Internally combustion engines have existed in all kinds of arrangements, spanning all the way back to the start of the 19th century. From Carl Benz’s 0.75 horsepower, one-cylinder engine that powered the world’s first automobile to the 860 horsepower Ferrari V12s in the back of the Formula 1 cars of the 1980s, internal combustion engines have come in all shapes and sizes.
One type of engine type stands out due to its curious placement in a large number of legendary sportscars. The inline 6 engine was introduced to the world in 1903, a few years prior to the implementation of a V6 engine in a vehicle. The inline 6 engine configuration is perhaps less common in modern vehicles due to the design challenges that arise from an engine so lengthy. However, inline 6 engines have multiple distinct advantages over their V6 counterparts. As a result, some manufacturers, like BMW, have chosen to stick with the inline 6 design despite their waning popularity.
With the internal combustion engine entering its twilight years, it is interesting to think that the best inline 6 engines to ever exist have already been made. In this article, we will cover the inline 6 engines that have made an impact on the automotive world and define the category as a whole.
What is An Inline 6 Engine?
In general, you can break internal combustion engines down into two arrangements. Those two types include inline engines and V engines. There are additional types of engines, such as flat, boxer, and W engines, however, those types are less common in than the previously mentioned two. While both inline and V engines function in nearly identical ways in terms of how they produce power, their design is very different.
Compared to a V engine, inline (or straight) engines are much less complicated. With inline engines, all of the engine’s cylinders are lined up in a single row along the crankshaft, as the name suggests. That means that there is only one bank of cylinders. A straight 6 engine utilizes a single-cylinder head, head gasket, and valvetrain. As a result, straight 6s are not only simpler engines, but they are easier to work on as well.
The straight 6 design comes with a number of unique characteristics as a result of only having a single bank of cylinders. For example, inline 6 engines tend to be much narrower than V6 engines while also being longer. Due to the increased length, straight 6 engines also need longer camshafts in order to actuate the single valvetrain.
Compared to other engine configurations that need counterweights and other measures taken to achieve a good engine balance, with inline 4 cylinders and V6 engines being good examples, inline 6s with the correct firing order are some of the most balanced engines available. That translates to extremely smooth engine function and power delivery.
Benefits and Downsides of Inline 6 Engines
As with any engine type, there are both pros and cons of the inline 6 engine design. Many of the benefits of the inline 6 design boil down to the fact that you are only dealing with a single bank of cylinders. Since there are fewer components required for an inline 6 engine to operate, there is less that could potentially go wrong as well. It also means that they are easier to perform routine service on, as everything is accessible from one side of the engine bay.
While we briefly touched on the fact that inline 6 engines are more balanced than many other engine configurations, it is important to understand why that is the case. It boils down to firing order. With a normal inline 6 firing order, pistons move in tandem with their counterpart on the other side of the engine. That pattern continues inward to the middle two cylinders.
To elaborate, pistons 1 and 6 complete their cycle at the same time, followed by pistons 2 and 5 then pistons 3 and 4. Since the pistons at opposite ends of the engine are in motion simultaneously, their forces cancel each other out. That leads to a very smooth engine with very little vibration.
While there are certainly some notable upsides to inline 6 engines, there are also some downsides. Most of the downsides pertain to the extended length of the engine as a whole. Due to the fact that straight 6s extend so far back towards the firewall, they are difficult to fit in smaller vehicles. Additionally, the increased length of many of the internal components, like the extra long camshafts, leads to increase flex meaning that they are more susceptible to breaking from stress.
The first entry on this list needs no introduction for many car enthusiasts. Not only is it one of the best inline 6 engines, but it is also arguably one of the best engines ever made overall. It is also featured in one of the most iconic Japanese cars of all time. The Nissan RB25DET has a lot going for it, to say the least.
The RB engine series was made famous by its placement in the Nissan Skyline. Prior to the RB25DET, the 2.0L RB20DET was the factory engine in many of Nissan’s flagship sportscars. The RB25DET introduced an additional 0.5L of displacement and other important advancements in engine technology. The RB25DET was initially seen in the engine bay of the Nissan R33 GTST in 1993. Its most notable uses include the R33 GTST, R34 GTST, Nissan Stagea, and Nissan Laurel among a few others.
Beyond simply being used to power some of the most iconic cars to come out of Japan, the RB25DET is also an incredibly strong engine with a lot of power potential left untapped by the factory. In stock form, the inline 6 RB25DET produces between 245 and 280 horsepower depending on the variant. In reality, its power figures are likely higher than what Nissan advertised.
