Trust me, Jukka knows how to tune the SU carbs, he did it to try and find more flow and it just wouldn't work. Obviously it's someone I'm bringing up out of the blue, so obviously the way I said it was vague and wasn't really giving him enough credit to put it in proper context.

There's real physics (air leaks primarily, but also the difference in pressure drop the venturi will make at atmospheric pressure vs double atmospheric pressure (i.e 15psi boost) and it's not linear. As such it's actually quite lucky that carbs work well at all in blowthrough config. I guess the issue becomes (more broadly) with regard to holleys and blowthrough that changing jets is easy, and cheap. It's ok to file one SU needle, but for any blowthrough requiring more than one SU carb, not many people could competently file two needles identically, which would therefore require either access to a lathe and or buying the needles, and that gets expensive quickly.

Technically the VE of the engine is maxxed out (and then some) with a turbo because it's pushing in more air than the engine can flow by itself. It might be interesting to note, however, that the flow is probably more accurately not described as being 110% (or any >100%) volumetric efficiency. In fact it's more accurate to say (and it pans out in power predictions) that the engine volumetric efficiency remains the same. So if a particular mini engine is 85% at peak torque rpm, then at 15psi boost and the same rpm it'll be close to 85% volumetric efficiency, just that that 85% of the engine volume is occupied by air at double the usual density..

Anyway, point being, that ANY carb acts as a slight restriction. It needs to increase velocity and create a pressure drop acros the venturi which is where fuel is metered through, and atmo pressure pushes the fuel from the fuel bowl into the carb throat. It has to have some pressure drop to achieve that. With a carb too small, you get more pressure drop than you require, and it's costing enough flow to cost hp. But it is good for signal strength and ease of fuel metering across the entire rpm range. A carb too big and you get more power at the top end, but if you aren't careful not enough signal strength down low to produce acceptable manners. For trivia's sake, the usual size recommendations from holley are EXTREMELY conservative and a bigger carb will produce more power across the whole rpm spectrum, but it does make for easier tuning for beginners, which is what sells the carbs to begin with. It's also because carbs are flow tested at 1.5 or 3.0" Hg and that's a higher pressure drop than you'd want on a performance engine, so if you actually flow tested the carbs at half that pressure drop (about what you'd want for high performance) then they 'actually' flow a lot less than their rating, and that's actually the flow measurement that 'counts'.

Obviously a trick with carbs is to have 'staged' barrels - where one opens for low speed running, and maintains good signal strength, low rpm manners and economy, and hte secondary opens up to meet flow requirements and keep the vac in the intake to a more minimal amount for good performance. Similarly the SU does it in 'infinite' increments as the piston rises. The problem occurs that the position the piston rises as might end up being the same (in a blowthrough setup) for higher rpm and little to no boost (say doing 140km/h on a freeway, where it's legal to do so) as it might rise for the amount of flow for mid range rpms and higher boost levels, and as such they'd actually have different fuelling requirements even though the piston (and hence needle vs jet0 positions are identical.. Be aware I'm really 'murdering' the physics behind it to make for a more accessible explanation.

On dellorto/webers, the emulsion tubes are (imho) works of art, unfortunately they just don't seem to 'like' air being force fed well above atmo pressure, and in a more constant flow stream rather than the stop start of individual carb throat per intake port layouts. I suppose, theoretically you might be able to put a hundred or so, and toil away with custom emulsions, but I suspect (esp given that the rings were used on commercial applications) it'll still not be perfect. The fact is carbs just aren't 'meant' to operate over such a wide range of atmospheric pressures (which is essentially what you are creating with a blowthrough under bost)

Plenty of people have tried it and sadly, the restrictor ring just happens to do the trick for the SUs and the weber/dellortos. Yes in both cases it will mean the carb is potentially acting as a slightly bigger restriction than is optimal, but we aren't talking night and day here, and boost will still be the ace up the sleeve - it will produce the power..

I'd also point out that there is probably more info available in text and online about SU carbs and their use on minis and technical info on that than there is on any other carb/engine combo in the known world.

Very clear John and a great point on their design capabilities
without going too astrophysicistical on the lingo :)

I used a Rochester Quadrojet on a 2lts draw through aplication with great results. (after starting with an SU and then a 45mm DCOE weber)

The advantage is small primaries and secondaries that work a bit like SU`s with a flap that opens only as far as the engine air requirement dictate. The fuel needles are atached to this flap.

So it works as a reasonably small carb on the primarys and opens up only to what the engine reqires on the secondaries. Secondary needles are also easy to remove and change or modify.

I was just messing around in the shed yesterday and had a look at the A12 carby and it seems to me that the float chamber has a tube leading to the inlet on an angle - kind of like a breather/pressure equaliser for the float chamber

So, if you made a carby hat/plenum that fit snuggly over the top of the stock carb - whats to stop you running a blow through setup seeing as those breather tubes will pressurerise the float bowl?

All you would need then for the carb to work is a higher pressure fuel pump, return line and rising rate fuel pressure regulator.

Am I over looking somthing here or is the stock A12 carb already setup for blow through? Only thing I can see is that the top gasket may be your boost limiting part but Im sure a new gasket with the help of some skiaflex or similar could help?

