire length is placed in the lathe and a light machining cut is taken off the OD of the tube.

Following that operation, the ram tube lengths are cut off a bit longer than what the actual finished length will be. Then each short length is placed in the lathe and a light cut is taken from the bore, this is to remove the ERW seam that is present in all exhaust pipe lengths.

You might think that this is a lot of work, just for a set of ram tubes when you can just go visit your nearest online weber shop and buy a set. Of course you can do that, but this is not about acquiring shiny auto bling simply by just handing over the cash, this is about engineering some nice parts for your own car, the personal rewards, the compliments from friends, we all know that practical hands on skills are dying, and if I can encourage you to become involved in a project such as this, then I will be happy… and that will be a win for me, and hopefully a success for you.

If you are keen to learn more about metal spinning, there are many DIY videos on youtube, the method is simple to learn.
There are two dies shown in the photo above, the die on the right is the taper forming die… smear a liberal coating inside the tube and on the die, then slip the tube over the die as far as it will go, then it is placed in a ten ton press and pressed into the tube, which will then form the tube into a taper.

To remove the tube from the die, just hammer it lightly with a wooden mallet, the tube will work its way down the taper, but be careful not to injure the metal… that’s why you use a timber mallet, repeat the operation with each tube.

The large, heavy round die is then mounted firmly in the three jaw lathe chuck, the tapered tube is tapped down into the die, then the lathe is started, with the die spinning, a long steel rod with a brass end is used to apply pressure to the inside of the tube, (As you will learn from metal spinning videos) the tube will yield, and will be forced outwards against the curve of the die, and that is how the bell mouth of the ram tube is formed, the radius curve of the mouth is one inch (1″), which is an ideal shape and size for a ram tube outer end, sometimes they are larger… but it depends on many factors.

The tails of the ram tubes are machined in the lathe from suitable size heavy wall pipe (The correct terminology is hollow bar). Once they are machined close to size, particular care must be taken to, machine a recess for the bottom end of the ram tube to sit in, which as we discussed, the parts can be silver soldered together.
The tail is also carefully machined to slip into the Weber body, this part is a bit tricky, the fit is a sliding fit between the carb body and the auxiliary venturi, you might need to study the pictures to understand.

To arrive at the dims for the ram tube tail piece, measure the ID of the carb throat, and measure the OD of the auxiliary venturi AFTER the outer part has been machined to create an allowance … see next photo, you’ll understand more.

Note the machined section of the venturi, the outer surface has ribs left from the die casting process, these must be machined off, and a nice flat machined surface which all allow the ram tube tail to slide neatly into the weber body without being loose.

The custom made ram tubes probably fit better than most Weber factory items, simply because of the attention to detail given during the machining and fabrication. Note also the that rim of the tail extends about .5mm beyond the face of the Weber body.

That is to ensure that when the cold air box backing plate is mounted, the tabs will press down on the ram tube rim and hold everything secure, and preventing any of the parts from vibrating or rattling while the engine is operating.

Above is an interesting view down the throat of the Weber Carburettor, looking at the venturis, and carburettor jets.

That’s it, the end of the project. If you get your carburation spot on, there is no doubt that you will have one very sweet performing Twin Cam Lotus engine. This part of the rebuild project is now all complete and ready to assemble back onto the engine when it is back in chassis, and the body has been replaced back on the chassis. Looking forward to the end result.

Oh, I almost forgot, there’s just one last detail I would like you to know. The seal on the fibreglass air box is a tappet cover gasket off a K series engine from an early Toyota Corolla, they are almost the correct size, all it needs is a short bit cut out and rejoin with super glue, and your good to go.