Downdraft SU new to me!

(Phil.Dobson) #1

hmm not sure I can get my head around this

(Old Ed) #2

News to me also. Found this link:

(Paul Wigton) #3

Not SUs, but a number of bikes used downdraught Mikunis.

(Robert Wilkinson) #4

I often wondered about the angled ones, but never saw this before. In our applications, the piston weight, plus a small contribution from the piston spring, opposes the vacuum chamber suction to determine piston height. In angled applications, only a vector component of the piston weight is downward. Here, the piston weight doesn’t contribute at all, except in creating some frictional drag in the piston bearing. So I suppose the spring must be pretty stiff. Unlike the constant force of weight, the spring force would increase as the piston moves away from the venturi.

I wonder if there’s a difference in performance?

(PhilW) #5

I was given one of those years ago and could not find what it fit. I gave it to another British car guy to use for parts on the “normal” carbs.


(Roger King) #6

There were also updraught SUs used by some manufacturers.

(Phil.Dobson) #7

indeed robert. i need to do some research on it as it just doest feel/look right to my tiny mind.

(Rob Reilly) #8

You guys don’t know your 1.5 Litre SS cars, though admittedly it was used only in 1938 so there aren’t many left. :smile:

So what’s the story on that Lagonda?

(Roger Payne) #9


Nothing at all unusual about these SU Downdraught Carburetters - at least in Australia, and obviously England, with the most prolific application being for various models of Wolseley.

A quick look at my SU Spare Parts Catalogue collection shows;-

WOLSELEY 8hp 4cyl 1947 had a single D2 to specification 440
WOLSELEY 12hp 4cyl 1938-1947 had a single D3 to specification 370
WOLSELEY 18hp 6cyl 1938-1947 had a single D3 to specification 354
SINGER 9hp and 10hp 6cyl 1938-1947 had a single D2 to specification AUC422 and AUC441
WOLSELEY (Series III) 12/48 had a single D3 to specification AUC370
WOLSELEY (Series III) 14/60 had a single D3 to specification AUC326
WOLSELEY (Series III) 18/85 had a single D3 to specification AUC354
DAIMLER Six-Cylinder had a single D4L to specification AUC454
DAIMLER DE 36/8 had a single D4 to specification AUC458
WOLSELEY (Series III) 8 h.p. had a single D2 to specification AUC440

There are others but the various Wolseley’s are the most prolific, and I dare say the source of the regular swap meet finds- I grabbed one for $5 last Bendigo Swap, mostly for bits and pieces. Not a lot of demand for these downdraft carburetters, with the D2, D3 and D4 sizes having same significance as the Horizontal SUs H2, H3 and H4 sizes.

The Lagonda photo not sure - pre-war, with all post-war Lagondas mostly having multiple H4, and I thought pre-war, usually HV4 - so maybe this is some special - often done. I don’t have a lot of pre-war SU catalogues, but pretty comprehensive 1940s/60s.

(Phil.Dobson) #10

Many thanks guys. I am interested in how they work in the Down draft config so will do some research. No story on the lagonda rob I just noticed it st a goodwood revival meeting.

With best regards

(Rob Reilly) #11

Well, with that clue, I found out that there were two Lagonda V12 racers built by W. O. Bentley for the 1939 LeMans, which finished 3rd and 4th, and also raced once at Brooklands, finishing 1st and 2nd. Near as I can guess, these are the cars registered as HPL448 and HPL449, though there are other replicas about.

Notice the horns in fairings. I wonder why racers would have horns; would the drivers use them to give signals to the pit crew?

(PeterCrespin) #12

Horns were compulsory for use on roads. Technically, the requirement was for “An audible warning of approach” . Most of my circuit and off-road race bikes were roadworthy for daytime use (no lights or battery) so I normally clipped a tiny bicycle bulb horn to the handlebars. It was completely redundant because a racing megaphone or crackling expansion chambers warned people just fine!

As for downdraft carbs, the 2.4 XK engine used a pair of them I believe, albeit not the CV type. Did the 2.4 come to the States?

(Rob Reilly) #13

Yes, we did have 2.4 Mark 1’s here for the first couple of years until the 3.4 Mark 1 pretty well replaced it in this market. The 2.4 had Solex downdraft carbs (also used on Porsche and VW).

( #14

Rob et all

The two V12 Lagonda race cars built for the 1939 Le Mans race had four downdraught SUs (D4L, 1 1/2”, Spec 435). A V1 rocket hit the factory in 1944 and badly damaged both cars but after the war they were rebuilt and raced. They are now rebodied to the original coach builders (Lancefield) designs. A third Le Mans engine was fitted to a special bodied coupe.

The replicas (or specials, if you prefer) are built up from standard V12 cars (that had two downdraught SUs) using original or recast factory competition parts.

The 2.6 litre, six-cylinder, twin-overhead-camshaft engined DB2 Aston Martins, that jostled with Jaguars in the early 1950s, of course have their engine roots in the V12. Both were designed by W.O Bentley.

I too struggle to get my head around the downdraught SU. The SU fitted to the Spitfire Rolls Royce Merlin engine beggars belief.!

(Lovell) #15

How about this…

(Phil.Dobson) #16

Yes, wonderful. I’ve seen a few of these. My interest in the down draft ones is just to understand how the pistons work in the horizontal plane. What keeps the jet entered etc etc.

With best regards
Philip Dobson

(Ian) #17

Do the dash pots hold oil ?
If they do how would you fill it ?

(Phil.Dobson) #18


With best regards

(Phil.Dobson) #19

(Rob Reilly) #20

Since this is the XK Engine forum…

I guess 3 HD8s weren’t enough for this XK120 owner.
Hmmm, makes it hard to see the distributor…