Anybody here knowledgeable of automatic transmissions? (Interchangeability question)

Hey guys, I don't know if this is the right place to ask since it's about a gasser, but it involves a TDI transmission. If it's the wrong place, please help me find where is the right place to ask. I already posted this over on VWVortex, but I get the impression of the people in the mk4 section there are not as helpful as TDIClub.

Anyways, here's the copy-pasta:


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My mom's mk4 is a 2001 2.0 automatic (01M). It gets pathetic mpg for how slow and small it is. I blame this mostly on the transmission. Yes, I'm aware swapping to a 5 speed would increase the mpg significantly, but my mom is getting close to retirement age and I doubt she wants to row her own gears.

The idea has been hanging around my head lately of swapping an 01M that has the tdi gear ratios into her mk4.

Information from http://zelek.com/diagram_charts/diagramlist.htm.

Tranny Code________1st______2nd______3rd______4th______Final Drive__Engine
ECN,ELT,EBP,FDB____2.714____1.441____1.000____0.742_______3.700_____TDI
EFB,ELZ,EPH,FDG____2.714____1.441____1.000____0.742_______3.700_____2.0L
DYG,ECV,ELY,EPG____2.714____1.551____1.000____0.679_______4.875_____2.0L
ECM,ENZ,EPJ,FCZ____2.714____1.441____1.000____0.742_______4.267_____VR6

I would be using a used TDI 01M if I could find one in decent condition. This car's not worth enough to spend a ton of money on this swap; the cheaper the better. Obviously I would have to get the TCM that goes with the TDI tranny to make sure it shifts right. If I also had to swap axles, that would be fine.

My question: What reasons do I have not to do this?

Assume: I can get the parts cheap enough, I do the swap myself, swapping in a stick shift is not an option, the reduction in acceleration is a non-issue (how much slower would it be to a stock TDI auto anyways?).

Her daily commute consists of 20+ miles each way on rural, flat, straight, 55mph highway. Also, she's kind of a slow driver so the lack of in-gear acceleration is not a deal killer. Every so often when I find myself not able to drive my own car for whatever reason (read: broken), I borrow this Jetta to drive to school (junior in college) and she uses their Buick for the week.
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Thank you!

Don't use the O1M, see this thread or one the hundreds of others.
Try car-part.com and find one for that make and model gasser

So what transmission do you suggest I use then? 5 speed swap is out of the question since it's my mom's car primarily and she's close to retirement age. The car is at 135k miles and on it's 2nd 01M. First one went out before 100k before we purchased this car, if you were curious.

Also, her current transmission is still going. It has not failed yet. This is pretty much a hypothetical question because I may never swap the tranny anyways .

I'm sorry, lack of knowledge. I didn't realize the gasser used an O1M.
You might try contacting user CoolAirVW here, he may be able to help you repair the one you got.
If you do source another, get one of the ones for the 2.0, the TDI model will not save on fuel with that motor.

Ok, well like I said, this one's still going with no signs of imminent failure. I was just asking out of curiosity in case this one did fail or in case the benefits were worth it. Driving at about 3500 rpm at traffic speed is just annoying and I figured lower rpm would help me get better than 25 freeway mpg.

My 01m on my TDI produces about 2800 rpm @ 75 mph. Maybe the ratios are different on a gas car?

You would at the very least need to have the ECU tuned to change the gear ratios in the cruise maps. There are probably other issues.

I think that a lot of people have a misguided understanding of the relationship between engine rpm and vehicle speed.
Lower rpm at speed means nothing in terms of fuel economy. The energy/fuel you are consuming while cruising on the highway is mainly used to push molecules of air out of the way as you move down the road. The faster you go, the harder the molecules push back (inertia/physics/Newton). The rotational speed of the engine has virtually no impact on this. Engine generates the power required to push the vehicle down the road until you reach it's maximum power output.
It takes a given amount of power to maintain a given vehicle speed regardless of rpm. What you really want is an engine with a higher volumetric efficiency. Like a TDI

It takes 4 times as much energy to go twice as fast.

TDI in MT said:

My 01m on my TDI produces about 2800 rpm @ 75 mph. Maybe the ratios are different on a gas car?

Click to expand...

Yes, the ratios are way different. Look at the red numbers in the chart I copied over, the gasser R&P is 4.8 whereas the TDI R&P is 3.7.

runonbeer said:

You would at the very least need to have the ECU tuned to change the gear ratios in the cruise maps. There are probably other issues.

I think that a lot of people have a misguided understanding of the relationship between engine rpm and vehicle speed.
Lower rpm at speed means nothing in terms of fuel economy. The energy/fuel you are consuming while cruising on the highway is mainly used to push molecules of air out of the way as you move down the road. The faster you go, the harder the molecules push back (inertia/physics/Newton). The rotational speed of the engine has virtually no impact on this. Engine generates the power required to push the vehicle down the road until you reach it's maximum power output.
It takes a given amount of power to maintain a given vehicle speed regardless of rpm. What you really want is an engine with a higher volumetric efficiency. Like a TDI

It takes 4 times as much energy to go twice as fast.

