transmission question

Originally posted by UberRoo
Then I disagree with a number of people on the network regarding this issue. The faster the parts are moving, the more heat they will generate, but also the faster the water pump and ATF pump are working so the better they are able to keep things cool. The net gain is roughly zero. However, there is some difference. By far the largest inefficiency in a car is waste heat. For any given amount of fuel, a certain percentage (about %25) will become useful energy for moving the car. The rest (about %75) will become heat. The more miles per gallon you get, the greater the percentage of energy being used to propel the car instead of being converted to waste heat. You get better mileage in higher gears, thus there is less heat.

He's blinded me with science!!!

That makes sense, I'm going to have to agree with you on that statement.

However, one thing I can't mentally resolve by this explanation:

Wouldn't the transmission generate alot more heat by being forced to downshift constantly? When I drive in "D" around town, the car allways upshifts to 4th like it perfers to be in 4th even at slow speeds. EVERY time I even touch the gas at all and it downshifts. During a shift, the TC is slipping and generating more heat than it would if it were still in 3rd right? I am no transmission expert (yet) and I admit I could be wrong but I think that the actuall downshift and how often it occurs would generate more heat than if it were just stuck in 3rd while driving at or below 50 mph.

I'm interested to hear more input on this matter, I'm just a curious lad

[lee]

Originally posted by SEA Sleeper

...snip... EVERY time I even touch the gas at all and it downshifts. ...snip...

Is this because you live in a hilly area? I live in flatland (Coastal Florida), and don't get this behavior unless the revs climb to around 3k rpm.

I've hesitated to post in these 3 vice D threads because of where I live. However, given my particular circumstances, I experience lower temps driving in D (most of my driving is long stretches at around 50 mph w/TC Lock-up). The only thing that causes my temp gauge to approach 200F is heavy stop & go.

[n00b on demand]

Originally posted by UberRoo
That's too bad, because one of the largest heat generators in the transmission is the torque converter. By allowing it to lock up, you are eliminating the most inefficient component in the drivetrain.

Also, leaving the car in 3rd still allows the car to make that third gear shift, which is the problematic one anyway. What's the point?

You make an excellent point UberRoo.....i understand it alot better now. Why does the torque converter lock up anyway? Ive never experience that on any other car....at least it isnt noticible. I am guessing it locking up is a good thing. I want to know what everyone else thinks about driving in "3" versus "D". But UberRoo you made some good points...thanks for lettin me know before i did somethin horrible. I am planning to get a tranny cooler anyway from summit racing or whatever its called.

[mbtoloczko]

The TC wastes at least a few percent of energy when its not locked, so locking it improves gas mileage. It is also somewhate like having another gear.

There are a few notions for driving around town (up to 50 mph) in 3rd gear. One reason is that the transmission does not shift as much so there is less wear on the gear clutch. Another reason is to have better responsiveness if you need to accelerate quickly. I always drove around town in 3 when I had my AT. I much preferred it to driving around in D.

[n00b on demand]

So then what do you think? Should I keep doing it or just drive in D all the time? Yea ive always said the TC lock up is like having another gear. I have noticed a little detrioration in my gas mileage from driving in 3. I get about 10 miles less per full tank.

[UberRoo]

Yes, mbtoloczko is correct about why the torque converter locks up.

I think the shifts themselves generate very little heat in the grand scheme of things, but they do generate wear. The proviso is that they really don't generate any appreciable wear unless they're under load, i.e., unless you've got your foot on the gas. Around town, I usually accelerate to a certain point and then coast. Sometimes it shifts into third while I'm still accelerating, but unless I'm at freeway speeds it never shifts into fourth unless I'm coasting.

The upshifts when accelerating are quite hard on the clutches, but the downshifts, even when accelerating, are very low stress and generate very little wear. All this considered, if the 3-4 upshift is under very little or no throttle (like mine usually is) and the 4-3 downshift just doesn't cause any wear anyway, those two shifts are pretty much harmless. It's that 2-3 shift that takes all the abuse because you're usually still on the throttle when that happens. ...and it shows, because that's the gear that always fails first.

SEA Sleeper:
During a shift, the TC is slipping and generating more heat than it would if it were still in 3rd right? I am no transmission expert (yet) and I admit I could be wrong but I think that the actuall downshift and how often it occurs would generate more heat than if it were just stuck in 3rd while driving at or below 50 mph.

