BMW 525e: Efficient Dynamics | STERN.de

In modern mobility, efficiency is the decisive lever. The following sentence hits the current zeitgeist nail on the head: “A concept like this is particularly useful at a time when the maximum speed is becoming less important for many due to restrictions and traffic density around the world.” These true words come from a press kit for the BMW 525e and accompanied the presentation of the fuel-saving variant of the E28 series. It came onto the market in 1983, two years after the introduction of the second generation of the 5 Series BMW.

The “e” in the model name stands for the Greek letter “eta”, which at the time stood for the efficiency variants at the Munich car manufacturer. This fuel-saving concept had already been introduced in the USA and Japan in 1981 and made the leap to Europe for the first time in the BMW 5 Series. The recipe for quenching thirst consists of several ingredients. The basis is the classic in-line six-cylinder (internal code M20), with a displacement of 2,693 cubic centimeters instead of 1,990 cubic centimeters for the 520i in-line six-cylinder. This was achieved by increasing the bore to 84 millimeters (plus four mm) and, above all, the stroke to 81 millimeters (plus 16 millimeters). This long-stroke design of the engine made it easier to call up the torque at comparatively low speeds. But that was not all in 1983 either. Many screws had to be turned so that the BMW 525e was a worthy representative of the Munich model range.

An important lever to get the engine used to drinking was then and now the internal friction losses. Classical physics also helped with the eta concept, according to which these friction losses increase squared with increasing speed, but only increase linearly with larger displacement. That required a high torque at low speeds in order to reduce consumption. So the maximum 240 Newton meters were already available at 3,250 rpm and the maximum speed is only 5,000 rpm.

As a result, the engineers used valves with less spring force, which in turn reduced the friction between the rocker arm and the cam. The whole thing was flanked by a reduced number of camshaft bearings (four instead of seven). This shows that with the fuel-saver BMW 525e, not one measure led to success, but that many small optimizations were necessary. The digital engine control gave the engineers the opportunity to reduce the idling speed and to provide the drive train with an overrun cut-off. They were also able to fill the combustion chambers with a perfectly dosed fuel-air mixture by closing the intake valves early. The intake manifolds of the eta engine were 49.1 centimeters long and exceeded those of the 520i counterpart by at least 20 percent with the same cross-section, thus further improving the degree of filling. In addition, there is a compression ratio of 11: 1. As a result of these changes, it was also possible to install a rear axle ratio that was 25 percent longer than the BMW 520i, which reduced the average fuel consumption to 5.9 l / 100 km. At that time, it was a real number of renunciation.

But simply putting a narrow-chested Sparhans on its wheels would have led to a revolt among BMW drivers. The benchmark was the 520i. The Eta-BMW 5 Series had to at least keep up with its brother. So the six-cylinder in-line creates 92 kW / 125 hp at 4,250 rpm. The performance is accordingly: after 10.7 seconds the 100 km / h mark is reached and the 525e managed a maximum of 185 km / h.

Driving in the decelerates and has a decelerated sovereignty even forty years after the debut of the BMW 525e. Where others are hectically pumping the accelerator, the older gentleman confidently draws his circles – regardless of whether it is a motorway or a country road. The classic in-line six-cylinder is a poem of smoothness and sophistication, regardless of whether its incisors have been ground or not. The pots, which are closely connected one behind the other, purr in a relaxed manner. Melodiously sonorous and not comparable to the pumped-up bout of strength of today, in which 300 hp are squeezed out of two-liter, four-cylinder units by means of forced ventilation.

The four-speed automatic fits into the low-speed concept and changes to the higher gear early. But that does not matter to the in-line six-cylinder with the larger lung volume. It is made for it and also bursts with enough power from the low rev range. Even after 38 years, driving the BMW 525e is a pleasure. The seats with the fabric covers are more like the living room armchairs, which were modern at the time, than sporty chairs. We feel like we would be committing sacrilege if we shooed this 5 Series around the corners. Tachometer? Nothing. Yes, there is also that in a BMW. The pointer that shows the current average consumption is important. It seldom dances beyond ten liters on our gliding tour. The cassette radio remains silent. We listen to the wonderful sound of the straight six-cylinder engine – that’s music to our ears.