Nissan RB25DET Engine Specs
|Engine||Nissan RB25DET Engine|
|Displacement||2.5L (2,498 cc)|
|Valvetrain||DOHC 4 Valves Per Cylinder|
|Bore x Stroke||86mm x 71.7mm|
|Weight||Long Block ≈ 608 lbs|
|Horsepower||250-280 hp @ 6,400 RPM|
|Torque (lb-ft)||235 lb-ft – 267 lb-ft @ 3,200 RPM|
Due to the RB25DET’s cast iron/aluminum head construction, it is an extremely strong engine from the factory. Combine that with larger injectors, reinforced injectors, and connecting rods, as well as a larger turbocharger than the previous RB20DET, and the RB25DET is truly a force to be reckoned with.
The RB25DET did see multiple revisions between 1993 and 2001. The first revision came with the Series 2 RB25DET which introduced small changes to the electrical system and a switch to a ceramic wheel turbocharger. Another, more significant upgrade came in 1998 with the introduction of the RB25DET NEO, which saw a notable performance increase from the Series 2. The RB25DET NEO came with a different head with solid lifters, upgraded camshafts, and variable valve timing, yielding another 30 horsepower and a similar amount of torque.
The Toyota 2JZ-GTE occupies a very similar legendary status as the RB25DET engine that we covered previously. The Toyota 2J, as it is often called colloquially, is the engine that cemented the MKIV Supra as one of the best sports cars ever made. Just as the Nissan Skyline and Toyota Supra battled for street supremacy in the mid-1990s, the straight-6 engines that powered them were also being tested head to head.
The Toyota 2JZ-GTE is a 3.0L twin-turbo DOHC engine released in 1991. Despite putting out a healthy 320 horsepower from the factory, like the RB25DET, the 2JZ is known for its ludicrous tuning potential. There are 2JZ-GTE engines out there pushing the 1,000-horsepower threshold with little to no internal modifications. That is how strong the cast iron block, forged rods, and forged camshaft truly are. The 3.0L 2JZ-GTE is also one of the first modern mass-produced engines with sequential turbochargers, meaning that they spooled at different intervals in the rev range. This means that the lower-end boost from one turbo feeds into the higher-end boost provided by the secondary turbo, leading to less noticeable lag.
The aftermarket community is where the 2JZ-GTE truly thrives, as virtually everything that can be done to a 2JZ-GTE has been done. With so much part availability and modding information out there, any power goal that you have is achievable with the 2JZ platform. Simple 2JZ-GTE bolt-ons can take the engine into the 400-horsepower ballpark. Other more intensive mods, like upgraded turbos, can see a 2JZ-GTE into the 600-800 horsepower threshold.
The 2JZ-GTE will undoubtedly go down in the books as one of, if not the, best inline 6 engines ever made. The combination of the 2J’s strength, aftermarket support, and cult fanbase makes it a very difficult engine to beat.
Toyota 2JZ-GTE Engine Specs
|Engine||Toyota 2JZ-GTE Engine|
|Displacement||3.0L (2,997 cc)|
|Bore x Stroke||86mm x 86mm|
|Torque (lb-ft)||315 lb-ft|
As a closed deck engine with a cast iron block and multiple forged internals, the 2JZ-GTE is a brute when it comes to handling power. That is compounded even further by its relatively low 8.5:1 compression ratio, meaning it can handle big boost. The 2JZ-GTE has an identical stroke and bore, making it a square engine. Square engines are perfectly balanced between providing solid torque and great high-end power. Combined with the smoothness of a straight 6 configuration, the 2JZ-GTE is silky smooth with its power delivery.
While the 2JZ-GTE’s pistons themselves aren’t forged, the important components around the pistons are. The 3.0L Toyota straight 6 features forged connecting rods and a forged crankshaft, both of which ensure safe modding to a point. These attributes of the 2JZ-GTE make it one of the most sought-after engine swap candidates ever. From Miatas to BMW M5s, 2JZ-GTEs have been swapped into nearly everything under the sun. Its versatility, smooth power delivery, and ridiculous power potential earn the 2JZ-GTE an unquestionable place on this list.
BMW B58 Engine
So far we’ve covered two engines that are both Japanese and from the 1990s. With this entry, the engine we chose is neither of those things. BMW has been building inline 6 engines since 1933. Where many manufacturers have moved away from inline engines in favor of V configuration, BMW has stuck to their inline guns and have become damn good at building them. As a matter of fact, they are so good at building straight 6 engines that they have won the overall International Engine of the Year Award 8 times with inline 6 engines. That’s an accomplishment. The BMW B58 straight 6 was one of those engines.
