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When Alex yon Falkenhausen applies BMW's engine department to turbocharging, watch out!
For those who thought the 2002tii was to be the ultimate BMW in the "02" series, the Munich factory had a surprise in store. Unveiled at the Frankfurt Motor Show last September, with 170 bhp DIN packed under its hood, the turbocharged version of the 2002tii lived up to BMW's sporting image by lifting the compact two-door which started life with a humble 1600-cc engine of half the power into the exclusive "over 125 mph" class. Not everyone gave it a hearty welcome, however. Part of the German press severely criticized the "provoking aggressiveness" of its racy- looking front air dam bearing the inscription "Turbo" in mirror script to warn those seeing the BMW come up in their mirror of what sort of car they had to cope with. The backward script was painted over even before the show opened, but the tension between the Bavarians and part of the press remained—with the result that no BMW 2002 Turbo was available at the Motor Show test day in Hockenheim. Then came the fuel "crisis," which gave BMW a chance to quietly solve the problems still giving them some worry at the time the car was announced. The most important one was a tendency for the exhaust mani fold feeding the turbocharger to crack under the strain of the 1650°F temperature of the exhaust gases. The solution is a manifold made of a special cast iron with high nickel content.
The turbocharging installation, applied to the basic 2002 engine with mechanical injection as used in the 2002tii, is straightforward. The turbo of K.K.K. manufacture is fed by the full flow of the exhaust gases and delivers fresh air through a long pipe to the intake manifold on the opposite (left) side of the engine. The throttle valve is located just before the point where the pipe from the turbo joins the manifold, with the blowoff valve just ahead of the throttle. This is controlled by a mechanism sensitive to intake-manifold pressure and opens when an 8-psi boost is reached to release the excess blower output into the atmosphere. This is admittedly a rather crude way of limiting the boost pressure; an exhaust bypass would no doubt be more efficient in terms of thermodynamics and reduce the exhaust back pressure. But engine development chief Alex von Falkenhausen says there is just no room to arrange for such a layout. No provision is made to release the pressure in the pipe leading up to the intake manifold when the throttle is shut: the blower, kept spinning by its own inertia, must work against the back pressure created by the throttle valve. Experiments made with a venting valve apparently did not show up any noticeable advantage in keeping the turbo spinning fast with no exhaust gases to activate it and thus in reducing the time lag before full power again becomes available when the throttle is reopened.
Apart from a different ignition timing curve and a reduction in compression ratio to 6.9:1, the engine is identical with that of other 2002 models and uses the same camshaft, though an oil cooler is included in the circuit mainly because of the heat transferred to the lubricant by the turbocharger of which the (plain) bearings are lubricated by a derivation from the main oil circuit. Maximum bearing pressures are the same as in the aspirated 2002tii engine, as they occur at exhaust top dead center.
As the maximum boost pressure of 8 psi (reached at 3500 rpm full load) is comparatively low. I asked Von Falkenhausen why BMW had settled for this figure, giving the mild engine output of 170 bhp DIN, and if it had anything to do with the reliability of the engine or of the transmission. "No," was his answer, "the engine could easily take more and the trans mission still has a small margin. We rather thought that 170 bhp was about the limit for the running gear of a car not necessarily driven by experts only."
He then handed over the keys of his personal car, which I was to take to the BMW test track. It's the only place where the Turbo could be extended in this period of blanket speed limits, though we also drove it with little respect for current legislation on some small secondary roads. Though the engine was said to be perfectly standard, this car had no air dam nor the ugly rear spoiler added to the trunk lid of production 2002 Turbos. It also had the optional 6-in.-wide (instead of 5 1/2-in.) light alloy wheels and a small Alpina steering wheel, but in performance it was probably fully representative of the production model. Starting from cold took some (but not excessive) churning and a slight whistle was noticeable over the entire speed range, though more so when going slowly. In town the engine proved extremely smooth and tractable but slightly gutless in the lower speed ranges—not surprising in view of the low compression ratio and the comparatively high exhaust back pressure. It was not before a short stretch of Autobahn was reached, on the way to BMW's test track, that the car showed its Jekyll-and-Hyde character by the way it rocketed up to the provisional 100-km/h (62-mph) limit. This speed, by the way, is reached in 2nd of the four gears in the standard gearbox.
