# Why are big-rig trucks always/frequently powered by an inline-6 engine? **URL:** \ **Category:** Factual Questions\ **Created:** [September 18, 2015, 2:31pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675 "2015-09-18T14:31:19Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [September 18, 2015, 2:31pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/1 "2015-09-18T14:31:19Z")
Reciprocating-piston internal combustion engines have a particular sound that goes along with the number and configuration of cylinders. A twin, inline 4, inline-6, V6, V8, V10, and V12 all sound different from each other. Judging from the sound, it seems that the vast majority of big-rigs on the road these days. Why so? The [cabover layout](https://en.wikipedia.org/wiki/Cab_over) was abandoned years ago in favor of the more aerodynamic configurations seen today, but the tradeoff is increased overall length. A different engine configuration could meet power/torque requirements while providing a shorter overall length. My guess is that the smaller cylinders of a V8 engine with equal total displacement would reduce efficiency, and the larger cylinders of an inline-4, while perhaps even increasing efficiency, would cause excessive torque pulsation (requiring a massive starter motor and a massive flywheel). But a V6 could provide exactly the same bore/stroke while providing a shorter overall package than an inline 6. In fact, the V6 is a very popular engine in passenger cars. So why is it not favored over the inline-6 for big rigs? ---
**Author:** ![dolphinboy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dolphinboy/32/330_2.png) [@dolphinboy](https://boards.straightdope.com/u/dolphinboy)\ **Post date:** [September 18, 2015, 2:33pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/2 "2015-09-18T14:33:37Z")
More reliable and simpler engine design? Just a guess. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [September 18, 2015, 2:39pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/3 "2015-09-18T14:39:27Z")
at least in north america, they pretty much are _all_ inline-6 turbodiesels, anywhere from 10 to 16 liters displacement. There have been other configurations in the past; Caterpillar had the 3208 and 3408 V8s and 3412 V12, Mack had the E9 V8, International and Cummins had a couple of V8s each, and the two-stroke Detroit Diesels were used in inline-6, V6, V8, and V12 setups. the best explanation I’ve heard for why they’ve settled on inline-sixes is: 1. they’re powerful enough for the job, and are inherently balanced unlike a four cylinder, 2. they can be packaged to sit lower between the frame rails, where a vee engine can’t, 3. they’re the easiest type to do an in-frame rebuild on. meaning, they can do a total overhaul without having to physically pull the whole engine out of the truck. ---
**Author:** ![troub](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troub/32/14802_2.png) [@troub](https://boards.straightdope.com/u/troub)\ **Post date:** [September 18, 2015, 2:53pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/4 "2015-09-18T14:53:55Z")
I don’t know anything about engines, but thanks for this thread; I’ve been doing a lot more interstate driving now (for nearly a decade prior I was only driving in the city) and I just noticed recently there are no more cabover semis (they were extremely common when I was a kid). I’ve been idly wondering what that was about, where they went, why they went away. Your Wikipedia link seems to explain that it was more about that there used to be laws about the length of trucks, which were mostly repealed. Length regulations also being the reason cabovers are still the “usual” configuration in many other countries. Anyway, cool; now I can stop speculating on _that_ when I should be thinking about driving 🙂 ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [September 18, 2015, 3:07pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/5 "2015-09-18T15:07:59Z")
oh, plus a 60° V6 would have a weaker crankshaft than an inline six. in order to be even close to balanced, a 60° V6 uses a [flying pin](http://cdn.shopify.com/s/files/1/0366/9149/products/IMG_4471_1031bda9-a20b-42f8-aa31-80089f036e2a.JPG?v=1416102690) crankshaft with two offset crankpins between each main bearing. they’re inherently weaker where one crankpin is next to another. in an I6, each cylinder has its own crankpin, and in most cases each one is separated by a main bearing. you _could_ make a V6 with a stronger crankshaft, where opposing cylinders share a crankpin (like a V8,) but it would require a 120° vee angle which means the engine would be wide and have to basically sit on top of the frame. ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 18, 2015, 3:13pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/6 "2015-09-18T15:13:39Z")
