FITTING WEBER DCOE’s TO A SPITFIRE, HERALD OR GT6

David Frew, Cape Town Triumph Club

This is a very popular conversion for Triumphs using the ‘SC’ 4-cylinder engine as well as its bigger 6-cylinder brother as used in the GT6 and 2000 saloon. Webers are most commonly associated with the tuning brigade, however, contrary to some belief, a set of Webers will not turn a daily driver into one that is an unmanageable beast to drive. In fact, because of their more efficient design, they will tend to produce a more flexible engine, with a smoother idle, when set up properly for a specific engine. A properly set-up Weber system will be on par with fuel injection, and they are quite popular with the home tuner due to their ease of setting up. They will also not lose their tune overtime (contrary to what is commonly believed), and I personally think they are better than S.U’s in this regard. 40 DCOE’s are the most common fitment for the Spitfire, Herald (single or dual 40’s) and the GT6 (triple 40’s) while many Spits have also use a single 45mm. (Some have also used the DGAV downdraught carburetor). 42mm DCOE’s were also made, but are quite rare. Obviously the dual or single set-up works best on Spits with the 8-port cylinder head, i.e. MkIII Spitfires onwards which use a modified version of the Works aluminium 8-port head. This results in the desirable ‘single feed’ system, i.e. one choke per cylinder. Mk1 and MkII Spits have the 6-port “siamesed” head, though a lot of early Spitfires have had 8-port heads fitted, as the first step, before any further tuning is done. All the GT6s used a 12-port head, so triple Webers fit the bill there. (For interests sake, the cylinder head on the MkI was not as good as later models – in 1968, the MkII was given the same head as the TR5/TR250, a much better breathing item. Also the manifolds for the MkI are different to the later models).
 

8-PORT HEAD OF THE SPITFIRE, MkIII ONWARDS IS ESPECIALLY SUITED FOR A DUAL WEBER D.C.O.E SET-UP…
  
 
...WHILE THE EARLY 6-PORT HEAD WILL HAVE TO MAKE DO WITH ONE.


MANIFOLD(S):
The major expense (apart for the carbs themselves) will be the intake manifold. Doing it the easy way, one would go and import a nice new manifold from the UK or USA, and you will be able to bolt the thing on in 5 minutes. The current cost of this approach will be in the region of R3000-R4000, which is why I investigated another approach. Aluminum manifolds for the Nissan 1400 (dual 40mm) are readily available, are relatively cheap (~R800) and the relative distance between centers of the 1st and 2nd (3rd & 4th) intake ports is exactly the same as the Spitfire. This was the closest manifold I could find off the shelf. The crunch comes when you have to cut the manifold in half (!) because it is too long (from 2nd to 3rd port). The oval ports of the manifold are also too large – the major axis is 34mm, compared to the Spitfire head port diameter of 31mm – so the top and bottom of each port will need to be filled with weld before the manifold is bored to the correct diameter of the head. It is best to drill the hole in the manifold for the locating dowel before taking it to the machine shop so that they have a reference to bore the ports to. The flange on the head side needs to machined to at least 1mm higher than exhaust manifold flange – you can take more off if you please, but things start to get a bit thin (and the aluminum is extremely soft. On my car the exhaust manifold flange is 9mm thick and the inlet, 10mm). Other than that it was just a lot of work with a file to get it to fit between the exhaust manifold. The outside of each half of the Nissan manifold needs to be taken back to the original profile (of the S.U. manifold), but on the insides not too much has too be removed to get it to fit. In some parts the profile of the manifold is larger than the S.U. flange, so some material has to taken from this area of the manifold to create a flange for the manifold lugs. A lot of work is needed to blend the machined ports nicely into the manifold, removing the dips and furrows created by the extra weld. Start of with a rotary file on a drill to get the most obvious blemishes out, avoiding not only sudden curves or dips, but sudden changes of cross-sectional area. After the basic shape of the port is defined, it can be finished of with finer and finer emery (also with an electric drill), concentrating on the entire flow of the manifold – as the surface finish straight out of the cast is not the best. (However, a polished finish is best avoided). The outside runners have a slightly longer and more restrictive path than the inner ones, but they can be improved by filing and smoothing out some of the outside wall, improving the path of the fuel mixture.


After this you can opt to weld the manifolds back together (and pray that everything stays nice and straight!) or just run two separate ones. They are also much shorter than the S.U. manifold – 90mm compared to 130mm. For the GT6s, I’m not at all sure how close an after-market Nissan 6-cylinder manifold would be, but I’m sure its worth some investigation. Manifolds for GT6s are available overseas.

THE TWO HALVES OF THE NISSAN 1400 MANIFOLD, WITH THE SOFT-MOUNT “DONUTS” FITTED.



TRIPLE WEBERS ON A (MUCH MODIFIED) MkIII GT6



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