Personally I would only us a DD as a blow-through.

Also, I love webers but there's no doubting the result is better with an SU if you're going draw-through.

Yeah i just bought a set of twin SU's off ebay with manifold and air box for $65
going to use the manifold to make what i call a plenum and if usable will kit one of the SU's and use it to so win win hopefully. What do you think shifty could i use a 180b sss SU carb for me setup ?

would probably have to use the pair for best results, or buy a 2 inch su so use as a single

how good are 175 CD stombergs for a turbo setup ?

O.k so maybe i can use one of the SU's on me A (A12)series draw through setup. Yeah Maybe ?

In a draw-through turbo setup, a given carb will support considerably more power than it would on a non-turbo motor. That said 'the bigger the better' so if you can go a a larger SU then that's definitely preferable as a smaller one may limit the amount of boost you can run without going lean.

At some point, each part of your system will become a restriction so it is just a matter of being conscious of that and not pushing the boundaries with a single small carb. I'm not an expert but I believe there are 1.75" and 2" SUs and I'd consider a 1.75" your starting point. I think the factory "SU" (Hitachi) carbs are 40mm which would be smaller again. Will certainly work but as to whether it causes a restriction or allows you to put enough fuel into it for decent boost, I couldn't say.

A good way to look at it (and with no real technical knowledge being applied) you might say the carbs came from say a 1.8lt engine. You're only using one, not two, so it's effectively off a 900cc engine and would be pusing its limits very hard on a turbo 1.6!!

Thanks again Yeah i kind of pondered that theory regarding each carb supporting 900cc but wasnt sure if that was relative. O.K so maybe i hunt for a 2 inch SU at some point. Meanwhile back to rust repairs on ute.

su manifold looks good for a pleumn too andy- ive got one for the lpg mobile, and plan on making a small pleumn with the r31 tb on the front and mixer on the front, needs to sit slightly higer than pleumn though as lpg heavier than air usually for will fall not rise in atmosphere.

get a 2inch off a rover, or even a 1.75 of a 260zz or similar

fwiw they used to run single 1 3/4 SUs on drawthrough holdens. they were definitely restrictive in that guise, but in all seriousness, a single 1 3/4 should be enough for practically any boost level you'll be likely to try on an a series - even an a15. People have managed about 220bhp from a single SU in drawthrough (defiitely that much in blowthrough) but the combo producing that much power in a drawthrough would probably jump 20-30 bhp just from a less restrictive carb at the same boost level.

But for an A series 1.5 litres, a single SU is enough. The 'trick' that happens when comparing carb size - i.e. 2 of them on a 1.8 litre motor - well if you actually separate each intake event, you'll see that with cams around std duration, there's very little overlap of each intake even on a 4cylinder, so a single SU feeding all 4 would still do the same as twin SUs split up that only feed 1/2 and 3/4. For each of those carbs, about 50% of the time, they are 'technically' not flowing all the time. Whereas with the turbo, it's flowing through the carb all the time.

Another example of this is comparing carb flow on an inline 6. believe it or not, the sort of carb cfm you'd need to feed a twin carbed six (with 123 fed by one carb and 456 fed by a second) would actually be the same sizing Per carb, as if you ran a single carb (or carb throat) on each individual cylinder. Since on there's no real intake overlap of those cylinders..Not that too many people bother looking at it that way, since there's other advantages of one carb throat per cylinder in NA setups.

Anyway point being - 1 3/4 will do the job, for sure. Not that anyone here (afaik) is trying this, but hypothetically if anyone wanted to run methanol, you can actually fairly easily adapt a holley fuel bowl and float (and they have off the shelf needle and seats that flow enough for alcohol, made specfically for them in fact) to cope with the amount of flow rate that an SU fuel bowl/needle and seat would struggle with. If anyone wants I'll do a few diagrams of it.