The Spun Bearing: The Case For the 90 Degree V6 (With Reservations)

Interesting | March 30th, 2012 by 6
BMW-M3-F80-F30-2013-Erlkoenig-Spyshots-02

The latest information (rumors) circulating about the next M3′s engine suggest that BMW is seriously thinking of placing a V6 (a 90 degree V6 at …

The latest information (rumors) circulating about the next M3′s engine suggest that BMW is seriously thinking of placing a V6 (a 90 degree V6 at that) into the F80. Heresy!! BMW, a company known for it’s silky smooth inline sixes appears to be treading into Pontiac territory (Hello Buick 3800!) with a 90 degree V6. At least that’s the fear.

Why A 90 Degree V6

The usual development path for a 90 degree V6 is through a cut down V8. Mercedes-Benz replaced its inline sixes with 90 degree V6s in years past, but has since developed a 60 degree V6 (allowing a common journal for a pair of pistons on either bank – more on this later) and a natural 120 degree firing order (one cylinder ‘fires’ every 120 degrees of crank rotation – all six cylinders fire within 720 degrees of rotation).

The Spun Bearing: The Case For the 90 Degree V6 (With Reservations)

However, the B8 Audi S4 uses a 90 degree V6 engine, using direct injection and supercharging. It has made it onto the Ward’s 10 Best
Engine list (which is no small accomplishment). And that engine is competitive with the N55 inline six that BMW produces. But whatever BMW decides to do with the M3 engine it will produce more power than the Audi 3.0 TFSI V6. (And make no mistake – you heard it here first – the coupe will be an M3, I believe there is too much history with the M3 label to introduce an ‘M4′, but could they introduce a 435i? That’s a question for another day – and a different set of styling cues too).

Benefits

First, the V6 (90 degree or 60 degree) puts less weight over the front axle centerline than an I6. And less than a V8 too, for that matter. The 90 degree V6 sits lower in the engine bay, reducing the center of gravity slightly. But from BMW’s perspective, the width of the V between cylinder heads allows for their ‘reverse flow cylinder head’-'short path from port to turbo’ plumbing.

A V6 should also have fewer friction losses than the BMW I6 since the V6 will utilize three main crank bearings while the I6 uses seven. Another plus is the shorter length of crank and camshafts which the V6 architecture offers.

BMW can ensure that each cylinder will fire at 120 degree intervals (like an I6 or 60 degree V6) if they use offset crank journals. That
reduces some of the odd noise that the 150/90 degree firing of a non-offset crank journal 90 degree V6 produces.

Reservations

One thing about offset crank journals for a 90 degree V6 bears stating, unlike a 60 degree V6 that connects two pistons (one from each bank) on a common journal of a given diameter, the offset crank journals – given the same diameter as the 60 degree crank journal – actually have less strength since the total connected area is offset. (FYI, the big end of the connecting rod attaches to the crank journal.) To see an illustration of that, I urge you to click through to this link – its from Hot Rod magazine (a really good value in a magazine from my perspective).

The other issue with a 90 degree V6 has been the miserable sound of the engine under power. Flatulence is a polite description for the engine note of some 90 degree V6s. This will bedevil the M engineers as much as the benefits for using a 90 degree V. It has to sound right, M has their work cut out for them.

What Is the End Result

The smart thing for BMW to do is reuse the geometry of the existing S63 V8 for the block and heads of the V6. They would be casting the heads, block, and crank girdle from new patterns of course. It would also have to use a different crank, cams, etc., but on the whole you can bring a new engine online with some good idea of what you’re dealing with.

Look for BMW to possibly reduce the bore of the V6 to 3.0L (from a 3.3L max that the V6 would carry over from the 4.4L S63 V8) – as Horatiu has already alluded to. There could be one, two, or three turbos. The three turbo setup would be two small turbos fed from each bank bypassed to a larger twin scroll turbo at higher RPM. The two turbo setup would be smaller twin scroll turbo bypassed to a larger twin scroll at certain RPM. The single turbo setup (the least likely in my estimation) would be a twin scroll turbo.

But, here’s the fun part, if the power gains between the outgoing M3 and the new M3 reflect the same gains that we saw in the latest M5 (from NA V10 to turbo V8), the HP number would be something north of 440 (think 460) and the torque number could be right at 390 lb-ft. And that’s a ton more torque than the outgoing V8. Should be fun. The other angle to remember, this engine should be considerably more efficient than the V8 it’ll replace. That’s the other big piece we saw with the new M5.

So, can we embrace a 90 degree V6? Possibly, but the proof in the pudding will be how it sounds. Get that wrong and and maybe they’ll have to pipe some artificially generated noise into the cabin to mask it.

Time for a poll:

[poll id="107"]

Andrew Murphy and Misha Nikolich contributed to this article.

  • Otto

    I seriously doubt BMW is still thinking about it. The choice is certainly made. If the M3 is due by the end of 3013, a brand new engine can’t be developed and tested in that timespan, even if it’s derived from an existing powertrain. But you guys have to get used with the idea that BMW is not “your BMW” anymore. FWD branded BMWs are coming soon, no more N/A I6, and so on.

  • Anonymous

    You missed the negative that a V6 doesn’t run as smoothly as an I6 and that it has to use a power robbing balance shaft.  There could also be more friction losses due to having two heads.  

  • Giom

    Very insightful Hugo! I’ve said it before, I’m not crazy about the idea of a V6, simply because of tradition. If BMW can make a very good case for one, I’ll accept whole heartily. What I don know, is that BMW will not do something if they can’t make it better than anybody else has done before.

  • v6 guru

    60 degree v6 pistons dont share a common crank journal

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