Transmission tunnel heat

Replace the two rubber inner shift boots (under the cosmetic vinyl boot). Instructions in the miata.net Garage section.

While driving my 90 (which I got last year) I notice a good amount of heat from the the tranny area inside the car and the stick itself gets pretty warm.. Replace the two rubber inner shift boots (under the cosmetic vinyl boot). Instructions in the miata.net Garage section.

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It seems some miatas will still have an issue with this even with good boots, not sure why the difference. One possibility is that the clearances are so tight between tranny and tunnel side wall on some cars that it still transfers thru. I believe it could be producing extra heat from old crappy and possibly low transmission fluid. I would highly suggest that you drain and refill the tranny with Redline synthetic or equal. This will generally smooth the shifting action as well.

Chris

92BB&T

Next thing to check is the heater air mix door adjustment. The shop manual says to set it so it opens fully, but on many Miatas that means it won't close all the way. Remove the glovebox, wiggle the temp lever, and it's obvious what needs adjusting. It's a 10-minute job.

Maybe driving style. Having just driven a few thousands of miles with torn boots, I find that the amount of heat generated by the gear box is much larger in low gears than in higher ones. It confirms the usual assumption that gear box power losses are almost nil in 4th, but considerable in 1st.

Leon

Telegram for Leon of Florida from William of Ockham:

The higher the road speed, the more cooling air flows between the transmission, catalytic converter, etc., and the tunnel.

Someone mentioned that they'd gotten a "lemon" 626 (It was never made clear what the year was, but the conversation we were having centered around my '82), and one of the "signs of lemonhood" this guy complained most (and quite bitterly) about was the amount of heat transferring through the passenger side floorboard and tranny tunnel area. I've experienced some of that in the 626 - The heat can get *INTENSE*, sometimes enough to make reaching for the shifter and coming up with a handful of metal shaft rather than the plastic knob QUITE unpleasant, to say the least. I'd been working on the idea that it was somethign in the tranny "being sick", and somehow dumping a mega-mess of heat into the metal of the shift lever, and was worried about what might be getting ready to die on me. The guy I was talking to claimed that the heat was coming from the two cats that are right under the passenger floorboard. Since hearing about this, I've been trying to figure out a way to insulate between the cats and the floorboards (There's already a heat shield - pretty cheesy, but at least it's there) in a way that won't get rattled loose/blown away/pulled out, or otherwise "lost", or stop working once it gets soaked (which it will, no question about that, when wet season rolls back around again).

Perhaps this same thing is applicable to the Miata?

Telegram for Lanny:

1) I have seen authorative sources that put the gearbox losses at a significant percentage in lower gears and at *zero* for fourth. Unless you have solid data to show those sources wrong, I have to go with the established authorities.

And it unavoidably implies that heat generated is an order of magnitude smaller in 4th than in 1st. Unless you have solid data to show the 1st law of thermo wrong, of course.

2) I did not say that the gearbox *was a bit cooler*, I said that "the amount of heat *generated* by the gear box is *much larger* in low gears than in higher ones." This was based on the observation that the gearbox's excess temperature was *an order of magnitude smaller*. The difference in driving speed cannot explain that.

3) Moreover, I am sorry to have to say that the high gear cool gearbox was with the top up (interstate). The low gear was with the top down. Little cooling air passes through the top. That there would be a big cooling flow along the bottom of the car seems to violate both mass conservation (of course, maybe some air was converted into energy, raising temperature,) and the fact that the flow would likely separate rather than enter the tunnel to a significant amount.

4) Please do not use mumbo-jumbo voodoo magic like "Ockham's Razor" and other nonscientific claptrap on me. While even scientists use that sort of thing to win discussions, they have absolutely no evidential basis. Quite the converse; if you think that the simplest/shortest/whatever solution is the right one, you simply indicate that you have absolutely no idea what has been happening in science in the 2000 years since Aristotle.

Leon

I wasn't referring to the flow under the car. Air enters the engine bay through the radiator, and some of it exits through the tunnel, past the transmission (perhaps less if you've removed the splash pan). You're thinking too hard about this, Leon.

I'm having a hard time imagining the transmission, in any gear, radiating more BTUs than the cat. I can see the tranny absorbing heat from the cat, though. Cats run hotter under lean conditions--maybe that's a clue?

Actually I am thinking very little of any significant amount of air moving along the route you indicate. ;)

Sounds reasonable, though I do not know. It appears to me that the gear box should be radiating a few kW on a curvy road, and almost nothing on the interstate. That is the sort of power produced by the electric heater in my office. Enough to heat things up considerably. I have no clue how much *heat* the cat is giving out, though it is certainly extremely *hot*.

