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in-line 6 Cylinder Engines, 'straight-six'

Discussion in 'Engines' started by brian eiland, Jun 26, 2012.

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  1. brian eiland

    brian eiland Senior Member

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    For years now I was aware that the in-line 6 cylinder engine was likely the BEST configuration for a 4-cycle internal combustion engine (the most naturally balanced configuration). I had been told this long ago by some car racing buddies, but I don't know that I ever looked it up in the library or whatever,...at least I may have, but I had forgotten the specific’s.

    Today I decide to see what I might find via google, and this site was the first to pop up:
    Straight-six engine - Wikipedia, the free encyclopedia

    Wow, this is pretty inclusive. I'm going to excerpt a few of the passages from that site for posting here on this subject thread that might ask a few more questions of more specificity as related to 6-cylinder Marine engines.

    The straight-six engine or inline-six engine is an internal combustion engine with the cylinders- mounted in a straight line along the crankcase with all the pistons driving a common crankshaft. The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.

    Balance and Smoothness
    An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs. That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero.

    An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed — for example, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (rpm). This characteristic has made the straight-six popular in some European sports-luxury cars, where smooth high-speed performance is very desirable. As engine reciprocating forces increase with the cube of piston bore, straight-six is a preferred configuration for large truck engines.

    Straight-Six Diesel Engines
    The straight-six in diesel engine form with a much larger displacement is commonly used for industrial applications. These include various types of heavy equipment, power generation, as well as transit buses or coaches. Virtually every heavy duty over-the-road truck employs an inline-six diesel engine, as well as most medium duty and many light duty diesel trucks. Its virtues are superior low-end torque, very long service life, smooth operation and dependability. On-highway vehicle operators look for straight-six diesels, which are smooth-operating and quiet. Likewise, off-highway applications such as tractors, marine engines, and electric generators need a motor that is rugged and powerful. In these applications, compactness is not as big a factor as in passenger cars. Reliability and maintainability are much more important concerns.

    Most of the engine components and accessories can be located along both sides, rather than on top of or underneath the cylinder banks, meaning that access and maintenance is easier than on a V engine in a truck or industrial configuration. In addition, a straight-six engine is mechanically simpler than a V6 or V8 since it has only one cylinder head and the overhead camshaft configuration has half as many camshafts
  2. Capt J

    Capt J Senior Member

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    The CAT 3406 is a good testament to this as is the DD 6-71. They're both a testament to durability and great engines, and inline 6's
  3. SHAZAM

    SHAZAM Senior Member

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    Never thought about this. I don't feel that any of the inline six Cummins can be described as "smooth" on the other hand there seems to be nothing smoother than an old 671.
  4. Ormond Bert54

    Ormond Bert54 Senior Member

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    But ... if you could find an old Dodge Dart:) So smooth you couldn't even tell it was running. Or the AMC 258 6 cyl:)
  5. Capt J

    Capt J Senior Member

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    I think that has more to do with Cummins injection system. The good old Perkins 6T-354 was a pretty smooth motor also.
  6. Pascal

    Pascal Senior Member

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    On the gas side, the old jaguar XK engine is a great example... Launched in 1948 it won Le Mans 5 times in the 50s and remained in production into the 90s

    One of the best gas engines ever designed...

    The big issue with a straight 6 is the length which simply doesn't work in modern stubby bonnet-less cars...
  7. AMG

    AMG YF Moderator

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    I think my Grandfathers Studebaker had a straight eight... :)

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  8. RT46

    RT46 Senior Member

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    How about the Ford 300, straigth 6 gas engine in their light trucks, that was a pretty decent engine with plenty of tk and half the maint and less fuel used than than a big block ford.

    On the diesel side, the 671 is iconic....

    with a very close second to the Cummins 6BTA for the smaller boat crowd

    the moder marine diesel of choice for the under 50 crowd seems to be the Cummins QSMs, also an in line 6.
  9. Fishtigua

    Fishtigua Senior Member

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    BMW seem have done quite well with the old M3 and M5 plus the 2.5 & 3.0ltr diesel engines. Lovely motors.


    Look how many straight sixes are on this list.

