What makes an inline 6 engine more reliable than a V6 or V8 configuration? 3

Another engine is the Nissan RB series in the Skyline which has a very good reputation in street/racing applicatoins.

Just a guess, but I think the single head design offers a lot of bonuses. You have simpler cams/timing (timing belt for the RB series), intake, and exhaust. The harmonics of the inline six means it's very well balanced. I would guess that all those features results in an arrangement that is pretty solid and reliable comparatively, because there's less things to go wrong (and something always goes wrong).

Of course the biggest drawback is packaging. They fell out of favor because it takes up so much space, you can have a very good V8 in a smaller footprint.

Where do you get that "general reputation"? There are plenty of current production automotive V6 engines that are extremely reliable. The reliability depends more on how well the design was done, how well the manufacturing was done, and how users use them, more than whether the cylinders are in a straight line or a V-configuration.

All of the factors that you give as an excuse for an inline engine being "more reliable" could equally apply to a V-configuration engine. The cylinder layout is irrelevant for that. (Most of your bullet points aren't accurate anyhow)

The Chrysler Pentastar 3.6 V6 in my van makes three times the power of an emission-strangled 225 Slant Six (displacement is almost the same) and meets emission standards that the old engine never could and uses less fuel than the old engine, and it weighs less, and it fits transversely which means the vehicle as a whole can weigh less, AND there is a fair chance that it will last longer with less fuss along the way. (So far, so good.) Sure, it's more complicated. But it's built to better tolerances and with better materials. It's got nothing to do with the cylinders being in a V or in a straight line. Technology has come a long way.

The old engine having less power meant that in a comparable application, the accelerator pedal was bolted to the floor ...

Now with that said, there are good and bad things about both.

The in-line engine allows for wider crank and rod journals. The V-configuration engine has two rod journals in the space where the in-line engine only needs one. This favors the in-line configuration in transport-truck engines.

But on the other hand, the long crankshaft of the in-line engine is more prone to torsional vibration.

The V-configuration engine requires two sets of cam drives if it is an OHC design. That favors in-line IF it will fit in the chassis. Transverse inline 6 is possible (it has been done) but it is hard to package.

Modern vehicles might be more tightly packaged and be more complex than they were in the old days ... but in the old days, cars were frequently junk after 60,000 miles. That's not the case any more, and at least in these parts, it's seldom an engine failure that sends it to the junk pile.

When I worked at a heavy duty class 8 truck manufacturer in their engine development lab, there was a very strong bias among the customer base that a straight 6 was more reliable than a V8. This was contrary to all the field testing, warranty data, etc. I think these biases may have had some truth to them back in the dawn of internal combustion. Once something like that gets ingrained in popular culture, it lasts for generations.

The only grain of truth I'm aware of is that straight 6 cranks are comparatively long and whippy and therefore limit the maximum safe RPM. So if the engines are built for optimum performance at lower crankshaft speeds they may be somewhat less prone to some of the wear and tear that happens to higher speed engines. There have been a few straight 6's with the power take off in the middle of the crank to get around this problem, turning them into two triples in effect. I think the Honda CBX did that but I haven't taken the time to google it to be sure.

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"The Chrysler Pentastar 3.6 V6 in my van makes three times the power of an emission-strangled 225 Slant Six (displacement is almost the same) and meets emission standards that the old engine never could and uses less fuel than the old engine, and it weighs less, and it fits transversely which means the vehicle as a whole can weigh less, AND there is a fair chance that it will last longer with less fuss along the way. (So far, so good.) Sure, it's more complicated. But it's built to better tolerances and with better materials. It's got nothing to do with the cylinders being in a V or in a straight line. Technology has come a long way."

This isn't a discussion of old vs new because that is something different entirely. To compare apples to apples you'd have to find an engine of the same vintage. Every engine back then had significantly less power than modern engines. For the most part there was a pretty direct correlation between displacement and power. The slant 6 was one of 6 engines that made it into late 60s mopar muscle car line. Looking at cars that are 50 years old now there are plenty out there where the original slant 6 hasn't been touched and that is very hard to find with an original V8 and to this day have a reputation for being very reliable. Plus the longevity of an engine doesn't really come into question all that often with modern engines. In older cars when the engine went bad that was usually a sign to replace it. Now days when it's time to replace your car it's usually because it's just too expensive to all the little stuff that starts to go wrong. So it's hard to compare the longevity of any modern engine.


