Practical Turbo charging
By Ron Schmidt

Pat Thomas has been giving me grief about not submitting any articles recently. I wish to state for the record that you as Beehive Beemers have only yourselves to blame. You have kept me so busy at work that I have no time to write! Thanks for your support; it means the world to the boys and me.

I have had a number of folks ask about the turbo charging project on my GS. The first question most folks ask is “WHY?” Well, there is no practical answer to that question, thus the tongue-in-cheek title to this article. The truth be known, there were two instances that caused it to go from a stupid idea to a formed plan. The first was on a ride with Troy the Welder. We were on an uphill grade in some state not to be named (state as in one of the 50, not as in altered) and we were passing a truck. As I pulled out to pass and opened the throttle I realized that it was already pinned to the stop. It was very disheartening to know that the mighty GS was wide open at a speed that wasn’t what we would like to brag about. The second was on a different ride when Mike Thompson on his Suzuki V-Strom 650 passed me on an uphill turn and I once again found that the mighty GS already had it’s tongue hanging out all the way. In that instance, I could have downshifted and passed the 650, but my gawd, an R1150GS should be able to put that 650 to bed regardless of what gear it is in. I joked with Troy,” I guess I’m gonna have to put a blower on the old girl.” When I saw the look of enthusiasm in his face I knew this project would have to be done, if for no other reason than just to do it.

To give an over-simplified explanation what turbo charging is, we need to know some of what is going on inside the engine. Please refer to my previous article about how a four-cycle engine works if you need to refresh your memory. If we think in the most basic terms, our BMW motorcycle engines are just air pumps. It takes a given amount of air to make a given amount of fuel burn well, so to burn more fuel and make more power, we have to be able to put more air into the cylinder. On stock engines, when the intake valve opens and the piston goes down, the ambient air pressure fills the cylinder the best it can. Because the air going into the cylinder has mass, it takes a bit of time to get moving and as a result the cylinder will not be fully filled. Under ideal conditions, we might get about ¾ filling, and at many RPMs, the fill amount is even less. As a result, we can only inject an amount of fuel that will burn with the available air. Additionally here in Utah and surrounding areas, we often ride at high altitudes. At higher altitudes the filling amount is even less, so our engines make even poorer power.

Super charging is a method of pushing more air into the cylinder each time the intake valve opens. There are a number of methods to do this, including turbo charging and mechanical compressors. While there are plusses and minuses to any version of supercharging, turbo charging, in my mind at least, made the most sense for my GS. Turbo charging systems are relatively easy to develop, make lots of power over the wide RPM ranges, and are somewhat less expensive to produce than some of the other alternatives.

The main turbo charger unit can be viewed as an exhaust driven compressor. The exhaust is run through a chamber with a wheel in it that is closely akin to the pinwheels we all played with as kids. Recall that the harder you blew on the pinwheel the faster it would turn. Well, the “pinwheel” in the turbo on my GS will often turn in speeds over 100,000 rpm! Whew, wouldn’t want to be near that critter if it decided to loose a vane! Imagine that instead of the little nail that held the pinwheel to the stick that you waved, it was connected to another pinwheel, so that the two had to spin the same speed. In the turbo, the second pinwheel is in another chamber, which allows air to be drawn into it. The spinning pinwheel then compresses that air. The compressed air is routed to the cylinder heads through a series of pipes. So, now when the intake valve opens, air is pushed into the cylinder under pressure. We get more air in there, so we can add more fuel and make more power. Simple indeed.

The simple stuff gets a little more difficult when you actually design a turbo system. Where will the turbo go that will not cook the rider (it can get too hot to be comfortably within a foot of), how are we going to make all the tubes fit together and in the available space on the bike, how will we supply oil to lubricate and cool the turbo unit, how are we going to get that oil back into the engine, what size turbo will be needed, how are we going to supply just the right amount of extra fuel for the ever-changing extra air, how are we going to make it quiet enough to not be a nuisance on 1200 mile days, and how are we going to tune it on slippery roads in the winter or pay for very expensive dyno time? Add to this, how am I going to fit it in a schedule of a divorce, purchasing and moving into another house, getting my elderly folks into a nursing home when Dad fell and broke his hip, all while running a small business? Yes, it took almost 6 months to do. Under normal circumstances, the time would have been greatly reduced. So, Col. Bob, cut me a little slack here!

How does it make the Mighty GS work? Darn well, actually. Because the engine is relieved of it’s intake pumping duty, that chore being taken over by the energy that would normally be wasted in the exhaust system, it gets better fuel economy under cruising conditions than it did stock. I often see 50 plus MPG during the rallies. The power is linear and predictable, actually feeling much like an R2000GS instead of an R1150GS. There is no time that the power hits hard like a racing engine; it just pulls like a freight train. It makes very usable power from 2500 rpm (because there is pressure at the valve when it opens, the power comes on earlier than it did stock) and continues to build power dramatically as the RPM increases. I have seen 125 mph on the GPS, (the speedo on the bike showed some really big numbers!) on level ground at over 6000 feet elevation. That is no small accomplishment, as the aerodynamics on my GS are more like aerodramatics. With its big square saddlebags, giant windshield, huge driving lights and high profile, it has the aerodynamics of a barn door. Put this engine in something more aerodynamic like an R1150RS or R1200S and I would expect perhaps 150 MPH to be available with proper gearing. You can slow down to 50mph in 6^th gear going up Parley’s, open the throttle on the steepest part and see 120MPH at the top without shifting, according to Troy. I’ll never know that by seeing it myself!

The official power numbers are not ready for publication yet, but here are some interesting stats for you. Before the project, I had a dyno run with the bike totally stock, showing 67 BHP and 62 lb-ft torque at the rear wheel. Later, with a smaller turbo than what is on it now (yes, we chose a turbo that was a little small the first time) and with the fuel management quite a bit off key, we saw over 100 BHP and 100 plus change lb-ft torque at the rear wheel. BMW rates the GS at 85 BHP at the crank, so we are seeing an advertised crank horsepower to measured rear wheel horsepower ratio of 1.28:1. Applying that same ratio to our measured numbers with the small turbo, we were making roughly the same BHP as a K1200RS. We are making more now.

I do need to thank some people for help in this project. Andy at Shandy Turbos in Midvale made his dyno available, and offered a lot of tips he has learned during his years of turbo charging Japanese sport bikes (he had a drag racing Kawasaki on the dyno one day when I was there that made 380 rear wheel horse power, holy cow!). The boys at Gillette Diesel helped us with an educated guess for the sizing of the second turbo (it is NOT an exact science, although there are guides that are helpful). Above all others, though, I wish to thank Troy the Welder, for his part in making all the air tubing which I do not have the skills to do, but more importantly for not letting me light a match under the GS and burning it to the ground several times when we ran into difficult engineering problems!

So, if a very dirty dull gray GS ever passes you on an up-hill climb, you might not want to bother trying to catch it. Troy might be the one holding the throttle open.