For quite some years now, my main mode of transportation has been a 1982 Mercedes-Benz 300SD, a base model-plus-sunroof 3 liter turbo diesel. This represented the budget version of the Mercedes flagship sedan, if you will, but still cost as much as a small house. I am finally moving up to something slightly newer, a 1997 Audi A8 Quattro. Like the Mercedes, when it was new, its price was similar to that of a small home. There are other similarities and differences, and only recently have I had the opportunity to make any kind of comparison.
The ideal comparison would actually be between the Mercedes 500SE and the A8, since that would be the equivalent engine, so there's not too much point making comparisons where power is concerned. The A8 is obviously a rocketship compared to the 300SD. What's more interesting is comparing the feel, the functionality, and so on. Since both vehicles have more or less base equipment, that's a more reasonable comparison. My A8 has the warm and cold weather packages, which vs. the standard equipment only means that it came with a ski bag passthru in the rear seat arm rest, a heated steering wheel, and a solar sunroof that runs the blower motor to keep the car cool on sunny days. Everything else listed in those packages is standard for all US models, at least in 1997.
The first things that are immediately noticeably different are the seat, parking brake, shifter, and steering wheel. The Mercedes wheel is massive, and really dominates the seating area. The rim is thicker than most American cars of the eighties, but it's not really fat enough to inspire confidence. That's fine, because the seats have no side bolsters to speak of. You should have your own confidence if you're going to push it around corners. The seats have three power adjustments, and a manually adjustable headrest with elevation and tilt. The Audi wheel is fairly small, and quite fat. The seats are 11-way power Recaro touring car types (there's an even fancier Recaro sport seat available) and they are extremely supportive in every direction. The upgraded Mercedes seat has got an adjustable lumbar support with a hand squeeze bulb. It was specified in 1978, after all.
The next thing you will notice, probably about the time you try to set off, is how much better the visibility is in the Mercedes. It's not a problem in the Audi when going down the road, forward visibility is perfectly adequate, but the rear visibility in the Mercedes is dramatically better. The C-Pillars in the A8 are really and truly massive, and the trunk is very high to promote rear impact safety. The result is not conducive to escaping impact. A four-sensor rear obstacle detection system was available, and it is a highly desirable option.
Once you really get moving, you can pick up on the differences in handling between the two cars. Although the Mercedes has a recirculating ball steering box and a massive steering damper (for a car, anyway) the S-Class front suspension of the day is actually quite fantastic. Although it does exhibit slight bump steer, there seems to be plenty of recovery built into the system and it never becomes fatiguing to drive, yet is as communicative as anything else I have driven. The Audi front axle has some significant positive features, like a pivot point only a whisker away from the wheel centerline, but it is designed to take most of the bumps out of the road, and it does. Thankfully, it is nowhere near completely dead, since the vehicle retains a hydraulic power steering system. It uses a Bosch Servotronic system to control steering rack damping, providing speed sensitivity. For my taste, it could have a little more resistance, but the goal was clearly to produce a vehicle which is easy to drive, and they succeeded brilliantly.
The two cars also have another big difference in their characteristics over bumps. The Mercedes basically erases all of the smallest bumps from the seat of your pants; you can still feel them through the wheel, but they are purely informational. The Audi transmits a lot more of the small vibrations through to the occupants, but major bumps virtually disappear. Not only do they not upset the car, but to the extent that they do, the car recovers more or less instantly, and without driver intervention. Driving the Mercedes is a far more active affair. It might be possible to improve some of these characteristics on the Audi, however; while the Mercedes came with stock Bilstein shocks, the A8 came with Boge and while they're not junk, they're not Bilsteins. You can get Bilstein HD or sport shocks for the A8; I have in mind the HDs for when the stock shocks wear out. They haven't so far, but I live in one of the bumpier counties in California, and that's saying something.
Between the Audi's steering system and the impeccable smoothness of the ZF5HP42 transmission, as well as the extremely progressive brakes, it really requires very little effort to operate the A8. What is more impressive, actually, is how close the Mercedes is to being just as easy to operate. Its transmission has a distinct tendency to clunk into gear, and into other gears any time you're not on the power, but it does have a hill sensor so very little accelerator pedal input is required. The brakes are excellent - both cars use front and rear discs, with ventilated fronts, and the Mercedes actually seems to have more brake for its weight. However, mine does not have ABS, which is a standard feature on the A8. It is also used for EBD, and on later vehicles than mine, ESP - it was instituted partway through the 1997 model year, with a switch from Bosch ABS 5.0 to 5.3.
