By on October 10, 2007

bmwbiturbo-overview.JPGThe American automotive market offers just three inline six (I6) engines. First and foremost: BMW's 3.0-liter unit. Devotees will find the propeller people's sublime I6 in the 328i and 528i, and in turbocharged form, in the 335i and 535i. Otherwise, there's GM's excellent 4.2-liter I6 found in the Trailblazer, Envoy and Saablazer 9-7x. And don’t forget Volvo’s new 3.2 and 3.0L I6 engines, used in the 2008 V70/XC70/S80/XC90 and the baby Land Rover. These sweet, smooth, silky engines are all that's left of a once-proud breed. The GM engine will probably die along with its host SUV in a few years, just as Jeep dropped its 4.0-liter inline six with the passing of the Wrangler. Mercedes ditched its straight six over a decade ago. Jaguar used to sell a six cylinder inline engine in the classical XJ6. And of course there were three outrageous examples from Toyota: the 4.5-liter straight six in the Land Cruiser and two different 3.0- liter six pots holstered by the Cressida, Supra, Lexus SC300 and IS300. Why has the I6 gone by the wayside? Lots of reasons. A V6 is a lot more compact, and most manufacturers use engines across their entire brand lineups. That means a modern V6 engine has to fit both transverse and longitudinal applications; Nissan, Toyota, GM, VW/Audi twist their V6s by 90 degrees. Hopefully the future will hold more I6 engines, if only because they tend to be so full of character, well balanced and smooth. Until then, you can pick up your own straight six on the cheap in a used Suzuki Verona (which even mounted it transversely).

[Click here for the technical differences between I6 and V6 engines]

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52 Comments on “Straight Six RIP?...”


  • avatar
    Sajeev Mehta

    The Suzuki Verona also has an I-6.

    I’ve had two friends wind up with $20,000 Bavarian heaps (dealer serviced) because their coolant overflow tanks burst and the I-6 overheated/warped the head. I like I-6s but they are heavy, bulky and can’t take the heat. And replacing the motor on a used BMW is not feasible for the average BMW owner.

  • avatar
    Alex Dykes

    I thought the Verona was axed after 2006?

  • avatar
    Virtual Insanity

    Could we call the VR6 a “metro 6″ perhaps?

    And Sajeev, I’m sure there are plenty of Supra and Skyline owners that would argue about I6′s not being able to handle power. Unless you are just talking about the BMW I6s, which, as a former BMW owner and dealership service drive employee, yeah, your right.

  • avatar
    Justin Berkowitz

    Alex: Yep, you’re right. They’re cheap as a sack of flour to pick up used though (and less useful).

  • avatar
    Sajeev Mehta

    AD: My bad.

    VI: Oh I know the Supras are insane HP makers. Who knows, maybe they overheat -> warp heads real quick too. No experience with them.

    I’m only referring to BMW I-6s and the frustration the dealers heaped upon my friends. I hate seeing that happen to people.

  • avatar
    Martin Schwoerer

    The other day, I drove a Chevrolet Epica that had a 2-Liter straight six. (From the company formerly known as Daewoo). DOHC, 24 valves.

    It was smooth but not strong and I failed to see the advantage to a V6. Chevrolet/Daewoo, I think, originally intended this design to have some BMW cachet but they gave up rather quickly.

  • avatar
    Alex Dykes

    The VR6 in german literally means “Inline V” so I’m in for a .5 maybe on that, depending on the exact angle of the cylinders. I would argue that in spirit an engine is a V if it actually has two heads. The VRs use one. But… The cylinders are offset by what, 10-15 degrees?

  • avatar

    Don’t forget – the BMW straight six is also in the X3/X5, and comes in three different versions (215, 255, 330 hp) in the absolutely sublime Z4 and Z4 Coupe.

  • avatar
    Sid Vicious

    Are you forgetting the superb I6 Cummins in the Dodge trucks, unless you meant to exclude HD pickups?

    As far as power – there’s nut jobs out there claiming 1000HP. Of course, it’s usually not long before they frag their tranny or blow the head gasket. But I believe that motor’s dead reliable up to 500HP or so.

  • avatar
    AGR

    Certain engine designs stand out through time, the inline 6 is one of them, its a timeless desgin, although challenging to “package” in anything but a RWD application.

    A V6 be it 90 degrees or 60 degress, are easy to package especially in a FWD application, the actual engine is rough, needs a balance shaft in certain configurations, and does not have the sound.

    The majority of HD trucks in North America have inline 6′s be it from Caterpillar-Cummins- Detroit Diesel

  • avatar
    Alex Dykes

    FWD? Eh, just whack a cylinder off and go funky with an I5.

