Heating the transmission with engine coolant

[fjasper]

Redline MTL 75w80 gear oil will probably work fine. It's a little lighter, and works nicely in cold weather, but it still has good friction properties to keep the synchros happy so shifts are smooth. I had good luck with it in an older Audi and an '89 Tercel 5-speed.

Seems like an advantage of using coolant to influence trans oil temp is that it should also function as a transmission cooler in hot weather.

[renault_megane_dci]

What about using a length of copper tube wrapped around the exhaust ?
Then you get free heat back and you can direct it into the engine and trans.

Of course you have to thermostat it for when you hit peak temp on longer trips.

This is only suitable for older vehicle without a cat' obviously.

Also it would call for a a oil to water heat exchanger somewhere wich would be the real killer engineering wise ...

[paulgato]

OK, I may have misread something. You could, I guess, 'T' off from the feed to the heater matrix, and fit an ordinary wax thermostat (grab a suitable housing from some car or other). Choose a thermostat with a rating of say 80 degrees C, so engine will heat up to a reasonable temp first but heat will go to gearbox before going to radiator.

But I still reckon you should first exhaust the possibilities of reducing friction by using the best, low-temperature-friendly gear oil you can find. That is SO much cheaper and easier, and my experience of doing that was very positive.

(The other possibility, which to my mind may be easier to implement, if a little less efficient in absolute terms, is to fit a 12v electric heater pad to the gearbox. The advantage of that (apart from ease of installation) is that it would start warming the gearbox as soon as you switch the engine on, and would not delay engine warm-up. It would be easy to fit it with a voltage-sensitive switch so it comes on only when alternator is running (or if you have a mains charger plugged into the car at night? Maybe then fit a similar heater pad on engine block as well?) Also maybe fit a thermostat on the gearbox housing so it switches off at a given gearbox temperature. Generating electricity with an alternator is not that efficient, but the losses mostly end up as heat in the engine bay, which is kind of what you want, so maybe not that inefficient overall.)

[stoatwblr]

This whole thread has been "interesting" and there are a bunch of assumptions in it about the kind of oil that's already in the gearbox and viscoscity of 'em.

Bear in mind the OP's wagon is a Toyota Tercel, with a low-midrange engine and a lightly loaded geartrain, not a truck or a high performance vehicle.

FWIW, my current Toyota and Nissan (02 and 03 models) BOTH use standard engine oil (5w50) as standard in their manual boxes. I use fully synthetic anyway. This isn't unusual - my ancient CH250 scooter (85 model) also used normal engine oil in the transmission casing.

First step, determine what's there. If "normal" oil, then switching to fully synthetic would give the best gains all round without having to mess around with heaters, intercoolers, etc. As a first step it may be the only step required.

Synchros don't rely much on oil drag - and excess drag will result in heavy shifting. The main reason for high viscosity oil in transmissions was to maintain gear face surface wetting under loadm Most modern synthetics are extremely good at this anyway, which is why manufacturers have shifted away from high-viscosity near-grease in transmissions over the last 20 years.

[redneck]

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I don't think that this is going to work. With the majority of your trips being short and with the large mass of the transmission that would need to be heated, the engine would remain in open loop longer. Negating any fe returns.
Possible causing a loss of fe.

A engine block heater along with a silicon pad heater attached to a insulated transmission filled with the lightest approved hypoid gear oil would be best. Just put it on a timer so it's preheated before you leave in the morning. You could also put a small 110 volt ceramic heater on the floorboard inside the car to pre-warm and defrost the windows. This would help allow the engine to reach closed loop faster by not using the heater core to provide these functions.

If however, you want to heat the transmission quickly. Just attach a pair of welding leads to each end of the transmission case. Set the dial to 250 amps, turn it on and in no time your transmission will be fully preheated and ready to go...

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[Sam Powell]

The new Dodge trucks have a thermostat on the trans fluid coolant lines to block its flow until the trans reaches operating temp of around 180. Now this is an automatic, which makes controlling the coolant temp simpler. I put a similar thermostat purchased from Jegs on the automatic coolant lines on my old Dodge, which has a standard 904 Torqueflight, but would be hard pressed to tell if it improved the mileage. However, Dodge (now Ram) claims the thermostat on the trucks did improve mileage. For what it is worth.

Before putting the 904 back in the Dodge, I had installed a1987 Ford T-5 five speed, which used automatic transmission fluid, and not heavy gear oil. I think the advice about synthetic is probably a good one. I went back to the 904 when my old knees got balky, and I stopped driving it with the stick shift.

