Single or twin Carbs?

[Shawn]

G'day All,

Just got the June issue of Practical Classics (yes we're a month behind here!). In the Helpline section there is a question from a 1275 Midget owner wanting to change from twin 1 1/4 SU's to a single 1 1/2 carb. In the answer it is stated that the airflow through the single carb would be slower and that there would be no benefit to be had from a single carb in the place of twins.

I've just done the exact same twin-to-single swap on my 948 and find the single to be heaps better. Feels like much much more power low down in the revs, only slight difference at the top end. I know that there are heaps of other variables between second hand carbs (wear, needles etc) and I'm not about to change back soon, but what is the general feeling?

What I'm asking is - On a standard mog engine are twin 1 1/4 SU's a better option than a single 1 1/2?

And while I'm at it, how could the airflow be slower through 1 carb than it is through two? By my calculations the single would have a throat area of 1.766 square inches and the twins combined would be 2.453. So, at any given point in the rev range wouldn't the engine suck the same amount of air and wouldn't that air have to move faster to get through a smaller area? (or would it flow at a lower pressure through the single..?)

Or am I just dumb, and should quit messing with things I don't fully understand and asking stupid questions...?

Confused!
s

[Stig]

Yeah, I saw the same thing in PC and though it was rubbish. I seem to remember the revered Mr Vizard saying to use a single carb as the airflow is faster than twins. Makes sense as it's got twice the flow and it's also more continuous which will give a better ram effect (hence improved low rev power). And it's less faff than balancing two carbs, I've got a Sprite with twin 1 1/4 SU's and it's not as smooth at low revs as the Minor (single 1 1/2 HIF) and easier to stall if I'm being too gentle with the right foot.

I'd put a lot more faith in Vizard than from some bloke that sells MG parts (was it B&G or Moss?)

[Alec]

Hello Shawn,
one thing that you may not have takeen into account, as it seems have Practical Classics, is that the S.U. is a variable choke carburettor and the engine air flow demand lifts and lowers the piston in the carburettor modulating the air flow speed over the jet.
I belive that a twin set up, all other things being equal, will give a fraction more top end power but that is offset by the extra complication and maintenance required.
Certainly, for a road car the single (of adequate size) is fine. I wouldn't change a twin set up for a single however if that is what I happened to have.

Alec

[Willie]

It is a well known fact that the twin carb set up on the Midgets etc
were just about the most inefficient you could ask for! The design of
the 'log' inlet manifold is awful and a single 1 1/2" rig will be a
worthwhile,and less fussy to set up, answer.

[Peetee]

Makes sense as it's got twice the flow and it's also more continuous which will give a better ram effect (hence improved low rev power).

Exactly what I was about to type.

Vizard is God.

[bmcecosse]

Errr - thereis no worthwhile 'ram' effect on an A series 5 port head ! The thing is - the engine only breathes through 1 inlet valve at a time - follow that back to the inlet port and back again to the carb. So although there is only 1 x 1.5" SU - it is BIGGER than the 1.25" SU - and the only only breathes through one at a time - there is NO worthwhile crossflow from one carb into the other port - in fact - the so called 'balance pipe' actually hinders the air flow - a simple straight pipe twin inlet manifold with no balance pipe works much better ! Compare directly with a Weber - no balance pipe ! If the single 1.5" carb is on a really good inlet manifold - like the MG Metro alloy one WITHOUT the daft water heating connected up - then it will easily beat twin 1.25" SUs on the standard BMC minifold - however those same twin carbs on a good manifold (see Maniflow) with the carbs 'tweaked' as per Wizard's ideas - will out run the single carb. Better still of course is a single 1.75" SU - or twin 1.5" SUs !!