I have been experimenting with twin SU carbs on my high compression Reliant 850 cc engine, used in a little road sports and off-road trials car (aiming to get up steep muddy hills).
The SUs are fitted to a pair of "experimental" siamesed inlet manifold castings, cast in aluminium, the engine intake ports being in arranged in two adjacent pairs (port configuration is: Ex,In,In,Ex,Ex,In,In,Ex).
The carb end of each inlet manifold has a round port (1.25 inch diameter), to match the SU, this changes to an oval shape mid section, tending towards a figure of eight profile at the cylinder head, where the adjacent intake ports suck at the whole diameter of the small "plenum" this profile gives, there being no dividing wall inside each manifold. The manifold length is about 4 inches from carb face to inlet port face and is about as close to "straight in" as possible, with very little charge turbulence occurring.
The original setup was a single semi-downdraught 1.25 inch SU, sitting on a water heated plenum casting above a 1 into 4 ally manifold. The tortuous path the intake air followed with this has to be seen to be believed, possibly a major restriction on power output, only beaten by the appalling cast iron exhaust manifold and single downpipe. My engine has a 4 into 1 tubular exhaust manifold of increased diameter, to a straight through muffler.
Since fitting the twin carbs it has suffered from poor fuel economy and produced less power than I expected, bearing in mind other mods on the engine. I have found some difficulty in getting the correct mixture. The carbs spit back at mid to high revs, a symptom of weak mixture, despite trying progressively richer needles. The strange thing is, the tailpipe is very sooty and the exhaust blows black smoke but the colour of the plugs tend to show too weak, which ties in with the spitting back.
I suspect that the little intake manifolds are too short and too cold, not allowing efficient vapourisation of the fuel droplets issuing from the jets. This possibly results in large fuel droplets and inefficient combustion, so much of my precious UK fuel is going down the tailpipe as soot.
I am about to change to a single choke downdraught Weber carb on a modified original manifold (discarding the separate heated plenum). This setup has reportedly given very good results on this type of engine, with up to 25% improvement over the single SU claimed by one very reliable exponent. No-one has so far explained why this should happen, but it is possible that the longer path from carb to inlet port gives better fuel vapourisation than my setup. I think that my setup is great for gas flow but poor for fuel atomisation.
I will report the results.
