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Atomizing Fuel: The SU Carburetor

September 9, 2011 by Matt

SU Carb Carbs Carburetor Carburettor E-Type XKE Jaguar

Today we begin a new series upholding the rapidly-fading knowledge of an obsolescent technology: the carburetor. Most modern tuners would say I’m wasting my time, but I maintain there’s a certain je ne sais quoi to carburetion, a patina, a mixture of art and science, that we’ve all but lost in our transition to computerized, sterile, efficient fuel injection. The latter may be superior in every respect, but as with record turntables, Polaroid cameras or arcade games, we do lose something in the progression to new technology: the attendant experience. There’s value in that.

Engineers have devised innumerable methods of combining air and fuel via engine vacuum, the principle behind carburetion. My Datsun 240Z project car features a pair of SU carbs, so it’s obviously the particular design I’m most familiar with. I could get James May-ish and wade deeply into the technical particulars of the design, its operation and the like, but I’ll spare you and summarize up the pros and cons of the fabled Skinner’s Union carburetor, exhibited by many vintage British, Japanese and Swedish cars. The highlights:

  • Simple design. In contrast to most other carb designs, with their multiple barrels, venturis, circuits and jets, the SU has exactly one barrel, one venturi, one float bowl and one jet. As an engineering solution to the problem of providing a correct volume of fuel and air throughout an engine’s rpm and load range, it’s uniquely elegant. Instead of optimizing a carb for one particular set of conditions and band-aiding the rest of the operating range, the inventors of the SU looked at the big picture of the engine’s requirements and worked from there. The result is a simple yet effective design that excites my inner engineer.
  • Fuel metering precision rivaling fuel injection. Another consequence of the developers’ design approach is the incomparable precision of the fuel metering. The SU carb constantly adapts to whatever conditions the engine is experiencing seamlessly, whereas other carbs have discrete circuits for different sets of parameters, and transitioning between them can be less than smooth. So whatever demands are puts on the engine, the SU accommodates them perfectly and delivers just the right amount of fuel.
  • Easy to tune. A side effect of the carb’s simplicity is its ease of tuning—if you know what you’re doing. Armed with a basic understanding of its principles of operation and the knowledge of a few of its quirks, a tune-up is a quick and easy affair, often no more complicated than a few readings with a synchrotester and a few screw turns. No swapping out jets, no delving into the carb’s innards. Simple.

And the drawbacks of the design:

  • Airflow. The secret to the SU’s adaptability and precision is the variable venturi created by the moving piston housed in its signature dome. Unfortunately, a cylinder isn’t the most aerodynamic shape, so the airflow entering the SU invariably strikes a vertical wall before accomplishing anything else for the engine. The air loses energy it could otherwise use to draw fuel from the jet, and the engine expends power overcoming the drag.
  • Sidedraft only. Another downside to the presence of the piston is that it limits the SU to a sidedraft orientation only. Unlike the rival Weber carb, available in sidedraft (DCOE) or downdraft (IDA) configurations, the piston of the SU requires gravity to operate, and thus airflow through the carb is limited to the horizontal. For some engines, say, a Porsche flat-6, this limitation renders the SU all but unusable.
  • Limited aftermarket support. Within the corners of the automotive world where the SU was offered from the factory, there is a great deal of tuning lore, secrets, techniques and the like. But the inherent limitations of the carb (airflow, etc) for performance applications means the SU never really caught on in the wider performance scene. So tuning support lags far behind, say, Holley. It’s a shame, really; as lovely a design as the SU is, it deserves wider recognition.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

Filed under: Atomizing Fuel, Technical


  1. John D says:

    It sounds like the SU carb is perfect for smaller displacement engines with limited hp potential. Ease of use, maintenance, and tuning is a huge plus for a carb. Unless you are looking to get monster power from an engine, it sounds like it would be the carb of choice.

    Has anyone ever tried to use one with a SBC? My guess is it would take a pretty interesting intake manifold, if it’s even possible. Seeing as how the small block chevy engine probably comprises half the market for carbs back in the day, if it doesn’t work with one it probably won’t see much aftermarket support. ;)

    • Matt says:

      You’d be surprised at how many guys race with SUs. Like I said, where the cars were originally equipped with them, their owners have found ways to extract very respectable amounts of power. But yeah, nobody’s going to do a 600 hp blow-through supercharger build with an SU. :)

      I’m sure guys have used them with SBCs, probably because it’s just what they had on hand at that point, and/or they’re more familiar with them. But with the myriad of manifolds and four-barrel Holley-style carbs available for the SBC, trying to get an SU to work is kind of silly.