Something I noticed today in particular is that it is easier to get in and out of the A8 with the door only partially open, that is to say, not opened even as far as the first detent. Not only is there more room to get in or out, but it's easier to swing your foot in after sitting down, probably due to the seat position and a slightly lower sill in the A8. The A8 also has soft-retracting handles above each doorway, which is a lovely feature. Another trait I noticed today (about the same time) is that the A8 actually has a superior turning radius, in spite of being AWD. The specifications claim otherwise (11.8 for the W126 vs 12.3 meters) but I've parked both cars, and I say bollocks. Perhaps if you keep one foot on the brake and use the other for the accelerator, that is true. I, for one, do not want to have to do that while also turning to look behind me.
The sound systems really can't be compared; At least in 1997, all A8s came with the 8 -speaker Bose sound system with a mid in each door, subs in the rear shelf, and tweets at the top fronts of the front doors. The four speakers in the Mercedes are all of odd sizes, and they were all shot when I got my car. I got some serviceable poly cone speakers at a flea market which are holding up very well with my ultra-cheapo JVC head unit. It was the cheapest thing I could find with bluetooth support, which never worked very well until about Android KitKat. The A8 also has a changer, but sadly it is literally specific only to this exact head unit and emulators are not readily available. There is no line in available, so you have to hack one if you want it; you can excise the tape deck and use its input, or if you have a changer you can tap into its wires and put your own audio signal there. Thus, so far the Audi sounds much better, but I'm limited to CDs and not what I've got on my phone.
The Audi obviously has far more power, and more traction than the Mercedes. But it also has a much longer nose; Because of the placement of the front wheels on the W126 platform, much of the weight of the engine and all of the weight of the transmission is behind the front "axle". This makes the Mercedes actually rotate much more easily, which can be done without getting on the power. The A8 does not understeer as much as people imagine it does, but it is a real thing, especially in the wet. You have to decelerate more going into the tightest turns in the Audi, but then the AWD will take you out of the corner just as fast as you like. The Mercdes is some four or five hundred pounds lighter in spite of being made out of mostly steel, and carrying a diesel engine, but it does have an Aluminum hood and trunk lid, and it is made entirely out of high strength steel. That means that it too is absolutely rigid, just as the A8 is with its "Audi Space Frame" construction. It's not really a space frame, but parts of it are, and they chose to make the vehicle heavier than it could have been in order to make it more rigid.
The Audi also has a bunch of other fancy equipment as compared to the Mercedes, but it's worth noting that most of it is having problems. Climate control works properly in neither vehicle, so that's a rather embarrassing tie. The defrost flap seems to be stuck open in the Audi, and the Mercedes is completely confused about what the interior temperature is. Neither vehicle's A/C compressor is working, and both systems are empty. The Mercedes sunroof sticks (I need to clean it one or two more times) but the Audi sunroof is possessed. It's only given me a hard time once, but I had to drop the control unit and actuator and manually move the roof, then reinstall them. The Audi has a standard power rear sun shade, but it only goes all the way up sometimes and the shade does not retract, but instead falls on the rear deck. Both vehicles are also having problems with the combination lever, aka the turn signal and high beam switch; the A8 can flash-to-pass but can't turn the brights on, and the switch is shorting (or breaking?) in the Mercedes and causing the cluster to go dead. It's a replacement for the original unit which was doing the same thing, although it took thirty years to go bad and the replacement only took two.
Ahem. Back to the comparo. The 300SD is simplicity itself, and it has essentially no emissions system save for an EGR which is easily capped. The A8 is a rolling computing farm, and about the only emissions equipment it doesn't have is an EGR; instead, it has a crankcase evaporation system which always feeds back into the intake, immediately before the throttle butterfly where it can make it nice and dirty. Crankcase ventilation from the right side is passed into the front of the left side head before being passed out its rear and into an expansion chamber under the cast valley cover, from which it goes to the intake. The fuel injectors are air- shrouded, meaning that they are supplied with auxiliary air during some driving modes. There's a crank sensor, a cam sensor, two knock sensors, four O2 sensors, and the PCM, TCM, and ABS all communicate with one another. The 300SD has a fairly expensive but relatively bulletproof Bosch Jetronic mechanical fuel injection system with turbo boost and altitude compensation, and a somewhat fiddly fuel return system that can be tricky to get to stop leaking. The A8 has an expensive and failure-prone Bosch Motronic system with intermediate driver stages for the coil on plug ignition units and a failure-prone fuel vapor evaporation system, as well as a failure-prone and model-specific Bosch in-tank fuel pump... and two fuel level senders, as well as three seals at the tank top. If the electrical system goes out on the 300SD, you can finish your drive to work, or home. If the electrical system goes out on the A8, the world stops.