V-6 engines vibrate too much. > [@](#): > > [Wikipedia](https://en.wikipedia.org/wiki/Straight-six_engine) > 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. They also have two connecting rods between each pair of main bearings, which may increase wear. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 18, 2015, 3:30pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/7 "2015-09-18T15:30:07Z")
The “cabover” design is almost universal in Europe due to strict length limits. Operators want the maximum load space within the rules. 26 pallets (4’x2’) has been the norm, but we are now experimenting with 15.6 metre trailers that can take 30 (that’s on the floor - these trailers are frequently double stacked up to the maximum 44 tonne (97,000lbs gross weight). Driver comfort is not the first consideration, although modern trucks are fairly comfortable. There are a good few V8s in operation but fuel economy is poor and maintenance costly compared to the standard straight six. [Youtube video](https://www.youtube.com/watch?v=zpzw4WEvNV4) ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [September 18, 2015, 4:06pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/8 "2015-09-18T16:06:50Z")
The inline 6 is inherently balanced, so there’s no additional engineering required to damp out vibrations or enable high RPMs, which is most emphatically NOT true of a V6- they’re all sorts of unbalanced, and require balance shafts and other mechanical gew-gaws to make them run smoothly. So inline 6 engines can be inherently simpler than other designs. They’re used in trucks, cars and even ships, because that inherent balance makes them really easy to scale up and manufacture. > **[Straight-six engine](https://en.wikipedia.org/wiki/Straight-six_engine)** > > The straight-six engine (also referred to as an inline-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balance, resulting in fewer vibrations than other designs of six or less cylinders. > Until the mid-20th century, the straight-six layout was the most common design for engines with six cylinders. However, V6 engines became more common from the 1960s and by th... ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [September 18, 2015, 4:10pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/9 "2015-09-18T16:10:33Z")
> [@jz78817](#): > > 1. they’re powerful enough for the job, and are inherently balanced unlike a four cylinder Yes, straight sixes have perfect primary and secondary balance. > [@](#): > > 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 One piston in each pair is always on its power stroke. > **[Straight-six engine](https://en.m.wikipedia.org/wiki/Straight-six_engine#/media/File:Inline_6_Cylinder_with_firing_order_1-5-3-6-2-4.gif)** > > The straight-six engine (also referred to as an inline-six engine; abbreviated I6 or L6) is a piston engine with six cylinders arranged in a straight line along the crankshaft. A straight-six engine has perfect primary and secondary engine balance, resulting in fewer vibrations than other designs of six or fewer cylinders. > Until the mid-20th century, the straight-six layout was the most common design for engines with six cylinders. However, V6 engines became more common in the 1970s. By the 20... Ford’s F-150, for many years the best-selling truck, had a straight six for a long time. ETA: ninja’d by **bump**. Or is it bumped by a ninja? ---
**Author:** ![Bullitt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bullitt/32/5725_2.png) [@Bullitt](https://boards.straightdope.com/u/Bullitt)\ **Post date:** [September 18, 2015, 4:21pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/10 "2015-09-18T16:21:26Z")
Reading that page, this paragraph gives a good explanation. > [@](#): > > Because it is a fully balanced configuration, the straight-six can be scaled up to very large sizes for heavy truck, industrial and marine use, such as the 16 L (980 cu in) Volvo diesel engine and the 15 L Cummins ISX used in heavy vehicles. The largest are used to power ships, and use fuel oil. The straight-six can also be viewed as a scalable modular component of larger motors which stack several straight-sixes together, e.g. flat- or V-12s, W-18s, etc. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [September 18, 2015, 4:26pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/11 "2015-09-18T16:26:06Z")
it’s also why Ferrari’s flat 12 in the Testarossa was simply a 180° vee; there was no need to make it an actual boxer. ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [September 18, 2015, 4:53pm UTC](https://boards.straightdope.com/t/why-are-big-rig-trucks-always-frequently-powered-by-an-inline-6-engine/731675/12 "2015-09-18T16:53:13Z")
An inline engine offers good access for repair and accessories on its sides plus each cylinder may be supported by two main bearings, important for a diesel. Before the small block Chevy V8 was developed in 1955, the GMC 302 C.I. inline six truck engine was popular in racing because of its 7-bearing crankshaft. This is one of those mature technologies where, for many applications, the proportions of an I6 just work really well for the displacement (power capability) required. And they can be balanced perfectly for smoothness.