SU Dashpot Oil Recommendation

I am frequently asked what oil to use in SU dashpots.  To me, the answer is clear: Use Mobil 1 15W-50; nothing else works as well.  Nothing.

The earliest dashpot oil recommendation that I can find is in the Owner’s Manual for my 1960 Volvo PV 544, with B16B engine and Type H-4 carburetors.  At the bottom of Page 32, it says: “Use thin engine oil (SAE 10W).”

The next recommendation that I find is for my 1965 122S with B18D engine and Type HS-6 carburetors.  On Page 62, Note 10 says:  “… top up with light engine oil (SAE 20 — not multigrade oil).”

Later recommendations seem to gravitate toward ATF, which I believe to be a totally inappropriate oil for SU dashpots.

In 1972, I purchased a 1-quart supply of SAE 20 oil, and I used that in both my PV-444 and my 122S, with acceptable results, until I ran out sometime about 1990.  When I ran out, I couldn’t find any more SAE 20 that was not multigrade.  By that time, I had junked my 122S and was running my PV-444 with B16B engine and Type H-4 carburetors.  I then went looking for a suitable dashpot oil and tried several different types and viscosities.

Among the oils I tried were: ATF, Mobil 1 5W-30, Mobil 1 10W-30, Mobil 1 10W-40, and Mobil 1 15W-50.  Of all those oils, Mobil 1 15W-50 is clearly the best.

Note that, at 0 deg F, the Mobil 1 15W-50 has a viscosity similar to that of a straight SAE15 oil, whereas at 212 deg F, it has a viscosity similar to that of a straight SAE50 oil.   Somewhere between those two extremes of temperature, the viscosity curve of Mobil 1 15W-50 must cross the curve for straight SAE20 oil.  That is to say: at some normal under-hood (carburetor) operating temperature, Mobil 1 15W-50 will have a viscosity essentially the same as straight SAE20 oil, which not coincidentally, is exactly what Volvo recommended for my 1965 122S.

Not only that, but because the Mobil 1 15W-50 is a multi-grade oil, its viscosity is much more stable than the SAE20, so its viscosity tends to stay near the optimal value in both hot and cold climates, summer and winter.

Although I have not personally used the “Special” SU damper oil sold by several different retail outlets, I have consulted with several people who have.  Those who, on my advice, switched from that rather expensive special SU damper oil to the Mobil 1 15W-50 that I recommend have been very glad that they did.

ATF is a lousy product for this purpose.  It’s not thick enough.  You need a higher viscosity oil to provide greater resistance to upward movement of the piston and needle on acceleration.  That causes a bit of “choking” action on acceleration which reduces the hesitation often noticed on acceleration with cars that have SUs.  ATF also seems to disappear from the dashpots at a rapid rate, necessitating frequent replenishment.  Those who have used the special SU oil have told me that it also requires frequent topping up.

Also, if the dashpot oil is too thin, there will be insufficient damping of the piston on acceleration, and the piston will rise too far too soon, which decreases the vacuum at the bridge (that is, at the jet and needle) which decreases the fuel coming through the jet, resulting in a lean mixture and hesitation on acceleration.

SUs really do benefit from using a somewhat thicker (more viscous) dashpot oil than ATF or similar oils.

I highly recommend Mobil 1 15W-50 for use in dashpots.  It provides the damping needed, and it doesn’t seem to disappear from the dashpots as quickly as lighter oils (especially ATF) do.  So, much less topping up is required.

By using Mobil 1 15W-50, you can adjust your jets a bit leaner and still avoid hesitation on sudden acceleration.  Whereas, with ATF, you will have to run a richer mixture at all times to keep the hesitation down to an acceptable level.

One of my carb rebuild customers (James Schoster, who I think missed his proper calling of Mechanical Engineer) took some videos comparing the viscosities of Mobil 1 15W-50 and the special SU damper oil.  Unfortunately, I cannot post videos on this blog, so I have to settle for a couple of still shots, captured from my computer screen, which still give a pretty good idea of how different the two viscosities are.

First, the “special” SU damper oil:

Dashpot-A

Notice how the special SU oil runs through the syringe in a steady stream.  Timing the video with my stopwatch indicates that the oil level drops from 4 ml to 2 ml in about 8 seconds, or roughly 0.25 ml/second.

Next, the Mobil 1 15W-50:Dashpot-B

Notice how the oil is falling as discrete drops.  Visually, it appears to be roughly 2 drops per second.   My stopwatch said it takes 10 seconds for the level to fall from 4 ml to 3 ml.  That’s a rate of 0.1 ml/second.

Clearly, the viscosity of the Mobil 1 is considerably greater than that of the special SU oil.

My customer (James Schoster) switched from the special SU oil to Mobil 1 15W-50 and was very pleased with the results.  Several others have done the same and all of them have reported improved performance (less hesitation) with Mobil 1 15W-50.

