OMG is that a real installation Rob?
Why would you do such a thing? Balancing SIX SU carbs? Fuel consumption? Potential for horrendous borewash? I think Iāve found a new definition of āmasochistāā¦ If youāre going to that much trouble and expense, wouldnāt you go for Webers? At least theyāre fixed jet, and have accelerator pumps.
I have seen these siamesed SUs on Minis occasionally as a single alternative to twin carbs.
Whatās the manifold like? If itās separate runners for each cylinder (like Webers) I think itās cool. Remember that SUs only open up as neededāso thereās no requisite increase in fuel consumption or richness. OTOH, having a separate carb for each cylinder in principle optimizes fuel distribution, which should increase both efficiency and performance. There is the balancing issue for sure, and a weight penalty.
The main advantage of Webers is one venturi per cylinder, but there is the concomitant penalty of lack of versatility, particularly when not running flat out. I would argue that six SUs, with a proper manifold, would be superior. IMHO. No need for each to have a two inch throat, obviously.
Iāve never fitted a Jag engine with Webers but Iāve converted three of my Healeys over the years, also six-pots. The main difference Iāve noticed is down to the accelerator pumps, which give much quicker throttle response than SUs. However those were competition cars, and I would agree that for road use the SU is perfectly adequate. But Iād be perfectly happy with two singles, not three siamese jobs!
Probably why you donāt see these double SUs very often on Jags, must not have been a popular idea even for racers.
Perhaps a little overcarbād but the bottom line is they are gorgeous!
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Maybe theyāre just getting bulk discount on the SU rebuild kits?
A LITTLE overcarbed??
They are pretty!
Reminds me of back when I was a kit. Guys would bolt a big holly 750 double pumper on their stock small block expecting magic to happen. Then they would wonder why it did nothing but bog down whennthey hit the gas.
Did your parents opt for any special options?? 
That makes sense. The SU achieves acceleration enrichment by temporarily āstranglingā the airflow and causing the increased vacuum to suck more petrol than air because the piston movement is slowed by oil in the dash pot. The fixed jet carbs simply pump fuel into the immediately higher airflowā¦so quicker throttle response at a richer mixture for full power. But this little quirk asideā¦the cleverness of the SU would seem to be gone with the downdraught versionā¦in the normal position the weight of the piston essentially determines the pressure drop between the inlet and outlet of the carb. And this stays constant regardless of the throttle setting ā¦so the designers can design a fuel flow rate at any position of the piston by setting the needle in the jet in a test rig and applying the fixed pressure. Pull the needle out and measure the fuel flow rate and the air flow rate at each piston height at the fixed pressureā¦and you can set the air to fuel mixture. Clever ideaā¦but turn the arrangement on its side and the constant force of the piston weight is gone so the constant pressure difference between inlet and outlet is gone.!!! I suspect the really soft spring was added to compensate for compressibility of air at higher flowsā¦but Iām guessing thereā¦
My first car, at 17, was my Dadās old Morris Traveller (yes, the half-timbered car), with its mighty 948ccs of brute power. I decided it would be a good idea to remove the standard 1 1/4" SU and replace it with a 2" SU from a Jag. It looked fabulous, dashpot all polished up, big polished aluminium air trumpet and all, looking bigger than the engine. The induction hiss was fantastic - probably not helped by the fact that I had managed, somehow, to fit it to the stock 1 1/4 inlet manifold. I was a bit surprised that the performance level, such as it had been, fell off dramatically. After the second visit to the filling station in the same day, it went back to the original carb. Kids (or kits) are the same the world over, they just work with whatās available to them.
As for the oversized Holleys, Geoff, there are still plenty of folk doing that!
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Downdraft SUs were fitted to a number of cars the big 6 cylinder prevwar MGs among them , post war T Type MGs used semi down draft. The twin choke SUs were for Coventry Climax engines in Cooper F 1 cars, Imwas recently offered a pair, not cheap but they doonāt have a choke ā¦ as standard.
But yes the down draft idea with a piston was not their best idea, WOBentlets had brass sloper SUs but I donāt know much more about them except Bentley owners like thhem.
This was discussed a while ago, I think at the beginning of this thread. Early sidedraught SUs had brass pistons whose weight precisely opposed suction, to create the CV effect. When alloy pistons replaced brass, a spring was added to make up for the missing weight. Presumably, with horizontal pistons in the downdraught versions, piston weight disappears altogether, but could be replaced with a relatively stiff spring. The force opposing suction chamber vacuum would be Hookean rather than constant. Maybe not a concern, or perhaps the needle taper was modified to account for it.
It would be interesting if someone could report on what the springs inside these carbies are like. IMHO.
Ha Ha. Yes they opted for the fat fingers.
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Ah HAAAH. So thatās where the springs came fromā¦I was reasonably certain the SU on my 1953 Morris Minor (903cc OHV beast of a carā¦) didnāt have a spring on the piston. Must have been the heavier piston versionā¦The springs added would have to be āsoftā with a low ākā value to try and ensure almost constant force over the distance the piston travels. Why didnāt the SU designers just reduce the diameter of the piston to allow for the lighter assembly??? . They just added an extra part for everyone to lose or damage for the next 40 years!!!
Over my head, but I think diameter is determined by the size of the throat, which determines the shape of the Venturi. But diameter doesnāt matter because the weight is proportional to diameter squared, while the upward force is the vacuum transferred to the vacuum chamber x the surface area of the piston, which is also proportional to diameter squared (think brakes). The height of the piston could have been increased to make it heavier, though, or the walls could have been thicker. Maybe they did that, but still needed a spring. Or, as you say, just add another part rather than redesigning the piston (more Jaguar like for sure).
The brass pistons of prewar were replaced by alloy for ease and cheapness on manufacture (surprise) At least thatās what I was told by one of the makers of them.
For anyone interested in unusual SUs , a couple of pics. A HD8 Sand cast and what I suppose would be a H 12. a 2 1/2 " twin float bowl, magnesium carby [ still not finished and assembled when I took the pic. But these were made for UK historic racing for Bentleys, ERAs, ALTAs etc. a serious carby. Also available in aluminium for those using funny fuels and not wanting to rain the carb after each raceā¦
The side draft SUs are easy to identify by the oiler in the side of the suction chamber cover.