When I worked at a heavy duty class 8 truck manufacturer in their engine development lab, there was a very strong bias among the customer base that a straight 6 was more reliable than a V8. This was contrary to all the field testing, warranty data, etc. I think these biases may have had some truth to them back in the dawn of internal combustion. Once something like that gets ingrained in popular culture, it lasts for generations.

Interesting point, I'm guessing perception and bias has some to do with it. But then perception and ease to maintain can go a long way. I'm sure as you know field testing and warranty data can be helpful in showing signs but can't tell the whole story. I take it as something pretty accurate for what happens under 60,000 miles. After 60k miles that kind of data starts to be less accurate. I do think lower RPM, less power, and generally being ran less hard has something to do with it. But there are a lot of examples of high performance straight 6 engines (supra, BMW, skyline, Australian fords) that are well know for how well they hold up in high performance applications.

eouedu said:

Looking at cars that are 50 years old now there are plenty out there where the original slant 6 hasn't been touched and that is very hard to find with an original V8

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It is true that there are a lot more original slant sixes out there than V8s- because they sold a LOT more slant sixes than they did V8s.

The reliability of the slant sixes is not present simply because they are slant sixes- there are other factors.

No one with a clue would ever argue that the venerable small block Chevy V8 was 'unreliable'. Architecture is not the reliability-defining variable.

If I were to put money on which has the better statistical L10 life in normal average use, between a 225 Slant Six and a 3.6 Pentastar, which are both 6 cylinder engines of almost the same displacement but one an old in-line design and one a new V6 design, I'd put that money on the Pentastar. Not because it's a V6 but because it's a modern design with better materials and better tolerances.

If I were to put money on which has the better L10 life between a modern BMW inline six and the Chrysler Pentastar V6, I'd put that money on the Pentastar, simply because BMW. (I've owned German cars.) It's not that it's V versus inline.

It's not the V versus in-line that makes the difference. It's everything else.

If I were to put money on which has the better statistical L10 life in normal average use, between a 225 Slant Six and a 3.6 Pentastar, which are both 6 cylinder engines of almost the same displacement but one an old in-line design and one a new V6 design, I'd put that money on the Pentastar. Not because it's a V6 but because it's a modern design with better materials and better tolerances.

Well again this isn't about old vs. new and I'm not sure why you think you need to make the same point you made before. This was a discussion I am trying to avoid. If you would like to discuss this in a separate post feel free but I don't believe this discussion is relevant to this post.

It is true that there are a lot more original slant sixes out there than V8s- because they sold a LOT more slant sixes than they did V8s.

I guess at this point we are both just guessing. It would be something that is pretty hard to track from either direction. How many are sold, see how many still exist and subtract what engines weren't swapped out in favor of a V8.

My point of this post was trying to figure out if there is something to a design that makes the nature of an inline six more reliable. I just find it more than coincidence that most inline 6 engines in passenger vehicles have a widely regarded reputation of being extremely reliable. I was trying to put a science to why. A few ideas along with a few point people brought up might be part of the reason why but no smoking gun answer as of yet.

The nature of your original post, and the nature of the specific examples that you cited, implied some degree of old-versus-new, "they don't make them like they used to".

-Cummins diesel all years but specifically the 89-98 "12 valves" are known to be very reliable (OLD)
-Ford 300 straight 6 (OLD)
-Dodge 225 "slant 6" (OLD)
-Jeep 4.0/4.2 (OLD, replaced first with one of a few V6 designs and now eventually with the Pentastar 3.6)
-BMW inline 6 (Some old, some new)
-Toyota supra inline 6 (OLD)
-HJ60 landcruiser diesel (OLD)

Runners up:
-Toyota fj40 gas straight 6 (OLD)
-Mercedes 300D (inline 5 so doesn't get full credit) (OLD although Mercedes still makes plenty of inline sixes)
-Chevy 250/292 (OLD)

... and yet, you failed to mention any counterexamples, of which I will provide just one: the Buick 231 / 3800 90 degree V6 has a well deserved reputation for being bulletproof.

