Heating the transmission with engine coolant

[Daox]

The thing is, I don't have short trips 100% of the time. I occasionally have to jump on the freeway and drive an hour away. Thinner oil at that point is either going to need to be cooled, or damage will be done (the last paper goes into how thinner gear oils do in fact reduce trans life). So, if I went with thinner trans oil, I'd have to find a way to tap into the trans case, and pump the oil through a cooler or something.

The trans is designed for 75W90. I know that that is safe to run. I could experiment with lowering the viscosity, taking oil analysis to see whats going on. Or I could try this heating mod.

[jakobnev]

70w-90?

[Daox]

Hah! I didn't even know 70W-90 existed.

From here:
Vis @ 100°C, cSt 15.6
Vis @ 40°C, cSt 90

EDIT:
Eh, after looking up the 75W90 on AMSOIL's site again, it also says 87 cST @ 40C. Here is the datasheet. The 129.7 cST came from their "long life synthetic gear lube" product, not their normal 75W90 synthetic.

That, and I can't seem to find a good source that actually sells 70W90 with a quick google search...

[darcane]

Originally Posted by Daox

I'm still digging for info. In this paper, I found this info. This only pertains to the bearings in a gearbox, but it shows substantial reductions in power loss due to increasing the oil temperature, and that is with the gear oil starting at 40C / 104F, and compared to 80C / 176F.

Maybe I missed it, but I didn't see anything in that study saying which oil is used in that test. An oil with a high Viscosity Index (VI) would show much less change in power loss due to the temperature change.

While it doesn't chart viscosity change over temperature (It does report VI though), this is an interesting study of a variety of gear lubes:
A Study of Automotive Gear Lubes

It includes both Synthetic (75W-90) and Petroleum (80W-90) lubes. While it's published by AMSOIL, after reading it I don't see that that invalidates the results.

[sqidd]

You have me thinking about this now.

I don't think you could ever get some sort of "hot water" circulation situation that will do you any good on cold days/short trips. Even if you bypassed the radiator and ran the coolant directly to the trans you would barely have enough time to get things up to temp. That trans will suck a ton of heat out of the coolant. It's a pretty big heat sink.

Now on the other hand if you had some sort of bypass setup that you could switch you could at least heat the trans up as fast as possible and you would also be able to run the trans at a hotter/more consistent temp on longer trips which would improve your mileage. For example, on a 60deg day if you drove it 1hr you could keep the trans at 200deg for the entire trip.

For really cold days/short trips I don't see any solution that is "worth it" aside from using a block heater/coil to pre-heat the case before you leave.

Or you could do both.

[darcane]

Maybe a little outside the box thinking... (easy when it's someone else's tranny).

Honda requires their special Honda MTF in their manual trannies... but if it isn't handy, they say just use 10W30 engine oil. I've seen reports of people using 10W30 in Honda trannies for 200k+ miles with no problem.

Why?

Well, the viscosities aren't too different between engine oils and gear lubes, but it's hard to compare directly at cold temps. The applicable standards are set up slightly different, SAE J300 covers engine oils, and SAE J306 is for axle and manual transmission lubes. Both have specs for viscosity at 100°C and there is a lot of overlap. A 90 grade gear lube will be between 13.5 and 18.5 cSt at 100°C. A 40 grade engine will be between 12.5 and 16.3 cSt at 100°C.

The specs for cold temps are not directly comparable. Based on the Honda MTF (which is thinner than most gear lubes) being comparable to 10W30, I suspect 15W40 to be pretty close to 75W90.

So...

maybe try a 0W-40 or 5W-40 engine oil to get thinner oil at cool temps but match the viscosity of the 75W-90 at high temps.

Granted, there is more to lubricants than just viscosity.

[Daox]

Seeing as how I am keeping this car for the foreseeable future, I'd rather not experiment with questionable oils. The transmission in the car is going to be modified and I'd like to not jeopardize the longevity of the transmission. Id rather take a couple percent hit on FE than shorten the life of the transmission.

The whole point of this mod is to improve short trip mileage. Putting excess engine heat to use is the method. I should have plenty of extra heat once the engine block is insulated. Super fast warm up is what I'm after for the engine and drivetrain to eliminate the cold start losses. The engine is going to be insulated either way just to have super fast warm up times. So, why not put the extra heat to use for the transmission?

[JBarr]

I would wonder what this will do to your engine temperature. My thought is that the efficiency of transferring heat from the coolant through the transmission case to the oil will be low and the heat sink will act like another radiator so you will result in slightly warmer oil and cooler engine temperatures.

[Daox]

The coolant circuit to the transmission would be thermostatically controlled. It would not open the loop until the engine is up to temperature, and it wouldn't close if the engine cooled down below a set point.

[JBarr]

So would you set the thermostat so that it will dump heat to the transmission before the thermostat to the radiator opens? If you transfer the heat through the transmission case it won't be the most efficient method, but it will be simple and not require serious modifications. If it is done cheaply the improvement may be worth it.