Originally Posted By: Shannow
Originally Posted By: Shannow
Originally Posted By: CT8
The starting viscosity is most important when considering how long the time it takes to travel from the sump to the oil pump.
Pretty close, and way closer than those who say it's to do with pumped oil flow.
Per the ASTM...
Quote:
The low-temperature cranking viscosity is measured according to the procedure described in ASTM D5293 and is reported
in milliPascal·seconds (centipoise). Viscosities measured by this method have been found to correlate with the ability of
engines to start at low temperature.
The pumping viscosity is a measure of an oil's ability to flow to the engine oil pump and provide adequate oil pressure
during the initial stages of operation.
.
.
.
Because engine pumping, cranking, and starting are all important at low temperatures, the selection of an oil for winter
operation should consider both the viscosity required for successful oil flow, as well as that for cranking and starting, at the
lowest ambient temperature expected.
Supportive of the above quotes, and my statements re pumping/protection, are the results from some cold weather multigrade testing.
Columns to the right are time taken to see rocker arm flow, and the time to full oil pressure.
SAE30 at freezing, took 3 seconds to have flow out of the rocker arms, and 6 seconds to full oil pressure.
at 20F, still 6 seconds for full oil pressure, 15 for rocker arm flow.
The 5W20 at 12F produced rocker arm flow in 5 seconds, and 6 to full oil pressure...
The 5 and 3 for rocker arm flow are all but the same, and there's no way that at 30F the 5W20 is going to outflow the SAE30, as I say, it's a Positive Displacement Pump, espacially in the gallery filling phase.
Drop the temperatures some.
SAE30 at 20F, 10W30 at 9F, and 5W20 at -11F all offer about the same levels of "start-up protection". As per the ASTM quote, pick a viscosity suitable for you ambient.
SAE30 would be OK in my environment, which has probably only seen three 20F mornings in two decades...in the oft quoted Floridan context, the 5W would offer noting in terms of protection over either the SAE30, or the 10W30 that those debates usually pertain to.
(As an aside, I use 5W, 5W30 A3/B4 is my favourite ATM, with less VII than the more popular 40s, and still an HTHS over 3.5).
As the the other part of the quote, "cranking and starting", I've never pulled apart a dry engine, there is always copious oil in the bearings, and pistons/rings.
This is thick, and at the extremes of temperatures, can have the term "breakaway torque" applied to it - my old physics teacher from Calgary described a beetle, SAE30, and a hill as the only way to overcome breakaway torque in his youth.
OVERKILL has mentioned many times "ease on the battery" with the lower "W"s
Same engine as the above tests, this time cranking speed and startability (diesel engine). Diesels need speed to get the compression temperatures high enough for ignition.
The lower "W" grades, as expected give better cranking speeds at colder temperatures....above freezing, VI probably comes into it.
Could the faster cranking speed skew the flow/time results in the first graph?
