BMW R 100RS Turbo


Cycle Guide 1978

You may have been puzzled in recent months as to why motorcycles which already go' faster than anything else on wheels have received large amounts of new horsepower by way of the hysterical magic of turbocharging. Well, there certainly may be no rational reasons, but there are a whole host of perfectly sound irrational reasons, beginning with the most obvious: Fast, in America, is never fast enough. Hence fearsome turbo-monsterbikes ripping off eleven- and even ten-second quarter-miles. And hence, naturally, the inevitable spread of the turbo's appeal, which now extends even into the upright ranks of BMW owners.

Admittedly, nursing secret desires for blitzwagens is hardly what one would expect of Beemer operators, but the fact is, according to Lou Nauert, Vice President for Research and Development at American Turbo-Pak, that's just exactly the way it is. Ever since a spy photo of a BMW mit turbolader appeared in a national magazine, Nauert says that BMW owner interest

has amazed him; people have ridden, he says, thousands of miles just to have a look at the technology that could turn their civilized and finely refined motorcycles into ravening beasts. So Nauert and American Turbo-Pak have happily pursued turbocharging for the German opposed-twins, and you see the result here: a brilliantly painted (Gotterdamerung Gold?) BMW R 100RS with ATP turbo kit, the prototype for what Nauert hopes will be a long line of Beemer turbos.

Considering American Turbo-Pak's standing in the infant aftermarket turbo industry, his hopes just might materialize. ATP, you will recall, was chosen by ex-Kawasaki marketing maven Alan Masek to be the prime contractor for the ferocious Z-1R TC. And, of course, ATP's own drag racer, Pee Wee Gleason, has been spreading the turbo gospel himself at drag strips all over the country. So if anyone is likely to make a turbo-Beemer fly, it will probably be Lou Nauert and ATP.

There are, of course, both philosophical and engineering questions to be answered about the whole idea of an aftermarket bolt-on turbo kit for something like the BMW R 100RS. BMW, after all, has made its position known simply by producing the bike and then asking over $6000 for it; as it stands, it must represent what BMW thinks a fast road bike should be. And doubtless many owners and marque-lovers feel the same way. But then there's the nagging question of sheer speed; an R 100RS is no match in a straightout drag race with a Kawasaki KZ1000, Yamaha XS11 or Suzuki GS1000—and that has to rankle Beemer owners, especially those who paid over six grand for their bikes. So another $1095 for a turbo kit (the price includes the turbo, all plumbing, a boost gauge, a mount to relocate the clock, a fuel pump and a deeper sump with one more quart capacity), to erase the irritating itch raised by being blown off in stoplight races could well seem a worthwhile investment. And not to be overlooked is the air of exclusive sophistication a turbocharger brings to any machine. Would it be too much to suppose that a BMW R 100RS owner with a turbo kit would think of his bike the way a Turbo Porsche owner thinks of his megabuck car? Probably not.

The engineering problems, as opposed to the philosophical implications of turbocharging a BMW flat twin, were quite another matter; Nauert and American Turbo-Pak didn't have to suppose that getting a satisfactory product would take more than simply bending up some new tubes and bolting on a standard Rajay turbine, they knew it. Early experiments seemed to confirm the conventional view that a turbo would be a waste of time on a BMW, but ATP persevered in the traditional cottage-industry fashion—painstaking trial and error. Nauert says that his first inclination—to lower the compression ratio substantially—produced an engine that ran well, but with no particular increase in horsepower. Huddling with Butler & Smith (the North American importers of BMW motorcycles) led to raising the compression ratio back to standard levels. And that change, along with the others implemented throughout three test units and unknown man-hours, resulted in the final kit now offered for sale.

The kit itself is fundamentally different from that for the four-cylinder Japanese bikes. The Rajay turbine, the heart of the unit, is entirely changed. Whereas the Z-1R TC uses a model 370 F40, the R90 and R100 series (all 900- and 1000-cc bikes) demand the more-efficient and smaller 377 F25. This lighter turbine spins with what Nauert figures to be a 50-percent increase in oil delivered to the impeller shaft. And the turbine fan itself is only 88-percent the diameter of the big Kawasaki's. These differences are mandated by the way the exhaust pulses reach the turbine; in the Kawasaki, with four cylinders, a steadier flow spins the turbo than in the two-cylinder BMW. And since a turbo has to turn very high rpm to be effective, a smaller turbine was required.

