SU’s and those dashpots – what oil do we use in the bloody things?
I think most people would be aware the original specification for the damper oil is SAE 20 (mono-grade) motor oil, but try asking at the local servo for a thimble full of that now-day! The SU is designed to utilize the dramatic temperature/viscosity variation of SAE 20 oil (known as ‘Viscosity Index’ or ‘VI’) which results in good enrichment for throttle fluctuations when cold to fast response when warmed up. It is the ‘VI’ that needs to be closely considered when looking for an alternative damper fluid.
Early mono grade oils had a ‘VI’ in the 90’s which means there was a large viscosity variation over the temperature range. SAE 20 starts out at about 200cSt @ 20°C to about 7.5cSt @ 100°C, so it’s easy to see how it will have a dramatic effect on the operation of the carburettor as it heats up. This is why some cars use different spacer/insulation blocks between the intake manifold and the SU’s, or various types of heat shields to achieve the correct heating ‘rise-rate’ and operating temperature into the carburettor bodies.
Most motor oils today are a Multi-grade (i.e. SAE 15w-50) and can have a ‘VI’ as high as 160, so they will not have the correct viscosity variation to provide the desired result. Multi grade oils use ‘long chain polymers’ to provide the multi-viscosity characteristics and under some circumstances (like squeezing past a damper piston at high speed) may experience ‘Temporary Polymer Shear’ where the polymers can untangle & align (like nesting spoons together) resulting in a momentary, but significant drop in viscosity (in this case – when it is needed most not to!)
For SU dampers, it’s the viscosity characteristics, not the lubrication properties that are important, so industrial fluids can also be considered. I have included some charts we use for comparison of various oil viscosities that may help. I prefer to use the ISO rating which is measured at 40°C instead of 100°C so it’s more indicative of the temperatures that SU’s actually work in. SAE 20 falls into the ISO 68 range at this temperature. Automatic transmission fluids tend to fall into the ISO 32 range.
Some promising industrial oils are: (Viscosity Index in brackets)
Castrol: Alpha SP68 (98), Hyspin AWS68 (98), Aircol PD68 (96), Magnaglide D68 (95), Pefecto T68 (96)
Shell: Tellus 68 (97), Vitrea 68 (95), Turbo T68 (101) – VI getting a bit high though
Mobil: EAL Arctic 68 (95), Hydraulic AW68 (95),DTE Excel 68 (97), DTE Heavy Medium (95), Mobilgear 626 (98) – but not the Mobilgear SHC 626 series
The diagram below indicates viscosity equivalents.
– by Grant Rodman, JCCT