Carbs Question

[JPSmit]

I'm trying to understand the concept of dual carbs. Am I correct that one carb feeds the front two cylinders and the other the rear? Is the advantage therefore quicker flow or better flow or both? I was at the Bronte Show last month and there was a Spitfire with 4 carbs - one per cylinder? what would a manifold look like for that?

 

[RickB]

Hi JP,
Yeah that Spitfire setup is pretty cool - my friend Pete is thinking of doing that with his. From my understanding it's 4 Harley SU's - I really don't know how they hook up though.
But the picture sure looked cool.

 

[Sarastro]

Keeping four SUs working, adjusted right, and synced--now there's a scary thought!

The use of two carbs is just to get enough air/fuel mixture into the cylinders with minimum flow resistance, which means the straightest line. I suspect there may have been a nod in the direction of satisfying people's expectations, too, when these engines were designed. Car enthusiasts felt that two carbs were better than one, end of story. At least in theory, it may be possible to do as well with one big carb instead of two smaller ones (e.g., the Leyland 1500 on Spitfires and late Spridgets) and probably would be less expensive and more reliable. But two carbs is way cooler.

Oh, yes, the front carbs mostly feed cylinders 1 & 2, and the rear 3 & 4, but the two pipes are linked by that cross piece in the manifold, so it's not quite that simple.

 

[JerryB]

The BMC a motor is a two port intake cyl head and lends it's self to either one carb centrally placed with a plenum and two runners or two single throat carbs (SU or other 1 bbl) each having a runner and a balance tube connecting the runners.

The Triumph you saw with 4 carbs had a 4 port intake cyl head and was a 1300 or 1500 cc motor, and probably used 4 Amal or other motorcycle carbs or possibly 4 throttle bodies and injectors. A 4 port intake lends itself to a single carb with a plenum and 4 runners, 4 x 1bbl, 2 2 bbl (DCOE for example) and more.