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1/2 a DCOE in use flows 98% of what a whole DCOE does. Because the whole DCOE would be ITB, but the 1/2 is on a common plenum.

50mm throttle body can make 300 HP.

Weird concept DD, I thought the complete carb
would do better than halve but now reading the
ad again I see its detuned for Formula ford from
formula 3 by going from double to single?
Maybe he has two manifolds for it?
The remarkable thing about it, is that it looks
like an early A series but crossflow.
All the parts are in the right place which could
be one of the world engines Nissan looked at.

coz youve got twice the vacuum across it
engine sucks on it harder

This setup would probably be because of class rules. Ive seen it a bit(DCOE`s only using half). It is often because the car doesnt require the extra carburation but the Weber is easy to tune and get parts for.

It is interesting when comparing ITB`s (IR) to single point delivery. An engine with twin 40 dcoe`s and 34mm chokes would perform reasonably well swapping to one 34mm choke on a common plenum. This is because each cylinder takes its turn at using the carb. its just used four times as often, albeit, with a greater retriction right at the top end.
The advantages with Inividual runners is the freedom to alter their lengths easily, having less bends and to prevent charge robbing on cars with high overlap cams.

A car will often perform better( higher average torque) with a carb/s that is/are a bit too small than way too big.

I like its simplicity and thank you for clearing this cast iron contraption up in laymans terms. It increases my enthusiasm to build a simple single su manifold for the A.
Reading the history of Nissan looking for an engine to power the datsun b10, this engine is very similar to the early A10/12 and even the head although Nissan went for the cheaper non-crossflow and wedge CCs, I feel the exhaust side is very similar to the A. Not to mention all the other block featured items.

They did it with race XU1's too... ran triple webers but only one choke off each
As they would flow more than triple single throat SU's

that really is an odd shaped manifold. Round edges between 1 & 2 and 3 & 4 but square between 2 & 3.