Click to expand...

I understand exactly what you're saying. The amount of horsepower required to push through 70mph wind will be the same regardless of whether it's at 3400rpm or 2500rpm. But will a reduction in rpm not increase the efficiency of the engine at that horsepower? There are less combustion cycles per second, but more fuel per combustion cycle. Don't higher gears increase efficiency, maybe by giving the fuel more time to fully combust before being pushed out of the cylinder?

These are all legitimate questions. I just don't understand why a lower rpm won't increase efficiency, given everything else remains the same.

Why, then, are manufacturers developing transmissions with more and taller gears if the car's speed is not going to be reduced? Why do mk4 2.0 automatic owners get barely 28 at best under ideal conditions when manuals can get 32+ all the time? I'm aware that automatics suck power through the hydraulics needed for the torque converter, but the final drive ratio is also reduced in the manual. I've not heard of a larger difference between automatic and manual fuel consumption than in the mk4, and the mk4's gear ratio differences are huge. Heck, some automatics of today are getting higher economy than their manual counterparts, because they have more gears and a lower final drive ratio.

If you're saying different rpms at the same speed do not change economy, why are people on here who swap their 5th gear getting better mpg? Can't this theory work for gas engines, too?

Don't take this as an argument, I'm not trying to berate you. I'm just curious why an 01M from a TDI won't be more efficient than an 01M from a 2.0 at, say, 70mph.

The reason that you can get a 6 speed transmission in a new Jetta vs a 5 speed transmission in an old Jetta is because the new Jetta has a more powerful engine.

It can go faster. That is the only reason why.

Guess how many gears the transmissions had when Jettas had a 1.6L 58hp engine. That's right; 4.

Guess why people who modify their 1.9L 90hp TDI engines sometimes also use a 6 speed trans; because they want to go faster and they have the power to do so.

I'm not trying to be snarky or make you feel stupid I just don't think that you will see any meaningful improvement in FE to warrant putting in a different trans.

Perhaps swap in a TDI and it's accompanying auto trans?

Get her a hybrid?

Yeah if there's no advantages, I probably wouldn't bother with it. The only reason I'm considering it is if it can be done for very cheap overall and provides only positive side effects as opposed to negative ones.

So long as I don't find any reason not to, I may keep it in mind if (when) the 01M she's currently on fails and we need to replace it. When it does fail, if she'd like a new transmission instead of scrapping the car, I might just put an 01M from a tdi in, since it wouldn't cost any different.

Though, if there are incompatibility issues, then I'd just replace it with the correct transmission. I came here to ask about the compatibility issues.

runonbeer said:

The reason that you can get a 6 speed transmission in a new Jetta vs a 5 speed transmission in an old Jetta is because the new Jetta has a more powerful engine.

It can go faster. That is the only reason why.

Guess how many gears the transmissions had when Jettas had a 1.6L 58hp engine. That's right; 4.

Guess why people who modify their 1.9L 90hp TDI engines sometimes also use a 6 speed trans; because they want to go faster and they have the power to do so.

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Explain this to me then.









The manuals have a overall top gear ratio of 3.0942, while the automatics have an overall of 2.695. Therefore, the automatic is turning at lower rpm at a given speed when in its final gear. There is more drivetrain loss in the automatic, because, well, it's an automatic, but the automatics with the same engines still get better mpg. Everything else about the car is the same, the gear ratios are the only difference.

I don't know anything about that ford auto trans but it appears to be a DSG type box (two separate final drives given). That's not your run of the mill auto trans.

Yes, but a DSG wouldn't be any more efficient than a manual. Even if they have the same parasitic losses, one allows the car to achieve better fuel economy through taller gear ratios. Most other compacts and subcompacts with manuals offered show automatic fuel economy numbers that are the same or higher than the manuals.

I thought of another point today. If you're saying your fuel economy is only determined by speed, not at all by gear ratio, then why don't we drive around in 4th all the time? Why do we even have overdrive gears? As long as we're below redline and only on partial throttle, the engine's only producing the horsepower needed to push through the air resistance.

Now I'm not saying I would expect to see a huge increase, but there's no reason an engine that can otherwise get 32+ mpg wouldn't get better than it's current 25 if it had better gear ratios in its transmission.

Gasoline and Diesel engines produce their power at different parts of the rpm band. This is why TDi cars have different gearing in the Trans. You've honed in on the wrong factor. The gearing of the transmission is not what makes a TDI mor efficient than a gas car.

Man, I think you should do it. Talk to a tuner and tell him what you want to do. I've got a runner FDB 01M on the ground in my shop. I'll sell it to you for $300 plus shipping. I'll even throw in the TCM.

