Horex' VR6 15-Degree Engine, Compact Engineering at its Best

Horex is a name we haven't heard of since the early 1960's. Back in 1920, the German glass manufacturer Rex in Bad Homburg was going moto, and that's how Horex was born, a brand named by welding together the names.
A 15-degree VR engine with 6 cylinders from Horex 1 photo
Photo: Horex
40 years of bike manufacturing have gone, interrupted by World War II until 1948. However, 1960 saw the burial of Horex motorcycles, right after Daimler-Benz took over.

And Horex has remained in a dormant stage until the announcement made in June 2010: the brand was to be revived, with fresh and solid financial backup, and even more, they were already working on a completely new, modern 6-cylinder beast of a motorcycle, due to be unveiled later in the autumn of 2011.

The official release of the new machine was delayed with one year, and after we had a look at what's inside it, we can understand why. It’s not every day that we get to see a V6 loaded on a bike and a “staggered six” is even less common.

This VR engine is not new, as it has been developed by Volkswagen back in the late 1980's. Basically, it mixes the character and architecture of a classic V engine with an in-line power plant, but it gets the best of the two worlds in a new package with smaller dimensions and offers multiple engineering possibilities as far as frame and orientation are concerned.
The VR denomination comes from two German terms: Verkürzt and Reihenmotor. A rough translation for these two literally means a "shortened in-line engine", which is a very good description, too. In fact, one of the main reasons leading to the invention of the VR engine was that an in-line power unit with more cylinders would be huge and engineers would definitely have a very hard time fitting one in any vehicle.

Prof. Dr.-Ing. Werner Bauer, head of the Institute for Internal Combustion Engines and Vehicle Drive Systems at the Munich University of Applied Sciences, is one of the guys behind the engineering team developing the Horex VR6 engine. One of the ever-present questions concerns the similarities between the VR6 and the initial VW design.

“That's understandable, because most people interested in this technology initially became familiar with the Volkswagen VR engine design. Strictly speaking, the basic idea of merging the advantages of inline and VR cylinder configurations is the only thing the HOREX engine and the different VR and W engines from the Volkswagen Group have in common.

“In fact, we had to start from scratch with the HOREX engine design. This is because the VR engine layout for a motorcycle is very different than the one used for car engines. The cylinder head is a good example. On the HOREX engine, we implemented a textbook solution for the intake duct, which runs straight up into the air box. This would not be possible with a car, because of the hood over the engine", Dr. Bauer adds.
On the other hand, a classic V engine had huge angles: 46, 60 and even 90 degrees (just take a look at the Ducati L-twin engines). V6 engines had already been around for a long time, but they were destined to power cars and other large vehicles.

Their bulky architecture was simply too big for a production motorcycle, and they were also still very wide. Rotating them 90-degrees was not such a big win, as the width and length were almost on the same “we cannot fit it in” side.

That's how the VR6 engine ended up with the peculiar 15-degree cylinder angle; it may seem strange, but this solution came with a lot of advantages that make the new Horex Roadster an extremely cool bike.

Now, take a look at the Honda Magna or the VFR: that is a V4 engine. Then, cut it in half and add one of the halves to it. A tad too much, indeed. So, here's where the VR6 engine choice for the new Horex was decided.

Horex wanted a big, powerful bike, with an exclusive design and even more exclusive character. That's why the engine had to be different and the frame of the new beast needed to match it. The Horex VR6 Roadster sports a mixed aluminum bridge frame and steel steering head to offer rigidity for the powerful engine, a lightweight construction, and last but not least – good looks to match the rest of the build.
On the tech side, the VR6 engine is a rather conservative one, with an oversquare 68/55 mm bore-to-stroke ratio. This rather balanced cylinder-piston architecture allows a better tuning so that the VR6 Roaster can deliver good amounts of torque in the lower rpm range, while not sacrificing too much of the peak-rpm power deployment.

Another constructive consequence of the “angled in-line” engine was the need to have different pistons. Of course, the VR6 unit could have been designed to run on casual pistons, but that would mean two separate 3-cylinder heads, since engineering a one-piece unit would have been hilariously large and too heavy, even if made in the favorite modern-choice magnesium.

The 15-degree angle would have resulted in too big a distance between the valve positions of each pair of cylinders, thus causing the top part weight and size partially negating the rest of the advantages.

So, the engineers came out with the best solution: slant-top pistons, each opposing two running on its axis angled at 15 degrees, but coming with a 0-degree axis offset in the top position.

Having the pistons positioned just like those of a classic in-line engine when reaching the top of the stroke finally allowed for an on-piece head to be used, granting a lighter engine and minimizing the number of parts, too.

As a fun fact, the same slant piston and staggered cylinder positioning has been used to create the monstrous W16 engine powering the Bugatti Veyron, only on a sensibly larger scale: the W16 comprises two VR8 rows of cylinders and angled in a “traditional” V manner.

Therefore, you see how the carefully-calculated cylinder offset and angles work in the Horex VR6 engine, offering a V6 unit whose width is comparable to that of an in-line 4. This space-saving solution allows a cylinder head with a 429 mm (17.87”) width. Even more, given the engine architecture, the Horex VR6 Roadster will be extremely fun to ride as it can deliver a remarkable engine speed range with plenty of brawn well below the red line.

The new Horex engine also makes the VR6 Roadster the first TOHC motorcycle to prowl the streets: the cylinder head contains three overhead camshafts setting in motion the two intake and one exhaust valves for each cylinder.

The Horex VR6 offers a max 161 hp output and a 137 Nm (more than 101 lb-ft) of torque. Once again, thanks to the cylinder positioning the VR6 will be able to offer huge thrust even in the low-revs range – a solid advantage for street riding, when top rpm are never reached, actually.

Despite Horex planning to release the supercharged version of the VR6 Roadster, it looks like they've chosen a more cautious approach and see how the customers and the rest of the world will welcome the new machine.
As for the build itself, the Horex VR6 Roadsters will be manufactured according to the vintage (and awesome) “one man – one bike” principle, with only one mechanic touching the machine from the very first bolt to the final dyno tests.

And with on-demand manufacturing, Horex makes sure each VR6 Roadster will suit perfectly its owner. We should expect only a few bikes a day, with a rumored price around €25,000 ($33,130), which is even cooler!

Here is some  visual explanation from Horex:
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