The BMW B58 engine is a 3.0L turbocharged inline 6 with direct injection, introduced in 2016. The B58 was designed to replace the beloved but aging N55 engine used in top-tier, regular-production BMW models. The B58 shares a few design features with the BMW N55 engine, but also differs in several notable ways. The 3.0L B58 is a closed-deck engine, while the N55 has an open-deck design. The B58 is also water-air charge cooled while the N55 is air intercooled.
Strength is a common theme between the inline 6 engines that we have chosen so far. The BMW B58 engine is no exception. Like the Toyota 2JZ-GTE, the BMW B58 has both forged connecting rods and a forged crankshaft. Also, like the 2JZ, the B58 can handle much more power than it produces in factory form. Beyond its strength, the BMW B58 delivers power more smoothly than almost any other engine available due to its twin-power turbo technology. In general, there are very few downsides to the B58 and it is already becoming a cult classic because of that.
BMW B58 Inline 6 Engine Specs
|Engine||BMW B58 Engine|
|Displacement||3.0L (2,998 cc)|
|Aspiration||Single Twin-Scroll Turbocharged|
|Valvetrain||DOHC Double VANOS VVT|
|Bore x Stroke||82.0mm x 94.6mm|
|Torque (lb-ft)||369 lb-ft|
The 3.0L BMW B58 has quite a bit in common with the Toyota 2JZ-GTE that we covered above. In fact, many people call the B58 the BMW 2J because of their similarities. The two commonalities that truly stand out are their unparalleled strength and ridiculous power potential. Most of the B58’s strength comes from its closed deck design and features forged connecting rods and a forged crankshaft. The B58’s forged crankshaft is truly the standout component, as it is said that it has 36% higher fatigue resistance which equates to a longer service life with stock power.
Compared to other high-performance turbocharged engines, the BMW B58 has a comparably high compression ratio. Generally speaking, it is a better idea to combine forced induction with a low compression ratio, but modern technology allows the B58 to retain an 11.0:1 compression ratio without putting too much stress on the internal components. Higher compression is known to boost baseline horsepower as well, meaning the B58 is the best of both worlds.
With stock internals, the B58 is capable of handling upwards of 600-650 horsepower and a similar amount of torque. As a result, the B58 is common in the aftermarket community. There are a ton of aftermarket parts available for the BMW B58 that can boost performance dramatically, including upgraded turbos, a quality tune, an upgraded downpipe, and many more.
AMC 4.0L Inline 6
In comparison to the other engines that we have chosen for this list, the AMC 4.0L I6 is an outlier. The Nissan RB25DET, Toyota 2JZ-GTE, and BMW B58 are all performance engines that can be found under the hood of legendary sports cars. The AMC I6 is anything but a performance engine. With that being said, it epitomizes everything that a great inline 6 should be. Not only is the AMC I6 one of the longest-running engines in Jeep’s history, introduced in 1964, but it is also one of the most recognizable American engines ever made.
While there are a number of engines in the AMC I6 range, the golden boy is the 242 AMC I6. The AMC 4.0L inline 6 balances all of the characteristics of the inline 6 configuration that makes it such a good format. For instance, the AMC 4.0L straight 6 is known for its hefty torque, smooth power delivery, and extreme longevity. The AMC 242 I6 was introduced in 1987 in the Jeep Cherokee XJ and Cherokee MJ, as well as the Wagoneer and Commanche. All four of those vehicles would go on to be known as some of the best Jeeps ever made, helped in large part by the AMC 4.0L I6.
Ultimately, Jeeps that utilized the 4.0L AMC straight 6 have held their value very well over the years, which is due in large part to the 4.0L straight 6 itself. Since they are simple engines that are easy to work on, maintenance is usually very cheap. Parts are also readily available, as there are millions of AMC straight 6s floating around.
AMC 4.0L Straight 6 Engine Specs
|Engine||AMC 242 4.0L I6|
|Displacement||4.0L (4,000 cc)|
|Bore x Stroke||3.88″ x 3.41″|
|Horsepower||185 bhp @ 4,750 rpm|
|Torque (lb-ft)||220 lb-ft @ 4,000|
At its core, the AMC 242 I6 epitomizes an American inline 6 engine. With a cast iron block and head, the 242 is an incredibly strong, yet heavy, engine. Despite only producing 185 horsepower, the AMC I6 is a 7-bolt-main engine, ensuring very little fatigue over time. The inline 6 replaced the outgoing AMC carbureted 258 CID engines found in older model Jeeps. Due to the fact that the 4.0L is fuel injected, it is the better option for offroading, as carbureted engines do not do well when they aren’t on level terrain.