There's another factor at work in the old-versus-new that was built into your original post. Engines that were designed decades ago for automotive applications were (almost) always designed for longitudinal installation. In the case of a 6 cylinder, this presents no particular issue, as long as the engine compartment is long enough, and it's cheaper to make one cylinder head and machine all the cylinders in a row ... UNLESS ... you are going to run the engine down a V8 machining line (like both Buick and Chevrolet did, and Chrysler later on) and create your V6 by, at least on paper, knocking two cylinders out of an existing V8 engine design. As an aside, the Chrysler pushrod 3.9 V6 that was one of the engines that replaced the Jeep 4.2/4.0 and the 225 slant six, is three-quarters of a 5.2 V8 a.k.a. 318 V8. But nowadays ... Most new engine designs for automotive applications are designed to accommodate transverse installation and that means it's almost sure to be a V6. So ... Inline almost equals old (or heavy truck) with a few exceptions, V almost equals new with a few exceptions.

Now, the second part of your post.

-less powerful so they aren't used as hard (There is absolutely no correlation unless you compare old versus new, or run-of-the-mill lump versus high-performance. And if you hot-rod the engine as you describe, this is out the window anyhow.)
-easier to access and work on therefore they are maintained better. (This is an application / installation issue, not an engine-layout issue. There is no difference in how you change the oil between a chrysler 225 and a buick 231, it's the same process. And the 225 required manual valve clearance adjustments whereas the 231 had hydraulic lifters ...)
-less moving parts (The only difference is in the case of overhead cam having more camshafts and a double set of cam drives. If it's pushrod, it is exactly the same. Also, SOME V6 layouts need a balance shaft to operate smoothly. 60-degree V6 with 60-degree offset journals, the current modern layout, doesn't need one.)
-Power peaks at lower RPMs so it takes less to do more (This is a TUNING issue, not an engine-layout issue. BMW inline sixes can spin pretty fast!)
-less prone to hot spots so less chance of detonation (Why? All else being equal - No difference.)
-being upright gives more lubrication around the cylinder walls because gravity isn't working against the high side of the cylinder wall (Bunk. The gravity forces are trivial compared to the oil being pumped in under pressure and squirted out and thrown around. And in any case ... a Chrysler 225 SLANT Six is tilted over 30 degrees, the cylinder inclination is exactly the same as that of a 60-degree V6! Many BMW sixes are tilted over as well. Same reason - hood clearance.)
-being upright applies the same theory for cooling and every other part where gravity could be fighting a v engine (See above)
-more bearings in the crankshaft (ford 300 has 7 main bearings) (This is the one thing in which the inline configuration can have an advantage. On the other hand, the crankshaft is longer and more prone to torsional vibration. By the way, the Chrysler 225 slant six only has 4 main bearings.)
-better balanced (Really? A 60-degree V6 with 60-degree offset journals - which is the usual modern arrangement - does not need a balance shaft. My Pentastar is a darn smooth-running engine.)
-more surface area exposed to air and more open engine bay for it sheds heat better (This is an application / installation issue, not a cylinder-layout issue. And in any case - in a liquid cooled engine, the heat rejected by convection and radiation from the cylinder block is peanuts compared to that removed by the coolant. And it's easier to get good coolant distribution on a shorter engine, i.e. V6 ...)
-less nooks and cranies for old oil and engine particles to hide when the oil is being changed (Really? Where's the difference?)
-less parts of the engine hidden that aren't as well lubricated and/or cooled (Really? What parts?)
-I don't believe any of the above mentioned engines have timing belts, it's either a chain or gears (again this is application specific and is not pertinent to inline or V ... it can be done either way with either configuration)

The title of this thread implied that, at least in your mind, it was a foregone conclusion that an inline-six was a better layout. I'm picking your post apart because ... I don't think so! There are too many other factors at work.

Thank you all for you input.

The perfect primary and secondary balance makes them pleasing from an NVH perspective. But that's unrelated to reliability.

Steve

I don't put much store in online popularity contests.

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I have heard the Subaru engine called bullet proof and it is neither straight, Vee, nor vertical.
Many Beetle engines outlasted the car.
Engines that have a reputation for early failure generally have a design flaw which is not related to cylinder orientation.
For years the slant six was the petroleum industry standard test engine for fuel additives to combat carburetor icing.
Most lists of the ten best diesel engines list both the six cylinder DT466 and the V8 Duramax.
I don't accept your hypothesis.

Bill
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"Why not the best?"
Jimmy Carter

"I have heard the Subaru engine called bullet proof"




Two of my friends worked at a Subaru dealership in the 1970s. As I recall they did a bunch of head gasket replacements. OR something that involved removing the heads. maybe valve train ?