Only the test track, however, enabled the car to reveal its full personality. There was little time, so we headed straight for the 4.5-mile high-speed course of two long parallel straights with big loops at each end, banked no more than a normal road would be. Undoubtedly the most impressive thing about the car is its smooth top-end acceleration and the complete lack of fuss with which high cruising speeds can be maintained. Compared with the 165-bhp 2002 Alpina I used to own, top-end performance is very similar but very much smoother and more silent (in the Alpina intake roar was the main offender). Thus high-speed cruising—around 110-115 mph—is much more relaxed than it was in that highly tuned, unblown car which had practically the same maximum speed. The turbo car uses a higher-geared rear axle (3.36 instead of 3.64:1,) and a slight period in the Alpina between 6000 and 6200 rpm is gone in the turbocharged engine.
On the other hand, partly because of the higher gearing, the turbo 2002 has definitely less guts in the lower-and medium-speed ranges and also "feels" less lively because of the time necessary for the turbo to reach its full output when the throttle is depressed, even if the engine is still spinning fast on the overrun. Assuming the throttle has been closed for a few seconds but the engine is still running above 4000 rpm, the time necessary to feel the additional push, coinciding with the moment the manifold pressure reaches its governed maxi mum (easy to check on Mr von Falkenhausen's car, which was fitted with a big, non-standard manometer) is anything between two and three seconds. This second push comes in quite progressively, however, and there is no problem whatsoever with controlling the engine output in delicate conditions as when cornering at the limit or (presumably) on a slippery surface.
When the engine is made to pull hard from low speeds, the manifold pressure becomes positive at something above 2000 rpm and the maximum governed pressure is reached at 3800, remaining steady up to the 6500 rev limit—when an ignition cutout takes care of unwary drivers. With such a difference between the power available below 4000 rpm and above, the optional 5-speed gearbox (not fitted to the car driven), in which 5th is direct but which has more closely spaced intermediates, should be a considerable advantage.
In the fast curves at the ends of the parallel straights of the track, the car felt beautifully stable and neutral with a slight tendency to tuck into the bend if the power was reduced when cornering fast and enough power to powerslide it out of the bend in 3rd when cornering near the limit. Things were easily kept under control by the precise and high-geared steering, which has strong self-centering action, excellent feel and very good high-speed stability. But the characteristic 2002 tremor around 50-60 mph is still with us, enhanced by the larger steering offset resulting from the use of ventilated front disc brakes and wider wheels. Brake fade was never a problem with that Alpina of mine, so it should not be with the Turbo either.
Unfortunately the test track was not available long enough for a full set of performance figures to be taken, nor are the straights quite long enough for the car to reach its outright maximum speed. Two runs in opposite directions gave an average of 121 mph at a tachometer reading of around 6200-6300 rpm and with the car still accelerating, so that the real maximum should be around 124 mph. It must be emphasized, however, that Mr von Falkenhausen's car uses a 1973-type body; the front air dam and small rear spoiler do nothing to improve the looks of the car but do improve aerodynamics and should raise the maximum speed by another two mph at least. But not to the 131 mph claimed by the makers, which is 'way beyond the ignition cutout point anyway.
Still, 124-125 mph is quite impressive for a full 4-seater of 2-liter capacity, and using the gearbox to the full we averaged 8 sec dead from 0 to 60 and 22.1 sec from 0 to 100 mph. The standing quarter-mile was reached in 16.2 sec elapsed time.
In the exhaust-emissions department, the Turbo is better than its unsupercharged equivalents as far as NOx is concerned because of its low internal compression ratio, and is about the equivalent of the 2002tii for HC and CO; the emission test program is such that the blower hardly comes into its own.
Wouldn't it be nice if BMW were planning this approach to meeting the tough 1975 American emission standards?