 

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8 Responses to SU Dashpot Oil Recommendation

  1. Being an “Old School” mechanic and Volvo guy myself, congrads on a very well done thr. shft. repair. Can I send you my spare pair of Volvo hs 6’s (throttle bodies only ) for you to install bushings and new shafts? I also have a surplus 19k mile b-20 for sale cheap. I can send photos. How much for the thr. shaft job? Thanks, Dave Cleveland (Ft. Worth, Tx 817/427-4049) I drive my 544 frequently and would like carb improvement.

  2. Stephen Phillips says:

    I was perplexed by the SU oil test versus Mobil 1 15w50 Synthetic multigrade so I thought I would check it with the descendents of the original company… http://www.sucarb.co.uk. They sell 2 types of oil – “SU Damper oil” which is straight SAE 20 monograde oil # BDR125S and “SU Dashpot oil” # BDP125S for pre-war SU’s without a damper ( SAE grade unspecified). The bottle shown in the test is labelled “SU Damper oil” so it should flow like any other SAE 20 at normal temperature – say 70 degrees Fahrenheit. So… it could be that the oil inside the bottle is not genuine SU Damper oil as supplied by SU Carb Co or maybe the ambient temperature was hot – say over 100 degrees F. The fact that the Mobil flows more slowly would indicate that its multigrade properies are working better at more extreme temperatures. The fact that Tom does not have any straight 20 oil left means he cannot do a back to back test test himself other than to say the Mobil oil does not need topping up as much as SU oil. I reckon that genuine SU Damper oil will work like any other SAE 20 oil and because it is monograde it will not be as stable as the modern 15w50 synthetic oil. On a nice summer’s day in England I am convinced that genuine SU Damper Oil will flow like the Mobil 1 15w50 racing oil. Some thoughts for all you experimenters out there!

    • Tom Bryant says:

      A few comments:
      1. The person who did the two tests shown in the photos has assured me that the oil is indeed genuine SU Damper Oil. I have no reason to think otherwise.
      2. The tests shown in the two photos were not done at “extreme temperatures.” Rather, they were made at normal ambient temperatures, roughly 68F.
      3. I agree that on a nice summer day, the flow characteristics of SU Damper Oil and Mobil 1 15W-50 should be similar. Assuming that SU Damper oil is indeed equivalent to SAE 20, oil, then what we know for “certain” is that at 0F, the Mobil 1 is slightly less viscous than the SU Damper oil, and that at 212F, the Mobil 1 is considerably more viscous than the SU Damper oil. Exactly where the two viscosity curves cross, I cannot say, but I would expect it to be closer to 0F than to 212F.
      4. At normal under-hood temperatures, which are probably approaching 200F, the Mobil 1 15W-50 is expected to be more viscous than the SAE 20 oil.
      5. One should remember that the carburetors themselves, due to the vaporization of the fuel, tend to run cooler than their environment. This lower-than-ambient carburetor and damper oil temperature should tend to make the viscosity of the Mobil 1 15W-50 closer to that of the SU Damper Oil while the engine is running.
      6. I recommend that doubters try the Mobil 1 15W-50 for themselves; the proof is in the pudding. I would be pleased to post comments, on either side of the issue, from readers who have actually made the comparison.

      Tom

  3. Pingback: SU Carburetor Tuning | Tom Bryant, Wiscasset, Maine

  4. Geoff says:

    Thank you been looking for a while will try Mobil 1 15w/50 let you know

  5. Matthew Ottaway says:

    3 in 1 Electric Motor Oil for 1/4 H.P. or larger motors is S.A.E. 20 and is widely available. Since 20 wt. is not as stable as 15W50 it provides more fuel enrichment at cooler temperatures, and less fuel enrichment at very hot temperatures (a variable timer influenced by temp.). The accelerator is very responsive (seems to read your mind) with S.A.E. 20. Multigrade 10W40, or Type F automatic fluid leads to an unresponsive accelerator. On very cold mornings, -20 C, the engine will die upon starting with 10W40. With S.A.E. 20, more vigorous damping enriches the mixture in extreme cold to keep the engine running past the point where the engine would normally stall (first 5-8 seconds after initial dry start). I’ve experimented with Lucas 20 Wt. fork oil, but it is evaporating rather quickly in the warmer spring temperatures and I’m nervous that it might gum up the rings as the flash point may be wrong, as is the case with Type F automatic fluid –I’ve heard there were warranty claims back in 1979 from those who ran shops in those days. The manual mentioned that 10W40 was an alternative, but, as mentioned, I was not pleased with the driveability. S.A.E. 20 makes for the engine feel more eager, a nicer driving experience.
    I own a 1979 Volvo, 244 DL, 2.1 L B21A engine, BorgWarner 55 automatic, in Ontario, Canada, where it was originally sold. The body is kept well preserved with Corrosion Free.
    Matt

    • Tom Bryant says:

      Having used both SAE 20 (for 20+ years) and Mobil 1 15W-50 I can assure you that the latter works better in all conditions, both low and high ambient temperatures. Try it and you will see for yourself.

      Tom

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