After the auto industry began to stabilize in the 90s, and car makers were in the process of making more standardized designs (ie boring), there was a smudge. Something really cool, really weird, and somehow extremely economical. Transverse mounted front wheel drive cars were in full force, but this was before everyone started throwing turbochargers on everything. As a result, high trim levels needed a hotter engine. More cylinders! More displacement! That was a problem, since the cars that this larger 6 cylinder engine was destined for were all small hot hatches and economical family chassis that would predominately have small (to very small) 4 cylinder engines. These engine bays wouldn’t accommodate a wide V6 due to the short hood lengths, and the suspension and steering clearance wouldn’t allow for a nice smooth inline 6 like BMW’s of the time. The solution?
The Vau Reihenmotor 6 cylinder engine, or the VW VR6.
The VR6 is effectively a 6 cylinder inline engine that is crushed down on both ends so that the cylinders are staggered. The end result is an extremely narrow vee, with the displacement and piston count of a 6 cylinder, and the form factor of a larger 4 cylinder. Double overhead cams are driven via a robust chain drive. The firing order is a very I6 style 1-5-3-6-2-4 (120deg firing interval) and some very interesting design implications presented themselves when it came to squishing this package all together.
First, unlike most V6’s the VR6 only has two camshafts. On the 12v version it’s not a big deal, each camshaft is dedicated to each bank and it all works like normal. On the 24v version, each cam is dedicated completely to either exhaust valve actuation or intake valve actuation. The valves in the center actually cross the center line of the cylinder head in a criss-cross pattern, resulting in asymmetrical valve lengths. That means the intake and exhaust valves on the outside are short, and the intake and exhaust valves that have to cross over in the center are fairly long.
Second, since the top of the block is flat to accommodate a single cylinder head, you run into an interesting issue. The cylinders are on an angle, but the top is flat. Hm.. In many versions of the VR6 (and there were many applications for this engine), the combustion chamber was a strange shape; it was kind of… crooked. The inside of the piston reached the top of the stroke, but on the outside there was a gap. In some VR applications, the pistons were skewed so that the crown aligned parallel to the cylinder head (Bugatti Veyron W16). In most production VW VR6 engines, there was just a gap and that was fine.
The single cylinder head is of a cross flow design, meaning there is one distinct intake manifold and one for exhaust on the other side. Because of the staggered configuration of the cylinders, the intake runners are different lengths depending on which bank the cylinder is. The near side bank will have a very short intake port, while the far side bank will require an intake port to travel across inside of the cylinder head. Same goes with the exhaust ports, which is most likely why the VW VR6 sounds so good. That would be in addition to the somewhat unusual 6-throw drop forged 7 bearing crank shaft (A flat-plane crank has 2 ‘throws’ and a cross-plane crank has 4 ‘throws’). The intake and exhaust manifolds are made, in part, to compensate for the different port lengths.
Don’t take my word for it, after searching for at least 15 minutes through YouTube trying to find a clip that wasn’t about shooting flames out of the exhaust pipe, I came across this clip, specifically detailing the VR6 in the R32.
VW VR6 Family History
The VW VR family has multiple generations and even a few different configurations. The first VR6 one debuted in the Corrado, when Volkswagen was attempting to take a swing at the Porsche 944 in 1991. It also made it into the B3 Passat as well. This first engine was a 12v design, which means each combustion chamber features two valves per cylinder for 12 total. In North America we got a 2.8 liter at launch, designated the AAA, where Europe got a 2.9l ABV. The extra few CC’s were gained in the bore diameter, and if anyone knows why VW would do it this way, hit the comments below. Ignition was your standard cap and rotor, and injection was your standard low pressure port variety. It was all controlled by Bosch Motronic.
The AAA was fairly undersquare, with a bore of 81mm and a stroke at 90.3mm. This is presumably due to the shape of the engine being fairly restrictive on bore size (especially at the bottom of the block where the narrow V comes together). Speaking of which, that angle was a very acute 15 degrees in order to fit in the small Mk3 Golf engine bay, a pairing which North America would finally get in 1994. The compression ratio is a modest 10:1.
As the 90s progressed and the engine proved to be a hit, VW wanted to expand the VR6. Well, maybe expand isn’t the right way to put it. By expand, I mean lop off a cylinder and a main bearing, and create an even more compact VR5 engine in ’97. Us yanks would never get the V5 (#morepower) but it was a stellar device to fill the gap between the 6cyl ABV and the somewhat wheezy 1.8t.
It took all the way up to 1999 until the VW VR6 got an upgrade to 4 valves per cylinder, for a total of 24. Putting that many valves in such a small space took some real thought. With the narrow width of the single cylinder head, fitting 4 cams was basically a no-go. Instead, Volkswagen came up with a series of rocker arms to activate all 24 valves with only two camshafts. Unfortunately, the North America market wouldn’t get the 24v 2.8 for a few years, and in the mean time the 12v got an update to tide us over, the AFP. The update included a variable intake, a small bump in compression ratio (to 10.5:1), and a new pair of camshafts for slightly better lift and duration. Topping off that somewhat odd choice is…when the 24v engine did finally show up, it was in a Eurovan.
The next year, the MKIV got the 24v 2.8l BDF and all was well and good in the world. in 2001, an upgraded 3.2l 225hp VR6 was developed for a hotted up New Beetle (The RSi, which never came to North America), but eventually ended up in the MKIV and MKV Golf R32 and the Audi TT. The 3.2 got another compression ratio increase, to 11.3:1, and had the signature undersquare style design at 84mm bore and 96mm stroke.
In the mid 2000’s FSI direct injection was added to the 3.2l and another VR6 emerged. With a narrower 10.5 degree Vee, a displacement bump to 3.6l, and direct injection, the top of the line VR6 put out an impressive 275hp. The 3.6l had a 12:1 compression ratio and an undersquare 89mm bore and 96.4mm stroke, making it very torquey and suitable for larger SUV’s and trucks such as the VW Toureg and Porsche Cayman. Interestingly enough, unlike all the changes mentioned above, North America got the hot new engine first, and it went into the Passat VR6. VW has strange ways sometimes.
The VW VR6 Phase Out
The VW VR6 was designed in a time where there was no replacement for displacement. The cap and rotor days have long been over. There’s no need to jam cylinders together with the new crop of high tech, direct injected, turbocharged 4 cylinders. Here in 2019, the VW Atlas is the last vehicle sold in North America to roll off assembly lines with a VR6 engine (The last generation 3.6l). Who knows what the future will hold, but one thing is for sure…. There will be VR6’s rolling around for years to come.
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