Quantifying drag from gearbox lube.

[jray3]

In trying to diagnose poor efficiency of a converted Miata, Seattle EVA members have zeroed in on transmission grease (which is actually heavy oil; 80w-90 in my converted Karmann Ghia). The i-MiEV uses ATF, which is generally thinner than what's found in manual transmissions, but I haven't found an actual viscosity spec for "Mitsubishi genuine Dia-Queen ATF SP III"
Point is, I wonder how much this stuff thickens up in the cold and how much that could contribute to reduced winter range. A stick-on block heater on the tranny may offer better 'bang for the buck' than some of the battery heating options that have been discussed. The only ways I fathom that heating the transmission up to say, 90 degrees F to start the day could cause harm would be if the hot spot gets too hot too fast and adding a daily thermal cycle could cause metal fatigue, or thermal variations within that transaxle could cause friction between close-tolerance parts...

Anybody collected data that could zero in on the effect of a cold gearbox (without the effect of cold batteries, tires, heater use, etc..)?

 

[siai47]

You could try a lower viscosity oil and see if that helps. I have used Amsoil "Syncromesh" transmission fluid in an older GM pickup to try and overcome some problems with the transmission in cold weather. It seems to have a lower or more stable viscosity then the regular fluid provided with the truck. You could most likely get away with a synthetic ATF in the transmission with no problems. If I recall correctly, ATF is a 20 weight engine oil viscosity equivalent.

 

[Don]

Amsoil says their synthetic ATF has a much power pour point than the non-synthetic stuff, so if you're concerned about losing winter range to the gearbox lubricant, swapping to synthetic would probably be a 'no brainer'

AMSOIL Synthetic Universal Automatic Transmission Fluid has a lower pour point, and is pumpable at much lower temperatures than conventional automatic transmission fluids as demonstrated by the Brookfield Viscosity Test. It is capable of providing greater transmission efficiencies and better fuel economy at low temperatures than conventional automatic transmission fluids.

Being a certifiable 'Miata Nut', I've looked into Miata conversions for several years - In fact, I bought the iMiEV largely because it had many more desirable features for the dollar (brushless A/C regenerative drive train, excellent BMS and battery pack, electric power steering and I could go on and on) than I could ever hope to have in a converted Miata for nearly the same money. The nicest Miata conversion I ever saw placed the drive motor where the transmission in the Miata used to be, using only the transmission tailshaft adapted directly to the drive motor, so the energy wasting transmission was discarded

That said, many heavy duty Ford trucks use ATF in their manual transmissions and some engineers recommend exchanging that for 10W30 motor oil which has proven up to the task, so if I had any conversion which kept the ICE transmission I wouldn't hesitate to use a lower viscosity lubricant better suited to the task. Your transmission will never get as hot as it did in ICE service, so either motor oil or ATf should do an excellent job and I don't think you'd ever have a lubricant related failure

A much bigger 'power hog' in a Miata conversion would be the terribly inefficient hypoid differential, which really does need it's 80W/90 gear lubricant, engineered to stand up to the wiping, shearing loads of the hypoid gearset in the differential - That puts much more stress on the lubricant than any transmission which has relatively straight cut gearsets

Don