Getting the air in

Ever since I first started playing about with old cars in about 1954 I have been intrigued by the design of manifolds. By 1956 I had a Lagonda Rapier, you know, the lovely little 1100 c.c. twin cam one with the engine designed by Tim Ashcroft and made by Coventry Climax.  As far as I was concerned this was the bees knees and the only thing I didn’t like was the manifolding.  My first attempt was a new four-branch exhaust and four Monobloc Amals  This made a world of difference but keeping the carbs balanced was a problem, so I thought that two bigger S.U.s, (1½” instead of 1 1/8”), on a better inlet manifold would be a good plot.  That was very successful, to the extent that today they are still made and fitted to many Rapiers, and are still known as “Barker” manifolds.

Much, much later I had a 30/98 Vauxhall, which had a carburettor on it that owed more to 1914-18 war aero engine practice than automobile engineering, so new inlet manifolds for that too, this time with two 1¾” S.U.s, much better fuel consumption and smoother all round.

Next came a Delage D8 engined special which I put together for competion use.  This had a Smiths Baraquand updraught originally and although that may have been fine for an imposing limousine it didn’t do much for my ideas for getting some performance out of the big straight eight, so again two S.U.s, this time 2” ones.  This was so good that a guy in New Zealand heard about them and wanted a pair and he reckoned that they transformed his car when he fitted them.

So to the B.M.W.s.  It seems that BMW themselves rather liked single updraught Solexes, or twin downdraught ones, or twin updraughts, or even triple downdraughts on the 328.  They certainly didn’t seem to have any firm idea about what was best! Fraser Nash usually fitted three S.U.s when they imported the 319/55.  But six cylinder engines with three inlet ports, like all BMs, have the disadvantage that the pulses in the inlet ports are not evenly spaced due to the firing order. You can think of it as “Suck, miss, suck, miss, miss, miss.” if you like. If you could link the first three cylinders to one carb it would be better,”Suck, miss, suck, miss, suck, miss.” Nice and even.

So which is better? Three carbs and uneven pulses or two carbs and even pulses? Obviously the latter, as evidenced by cars like the Austin Healey 3000, the ones with two carbs were much better than the ones with three, they even won more Rallies like that.  Jaguars were the same, if they didn’t fit three twin choke Webers, two big S.U.s were the favourite. But we cannot actually link the first three and the last three to a carburettor each with a BM, the nearest we can get is to have two carbs on what is known as a ‘log’ or ‘plenum’ type manifold. This has two carbs feeding a chamber from which the three ports feed, this also has he advantage that the mixture from the two carbs gets mixed to some extent in the plenum chamber resulting in all the cylinders getting a more or less identical mixture which is considerably better than any two out of six being too rich or too weak which can result if one of three carbs is badly tuned.  Tuning them is easier too as the engine will run on one carb at a time, the other one being left on tick over mode whilst it’s partner is checked.

Well that’s my theory anyway, so I have made patterns and core boxes and have had casting made and fitted two 1½” S.U.s on both EGH 904 and CMT 948.  These were both very successful and the latest version which is fitted to CMT uses horizontal carbs as I think they do look more in keeping than the semi-downdraught ones which have a more modern appearance.  It allows air cleaners to go under the bonnet as well.  I have also kept the original exhaust manifold this time as I didn’t think the Cabrio needed a four branch like the Sports.  Keeping the original entails cutting off the hot-spot bulges and welding blanking-off plates in their place, but that at least has the benefit that the exhaust gases now have a much smoother passage out.

It seemed a waste of time to only cast one manifold, so if anyone wants to get rid of their rotting Solexes (they do deteriorate don’t they? It must be Mazak Worm!) or old triple S.U.s and try some proper modern S.U.s, now is your chance, see the adverts at the back.(Historic BMW, Issue 4)

David Barker