The changes necessary to the standard R100RS (or any R100) are substantial enough to warrant more than afternoon shade-tree skill levels in assembly. First, there's the location and installation of the turbo unit, the battery (a thinner one must be used on the Turbo) and fuel pump—an operation which demands that the battery box be opened up in front to accommodate the turbo housing. Then there's the plumbing, which on the BMW is fairly complex. (We counted 12 welds in the system.) Tolerance for error is slight; the waste gate, for instance, is sandwiched between the left rear frame tube and the transmission case—a position that allows access for twisting the boost screw but isn't designed for sloppy installation.

The boost gauge (which displays vacuum in lbs/in.2 increments) is also different than the glycerin-filled gauge mounted on the Z-1R TC handlebar clamp. The "dashboard" of the R 100RS happens to provide just the right size hole for a VDO undamped gauge. This tidy gauge displaces the clock into an ATP-supplied and BMW-made nacelle which fits neatly below the handlebars.

American Turbo-Pak says that those are the only modifications necessary to the bike to get it running on turbo power. But Nauert has gone further on his bike. He's installed another clutch disc (with alternate pressure fingers removed) and has discarded the standard BMW remote-linkage shifter in favor of an old lever simply hung directly on the shifter shaft. The clutch pull is thereby dramatically increased—it would substitute for a bodybuilding machine any day—and the direct shifting requires a healthy and positive toe movement to select the gears.

Nauert says that at street boost—eight to ten pounds—modifications to the drive train, valve gear or pistons aren't required. But then he adds the classic caveat: Nothing is free, not even the magic power from a turbo. So any "hard" usage will undoubtedly cause much faster wear and tear than normal.

But what happens when you screw it to the stop and revel in a little "hard" use? A good deal. ATP claims that with Pee Wee Gleason aboard on a cool day at Orange County Dragstrip, the gold R 100RS you see here has turned a best ET of 11.65 seconds at 118 mph. Our experience, on another calm, cool day at Orange County, was slightly different, but still indicative of the new horsepower found in the Beemer. Our best run was 12.6 seconds at 112 mph, running at 15 pounds boost without the simple baffle in the baloney-sliced exhaust pipe. The same day, a lot of tweaking and Pee Wee himself only managed a 12.39 at 105, so it seems as though our times were pretty representative.

Staying within the realm of pure numbers for a moment, that figure of 12.6 seconds needs to be put into perspective. Granted, most testers have managed to squeak out mid-to-low 12-second times for all the standard Japanese 1000-cc fours, and even some 11-second times. But that same 12.6 represents almost an entire second (0.8 seconds, to be exact) lopped off the previous standard R R100RS's time—with the same rider doing the testing at the same race track.

Moreover, the real benefit of a turbo to the street rider isn't really in banzai drag runs, anyway. It's in mid- and high-speed acceleration while rolling—a concept fully realized in the behavior of the turbocharged R100RS.

In everyday use, the Turbo RS works just like the standard bike; no funny habits, and none of the jerkiness and nervousness at the throttle associated with some of the four-cylinder applications. The bike even sounds much the same. Cold starts are a snap; a few pumps of the throttle and it lights off without a hitch.

Running moderate (circa eight pounds) boost on the road gives the R100RS no more appreciable increase in the feel of its power delivery until about 4500-5000 rpm—and Nauert readily admits that getting the turbo to work below the first test unit's disappointing 6000 rpm was a major challenge. The smaller, more efficient turbine is, in fact, all that made a usable power band possible.

And it is usable. The famous tractability of the big Beemer seems complemented by the turbo, not destroyed by it. The same effortless mid-range power is on tap, but with the turbo, when the 'mixture is right and the boost properly set, the last 2500 rpm in each gear goes by in a satisfyingly lusty rush. You learn, on the Turbo RS, not to stir the gear lever like a frantic two-stroke's or even a four-cylinder four-stroke's; you come upon obstacles on the road with an easily-developed sense of timing for the final high-speed rush, the passing blitz. And when it comes, there isn't that awesome weight on your chest and fearsome lunge typical of the Z-1R TC, there's simply a slight impression of compressed time. Oddly enough, even considering the (to us) out-of-character cosmetics of the baloney-slice muffler, the awkWardly placed air filter (which sits near your right knee) and the bizarre exhaust plumbing, such a sensation seems right at home with the BMW.

The question is, of course, can those kinds of sensations and a little more speed be worth over a grand? And the answer is predictable: It depends. Not just on your view of the trade-offs involved (slightly lower fuel economy, an instantly voided warranty and unknown durability problems), but on how fast you want to go and how you want to go fast. Both are questions central to each enthusiast's personal involvement with the sport, and neither is entirely answerable here.

Lou Nauert and American Turbo-Pak, on the other hand, have no such problems. They have seen their future, and as far as they're concerned, the whole thing is turbocharged.

Source Cycle Guide 1978