  • avatar
    jkross22

    Other than V8′s, the inline 6 seems to have the best more revs = more power equation. Here’s hoping BMW, Volvo and GM don’t give up on this design!

  • avatar
    Redbarchetta

    I hope they soldier on and some other manufactures decide to start making I-6 again. Much prefered over a V-6. Unfortunately with this hybridization of everything it will probably mean the death of the I-6 because of packaging, shame.

    I would love to see someone come out with an oversquare under 2 liter straight six with a low boost turbo. Something that can fit into a v-6 sized hole putting power to the rear wheels.

  • avatar
    Emro

    AD: you’re right, the VR6 is a 15 degree vee

  • avatar
    philbailey

    ‘Scuse me if I weep no tears about the Jeep I6. A perfect example of how you can still screw up even the best engineering principles.

  • avatar
    willbodine

    Car shopping recently I heard from a BMW salesman that their straight six was “inherently better balanced” than a V6. I couldn’t resist asking him when they would be bringing out a straight eight.

  • avatar

    There are few engines as visually alluring as a DOHC straight-six. Think of those cathedral-like Jaguar, Aston-Martin, Benz, or Maserati engines of yore. Stunningly beautiful to look at, and silky-smooth wonderful to drive! The exhaust note of a big-bore, long-stroke straight six is wonderful to hear as well.

    All the V-layout motors have all that … stuff… plopped on top of, and under them so they look ungainly by comparison to the cross-flow elegance of a classical DOHC six. But manufacturers now wrap their mechanicals in shrouds and plastics, hiding the machine itself from view. Pop open the hood and… “it is a plastic box with a logo screenprinted on top!” I understand that this assists in making a quieter ride, but it also robs the aesthetic appreciation for the functionality of the machinery and its operation. Gearheads like to see the flow of fuel and air and how it mixes, moves, burns and exhausts. Not possible in these machines anymore… look, it is a black plastic box with a logo screenprinted on top. Sigh.

    Finally, front-engine, rear-drive cars are becoming a rarity and Detroit’s marketing muscle has killed the image of the I6 in the mind of the consumer (Detroit’s pushrod sixes were underwhelming anyway, in all aspects beyond reliability) so other than the rare Bavarians it seems all but dead.

    http://chuck.goolsbee.org/archives/307

    –chuck
    http://chuck.goolsbee.org

  • avatar
    tentacles

    Meh, the Japanese iron block I6s were overrated IMO. They did fine as Japanese competitors to the contemporary 3-series, and it’s nice that they eventually upped the power to what they did, but it was Toyota and Nissan trying to get squeeze the last life out of an obsolete concept (that started with their domestic sports sedans – the Toyota Chaser/Cresta/Mark II and the Nissan Cefiro/Laurel) since they had no V8 offerings, while everyone who knew better was going to aluminum V8s for their performance cars (Hello LS1). Now they do know how to make good V8s, so it’s no surprise that they’ve ditched the I6 like a washed up pop star.

    It’s no accident that the JGTC Supra race cars use either 3S-GTE I4s or 1UZ V8s, but not I6s.

  • avatar
    GEMorris

    The volvo straight six is mounted longitudinally in every volvo application. As is their Yamaha-designed v8.

    By seperating all of the accessories to a separate unit, they were able to fit the I6 into the same space as the old I5. Unfortunately it doesn’t look like there is room for turbos on the new I6.

    I6′s also have serious RPM limitations due to long long long drive shafts and camshafts which can lead to warpage at high RPM. An I4, V6, or V8 will, if properly designed, always be able to outrev a similarly designed I6.

    I6 does deliver some serious torque, but hey, so does a turbo slapped on an I4, and that gets better fuel mileage.

  • avatar
    Justin Berkowitz

    GEMorris:

    What about the T6 engine in the S80 sedan? Is that an older style block?

  • avatar
    melllvar

    Ford’s I6-in-exile is being retired in favor of RWD Duratecs. I would rather the Barra I6 replace the Cologne & Vulcan V6s in the US.

    A Mustang XR6 Turbo would be sweet.

  • avatar

    An inline-six IS intrinsically smoother and better balanced than a V6 of the same displacement, even a 60° V6. It also allows freer-breathing intake and exhaust manifolds. There’s a trade-off in that the crankshaft and drive shafts are longer, which increases their rotational inertia (thus limiting their ultimate RPM potential), although the relevance of that in a street engine is dubious. But the primary reason they’ve been replaced is just packaging.

