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Classic Engines, Modern Fuel

Comments on Topic: SU dashpot oil

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Submitted by BHaskell


The dashpot oil sold by Burlen Fuel Systems is SAE 20 oil. In the book, Paul states that normally 20W/50 engine oil is recommended (page 48). Why the difference (50 versus 20) from the manufacturer's recommendation?



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Submitted by The Author

SU Dashpot Oil viscosity


This is a very interesting question that covers two areas. I will answer them separately.

Oil viscosity in the dashpot.

The damper in the dashpot serves two purposes:

  1. Damp any vibrations of the suction piston. As these are driven by the induction cycle and occur very quickly, virtually anything would damped the oscillations even, for example water.
  2. Make the mixture richer during acceleration. (For those with fixed jet carburetors, the accelerator pump performs the same action). Imagine an engine running at relatively low RPM with the throttle partly open. There is a high vacuum in the inlet manifold. If you quickly floor the throttle, this vacuum causes the suction piston to rise very quickly – over weakening the mixture – until the airflow through the carbs “settles down”. The damper slows this process keeping the mixture rich. The greater the viscosity of the oil the longer the dampening effect and enrichment of the mixture.

    However, this is not an exact science and depends mainly on how you drive your car. If you are a “gentle” driver, pressing the accelerator slowly. You are in effect doing what the damper does. A “heavy footed” driver benefits from a more viscous oil to “smooth” their actions. Maximum power occurs with an AFR of 0.95 (weaker), normal running is best with an AFR of 0.85 – 0.9. Hence if you want rapid acceleration you do want some weakening of the mixture (a thinner oil). Too thick an oil could make the mixture too rich causing smoking or sooting of the plugs.

    The best approach is to try putting different viscosity oils in the dampers and seeing how they affect your car’s performance. In my experience, I have not noticed any big differences.

Oil Viscosity

SAE 20 is a single grade oil which means it gets thinner as the temperature increases. Single grade oils were all that were available until multi-grade oils were introduced around the 1970s.

When you see an oil rated as 20W-50, the oil is a multi-grade which simply means that the oil falls into two viscosity grades, in this case 20W and 50. This is made possible by the inclusion of a polymer, a component which slows down the rate of thinning as the oil warms up and slows down the rate of thickening as the oil cools down.

A 20W-50 oil has the same viscosity as an SAE 20 oil at (I think) 30C. The only difference in the dashpots is that SAE 20 will get thinner than a multi-grade as the engine warms up. Nowadays with multi-grade oils being available relatively cheaply. I use my normal engine oil in the dashpots.


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