The AMC 4.0L received multiple notable revisions throughout its lifecycle. Original 4.0L I6 engines utilized a groundbreaking Renix engine management system that was too advanced for most shops to pull codes from at the time. This system was later replaced with a more standardized Chrysler multi-point fuel injection system that made problem diagnosis easier. The 4.0L received another update in 1996. This revision strengthened the block even further with additional webbing and a stud girdle for the main bearings. These later reinforced engines received the “PowerTech” branding. Despite their bottom-end strengthening, early PowerTech engines were known to have faulty heads prone to cracking.
Ford Barra Straight-6
For anyone that isn’t an Aussie, this is an engine that you aren’t likely to have heard of. The Ford Barra inline-6 is an engine that was found exclusively in Australian-spec Ford Falcons. Prior to the introduction of the 4.0L Ford Barra in 2002, Ford Australia had an extensive history of developing straight-6 engines. The Barra evolved from the previous Ford Australia “Intech” straight 6 engines that were manufactured between 1998 and 2002. In many ways, the Barra was the swan song to their previous experimentation.
When the Ford Barra engine arrived on the scene, no one expected it to be a particularly interesting or innovative engine. In fact, it seemed rather primitive, with a heavy cast iron block and a relatively simple design. However, that would come to be the Barra’s biggest strength, quite literally. In fact, the Ford 4.0L Barra is said to be able to withstand close to 1,000 horsepower on factory internals. That obviously appealed to the tuning crowd rather substantially, giving the Barra I6 a cult-like following. With some internal modifications, there are Barras out there chucking out 2,000 horsepower plus.
Due to the fact that the Barra was only offered in Australia, it is somewhat difficult to find one stateside. Since the Ford Barra I6 was made up until 2016, there are still quite a few out there in good condition. Ford, and multiple other aftermarket retailers, offer the Barra as a crate engine that can still be bought new. While it might be able to withstand similar power figures as the Toyota 2JZ-GTE, the Barra doesn’t receive the same aftermarket support due to its regional specificity.
Ford Barra Straight 6 Engine Specs
|Engine||Ford Barra 4.0L I6|
|Displacement||4.0L (4,000 cc)|
|Aspiration||Naturally Aspirated / Turbocharged|
|Valvetrain||Dual Overhead Valve, 24-Valve|
|Bore x Stroke||3.632″ x 3.910″|
|Compression Ratio||8.47:1 – 12.0:1|
|Horsepower||209bhp – 436bhp|
|Torque (lb-ft)||276 lb-ft – 425 lb-ft|
While the base Ford Barra was originally offered as a naturally aspirated inline 6, it was also offered in turbo spec, found in the Falcon XR6 Turbo. Both variants are DOHC, 24-valve engines featuring Ford’s VCT version of variable valve timing. Beyond just being offered as either naturally aspirated or turbocharged, the Barra was also offered in petrol and liquid petroleum gas variants as well. In total, there are 11 different variants of the inline 6 Barra, with even more if you consider the V8 Barra variants as well.
Due to the wide range of variants in the Barra engine family, the specs of the engine vary quite significantly. For example, the compression ratio of naturally aspirated and turbocharged Barra engines range between 8.47:1 and 12.0:1. Turbocharged Ford Barra engine variants also received stronger pistons, higher fuel pressure, and a large Garrett GT35/82r turbo. As Ford began perfecting the Barra formula, power increased substantially over the years. Initially, the N/A Ford Barra inline 6 made 244 horsepower and the most powerful variant at the end of the engine’s lifecycle made 436 horsepower.
While the Barra might not be anywhere close to the most well-known straight 6 engine on this list, it is one of the most capable. There’s no doubt that the Ford Barra deserves a spot on a list of the best inline 6 engines ever made.
Best Straight/Inline 6 Engines Ever Made Conclusion
When it comes to engines, inline engines are often left behind for more modern configurations. The inline 6 has been left in the dust by V6 engines, for popularity’s sake at least. With most modern cars utilizing a front-wheel-drive layout, straight 6 engines are simply too long to be useful. With that being said, there are still manufacturers that appreciate the straight 6 configuration and are still building them to this day.
Most of the inline 6 engines on this list are here for their strength, smooth power delivery, and power potential. If you ask us, those are some of the most desirable traits that an engine could possibly have. The Toyota 2JZ-GTE, RB25DET, and BMW B58 are all gleaming examples of this. They all arguably transcend the inline 6 class, with some considering them to be some of the best engines ever made as a whole. While there might be other engines that are stronger and more powerful, inline 6 engines will always be underappreciated underdogs.