Thoughts on transmission life, and final-drive ratio

[maluckey]

OK,

This one has bugged me for years, but finally got around to posting it. The reason is because I'm more than doubling torque ratings on my car, and as it gets older, transmissions won't be cheap.

Let's say that you have two transmissions, for this exercise, rated at 166 pounds feet torque.

-One transmission has a 3.94 final drive
-One transmission has a 3.55 Final drive
-Both are straining against 370 pounds feet of troque.
-There is the same traction and a Quaiffe LSD installed for each

Which one fails first? My thoughs go back and forth. The higher numeric geared tranmission means less load on the input shaft., but more on the output The other would have more load on the input shaft I'm thinking, because of less output power?

Am I thinking about this all wrong?

M

 

[HarryLui]

Auto or Manual?

 

[Billb2]

As soon as you exceed the 166 pound feet of torque spec the transmission will fail.
It doesn't matter what's inside it.

 

[Yuriman]

My reasoning tells me they both fail under similar conditions, and that final drive has nothing to do with it.

 

[maluckey]

Auto or Manual?

Both manual transmissions

My reasoning tells me they both fail under similar conditions, and that final drive has nothing to do with it.

Other than the fact they are far outside of spec, which means that their lifespan is limited (to Billb's point, which is assumed) , are you saying that they will fail in the same manner? Can you expand on that?

 

[Yuriman]

So, here's my reasoning. In the same car, with the same engine, both transmissions will have the same torque on the input shaft and the same overall load on the output shaft; the car weighs the same and you're probably going to be in lower gear on the taller transmission to get similar acceleration, so internally things will be similar. I suppose it depends on where the point of failure will be, ultimately you're running the same power through them and the gear reduction will be in one place or another.

 

[jlee]

As soon as you exceed the 166 pound feet of torque spec the transmission will fail.
It doesn't matter what's inside it.

That's doubtful.

OP, by "rated at" do you mean that's an actual transmission rating, or that's simply the amount of power the corresponding engine makes from the factory?

 

[maluckey]

That's doubtful.

OP, by "rated at" do you mean that's an actual transmission rating, or that's simply the amount of power the corresponding engine makes from the factory?

Actual transmission rating. It's already assumed that the unit will fail sooner that if within the ratings. Just trying to figure out the better choice of the two

 

[freeskier93]

I'm confused, final drive ratio is the ratio in the axle. What does that have to do with the transmission? Torque output from the transmission will be the same, its torque output to the axle shafts that changes.

Transmission torque rating is based on max torque output of the engine.

 

[maluckey]

I'm confused, final drive ratio is the ratio in the axle. What does that have to do with the transmission? Torque output from the transmission will be the same, its torque output to the axle shafts that changes.

Transmission torque rating is based on max torque output of the engine.

Sorry! Forgot to mention that it's a transaxle. That was a BIG omission on my part.

 

[jlee]

Put a Toyota E153 in it.

 

[maluckey]

Put a Toyota E153 in it.

I like the line of thinking! I already run an engine/trans combo that was never intended to be there. Unfortunately, it won't fit (already checked!)

I did some thinking about the fact the the differential is said to be virtually indestructible, and that leads me to thinking that maybe the case itself will fail first by twisting and then the gearset will self-destruct? Still thinking about this

 

[Rifter]

OK,

This one has bugged me for years, but finally got around to posting it. The reason is because I'm more than doubling torque ratings on my car, and as it gets older, transmissions won't be cheap.

Let's say that you have two transmissions, for this exercise, rated at 166 pounds feet torque.

-One transmission has a 3.94 final drive
-One transmission has a 3.55 Final drive
-Both are straining against 370 pounds feet of troque.
-There is the same traction and a Quaiffe LSD installed for each

Which one fails first? My thoughs go back and forth. The higher numeric geared tranmission means less load on the input shaft., but more on the output The other would have more load on the input shaft I'm thinking, because of less output power?

Am I thinking about this all wrong?

M

I would guess the vehicle will blow alot of U joints before hurting the transmission itself. Usually they are designed with U joints being the weakest point, which is a good thing they are cheap and easier to replace than transmission internals. Be careful when upgrading U joints not to make them to strong, as you will want to keep them being the weakest link.

 

[jlee]

I like the line of thinking! I already run an engine/trans combo that was never intended to be there. Unfortunately, it won't fit (already checked!)

I did some thinking about the fact the the differential is said to be virtually indestructible, and that leads me to thinking that maybe the case itself will fail first by twisting and then the gearset will self-destruct? Still thinking about this

If the case is the weak point, you could probably brace it (same concept as billet differential covers, differential braces, etc). In the MR2 transmissions, the diffs are typically the weak point.

 

[freeskier93]

Sorry! Forgot to mention that it's a transaxle. That was a BIG omission on my part.

That clears up the confusion, however it doesn't really change what the weak points are going to be. It's still the "axle" part that is impacted by the increased torque.

I come from the world of off-roading, where the amount of torque at the wheels is way beyond any typical vehicle. Differentials are far from indestructible, but 99% of the time it's axle shafts or u joints that break.

In your case maybe case flex could be an issue? I only say that because my guess is the transaxle is made with weight in mind and is cast aluminum. I don't know though, in my mind 3.55 to 3.94 is an inconsequential change that I wouldn't blink an eye at. Again though, I come from experience where a typical gear change might be 3.07 to 4.88.

 

[maluckey]

That clears up the confusion, however it doesn't really change what the weak points are going to be. It's still the "axle" part that is impacted by the increased torque.

I come from the world of off-roading, where the amount of torque at the wheels is way beyond any typical vehicle. Differentials are far from indestructible, but 99% of the time it's axle shafts or u joints that break.

In your case maybe case flex could be an issue? I only say that because my guess is the transaxle is made with weight in mind and is cast aluminum. I don't know though, in my mind 3.55 to 3.94 is an inconsequential change that I wouldn't blink an eye at. Again though, I come from experience where a typical gear change might be 3.07 to 4.88.

Yeah, the transmission is cast aluminum, and It's certainly true that the axles are also outside of spec, though I found some axles from a higher-powered, heavier vehicle, and they last longer than the stock parts. As I upgrase one area, it tends to point out the next weakest. Now I'm running decent sized tires, stronger clutch, better axles and steel trans and engine mounts, the next thing is likely to be the transmission that goes. The transmission is an upgraded unit, with stronger gears and a few added tweaks from a former factory engineer/racer. Even still I'm expecting it to fail in the next couple of years, and just want to try to extend how often I buy new transmissions.