    The old Japanese iron-block sixes were something of a mixed bag because many of them were derived from inline-fours (in the same way inline-sixes and straight-eights were once related). The Nissan straight-six in the Datsun 240Z/260Z/280Z, for instance, was basically the iron-block four with two extra cylinders. Doing that is economical from a production standpoint, but it’s hardly a pinnacle of efficient engine design.

    The cost rationale is compelling to a lot of manufacturers, though — the Buick 90° V6 is 90°, for instance, because it was derived from the old aluminum V8, and Buick engineers wanted to be able to build it on the basic tooling. A lot of engines exist for that reason; it’s a lot cheaper to revamp the design of an existing engine than to come up with a genuine clean-sheet design.

  • avatar
    guyincognito

    The bimmer I6′s can take heat just as well as any motor, its the cooling system thats the problem. Plastic tubing on the radiators, plastic impellers on water pumps, and plastic expansion tanks are prone to failure. How many engines can run for long with no coolant? Of course these issues are easily fixable. After that the engines are rock solid. I’ve seen plenty of people running 300 supercharged whp in daily driven E36 M3′s with over 100K miles. Soon you’ll be able to count me in on that category. There are built turbo I6′s (dyno queens though they may be) putting down ~ 1000 hp.

    There is no substitution for the smooth torquey power of the I6 IMHO. With a properly tuned exhaust only a 12 cylinder could top its note. And not high revving? Tell that to all the E46 M3′s out there.

  • avatar
    Justin Berkowitz

    @melllvar:

    Apparently the Barra I6 can’t meet Australian emissions regulations. If that’s true, it definitely won’t meet American ones. Pity, that. But the 3.5 liter V6 is a sweet engine and headed for, well, pretty much everything. And it takes a turbocharger, too.

  • avatar
    Vega

    My father’s W124 E320 convertible is the best advertisement for straight 6s you could imagine. Smooth, strong, classy. On the other hand, my M20 powered E30 320i convertible is completely gutless below 4000 rpm and is running on its second cylinder head. But the lack of vibration and smooth, turbine-like sound is still addictive. V6 engines are a victory of packaging necessity over engineering excellence.

  • avatar
    tiger260

    BMC in the UK used to make versions of their Austin/Morris 2200 “land crab” car in the 1970s with a straight six engine fitted transversely across the front. There was even a fancy badge-engineeered Wolesley version called the “Wolesley 6″.

    They were really quite nice cars (if you like that Issigonis-front-wheel-drive-wheel-at-each- corner type thing). But a bit odd-looking and they never really were much of a sales success…

    We had one of the more standard 4-cylinder Morris 1800 versions as a family car and still have very fond memories of it. I doubt if there has ever been a family car built since with such a high ratio of usable interior space to outside dimensions……

  • avatar
    stuntnun

    pretty sure this is even more true on the skyline but i know the supra can handle alot of bolt ons and turbo upgrades on stock internals and handle 450 hp easy with no warpage or heating issues if the radiator and inter cooler are set up correctly

  • avatar
    Alex Dykes

    The T6 is the same block as the 3.2L NA engine. They are loosly based off the Volvo modular engine family of 4-6 cylinder inline engines that has been around for some time. The length of the engine has nothing to do whith whether or not a turbo would fit untder the hood since the turbo (in Volvo’s applications) is behind the engine, and the intercooler up front. Infact, the inline design makes turbocharging easier in this layout, there is plenty of room under there.

  • avatar
    melllvar

    Justin,

    Didn’t realize, thought it was just a Mullaly OneFord move.

    I liked my Mazda6′s 3.0L V6. Moving to a common V6 makes sense, and I do like the engines.

  • avatar

    Your headline scared me. I thought that you were going to announce that BMW was going V6.

    Now I have nothing against V6 motors, I used to own one of the best (Yamaha-Ford 24V/3.0) but I’m one of those who think that “real” BMWs have I6s.
    I’d never have anything else in my Bimmer.

  • avatar
    Sajeev Mehta

    The bimmer I6’s can take heat just as well as any motor, its the cooling system thats the problem. Plastic tubing on the radiators, plastic impellers on water pumps, and plastic expansion tanks are prone to failure.

    guyincognito: Three plastic expansion tanks in my case. The third was a 540i, which survived the overheating with zero problems. The “flaw” of a long aluminum head is warping at high temperatures. Other than that, the I-6 is a nice motor.

    Nice, but an GEN-III GM small block is lighter, more compact, has a lower center of gravity, loves fuel efficient gearing and sports a much fatter powerband to go with much more HP out of the box.

    I-6′s are nice, but V8′s rule. (I can’t believe I went there!)

