Zen & the Art of Turbos

What finer improvement to a Ducati 900SS could there be than a wacking great turbo? Zen Motorcycles’ Laurent Dutreul explains

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Laurent Dutruel is wearing a smile a mile wide when he swoops off the Montlhéry banking and rolls to a stop. “Man, you’ve just got to try a V-twin turbo. There’s nothing like it!” Nothing like a V-twin turbo? There’s certainly nothing quite like the Zen Motorcycles’ 900SS. But then there’s nobody quite like Laurent.

After working for 25 years in the family’s agricultural machinery business, the Frenchman needed a change. “I repaired a lot of lawnmowers – enough to last a lifetime,” he laughs. “So I became a mechanic at the Harley dealership in Nice.” He wasn’t a big Harley fan, but they grew on him and it wasn’t long before he was building some impressive custom bikes – usually with a turbo to boost power.

But it was never just for show. All Laurent’s bikes are made to perform. “I never lose sight of the fact that a motorcycle is built to be ridden,” he says, before adding: “And ridden hard.”

You can’t put Laurent in a box and label it ‘custom bike builder’. He built a rigid framed, rigid forked bike powered by a Buell XB9 engine that weighed only 115kg. There was no gearbox – only a countershaft to the clutch – and so to start the engine he wrapped a rope around a pulley on the mainshaft and yanked. “It starts easy. Just like a lawnmower!” says Laurent. In August 2011 he was timed at 130mph on the Bonneville Salt Flats “on a rigid bike – the run was absolutely terrifying. But awesome.”

And then there was his turbocharged Harley XR1200S, which won the Street Performance class of the AMD custom bike world championship in 2013. He shipped that one to San Francisco and rode it cross-country to Bonneville, and then broke the record for the 1350 M-PBG class (Modified, Supercharged pushrod engine running on gasoline) with a speed of 159.54mph.

Now based in Massongy, on the south shore of Lake Geneva, Laurent calls his small business Zen Motorcycles. Of course, he’s still hooked on V-twins. “There is something about the simplicity and purity of an air-cooled V-twin that I find very attractive,” he explains. “And I also like turbochargers.”

A turbocharger is mechanically simple and is a cheap way to get more power, which is where Laurent’s latest creation comes in. It started life as a 1995 Ducati 900SS, Bologna’s sweet-handling sports bike that delivers 73bhp at the back wheel and a 136mph top speed. He stripped off the fairing, tank and seat and got to work. “I decided that the best place for the turbo was behind the engine and under the seat. So I cut away the rear subframe, put the turbo in position, and then bent the stainless steel exhaust pipes to fit – the front one runs under the engine and up to the turbo, while the rear one is quite short.”

Since a turbo has to spin at very high revs to be effective, the ones used on motorcycles need to be small so Laurent chose a Garrett GT2056. If he had used a four-cylinder engine he could have bolted it between the exhausts, intercooler and the carbs, and he would have been good to go. But a V-twin comes with its own special challenges. Because of the large exhaust gas pulses and the intermittent intake strokes, sometimes there is nowhere for the turbocharged air to move to and it stagnates in the feed pipe. That’s why you need a plenum chamber – a box which stores the air when the inlet valves are shut.

Laurent constructed a triangular plenum chamber from sheet alloy and mounted it between the V of the cylinders. “The volume of the plenum is important,” he explains. “Combined with the volume of the feed pipe, it needs to be five or six times the volume of a cylinder.” If you are wondering where he got that information, it comes courtesy of Honda. “That’s what they did with the CX500 Turbo,” he says. “I use one as my daily ride.”

Laurent wanted the intercooler (which cools the air compressed by the turbo) to be as big as possible and still fit under the petrol tank and behind the forks, so he bought a core from an American supplier and built one himself. While the 900SS used a pair of 38mm Mikuni carbs to feed the two-valve, SOHC Desmo, Laurent chose a single Mikuni HSR smoothbore with a 45mm choke. Of course, while a normally-aspirated engine sucks air through the carburettor, a turbocharged engine has air blown through and so the Mikuni had to be modified. “It is not essential to have multi-carbs with a turbo,” he says. “One carburettor works just as well, it takes up less room and it is much easier to tune.”

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To avoid over-stressing a turbocharged engine, it is best to reduce the compression ratio. Instead of pistons with lower crowns, Laurent recommends fitting a thick alloy plate between the barrels and the crankcase. Now the Duke runs with a compression ratio of 7.5:1, down from 9.2:1. “Don’t put the compression plate between the head and barrel – you might get problems with combustion pressure blowing past it.”

With the turbo, intercooler and plenum chamber in place it was obvious that the rear monoshock would have to be relocated. The turbo was going to define the construction of this motorcycle. “I tried different ideas, but finally decided to lay the suspension unit on top of the engine and connect it to the swingarm through a bell crank,” says Laurent. Oh, and that meant modifying the original alloy swingarm by cutting off the upper box-section member that wraps around the tyre and replacing it with one made from round tube.

A new rear subframe was constructed from a lattice of welded alloy tubes, while the polished alloy seat hump was designed to carry most of the electric components. “I don’t work from drawings,” says Laurent. “I just bend a piece of alloy sheet, and another, and then weld them together. Gradually the shape begins to take a form that I like, but not always – sometimes I have to start over again, and again, and again. I want my bike to be a perfect combination of beauty and rideability.”

That’s what happened with the petrol tank. “My first design covered the shock as well as the bell crank and the pipe from the turbo, but then I thought... why hide them? So I started again. Now the front and left side of the tank holds the gas, but the right side is a lattice fabricated from alloy rods that have been welded together and polished to perfection. You can see the construction.”

And that construction is rather special, with superb attention to detail. Check out the dinky Motogadget digital speedometer, which has been crafted onto the turbo pipe so that the rider has to peek between his knees to check how fast he’s going. “I put the turbo boost gauge where the speedometer is normally mounted,” adds Laurent. “It’s more important to see that than the speedo!”

It might seem a bit anal to keep re-making components, but you can tell that Laurent doesn’t take himself too seriously as soon as you see the ‘silencer’. “I call it the chip cutter,” he laughs. “You don’t really need a silencer if you have a turbo because the exhaust pulses have been absorbed and that cuts down the noise. But a motorcycle doesn’t look right if it doesn’t have a silencer, so I made this one.” He didn’t say if he melted down old chip pans to get the alloy to cast it...

To keep the Zen turbo Duke looking as clean as possible he has hidden most of the wiring inside the frame tubes, and the battery – slung under the back of the engine – uses the earth terminal to bolt it to the chassis.

Not even the most passionate Ducatisti could claim that the 900SS has stylish wheels, so they were junked and replaced with alloy rims and stainless spokes and Laurent decided that he only needed a single disc up front. “It has enough stopping power and more feel, and looks a lot better as well” He is making a pair of mudguards, rolling them to shape on an English wheel, but they weren’t finished in time for our test ride. He’s already clocked up 800 hours on the project. No wonder he was so keen to try it out on the Montlhéry banking.

Reduced weight and a turbocharged V-twin make an awesome combination. You can choose as much boost and as much horsepower as you want. When the orange needle on the gauge indicates 0.7 bar, the Ducati engine is delivering 120bhp, but Laurent can play Dr Jekyll as well as Mr Hyde. “For most road conditions 0.4 bar is enough,” he says with a devilish grin that shows the temptation can sometimes be too much to resist. But which is more important, power or torque? “If you want to use a bike on the road, torque beats power every time,” says Laurent. “I love torque – not high revs!” But don’t take his word for it – you’ve just got to try a V-twin turbo....