Sam

[Daox]

I love all the brain storming guys! Thanks.

A reminder of my specific circumstances though. This car will be fairly modified by the time I get to this modification.

Car:
- 1993 Toyota Tercel
- 1.5L gas engine with an insulated block for super fast warm up
- modified 5 speed transmission

Trans Oil:
- Synthetic 75W-90. I'm not willing to test different oils due to using a modified transmission. I don't want to toast it.

Other:
- I will be using a 1000W block heater at home.
- The car will likely be equipped with an electric water pump. This allows me to turn the pump on when plugged in to preheat the trans using the block heater as a trans heater.
- Other methods of warming up the car faster are being considered (using exhaust heat, etc.)


I love the simplicity of a wax thermostat. The only thing I'd worry about is overheating the transmission. It wasn't designed to have 180F coolant being pumped into it. It doesn't have active cooling. Thus, I'd like to have a way of stopping the coolant flow once the trans gets up to 180F or so. Also, remember most of my trips in the car will be under 10 miles, but the car also has to be able to go on trips of much more than that and at freeway speeds.

For those with an automatic trans, this would probably work perfectly though as the trans cools itself via the radiator anyways. You'd just let the trans soak up the heat before it gets to the radiator.

[paulgato]

Thanks for the re-cap Daox.

Check and double check of course, but I really doubt the transmission will be troubled by running a bit hotter than 180F (82C). I'd imagine it would be more than happy to run at 100C (212F).

The way I see this temperature issue with engines (and gearboxes are similar) is thuswise...

Vehicle engines have been designed to run at about 92 C simply because that is just comfortably below the boiling point of water.

Water happens to be one of the very best coolants available at any price, but only in its liquid phase. Even minor local boiling in cylinder heads can be very damaging, hence running engine coolant systems under moderate pressure to raise the boiling point a bit. But basically a nice, nominal 10 degrees below boiling point is a good safe temperature to aim for.

There's nothing inherent in the materials that engines are made of that makes 92 degrees C a particularly good temperature to run at, EXCEPT that it is just comfortably below the boiling point of water. Those people who have used Evan's Waterless Coolant, which has a boiling point of about 180C (360F) have had good results from raising their engine temperatures by several degrees. I think it's fair to say that higher temperatures are better for engines in general, provided that the temperature is relatively even. Overheating is really bad, but not because things get too hot in an absolute sense. When the temps rise above the boiling point of water then the heat dissipation mechanism breaks down in a catastrophic way, with local boiling in cyclinder heads that 'runs away' as those spots are no longer in contact with water, but only steam, which is a poor conductor of heat.

As I understand it, synthetic engine oil doesn't even begin to break down below about 150 degrees C (300F?). I did a little research on that when I was contemplating fitting a sump (oil pan) heater, and was concerned that a powerful sump heater pad might locally overheat the oil in the sump. I would imagine that synthetic gear oil behaves in a very similar fashion, and that therefore the practical temperature limit for a gearbox is 150C. Unlike engines, gearboxes don't have localised hotspots, so I'd imagine they are more tolerant of elevated temperatures, although they don't normally get regular oil changes, so any gradual heat-degradation might accumulate over a period of years. Still, running at 100C for example ought to present no problem at all.

The hotter the gearbox, the more heat will be transferred down the driveshafts to the wheel bearings and so on. Win/win/win, as far as I can see.

As for "active cooling", well if you are happy with a temperature of about 100C, then the gearbox will be actively cooled by the coolant running through it, through the engine block, the cabin heater matrix and the radiator.

But as the car is driven, heat is naturally transferred from engine to gearbox anyway, just through conduction and air convection, so if you can start off the day with a warm gearbox then maybe you can rely on the existing heat transfer from the engine to keep it warm. If you can plug into the mains at home then maybe all you really need is a gearbox heater pad.

[Daox]

You make very good points and I don't really disagree with any of them.

If I were to go the thermostat route, I'd probably use a lower temp thermostat though, say 160F. This would ensure that the trans thermostat opens before the radiator thermostat even begins to open up.

That all being said, where could one get a stand alone / inline thermostat housing?

[renault_megane_dci]

Dedicating a car to FE is a really cool project.
What's the trick 5 speed gearbox project ?

If riding very short distance I wonder the advantages of warming up temp versus a lighter vehicle ?