  • avatar
    tentacles

    Well, BMW seems to agree with you, as does Audi, MBz, and Toyota. I guess the rumors of the V8 Nissan GTR were unfounded, but they don’t use I6s either.

  • avatar
    melllvar

    I-6’s are nice, but V8’s rule.

    +1

  • avatar
    SunnyvaleCA

    Are the Porsche 6′s inline designs lying on their side? Or, are they horizontally opposed engines (180-degree V’s if you will).

  • avatar
    tentacles

    Modern Porsches all use horizontally opposed engines, except for the Cayenne.

    If I’m not mistaken, 180 degree V engines are just that, with 2 pistons per crankpin. They are not the same thing as a horizontally opposed Subaru/Porsche/Corvair boxer engine, where each cylinder has it’s own crankpin. I think the Testarossa was the last street car with a real 180 degree V engine.

    I’m ready for a return of the Inline 5 for sports cars. Where is the modern Sport Quattro?

  • avatar
    DearS

    An Inline 6 is the most practical ie. best engine for many applications I favor. Its more practical for a small-medium sized couple or sedan for everyday relaxed/responsive enjoyment. A four is too tedious and a big V8 too on and off for me in the long run/everyday. Still the engine is only a part of what I like about my cars.

    btw, I drive an E30 325is.

  • avatar
    GEMorris

    The Volvo I6 I was speaking of took the alternator, ac compressor, power steering pump, etc, and put those accessories in a separate package attached to the motor, very near to where the turbo would be. However, upon further research, it seems that volvo has planned turbocharged versions of this engine from the start. The engine designation is SI6

    The big deal about the engine is that in n/a trim it fit in the same space as the turbo 5, and also provided more power.

    http://en.wikipedia.org/wiki/Ford_SI6_engine

    As a previous poster mentioned, the rotational mass per cylinder on an I6 is very high, making high rpm race-style engines problematic (so problematic the E46 M3 was the last I6 and had a questionable reliability history, the only way they were getting more power was either turbos or v8 and they went v8). All of this is of course, as it has been said, of dubious use in a street engine. Straight sixes are smooth beasts.

  • avatar
    Virtual Insanity

    Stuntnun:

    From someone far more knowledgable on the Poopra than myself (and I know a decent bit about them),the 2JZ-GTE can handle upwards of the 600 mark on stock insides. Nissan’s RB26DETT is just as strong.

    On a 2JZ-GTE, 450hp is a turbo back, intake, BCC and touch of more boost away. Thats the famed “BHP” term thats actually been copywriten. The Supra and Skylines can be a beast just as easily as any Corvette, Mustang, or FBody. Its not about displacement, boost, or cylinder count. Never has been.

    Its all about the cash.

    Modifying cars is the art of taking money and making it into noise.

  • avatar
    Alex Dykes

    GEMorris: Volvo hasn’t just planned turbo versions of them, they have been shipping them for a while now in Europe and the USA. Just checkout your local Volvo dealer and ask for an S80 T6. 3.0L, low pressure single turbo, 285HP 400nm, excellent torque curve, nice engine note, wish it was 50HP more powerful.(stay tunned for the high pressure version)

  • avatar
    GEMorris

    Alex: True true, the previous T6 was twin turbo, Do you know if the turbo SI6 is twin or single?

    I’d like to think I know what volvo is doing but I really only care about (and pay much attention to) the v50/c30 now that they killed the R-line.

  • avatar
    tentacles

    The problem with the Japanese ironblock engines is that they are massively heavy to begin with (hence their resistance to stress, there’s nothing particularly clever in their construction), and the addition of turbochargers and all the accoutrement equipment means that they are simply not competitive with aluminum V8s from a power/weight ratio perspective. The turbocharged Supra weights over 300lbs more than the NA version. No link off hand, but IIRC the entire 4.2l engine in the Audi RS4 weights around 300lbs. If some of the stats for engine weights around the net are to be trusted, the 3.0l 2JZ engine on it’s own weighs 594lbs, and ~750lbs with the T56. Compare this to 575lb for the old iron block 5.7l LT1, and for such a behemoth of an engine (in terms of weight), 600hp doesn’t sound all that impressive. I think the aluminum LS1 WITH THE TRANS weighs in at a hair over 500lbs.

    It’s not surprising, since the JZ and RB series engines had their roots in much more sedate luxury cars, and were never meant to be the high performance powerhouses that they eventually became.

  • avatar
    jthorner

    Am I imagining things? Didn’t this article talk about two remaining I6 engines this AM? I must be hallucinating.