OP said:

Ford 300 straight 6...

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The Ford "Barra" engine, descended from the above, is highly praised on the various Aussie motoring YouTube channels (such as Mighty Car Mods and Skid Factory).

eouedu said:

I don't want to start an engine debate but I know enough that generally straight 6 engines have a reputation for being more reliable than a V6 or V8 configuration. Just to simplify this discussion I want to stick with a discussion of older pre efi passenger vehicle and pickups engines because that is what I'm interested in. There aren't that many inline 6 engines out there but most of them have a very good reputation for being reliable.
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It might be as much a social issue as a mechanical one.

Whatever truths may be contained in the original post is probably more a consequence of the kind of drivers who bought them, tending (as a group) to drive more mildly than those who bought more powerful engines. IOW, it wasn't the IL-6 configuration that gave rise to reputations for reliability so much as it was in the nature of most people who chose them to drive more gently. 60 years ago, people were less inclined to buy more than what met their needs; these people had lived through the Great Depression of the 1930's and WWII, and frugal attitudes were far more prevalent as a result.


Norm

If you limit the discussion to the classic inline six vs v6/v8 without efi, then we are talking about rather old designs with a rather limited power output designed in a era where the knowledge of construction materials was limited, calculation was essentially done by sliderule and manufacturing precision still was rather crude. That design method resulted in several designs, some in hindsight rather good, others not so.

On the basis of what were some engines better then other? Maybe to a certain extend by design - but some most likely where engineered better by sheer luck without the designer knowing it at that time. Compared with modern design and production, those "classic" engines were developed and produced on the basis of "hit and miss" and some performed remarkably well.

But none of them comes even close to modern designs that are well designed and produced to very tight specs delivering a lot more power over a longer time with less fuel and emissions with much larger maintenance intervals.

I think it is not really possible to rate the ancient engines in terms of reliability (whatever definition you use for it) because of the lack of reliable data. What we "see" as reliable most times is based on anecdotal evidence that subsequently is accepted by a large number of people - but not backed up statistical data.

VE1BLL said:

The Ford "Barra" engine, descended from the above, is highly praised on the various Aussie motoring YouTube channels (such as Mighty Car Mods and Skid Factory).

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Thumbs up to MCM and TurboYoda.. One test of the strength of original designs especially the bottom end, block rigidity, choice of internals e.g forged crank, beefy rods, closed deck for which the 2JZ is a legend and also the Barra (after beefing up the valve train and head bolts) is the degree of turbocharged boost the engine can be take stay in one piece.

A raw 2JZ block looks more like a casting for heavy duty diesel application than automotive gas engine applicaiton.

Another thing that really helps engines of the modern era rack up the miles without overhaul is good crankcase ventilation, along with low blow by (cylinder stability and finish, piston ring technology, valve stem sealing, and probably a bunch more) and accurate fuel delivery (Fuel injection and engine management).

As "proof" when the EOM gets PCV wrong, the other two aren't enough by themselves. Witness Toyota, Mopar, Audi, VW, etc sludging.

Then again, if cooling system maintenance is neglected for 60 kmiles or so, radiator and rotted head gasket issues can kill an engine right quick. Just like the olden days.
As can a simple radiator cap that doesn't hold pressure if the engine design or operating conditions permits localized hot spots that spawn severe nucleate boiling and resulting cracked heads. Whee.

Some manufacturers are consistently WAY better at all this than others, making the statement that all manufactured products are designed to just survive the warrantee period ring pretty hollow.

If a person searches forums and engine failure sites you will see there are plenty of modern designed and manufactured engines that don't come close to the longevity of the old stuff. Then there are the cases of massive sludging problems that were occurring even with proper oil changes. IMO any reliability problems are not from the cylinder arrangement, but the materials and the design as well as how well the engine design is matched to the application and what percent of the power that the engine is capable of is used during its life as well as the rpm range it runs in, oh and then who does the manufacturing as well. A normal driven Geo Metro 1 liter will last for years, as will a 1UZFE both can go over 300K miles easy with a bit of normal maintenance, and neither one is a overly fabulous design.
I saw the mention of Pentastar, they had a huge rash of birthing problems with the goofy integral exhaust manifold in the head that didn't help with valve seat retention, this is from memory it has been awhile since studying that one. I did just watch a video of one that had a roller follower failure on a 2018 3.6 with 4,000 miles so?