bizmonq

OK maybe I've given this too much thought. Seeing as my allroad does mostly long highway trips between WA and AZ and gas prices ain't going down, I've been thinking if I could re-gear the transmission, couldn't I be able to take advantage of my 30% APR chip-induced torque increase (366 ft lbs @2500 RPM vs stock 280 ft lbs or 30%) and get better gas mileage via lower RPMs?

I researched some info (link below) showing the TDI and gas gear ratios, so I made up a simple Excel spreadsheet that compares the two. 1st and 2nd gears are exactly the same. 3rd and 4th are about 15% taller, 5th is 18% taller and 6th a whopping 22% taller than the gas version. The 5th gear of the TDI transmission is exactly the same as the gas 6th gear, making the TDI 6th gear a real "overdrive". Yes, the top speed is theoretically increased, but my main goal is better gas mileage. For example 75mph with the gas engine tranny has the engine turning at about 3000 RPM. With the TDI tranny the engine would be ticking over at a nice economical 2500 rpm. If the going gets tough and you hear (or in my case see with the boost gauge) the turbos work too hard, a simple downshift to 5th puts you back in the gas range. I don't know about you guys but with the ar's power I rarely if ever downshift once I get to cruising speed - even over the big passes. With a TDI tranny you might have to shift more often, but is that such a big deal?

I've posted on this subject before wondering if it would be possible to swap out the gears inside the transmission. It was suggested that it might be easier to locate a used TDI transmission and simply swap out the transmissions. That would be relatively easy if there were any TDI's here in the US to be had. Turns out I'm going to Germany next week for a trade show and have contacted a friend who is willing to work with me to arrange the sourcing and shipping. Used TDI ar trannys are available!

It looks like the cost will be around 2 grand or so (and a lot of aggravation). With gas prices at about $3.30 per gallon, and assuming a 5mpg increase, it would take about 77K miles to break even assuming I can't resell the original tranny. Sell the tranny for a grand and the break-even goes to 38K... about 2 or 3 years based on my current driving habits.

So I'd like to ask the collective wisdom of the board if you guys agree with my thinking? Have I overlooked something obvious? Am I nuts? Talk me out of it...please!<ul><li><a href="http://www.elektro.com/~audi/01E/">Audi gear ratio info</a></li></ul>

magilla70

as long as the final drive ratios are the same. Which by your info they are. If they were different you would need the rear diff also.

QCRAZY

...to get an A6 01E tranny and rear diff. You can take advantage of the lower final drive ratio of the A6 tranny and get near the same ratio as the TDI (drops it to 2600 RPM). Except you'll have more options if you run into trouble down the road.

The cheapest way to do this conversion would be to buy a wrecked A6; salvage what you need and then part the rest. You'll find this will get spendy quick.

Rally_nutcase_1982

All of this sounds very convincing but have you taken into account that your Turbos might have to spool a bit longer to get to the desired speed?
Since your gear ratio will be longer towards the differentials to make the wheels actually moving at a certain speed, it could require to draw the power from the engine over a longer period of time.

Maybe I'm wrong with these assumptions but you could actually lose on this deal utilizing a TDI tranny.

I don't know if you have to transport anything in the allroad on your long trips but have you considered the Jetta TDI? That is (unfortunately) the only alternative that comes to mind, not even the new Passat Wagon comes with a Diesel engine - it simply sucks.

I can also imagine that the ECU will need some work and fine tuning, not sure how many sensorsand monitors are involved that give info from the transmission/differentials to the ECU......just thinking out loud....

mustbeaudi

5 mpg gain - dunno 'bout 'dat, but fun to consider the issue.

paulroad

..the power curve of our engine, especially with the APR juice, we do have turbo boost at your 2500 RPM cruise, but it is a minimal boost. I would think at this speed the gas engine is going to need a lot of gas to make any kind of move. Flat driving, long distance, cruise control locked on, I'd say you win. But in normal daily use I think you will loose out.

Check your figures, look at how often you use 3rd and 4th for day to day driving, now those gears will be taller. Your "in town driving" will not only suffer from a performance standpoint, but perhaps in economy as well.

The diesel has its max torque at low RPMs. The gas 2.7T, even with the APR, is still looking at the max at 3500 or so.

However, the thought did come to mind that this mod may be gold, if you also do the downpipes and exhaust. This will allows a bit more torque down low as the spool up reportedly is quicker with these mods.

Small Town Terrorist

I contacted Vagparts about the TDI sixth gear. Below is the response. A mechanic friend of mine agreed to help with gear swap. After thinking about it and running the #'s, I decided against it. The best way for me to save money on gas is to slooooww down. The ar has a Cd around 0.35, Frontal area = 2.30 m2, and Cx = 0.81 It has too move a lot of air(burning mucho amounts of fuel) when traveling above 80MPH.

*** VagParts response***

Thanks for the enquiry about the ratio change, 01E 311 362M is not listed in as an Allroad gear but is listed as a Quattro 6 speed according to my listings our price will be ?194.82 GBP, the matching gear for this will be ?121.95 GBP, the only other items I think you will need to change will be the bearing cage ?7.64 GBP and the thrust washer ?13.30 GBP synchros should be ok if they are smooth in operation now

I hope this helps let me know where you are and I can quote for shipping

***END*

bizmonq

Thanks to all for your opinions. I'm linking the APR graph to this message to show you where I got the torque / HP data from. I think the extra HP and torque will offset the taller gearing under acceleration.

I'll also definitely look at the A6 swap possibility...<ul><li><a href="http://www.goapr.com/Audi/images/dyno_allroad_stage_i.jpg">APR dyno graph</a></li></ul>

bizmonq

replaced at the same time as they are on the same shaft. If so the jump between 4th and 5th would be 47%. Might be too much. Also, I hear working on these transmissions require special tools that are dependent on exact models. I heard this from a mechanic who works on the almost identical Porsche boxes. Its not impossible, but could be quite a bear.

MachineMan

resistance tires, take out the back seat, heavy rubber mats, spare tire and jack, clean out your glove box, door pockets, arm rest &amp; ash tray and you've probably picked up half of your 5 mpg gain. Your mileage may vary.