Yes and no. During the upshift, yes there is more slippage in the torque converter (and the clutches,) but during a downshift there's virtually no slippage at all. Again, I wouldn't sweat the 3-4 shift, as it's not the problematic one. In my experience that shift typically happens at fairly low engine load unless I'm on the freeway, in which case I want to let it happen anyway, regardless of any wear.

As far as there being more slippage in fourth than in third, that is a valid point. I believe this is true, but I would reckon the difference is marginal. Modern torque converters tend to slip right up to a certain RPM, and then pretty much disallow any more slippage. The question is whether the marginal difference is made up for by other losses caused by using a lower gear. I just don't know, but I tend to think not.

A transmission cooler is clearly the way to go. I've always been somewhat skeptical about how effective they are at reducing wear on the clutches. (Transmission coolers are very effective at reducing heat - I just wonder if heat really is the main culprit.) I'm more inclined to think that the clutch disc friction material is more consequential, but that can't be fixed with an aftermarket bolt-on accessory.

[n00b on demand]

So UberRoo i guess i should just leave it in D and go? I shift up at 50 mph and downshift manually back to 3 when its around 30 mph or so or when im at a stop. Or when it goes above 3 rpms i will shift up to D. I did notice the gas mileage difference. I do mostly city driving which is why i started driving in 3. The tranny shifts to 4th at round 35 or 40 mph or so..maybe even lower. thats why i figured to keep the heat down i would keep it in 3 so i dont over heat or take the risk of doing so. Ive always been confused about this issue. People always tell me something different but you made some good points. Dont know what to do though.....

[UberRoo]

Basically, it just doesn't matter. It'll only matter if your gas mileage starts to suffer or you find yourself driving around with the engine at high revs. If that's the case, don't leave it in third. It sounds like you already know that, so all is well. I just leave mine in fourth unless I'm in "Maniac Mode."

[gl1674]

UberRoo,

I found this sentence quite puzzling:

the downshifts, even when accelerating, are very low stress and generate very little wear

It is clear that downshifts while coasting are harmless, but downshifts during heavy acceleration seem to be as bad, if not worse as upshifts during heavy acceleration. The engine generates lots of torque, lots of slippage and friction has to take place to lock the clutches in the new gear...

I just wonder if heat really is the main culprit

I'm with you here - there was never a statistically valid evidence,
just people really wanted to believe they could extend lifes of their transmissions...

[UberRoo]

Originally posted by gl1674
UberRoo,

I found this sentence quite puzzling:

...the downshifts, even when accelerating, are very low stress and generate very little wear.

It is clear that downshifts while coasting are harmless, but downshifts during heavy acceleration seem to be as bad, if not worse as upshifts during heavy acceleration. The engine generates lots of torque, lots of slippage and friction has to take place to lock the clutches in the new gear...

To be clear, I define an upshift as when the transmission goes from a lower gear such as third, and shifts into a higher gear such as fourth, for the purpose of reaching a higher overall speed, not for greater acceleration. It follows that a downshift is the opposite. I've heard people reverse these terms, and in some applications it actually makes more sense to refer to them in the opposite manner. Just so long as I'm on the same page...

When the transmission downshifts under load, the natural tendency of the engine is to increase rotational speed, which of course is fairly obvious because you're applying throttle. The natural tendency of the input side of the transmission is also to increase rotational speed, which is logical because it's changing ratios to allow the engine to rotate faster. During that brief moment when the clutches are uncoupled, both sides of the equation (the engine & transmission) are working towards the same goal; to increase rotational speed at the clutch.

There is undoubtedly a moment when the clutch is being engaged where there is a slight differential in speed, but again, because both sides are working towards the same goal, the amount of torque on the clutch should be very minimal. In fact, if the downshift were too rapid, the load would actually be reversed; with the transmission forcing the engine to accelerate.

Upshifts on the other hand, inherently create a large differential in rotational speed, and then require the engine and transmission to synchronize - all under load. Upshifts tend to be smooth because there's lots of slippage just like a skilfully driven manual transmission. Downshifts tend to be very firm because there is effectively no slippage at all.


Regarding the heat-related longevity speculation, the one factor I would consider is that getting the fluid too hot does change it's properties. The most noticeable evidence of this is the smell, (although even then I'm still speculating that heat causes the "burnt" odor when in fact it may come from the clutch discs themselves.) If the odor truly comes from burnt fluid, it is not at all improbable that the fluid itself could become a damaging substance. ...but again, I'm just speculating. I'm not comfortable accepting "excessive heat" as the cause with no explanation as to how heat causes these premature failures.