      On the other side of the pond, though, the SU was offered as original equipment on at least one V8 that I know of: The Rover 215 ci V8:

  2. John D says:

    Sure it’s a V8, but with less than 200hp I think it could still be considered to have ‘limited hp potential’ as I mentioned above. It would be great to see a big ol’ intake for a V8 with 4 SU carbs lined up side by side. That would probably get you some respectable power out of a large displacement V8…but like you said, it would be kind of silly. But I’ve seen worse…

  3. Dean S says:

    Size is irrelevant, with SU and Zenith~Stromberg CD’s, you can tailor a perfect set up from 60 to 600 hp.

    The US engineer who developed the Packard 352 oversaw the Rolls Royce/Bentley L-series 380 and 410 engine developement in the late 50’s. They easily ran just two SU HD8 derived carbs from 1959 to the mid eightees before 4-bbl Solex and then proper EFI systems got phased in.

    The SU and Zenith~Stromberg CD 150/175 carbs are highly adjustable, and as long as fuel atomisation is not biased between cylinders by poor adaptors or manifolding, they work fine on any engine. Two bigger SU or Stromberg Zenith CD175 carbs can flow 530 cfm rated at 1.5″Hg, enough for 330 hp technically, and under boost, probably well over that.

    The limits are main jet sizes, needles and tracking down a multiple matched set in good enough shape. The old days of 125 thou main jets and bigger Rolls Royce/Jaguar/Aston Martin needles allowed a smart engine tuner/dyno operater to make well over 340 hp with three HD8’s on the right V8, V12 or twin cam six so equiped.

    Like the old Jaguar XKE V12 with its four Stromberg Zenith CDS 175’s which were able to flow 1060 cfm , you can make any 1060 cfm equivalent carb from four SU HS6, HS7, HIF38,HIF 44, or Z~S CD 175 carbs. It’s really no different to tuning a four corner Dominator 1050 cfm 4-bbl carb on a hot Detriot V8.

    • Matt says:

      Thanks for the insights, Dean! Fascinating stuff. Love my Z’s SUs.

    • Hoffy says:

      Ive a mk1 cortina 1500cc, for years i was a volvo nut and loved the twin 175cd carbs on early volvos. My latest venture is im planning on fitting a sc12 supercharger to the 1500 and want to use a match set of 175’s i have, the head has been extensively worked, port polished, bigger valves etc, its got a 40/80 cam(yes lumpy). Ill drop the compression but im also hoping of about 20lbs of boost. Plans are to increase float/ capacity bigger jets, but my knowledge part number etc for the jets had left my mind along with my hair. Any help would be great.

      • Matt says:

        Hoffy, sorry for the late reply. I can’t speak to your challenges, but if you’re planning on using SUs with your setup I highly recommend checking out ZTherapy. There’s a wealth of SU tuning knowledge there.

  4. bangernomic Gav says:

    There are four SU set ups for Rover V8s. A company called Boxer makes the adaptor set up. Apparently they’re temperamental….

  5. Laurence says:

    Hello Matt,

    I am thinking of converting my ’69 E Type from twin Zenith-Strombergs (no exhaust gas recirculation manifolding) to triple Z-Ss (they fit easily on the triple SU manifold). In other words, I want to add a third 175 CD-2 Zenith-Stromberg. Some people seem cynical about such a conversion, but I have also read that an E Type with the original 9:1 compression, which is what I have, does run nicely with the Z-Ss as they atomise the fuel more finely than SUs. What advice might you have regarding such a conversion? Thank you for your attention. Laurence

    • Matt says:

      Hi Laurence,

      Jealous of your E-Type! Series 2, no?

      Regarding the carb situation, I wish I could advise you. Personally, from my limited knowledge of Z-Ss, they’re more complex than the beautifully simple SUs, which would seem an argument for the latter. Otherwise, if you’re familiar with Z-Ss and their quirks and can acquire an appropriately sized set, I’d say go with that.

      Good luck!

  6. John says:

    Hello Matt

    Not strictly true about sidedraught only. The original design had heavy pistons relying only on gravity, but these changed in the mid 1930’s to lightweight pistons assisted by springs. This allowed a downdraught configuration with horizontal pistons (e.g. Rover), but it didn’t seem to catch on. Many a Mini engine has used a 30 degree inclined setup to save space.

    Plenty of 6.75 litre V8 Rolls Royces and Bentleys out there with two 2 inch HD8 SU carbs, big enough for 250 horsepower. At full power the pistons lift out of the airstream and there really isn’t much obstruction to airflow.

    • Matt says:

      Very interesting info, John! I’ve seen the angled SUs on Range Rover V8s and Volvo 4-cylinder engines, but didn’t know they went all the way to horizontal. Would love to see a pic of that setup.

      True, at WOT the pistons do lift all the way up, but that regime doesn’t occur all that often except on the race track (and even then, not all the time). Furthermore, there’s still the turbulence-creating, pre-jet “step” that isn’t ideal for airflow either. I think the fact that no top-tier race teams ran SUs speaks for itself… Webers/Dell’ortos/Mikunis were always the carb of choice.

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