Only down side is the fact the surface area of the stock carb (20/26) venturis will only flow as much as a 34mm pipe. Which is a bit restricting.

34mm restricting? say that to the WRC car tuning guys, they get 34mm restrictor plates on
the compressor side of the turbo and still make 400psi of torque and 300+hp :)

Seriously you wouldnt want to use a blowthrough through such a small orifice.
drawrthough is easier and keeps temps down by petrol cooling the SC brades.

Very funny D lol.

Nah, what I was getting at is that you could then adapt this to a 32/36 DGV as they are nearly the same (they have those "balance" tubes for the float bowl pressure) which is good because they are simple carbs to work with/tune an cheap because there's so many of them.

The cross sectional area of both the DGV' venturis equals the flow of a 48mm pipe - so its a lot better than the "34mm' stock carb.

Ive seen plenty of 32/36 leaking without boxing up but if you are set on boxing
something up at worst, choose the nicest looking finish possible :)
Im not a fan of boxed up carbs, I would prefer to go single throttle carb at worst
to minimise wiring in the bay. LPG/Propane is my ultimate goal with water inj.
Petrol is going to go way up for everyday use.

Well if they leak I wont bother - it was just appealing to me at how it looked like a unboxed blow through was easy but the devils in the details.

Yeah, I'm not a fan of boxing up carbs either - to me a carb is like a scientific device used to meter fuel precisely with relation ship to pressure and I like to see that empirical scientific device type look with my carbs (probably why I like SUs over webers).

And as I found out earlier in this thread (and once again) "carbs + turbo = draw through"!!!

diymark can u send me a pic of that stock carbi on an a12 thats setup for blow through with the tube your talking about that would need to see vac/boost so the carbi dont have to be boxed please

thanks nick

One of the biggest issues with blowthrough is sealing hte carb bonnet. Some carbs are good in that respect, some just won't seal no matter what you do, and you are limited as to how hard you can tighten the carb hat before they distort and wreck the carb main body. (2 barrel holleys 'classically' get that on NA apps, because they use the same air filter base as the 4 barrels, but only one half of it has barrels/body underneath to support it, so that outlying 'balcony' warps somethng chronic. A good 50% of the 2 barrel holleys I've seen for sale second hand were stuffed due to that. Ironically to make them seal well for turbochargng, you can always run a second carb bonnet bolt, to the area where secondaries would normally be. it then 'clamps from under the carb air filter base, to the carb bonnet, and keeps it level, rather than trying to warp it out and downward.

The other issue that comes up is a lot of carbs are vac or airflow secondary, and although some or all of them can theoretically work in a blowthrough, in practice many of them don't work well at all, and don't fully open. This is bad enough in and of itself, but if you add to the equation the fact that the factory carbs are too restrictive for an NA 1.2litre (let alone the larger motors) then it's costing you power big time. Generally the 'go' is to look at the carb sizing you would use for a non turbo engine pulling the same rpms for upshifts. In very general terms, it'd be pretty hard to go wrong with a 350 holley for a blowthrough a series.

Another thing to look at for blowtrhough is the acc pump design. On a holley it's a sealed diaphragm pump, so works fine for blowthrough. On those that instead use a 'plunger' or pushrod type deal, often the linkage passes through the top of the fuel bowl cover area, and this can be a location for air leaking. It's (*obviously) leaking from teh fuel bowl area, which is the worst possible place, as you ideally want the pressure in there to be not just higher than boost pressure (post venturi boost pressure at any rate) but as relatively high vs that pressure as is practical without being a massive restriction. The better you can tweak this pressure differential, the stronger the signal under boost, and the less it'll lean out in practice/easier it is to maintain fuelling. If you leak air from the fuel bowl, it will cost big time, and require considerably bigger jets etc, which will just manage to keep it acceptable under boost, but then will be rich to buggery off boost, almost to the point of it being undrivable..

If someone had to run blowthrough with a carb that had a plunger type acc pump, it's probably the only time I'd personally choose a full carb box/enclosure rather than the simpler carb bonnet. Obviously the 'long' answer would actually be to seriously consider using a different carb altogether.

On carb fuel bowl vents - you'll find that practically ALL post 1970 (or thereabouts) carbs on production vehicles will have the fuel bowl vent integral to inside the air filter base or in the mouth of the carb up well above the venturi. The reason for that is pretty simple - emissions - that way any trace amount of fuel evaporating from the carn will be ingested. Then you will see the move around 76 (I think - I can look up ADRs if you want) where the vent will now be RE-routed - and instead of going to the air clenaer, it will be plumbed to a charcoal cannster. Obviously you could simply drill/tap and install a hose barb fitting onto a carb bonnet, then cut and re-route the carby fuel bowl vent to carbon cannister hose to the carb bonnet, and that would provide pre-venturi boost pressure to the fuel bowl and enable fuel delivery under boost.

It's a fairly 'nasty' issue with some carbs. You cna set em up right, then they get a small leak, and you'll get fuelling up to about 2-3psi boost, tnen nothing. and the fuel flow stops abruptly, the car lunges down/forward as if you've slammed the brakes on, boost drops below 2-3psi, and fuel is re-initiated, and it takes off hard, then drys out, looking like a learner driver that can't use the clutch properly.