Maybe. However, the cat has a heat shield, and is in a region with at least some reasonable amount of air flow. Nor does it have a direct path to the cabin like the gearbox without rubber boots. There is a significant air gap around the cat.

Nor does the cat explain the large *differences* in heating.

Leon

Why should a curvy road cause the gearbox to produce more heat than the interstate would? The chief source of heating in the gearbox has got to be the churning of the oil, and that's going on just as much (if not more) at highway speeds.

Two reasons:

1) Lower gear. A curvy road is driven mostly in second. The interstate in fourth or fifth.

2) More power is being transmitted through the gear box on a curvy road. You are constantly accelerating if you are not braking or cornering at high tire friction.

I doubt that. Churning from oil would not be that much different between first and fourth. It is the gearing *reduction* that causes the real power loss, though why that is you will have to ask a *real* mechanical engineer, not an aerospace one. I assume it is not that easy to make *different size* gears with teeth that match perfectly without slipping a bit past one another. Which would make the thin film of oil in between produce heat.

Leon

Note that Leon is engaging in a thought experiment here, since there are no roads that curvy in Florida.

OTOH, most of Missouri's genuinely-curvy roads are driven in fourth, or occasionally third if the yellow sign says "SAFE SPEED 20." I almost never need second gear, unless a turn is uphill and marked at 10 mph.

Gee, maybe that's why my tunnel doesn't get hot...I drive fast enough to keep it cool.

Then again, Leon does not just drive around the block in a big city like St. Louis, getting scared if he gets as far as the suburbs, as others might do, but actually sees some other parts of the US.

True. They are about as curvy as those in Florida and are driven in the same gear.

Since you are using fourth on your "curvy" roads, the gearbox will not generate appreciable heat. So there is no need to keep it cool.

Your "curvy" roads may not have much in the area of bends, but they sure are tall. Obviously, they are driven above the redline in third, which means that either you are not driving them much, or you are not driving them much since you are spending too much time in jail. Of course, we have the same problem with our "curvy" roads here in Florida, but at least here it is a decent temperature year round and sunny.

And yes, I have been in Missouri a few times and seen the "curvy" roads.

Leon

Sure. Lower gear. Now why should that result in more heat? The driving shaft will be turning at the same speeds in every gear, and the driven shaft will be turning more slowly. How does that add up to more friction?

Neither braking nor cornering result in any gearbox friction and in fact give the gearbox time to cool as it's transmitting no power during braking and less power during much of the cornering.

Actually, it's very easy. Check out this link:

The different size of the gears makes no difference whatsoever.

There is *some* sliding motion due to the gears being cut helically, but it is small and present in every gear (except -- usually -- reverse).

The point being that the things *you* mention are pretty much the same in ever gear. What's different in high gears is the speed of the driven shaft and thus of all the gears on it and of all the gears turning with those while not locked to their shaft. This would obviously be reduced by putting the dog clutches on the driven shaft, not the driving one, but it would not be *eliminated*.

This question was answered twice by me. Once in my *first* post in this thread (assuming you know the 1st law of thermo), then from another perspective in *my first post replying to you!*.

You think I keep answering it?

Anyway, the fact is very well established and does not come from me.

1) Does it not occur to you that the energy wasted in braking will have to be made up by something else? If so, do you have views on what that something else would be? 2) Don't you think the tire friction loss in cornering will have to be made up by something else? If so, do you have views on what that something else would be?

Pray tell me where on this page it lists frictional losses?

I think the claim of creating kWs of heat by "churning oil", in a normal sense, is ludicrous.

Maybe it is me, but sjeesh, Leon

Where? Quote the precise text to which you are referring.

Does the energy being lost to rolling and air resistance *continuously* while driving at highway speeds not have to be made up?

It explains that it is *easy* to make gears that don't slip past one another.

You think frictional losses make more sense? Why is it that manuals don't need cooling radiators, but automatics do? Could it be because automatics churn oil even more?

Ermm... I'll play... Is the energy wasted in braking made up for by being converted to heat in the brake pads, rotors, drums, or brake shoes (Apply whichever is applicable to how the vehicle is equipped) and connected hardware?

I think it probably is...

He's trying to say that you'll then have to expend energy to speed the car up again, and that that process will heat the gearbox due to its friction. And he's right.

But he hasn't show that it's going to heat the gearbox more than the higher speed motion of its components when traveling at highway speeds...