    10 Best Diesel Engines Ever - Diesel Power Magazine
  10. Marmot

    Marmot Senior Member

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    If 6 is good, 8 has to be even better:

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  11. Bamboo

    Bamboo Senior Member

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    Mercedes OM 617-9XX IDI 3.0ltr straight 5 (NA and FI) found in 1977-1985 MB models should be on that list.
  12. carelm

    carelm Senior Member

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    I had a couple of XJs with the straight six engine. They were really smooth and bullet-proof. Also they were pretty easy to work on. IMHO Jag made a huge mistake by going to a V8 before they worked the bugs out. The first V8 engines had major problems with Nikasil cylinder liners, timing chains and water pumps.
  13. Capt J

    Capt J Senior Member

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    Well 8 cylinder Gardener diesels are well known to go forever and ever.
  14. brian eiland

    brian eiland Senior Member

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    ....from another forum
    That appears to be in direct conflict with this quote from that weblink??...and I'm sure I could find many other praises for the natural balance of an inline 6.
    "An inline six engine is in perfect primary and secondary mechanical balance, without the use of a balance shaft. The engine is in primary couple balance because the front and rear trio of cylinders are mirror images, and the pistons move in pairs. That is, piston #1 mirrors #6, #2 mirrors #5, and #3 mirrors #4, largely eliminating the polar rocking motion that would otherwise result. Secondary imbalance is avoided because the crankshaft has six crank throws arranged in three planes offset at 120°. The result is that the secondary forces that are caused by differences from purely sinusoidal motion sum to zero."


    I would ask you to find some verifiable references to that statement.

    I do remember that Chrysler did some experimenting on their slant-6 and achieved some success in racing when engineers utilized the slant of the engine for very long intake ports to boost horsepower by tuning the intake system.

    Any breathing problems for a marine 6 would be overcome by turbo boosting which is a natural fit for a good strong 6 engine not worried about turbo-lag as in quick acceleration in a vehicle.


    I'm not quite sure what you are referring to here?....more wear or less as a result of the slant?

    I seem to recall that the Plymouth Valiants and Dodge Darts had quite a good record for durability....exceptional I believe?
  15. brian eiland

    brian eiland Senior Member

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    Wasn't Jag's major problem with valve adjustments?....often getting out of adjustment, ....and then not easily adjusted....shims or something like that??
  16. Grecko

    Grecko New Member

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    While a straight six is the smallest cylinder count that has primary and secondary balance, it isn't that great in terms of power to weight ratio. That is, for the amount of weight the displacement isn't as great as a V8. To put it another way, for the same weight you can get more power with a V8 than you can with a straight six. And obtw, a V8 also has both primary and secondary balance.

    Bottom line is that a straight six is less expensive to make, and it can be smooth, but the reason V8's dominate in performance cars is that the power to weight ratio for a V8 is a lot better.

    Also, if you start to make a big straight 6, the shaft torsional vibration can become issue at high rotational speeds.
  17. carelm

    carelm Senior Member

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    Brian,

    The 1990 XJ6 I had required shims as you noted but the clearances stayed fairly constant. The 1996 XJ6 had valves that were adjusted automatically. I had no problems with either one. Both were superb highway cars although a bit large. They were also fairly low-slung which aided handling. I compared my 1996 XJ6 to my brother's S-Class Mercedes and while they were about the same length, the Jag was about six inches lower.
  18. ddw1668

    ddw1668 Senior Member

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    My 1973 Matthews was re-powered in 99 with Mack Six cylinder turbos rated at 650HP replacing DD 8V92's. She will do 25 mph while well loaded with almost no smoke and 2000 hours.

    I had a 1990 XJ6 with the inline 6 and it required several valve adjustments until a drunk rear ended my wife and totaled it. A 97 XJ6 required no valve work. We now have a 1999 XJR and a 2005 Super V8 with supercharged V8s with the Nikasil liners. The only major problem with any of my four Jaguars over the years was a trans failure in the 99. They used a Mercedes trans for that model as theirs would not handle the 370 HP.

    Bottom line........I like Jaguars (please don't say "Jag") and I am very happy with six and eight cylinder power.
  19. K1W1

    K1W1 Senior Member

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    Hi,

    I saw that and was curious as to what these engines were.

    It seems MACK has diversified from land based vehicles some time ago.

    news: Mack expands engine lineup beyond trucking

    This is the first pair of them I have ever heard of being used.

    How has the 2000 Hrs been for those engines service wise?
  20. carelm

    carelm Senior Member

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    Jaguar went to the ZF transmission for the reason you stated. Overall an excellent transmission but replacing the fluid requires a trained technician and a Druid Priest.:D Have you had any problems with the air suspension? That appeared to be an issue for the early model XJ350s (2004-2008). I'm on my third Jaguar now so I'm now considered a lifer by my Jaguar dealer.