Keith Calver - Manifolds - twins
"We already know the benefits of twin carbs, and have already commented on the fact that the manifold type was responsible for the early horsepower gains. You only have to look at it to see why - the carbs are almost running a ‘straight shot’ into the intake ports; far more efficient than the early single carb counterparts despite pretty awful castings - especially internally - that really were not at all as efficient as they should have been.
Apart from the factory fitted manifolds of the Coopers and S, there are only a couple of other options. Those fitted to the Sprite/Midget, MG1100 and Austin 1300GT have a lower carb-mounting angle; so can cause installation problems in a Mini. The main problem was the manifold design - basic and not especially efficient. They hadn’t had the benefit of the development that the single carb one has in more recent years, simply because the demand is far smaller. Many of those using twins want it to look ‘original’ or where racing’s concerned have to meet homologation rules, so could be construed as looking original. Mini Spares took some time out when looking at reproducing the original 'ST' twin carb manifolds to make sure the internal finish and shape were maximised given the criteria of a seemingly 'standard' reproduction manifold. Consequently the 'off-the-shelf' performance is much better and more consistent.
For maximum performance out of a twin carb set-up, the best manifold is manufactured in steel by those manifold fabrication wizards at Maniflow. Unfortunately it is made to favour the bigger carbs, so isn't really suitable for twin inch and a quarters - but probably works better than the original cast aluminium ones! BUT - there is a big price penalty. If you decide to run twin carbs, then this steel manifold is a must to maximum performance potential."