Transmission Up/Downgrade

My 1989 BMW 325i has a ZF 4 HP-22 automatic transmission, and it behaves perfectly. I can find good, used ones easily for about $100, and the transmission weighs less than 70 pounds. I’m a girl and even so, I have personally, physically picked one up.

My 2000 Audi A6 with the 4.2 V8 has the ZF 5 HP-24A automatic transmission, and it is wonderfully high-tech, but it has some very puzzling problems. Finding a good, used one means spending $1,800 or so, and the thing is massive and weighs almost 320 pounds. I’ve personally removed one, and it scares me.

It’s sort of like the movies where person A is in love with person B, who is glamorous but difficult. Eventually person A becomes less and less enthused and finally checks out, and chooses person C: someone less glamorous, but more practical — and nicer.

You can probably see where I’m going with this.

I’m fantasizing about replacing my ZF 5HP-24A with a ZF 4 HP-22.  And yes, the former is an AWD transmission and the latter is RWD, so I’d have a Quattro without superpowers. Still, it’d be a lot better than it being dead in my driveway. And besides, I live in Nevada on mostly dry, flat, level ground. Do I really need AWD? No. Maybe I could just remove the front half-shafts and call it good?

Originally I’d fantasized about the ZF 4 HP-24 as used in the 300 HP BMW 750iL, but that also uses electronically controlled shifting and I need a vacation from electrical issues. A nice, simple  transmission with hydraulically operated shifting would be wonderful. The most complicated electrical component should be the back-up light switch.

Power comes in the front, power comes out the back, a shift lever controls the basic direction the thing is supposed to be going, and that is that. Beautiful and simple. I miss that.

Sadly, the Audi’s torque would probably tear the ZF 4 HP-22 apart. The torque rating for the Audi engine is: 300 foot-pounds or so, wow.  The most torquey engine used with the ZF 4 HP-22 is, as far as I can tell, The Jaguar XJS 4.2 liter engine, with 236 foot-pounds.

Next, I started wondering what life would be like if I took the Chrysler A727 transmission out of my old Jeep and stuck it into the Audi. How viable would that be?

Wikipedia says:

The … 1962 … A727 … was initially referred to in consumer-oriented publications as the “TorqueFlite 8” … Compared to the early cast-iron transmissions, many and various internal improvements were featured … A727 Torqueflites became — and remain — wildly popular for drag racing, off roading, and monster truck applications because of their controllability, reliability, ease/cheapness of repair and brute strength.

The Chrysler A727 transmission:

  • Could certainly handle the torque and power that the Audi engine would generate.
  • Weighs about half of the weight of the ZF 5 HP-24A transmission.
  • Electronic components: zero.
  • Has affordable parts. A master rebuild kit is priced at maybe $150, and many mechanics can rebuild one. By contrast, just the solenoid kit for the Audi is priced at more than $500.
  • Doesn’t need expensive ATF.

Suddenly, the idea has become VERY tempting. So, what would be hurdles, aside from getting my hands on a Chrysler transmission (which is easy; I own three of them)? I’d have to…

  • Mate a mechanical torque converter to the Audi engine and yet end up using the same ring gear so that the Audi starter still works.
  • Mate the Audi engine to the Chrysler transmission using a custom-made bell-housing.
  • Mate the rear of the Chrysler transmission to the Audi drive shaft, or have a custom one made. No problem; our local drive shaft shop rocks.
  • Deal with the back-up light wiring
  • Deal with shift lever issues
  • Deal with shift lever console issues, such as no gate for the Tiptronic.  At least one model of the contemporary Passat has a non-Tiptronic transmission so I might use a center console from that on the A6.

There’a a nice C5 Audi A6 being advertised in Walnut Creek, for around $10K. The main attraction is that the car was converted away from the ZF 5 HP-24A to stick shift by someone savvy, who did a nice, clean job. I’ll probably go have to pick their brain as to how to get the Audi wiring to be happy even without the ZF 5 HP-24A.

Imagine the ad for a Tanya-converted car:

  • 2000 Audi A6 with the 4.2 V8
  • Converted to RWD and Chrysler A727 Torqueflite aAutomatic transmission, rebuilt with 10-year warranty.

That should do it. I’m located in Nevada anyway, and Nevada urbanites tend to need a Quattro about as much as they need a high-lift monster truck to drive to and from the grocery store, or ten miles to work, on flat, level, dry ground.


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