[oab_au]

I have allways used 3rd for driving below 100kph/60mph.
When the transmission is in 4th the overall ratio is 2.45:1 and it will hang on to 4th gear for as long as it can, till its forced to change down. This is a very high gear ratio, and if driven around town, the engine will be pulling 4th gear, to around 1500 rpm.

When you accelerate normally without it kicking back to 3rd, the torque converter will start to multiply torque, to provide enough for the acceleration, heating the ATF as a by-product of the multiplication. At the same time, the gear train has the multiplied torque loading the gears as it goes through a 0.694:1 step up. All this is happening when the transmission oil pump is running only just fast enough, to produce line pressure.

If you are in 3rd, the overall ratio is 3.54:1 and the engine will be turning around 2000/2500rpm. When called to accelerate, the engine is all ready producing ample torque. So the converter does not have to multiply as hard, to provide the necessary torque, producing less heat. The gearbox ratio is 1:1, it is locked up into a solid shaft, no gears are running, no wear, no heat, no lube problems.

The torque converter produces the heat in the auto box. The actual gear changing produces little heat. The lower ratio suits the lower speed, keeping the engine running in the meaty part of the torque curve, where it is more efficient. The higher rpm is what the engine needs, its not a low revving 5.7lt V8, although it produces enough torque to rival one. The engine loves to run in the 3500/5000 rev range, not lug around at 1500/2000 rpm in 4th.

Do your car a favour, use third, you'll like it.

Harvey.

[gl1674]

Do your car a favour, use third, you'll like it

Mine definitely does not like it. It bucks when decelerating. I mean it engages and disengages engine braking every couple of seconds. Before you mention the dropping resistor - it's in place. At this stage I classify the bucking as "some valve body problem to be pondered at some spare time :-)".

The engine braking in 3rd, even when it works properly, is the annoying side effect of driving in 3rd on US spec SVXes. In 4th it does not brake that hard, it coasts nicely.

[oab_au]

Originally posted by gl1674


Mine definitely does not like it. It bucks when decelerating. I mean it engages and disengages engine braking every couple of seconds. Before you mention the dropping resistor - it's in place. At this stage I classify the bucking as "some valve body problem to be pondered at some spare time :-)".

The engine braking in 3rd, even when it works properly, is the annoying side effect of driving in 3rd on US spec SVXes. In 4th it does not brake that hard, it coasts nicely.

I don't know mate, I can only go off my model, that runs third as smoothly as top. As a fact, sometimes I have been running in third, for quite a while without noticing, that it was not top gear.

I have seen others that have said the same thing, acting like the resistor was disconnected. Have to have a think about it.

Harvey.

[mbtoloczko]

Mine's never bucked when driving around in 3rd gear.

[UberRoo]

I have also experienced the "bucking" during deceleration in third. It's very apparent when the gear selector is in third because normally the TCU would have the transmission in fourth during the greater part of the period of deceleration and it would pass through third gear only briefly. On rare occasion the TCU lingers in third long enough for it to be noticed.

I currently have my own electronic shift kit installed, which utilizes a handful of relays to operate the solenoids. When I remove the kit, this "bucking in third" problem goes away. Disconnecting the overrunning clutch solenoid (3) or manually applying power to it does not alleviate the problem. I know that solenoid three, in addition to controlling the overrunning clutch, also is used to dampen the downshifts. I've been unable to observe any difference when operating with solenoid three disabled, but perhaps this is a symptom of the problem that this function was intended to overcome.

What I suspect is happening is that in third gear during deceleration, the RPMs drop and the TCU signals the transmission to downshift, which promptly causes the RPMs to rise, which in turn causes the TCU to signal for an upshift, which causes the RPMs to drop, which again in turn causes... Well, you see where this is going.

The extent of my shift kit is that it gives full pressure from solenoid A except when shifting from first to second. I wonder if the reason for this bizarre behavior is because the TCU is not seeing the correct load from the solenoids. It is for that reason that it generates a trouble code, and perhaps under these conditions it also modifies shift behavior.

Regarding the third or fourth gear question, the Subaru SVX owners manual does suggest that you may use the gear selector, but recommends that you should just leave it in drive during normal driving conditions. As a general rule, the slower you can keep things turning, the lower the losses. The SVX motor does love to be wound up, chomping at the bit even, but the whole drivetrain was designed for steeeeep gears. If you prefer third, by all means, definitely use it. For prolonging the life of your transmission or to make your car happy, the benefits are dubious. Spinning things faster, even if there are fewer moving parts and all under less load, is still increasing the potential for generating heat. I wager that the net gain or loss is zero, or trivial at best.