An inline six (straight six) engine is a type of internal combustion engine that arranges six cylinders in a single straight row (in a line, hence the name), rather than splitting them into two banks like a V6. Mechanically, this layout is what sets it apart: All six pistons fire along one axis, on top of a single cylinder head and crankshaft, which gives the engine inherent primary and secondary balance without needing counterweights or balance shafts.
Because of cost, packaging, and the engineering complexity of casting a longer block, only a handful of automakers still build inline sixes today, namely BMW, Mazda, General Motors, Mercedes-Benz, Stellantis (through brands like Jeep and Ram), and Jaguar Land Rover. Still, what an inline six does inherently well is smoothness; the natural balance of the firing order means less vibration and a more refined power delivery than a V6.
This often comes along with strong torque characteristics and a simpler, often more serviceable design since there’s only one head and one set of accessories to deal with. However, the very same things that make the inline-six superior in some ways can also work against it in others. Here are four common problems with inline-six engines.
The inline-six’s size and dimensions lead to packaging challenges
Beyond the length itself, an inline six’s mass sits higher in the chassis than a V-configuration engine of similar weight would. Autoblog puts it directly: A straight-six “will typically have a higher center of gravity” than a V-engine of similar mass. This is because the V engine is notably shorter due to having two banks; that allows it to sit lower in the engine bay which helps with center of gravity.
This is also partly why BMW slants its engines. Rather than mounting them upright, the brand tilts them roughly 30 degrees in the chassis, a packaging choice made to clear the hood line on cars designed around shorter V8s and to place the center of mass lower. The long design of an inline-six engine creates packaging challenges that can also limit vehicle design. Its size generally confines it to a front-mounted position.
Another reason why V6 engines are often favored over older inline-sixes is that the length of the inline-six can be a problem for crash safety since shorter V6s could do with more favorable crumple zones. Where an inline-six also falls short is packaging and width versus length; the long block doesn’t fit well in transverse front-wheel-drive platforms, and it takes up significant longitudinal space in the engine bay, which is part of why it’s largely been confined to rear-wheel-drive and truck applications.
The inline-six’s longer camshaft leads to greater flex and torsion
When the car is on the move, all of its components feel weight shifts. Because of that, many of these components can also flex. Where a V6 basically splits cylinders with three on each bank, an inline-six lines them up in a straight row. The downside? The crankshaft is much longer on an inline-six. That extra length is precisely what a crankshaft’s torsional twist scales with, which is why the inline six’s one-piece crank fights more resistance than a V6’s shorter one ever has to. With every cylinder added, the flex is a more serious problem -– partly why seven-cylinder engines don’t exist.
An engineering paper published by Politechnika Krakowska (Cracow University of Technology) ties the severity of that twist to bore size. In other words, it flags anything above approximately 90mm as the point where torsional vibration turns into a real problem you need to counter. You can do so by first trimming the crankshaft mass. However, when that does not work, the paper’s prescribed fix is the torsional vibration damper, built around “increasing the work of friction (damping work) and thus decreasing the crankshaft torsion.”
Engine Builder Magazine puts the underlying problem more plainly, describing torsional vibration as “the end-to-end twisting and rebounding of the crankshaft caused by combustion,” which is why some form of damper ends up on nearly every multi-cylinder engine. Bore diameter and crankshaft length are just two of several variables engineers juggle to get an inline six smooth. The leftover, uncancelled vibration from all of this juggling also produces torsional vibrations in the camshaft, not the crankshaft alone.
Inline-six engines suffer from uneven cylinder air/fuel distribution
Crankshaft flexing isn’t the only inherent problem of placing six cylinders in a row instead of splitting them into two V-shaped banks. Another practical consideration of such a design is the fact that the intake manifold needs considerably longer, more convoluted runners to reach every cylinder evenly. In other words, the potential problem is the manifold not distributing air and fuel evenly to the first and last cylinder in line, and that can cause combustion problems.
Ford’s old 144, 170, 200, and 250 cubic-inch inline-sixes show what that looks like in practice. Those engines cast the intake manifold directly into the cylinder head as one piece, and per OnAllCylinders, “the log-style intake manifold suffered from air and fuel distribution issues.” Put plainly, that means certain cylinders along that single row pulled in a different air-fuel ratio than others, simply because of where they sat relative to the carburetor feeding them.
A 2016 SAE study using an inline six heavy-duty engine found that as intake valve closing timing was delayed, cylinder-to-cylinder power output varied, ranging from a 9% difference at one timing to a 38% difference at a later one. The researchers traced part of that variation to intake runner length and the direction fuel flows back into the manifold.
Some inline-sixes are notorious for timing chain problems
When we wrote our five inline-six engines you should avoid list, all but one of the engines covered had notable timing chain issues. Some BMW inline-sixes, like the N57, get hit with this criticism often, partly because the chain sits at the firewall side of the block. To replace the chain, cost depends heavily on whether it sits at the front or back of the engine.
RepairPal prices a timing chain tensioner job on the front-accessible N54 335i at $1,235 to $1,707; rear-mounted chains, like on the N47, cost more. One owner on BimmerFest was quoted $5,500 for a related N55 chain repair. The difference is mostly labor: BMW Insights puts dealer labor at $1,200 to $1,500+ versus $600 to $800 at an independent shop, with rear-mounted jobs adding hours since the engine often has to come out first.
Mazda’s new turbo inline-six in the CX-90 and CX-70 follows a similar layout. It runs three separate chains, one each for the camshafts, crankshaft, and fuel pump, all positioned at the rear of the engine rather than the front, per discussion on Mazdas247. That’s the same positioning choice that makes BMW’s chain jobs so expensive, since reaching it again means dropping the engine. It is worth noting that timing chain problems aren’t inherent to inline-six engines, but rather that some inline-six engines had to compromise due to length and place their timing chains where servicing them is a lot more difficult.
Stephan is the sports journalist for the Maple Grove Report.