    Anyway, I would expect that the current generation of I6 engines is probably the last we will see in automotive use due to the limitations others have already mentioned.

    It was kind of strange for GM to do a complete clean-sheet I4/I5/I6 engine family for the Trailblazer and Colorado families. It would have been much more cost effective for GM to continue sharing basic engine designs with it’s other cars and/or trucks. Between the Ecotec-4, two families of V-6 engines (some already fitted into Silverados) and the range of truck V-8s they could have covered the Colorado range of vehicles without all-new designs.

    Personally I love the smooth character of a great I6 engine, but they aren’t a good fit for today’s world. I do, however, hope to keep the I6 in my old F150 running for a long, long time.

  • avatar
    doctorv8

    @ GEMorris:

    The volvo straight six is mounted longitudinally in every volvo application. As is their Yamaha-designed v8.

    The Yamaha derived 4.4L V8 Volvos are transversely mounted, not longitudiunal…just as their 3.4L V8 predecessors were in the 96-99 Taurus SHO. Obviously, the I6 in the same car (Volvo S80) is also transverse.

  • avatar
    Justin Berkowitz

    @jthorner:

    You are not going crazy, no worries. The article was incomplete at first, then someone said “hey idiot, you forgot Volvo.”

  • avatar
    skor

    One of my high school buddies had a hand-me-down Ford F-150 with a 300 cube straight six. That truck soldiered on despite our best efforts to kill it. We ended up calling it “the thing that wouldn’t die”.

  • avatar
    guyincognito

    @Sajeev, oh no you di..ent! Seriously though, these comments are making me sad. One of the coolest I6′s ever just came out not to mention its dual turbo diesel cousin and yet its death bell is being sounded?

  • avatar
    Johnson

    I too like the smoothness of I6 engines as well as the way they look, but ultimately those of you who still think it’s a good idea for manufacturers to make I6 engines in the future are clinging to old ideals. That’s just like saying in the 1970s and 1980s that it’s a good idea to continue V12 development just cause you like all the old cars that had them.

    There is no meaningful technical or economic reason as to why any manufacturer needs to continue with I6 gas engines. BMW continues to cling to I6 gas engines just because they don’t have that much experience with V6 engines, and they wouldn’t be able to make a good V6.

    In terms of the I6 being a very smooth and balanced engines, modern V6 engines have balance shafts, and there are state-of-the-art active engine mounts that neutralize any vibration. I6 engines are physically large and apples to apples heavier than V6 engines.

    I6 engines in diesel applications on the other hand are very much alive. There are numerous reasons why an I6 design makes great sense in a diesel application.

  • avatar

    Johnson, why does I6 make particular sense in a diesel?

    No discussion of I6 is complete without an allusion to the Chrysler Corp slant sixes of the sixties and early ’70s. My parents had a 1970 Valiant. I knew, as soon as I test drove the thing, that it was going to last and last. It was silky smooth, and far more powerful than anything they’d ever owned. My sister, then 7, told me years later that she, too, knew that it would be around for a long time. It was still going when she graduated from college.

  • avatar
    Brock_Landers

    Massive iron block is not the only reason why 2JZ-GTE and RB26DETT are so easily modified. It’s in the advanced head design, intake, oiling, cooling, state of the art materials (internals). These engines were over engineered(which happens rearly these days, in the bean counting era that we’re in), factory showcases of japanese know-how and craftsmanship.

  • avatar
    Vega

    @ Johnson: I’m sure BMW would be able to make a fine V6. They developed brilliant new V8s and V12s in the 1980s from scratch without prior experience to build on, both to great sales success.

    It’s just they know I6s are part of their heritage (since the 1930s 328 racecars) and add tremendously to their brand cachet (something that many V6 producers are missing).

  • avatar
    Sajeev Mehta

    @Sajeev, oh no you di..ent! Seriously though, these comments are making me sad. One of the coolest I6’s ever just came out not to mention its dual turbo diesel cousin and yet its death bell is being sounded?

    It is sad, but progress is progress. Considering the inline engine designs are the oldest (and the OHV V8 is the newest) maybe today’s cars have outgrown it.

    I still think the I-6 is great for trucks new and old. Space and weight aren’t big concerns here and torque is paramount. I’d love to see Cummins make more diverse diesel truck motors and a reincarnation of the 300 six in an F-150.

    V6 trucks kinda suck, comparatively.

  • avatar
    BimmerHead

    Both of my vehicles are powered by I6 engines… a BMW and a Jeep Wrangler. I find little more annoying than seeing either of these vehicles advertised for sale with V6 engines (new Wranglers withstanding).


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