SU Carburettor Dampers

Oh, you push the damper in, and you pull the damper out, but the smoke goes up the chimney just the same…

Early TCs suffered a slight flat spot whenever you hit the accelerator after cruising. The twin H2 carburettors did not have dampers until they were introduced in August 1947 at TC3856. The same thing happened with the H4 carbs on the TF, which were modified in February 1954 at TF3495.

So, how do the SU carburettor dampers work, what oil should I use in them, and how full should they be?

1) When an XPAG engine is running at a steady slow-to-medium speed there will be a high vacuum in the inlet manifold, but it is “hidden” behind the (almost closed) butterflies so not much vacuum reaches the carbs. Therefore, the pistons inside the carbs will sit quite low.

If you now operate the accelerator sharply, the vacuum transfers to the top of the piston causing it to jump upwards, allowing a large gulp of fresh air to enter the engine. This has the effect of producing a temporarily weak mixture, which is not what you need for acceleration.

The sole purpose of the damper is to slow the rise of the piston under such circumstances. With the engine sucking hard and the carburettor pistons reducing the size of the hole, the airspeed over the jet is forced to increase, producing a venturi effect that sucks extra fuel. In other words, it generates a temporary rich mixture – just what is needed for instant acceleration.

2) The grade of oil in the dampers is critical. You need more damping when the engine is cold in order to provide a richer mixture while the car is warming up. So, you need an oil that gets thinner as it gets hotter. Therefore, you are better off with a straight SAE20 oil rather than a multigrade 20W/50.

Personally I use 3-in-1 or Redex petrol treatment additive, but you can buy special damper oil from Penrite. (Burlen recommend SAE20 engine oil).

3) Over-filling the dampers is a waste of oil. Each dashpot has a hollow tube inside that the damper fits into. It is only necessary to keep these tubes topped up with oil, leaving a little space for the damper. Any more oil will overflow and it will quickly get sucked into the engine and burnt.

In the case of SU’s without dampers, they still require oil every 1,000 miles, but the oil merely lubricates the guide tube. It has no damping effect.

Barrie Jones

5 thoughts on “SU Carburettor Dampers

  1. Richard Michell says:

    This is a very helpful article. However, it contains, I think, a perhaps common misinterpretation of oil viscosity grades. If an SAE20 monograde is the preferred oil grade then the more reasonable multigrade to compare it with would be something like a 5W/20, not a 20W/50.

    These two oils – SAE20 and SAE5W/20 – would have the same viscosity at 100C but the 5W/20 would thicken less at lower temperatures than the SAE20.

    The SAE20W/50 would be significantly more viscous than the SAE20 at all temperatures in the normal temperature operating range of a vehicle. They would only be similar at very low temperatures (below -10C). The presence of the “W” (or its absence) is important in interpreting the viscosity grade definitions.

  2. John James says:

    The following is posted on behalf of Barrie Jones:

    Perhaps I have not made myself clear.

    Many MG owners (like me) use 20W/50 oil in their engines.

    Therefore they often use the same oil in their SU dampers.

    I believe this is a mistake, because a 20W/50 would be much too thick once the engine gets hot.

    Therefore, I prefer to use a non-multigrade oil such as a straight 20 grade oil in the dampers.

    Remember, all oils get thinner as they get hotter.

    However, a 20W/50 only thins a little, whereas an SAE20 thins a lot.

    For SU dampers, that is precisely the characteristics that are needed.

  3. Eric Worpe says:

    Barrie has highlighted one of the subtle advantages of the SU carburettor over fixed jet carbs that use an accelerator pump jet.
    Fixed jet carbs inject a squirt of fuel from the accelerator pump that is independent of the running temperature, whilst as Barrie has pointed out, using a monograde oil such as SAE 20 allows the SU to enrich the fuel/air ratio in sympathy with the running temperature due to the greater change in the oil’s viscosity over say a 20W/50 multigrade.
    I imagine Barrie mentioned a 20W/50 oil as that’s what many tend to use as it’s easily to hand.

  4. Duncan MacKellar says:

    At the end of the article Mr. Jones mentions SU’s without dampers still needing lubrication of the guide tube every 1,000 miles, and how that provides no damping. Quite true, and he is referring to early TC’s, etc. To add to that, old sports racers like Lotus 15’s and 17’s with SU’s intentionally leave out the damping oil. Throttle response in such racing cars is quick– not so much because of the lack of damping oil as it is due to the extreme richness of the air/fuel mixture. The mixture goes a bit less rich on sudden throttle as the carb piston slams upwards, but very quickly reaches equilibrium in such racers. Always run damping oil in your SU’s in your road car if they were designed for it, especially if you like to drive spryly. I mention this as I have run across numerous internet sources as well as a youtube video or two that say damping oil slows you down. Other folks may misinterpret why racers leave out the damping oil.

  5. Mark Campbell says:

    Very interesting article. Whilst my carbs don’t have dampers they also don’t have any oil reservoir either. Even more strange is that it still has alloy pistons which don’t appear to have been weighted !!
    Any ideas on this one ?

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