Buyer's Guide: BMW E24 6 Series
For many, the big coupe is the ultimate of Munich's driving machines
From the February, 2009 issue of European Car
By Mike Miller
Robert Bosch to the Rescue
The significant revisions BMW brought to the E24 for the 1980 model were all overshadowed by the wonders of Robert Bosch innovations. Using Bosch's ingenious new invention, the oxygen sensor, allowed efficient use of a three-way catalytic converter. The positively horrible and universally hated thermal reactor and EGR emissions control systems were dispensed with at once in favor of this new technology. A new factory electronic ignition system also appeared. The darkest chapter in U.S.-spec BMW engine history was finally at an end.
This early Motronic system, wonderful as it was at the time, is less than desirable today from a performance-tuning standpoint. No chips are available to upgrade the ECU--what you have is what you have. Even though the engines will still respond well to high-compression pistons and camshaft upgrades, electronic optimization is not possible. Similarly, while the catalytic converter is far preferable to the dreaded thermal reactor (writer looks down, spits), the exhaust system on these pre-1985 Motronic cars is incredibly restrictive forward of the cat. The end result was lots of header installations and de-catting prior to that becoming bad juju, and later, installation of 3.5-liter engines with advanced Motronics.
A five-speed manual gearbox backed up the newly invigorated engine, but, contrary to popular close-ratio European fashion, fifth gear was overdriven to reduce engine speeds at cruising velocities. This allowed both increased fuel economy and use of lower (numerically higher) differential ratios to aid acceleration. Traditionally, BMW gears down cars when it wants to spiff up low-end zoot either as a result of engine size, automatic transmission or, in this case, emission controls. The automatic transmission option included cruise control starting in 1982.
Outside, the lower front valence panel was revised, and the rear bumper ends were extended to the wheel openings in 1982. A silly federal law mandated an 85-mph speedometer, and BMW stuck a red LED clock in it for some reason. Independent techs sometimes played speedometer baseball out behind the shop with these units, as owners just couldn't stand to look at them. By 1984, we were back to the 140-mph speedometer. Central locking appeared; it was now possible to lock both doors and the trunk with one turn of the key at the driver's door. But the little flap covering that driver's-side door lock would wear out, and you had to replace the entire cylinder--a problem never solved on the E24.
The 1982 model year saw a mid-production suspension revision to the now familiar wishbone front control arm design, which eliminated the front strut rods. No outward indications signaled this revolutionary advancement in BMW front suspension design. The wishbone design allowed far greater high-speed stability through increased control of caster angles and afforded greater control over ride quality independent of shock damping qualities. Suspension geometry under heavy cornering loads was also improved. However, the wishbone design is not a panacea, as technicians and do-it-yourselfers are well aware. The 06/82 production date marks both the beginning of the BMW wishbone control arm design and the beginning of the now-notorious BMW front-end vibration proclivities (previous vibration problems with the E21 3 Series were isolated to that model, which did not use wishbone control arms).
The problem, near as anyone can tell, is two-fold. First, there is extreme sensitivity to wheel balancing. Second, the front control arm bushings, aka thrust arm bushings, are not stiff enough. BMW used different control arm bushings for every model 6 Series, E28 5 Series and E32 7 Series. All are too soft--except for the E32 750iL bushings. These can be made to fit the E24 and E28, but they have to be machined professionally. Aftermarket companies offer E32 750iL bushings already machined and ready to install. Among them are BMP Design (www.bmpd.com) and Bavarian Autosport (www.bavauto.com). If your post-06/82 production 6 Series has a chronic front suspension vibration and other possible causes have been eliminated, use these control arm bushings. They are also a good performance upgrade for modified cars.
During this period, the now-dreaded Michelin TRX tire and wheel combination replaced the familiar 195/70-14 tires and 6x14-in. alloy wheels. The TRX promised revolutionary handling improvements but in order to deliver them required a specially shaped wheel bead area. In order to prevent TRX tires from being mounted on wheels without the specially shaped bead area, Michelin built it in a metric size, in this case, 225/55VR-390. In truth, the TRX did offer improved steering response when the technology was new, but it was quickly eclipsed by "regular" tire technology, after which the Michelin TRX became little more than a monumental pain in the ass. Drivers couldn't mount normal tires on TRX wheels. Instead, they could only buy Michelin TRX tires, which were very expensive and decidedly yestertech in terms of performance. Michelin TRX snow tires worked better for their intended purpose but were unspeakably expensive. Still, BMW would buy into TRX for the next five, long years.
Meanwhile, BMW owners sought aftermarket wheels and factory alternatives. Today, TRX wheels are routinely junked in favor of the same options--aftermarket and factory alternatives and normal-sized tires. Typical replacement fitments include 195/70-14, 225/60-14, 215/60-15, 225/50-16 and 235/45-17. The 17-in. size often requires body modifications for increased clearance, especially on lowered cars--most drivers considering this fitment will also have a completely tuned suspension with sport shock absorbers and shorter, stiffer aftermarket coil springs. Replacement Michelin TRXs are now considered "vintage tires" and are sold by specialty vendors such as www.cokertire.com at sky-high prices.
By 1983, 633CSi pricing was nudging the $40,000 mark in the U.S. The German-spec 635CSis could be personally imported to the U.S., so-called "gray market" cars so quick and fast they could suck a U.S.-spec 633CSi into the air intake and cough it out the exhaust while costing less money in the process. This was becoming a not insignificant sales threat. Moreover, the U.S.-spec 533i had nearly identical performance numbers at a savings of over $10,000. BMW of North America was confronted with the low power situation; something had to be done. BMW AG, typical of the era, could not understand why Americans wanted more power. After all, they reasoned, we could only go 55 mph--a speed considered laughably slow in Europe, where serious drivers had access to cars that could and did triple the U.S. speed limit. The notion that there could be wholesale disregard of speed limits here was inconceivable to the Germans, when in fact that was and still is precisely the norm in the U.S.