Yeah I understand what makes a tdi more efficient than a gasser. To be honest, I believe a gear reduction in a tdi would yield a higher gain than the same reduction in a gasser. But I do think there would be some gain in a gasser. The question of whether it's worth it or not is a whole different story. I didn't engineer the drivetrains, so I wouldn't know what to expect until I try it myself.

Thanks for the offer for the transmission, but like I said, this is a theoretical situation. I personally would do it if only for just the other benefits of reduced rpm. However, this is my mom's car that I'm asking about and she'd probably rather keep it the same as it is as long as it's still driving. Only if the current transmission fails would I consider swapping a tdi auto in since a new trans would have to go in anyways.

That's why this has always been theoretical. If her trans ****s the bed in the near future, I'll take you up on that offer. If it were my car and I for some reason couldn't drive stick, I'd probably do it just to see how it turns out.

I'll throw this out, I was sold a TDI trans but turns out its a gasser trans, I went from 2800 rpm at 75 to 3800 rpm, not happy and yes, lower rpm at cruise can relate to better fuel economy.

Obviously, this is a very old thread. I don't think people realize how much gain in MPG you have to achieve to make certain mods economically feasible. Even if you only dump $500 dollars in a vehicle it would take years to make up the cost if you only got a few miles per gallon extra.

Your better off not having a car at all. Then you save the money on fuel, repairs and the purchase price of the car!

All you guys that think fuel economy is so that you can "make up" for repairing your car are just looking at it wrong.

So yeah, a long delayed post on an old thread.

This was never about trying to save money on fuel by spending money on a different trans. This was always about whether I should put the TDI 01M in place of the 2.0 01M when/if the current one fails. Only then would it be economically feasible to put a TDI trans in if I had to spend money to replace the 2.0 trans anyways.

Also, the fuel economy gain is only part of the benefits. You guys are trying to say you'll get the same mpg in 4th gear at 60 as you would in 5th gear at 60. While it's true the horsepower required to push through 60 mph of air and friction resistance would be the same, the engine operating at a lower rpm will be getting better fuel economy. This is mainly due to the reduction in losses to Friction Mean Effective Pressure (FMEP), otherwise known as "pumping losses".

The other main gains to reducing rpm on the freeway are reduced engine wear and reduced engine noise.

If I could get three small benefits from a different trans with no ill side effects, then it makes sense to use a TDI trans if the cost difference is small. Again, remember, it would not make sense to do this while the 2.0 trans is still working fine. Only if the 2.0 trans were to fail and we were looking to source another transmission. This was always a theoretical question.

I know the 2.0L has less torque so its possible using the TDI trans would make it worse due to the lower ratio.

bherman13 said:

This was never about trying to save money on fuel by spending money on a different trans.

Click to expand...

Well, to be fair, at your first post it was.

bherman13 said:

It gets pathetic mpg for how slow and small it is. I blame this mostly on the transmission.

Click to expand...

I just pulled the 01M out of my TDI; it has 160k and a couple of issues, I don't know if they are serious or not. It was taking longer to start moving once shifted in D, and the 2-3 shift was rough. I read the thread on these transmissions by coolairVW, and maybe this tranny has a lot of life left.

Anyway, it's yours if you want it, just pay for packing and shipping.

On the topic of fuel efficiency, the TDI engine will get better highway mileage at lower rpms because it makes better use of the fuel closer to the rpm where it makes peak torque. The thread on the 5th gear swap is full of good information on that topic. From what I've read on ecomodder.com, most gas engines will gain mileage benefits turning lower rpm on the highway as well.

belome said:

Well, to be fair, at your first post it was.

Click to expand...

Ok, yeah I said that. But I did not mean I would remove a working transmission and swap a TDI transmission in. I just thought, if the 2.0 trans failed and I needed to get a different transmission anyways, I'd consider the TDI trans.

A lot of the people on here think I was wanting to throw out a working transmission and spend hundreds of dollars solely for the sake of mpg. What I really wanted was to know if I could put a different trans in when the time comes that the stock trans fails.

Thanks, UFO, for the offer. If this car's gas transmission fails soon, I'll contact you.

If you feel the 2.0 is slow now, wait until you install the TDI trans, it will be that much more of a dog.

turbovan+tdi said:

If you feel the 2.0 is slow now, wait until you install the TDI trans, it will be that much more of a dog.

Click to expand...

Honestly, I personally think it's a bit more "peppy" than it needs to be. It doesn't have a problem getting out of its own way due to the really low ratio. I wouldn't have a problem giving up some of its acceleration for an increase in mpg and all the other theoretical benefits.

turbovan+tdi said:

If you feel the 2.0 is slow now, wait until you install the TDI trans, it will be that much more of a dog.

Click to expand...

Ding ding, correct answer. There is a reason the 2.slo is geared so short.

I really doubt there is any benefit to running a TDI trans on a 2.slo. Only thing it will do is make it a total dog by having the tall gearing.