The Differences Between Inline Four & Boxer Four Engines


Hello everyone and welcome! In this video we are going to be comparing inline four-cylinder engines with boxer 4-cylinder engines and talking about the individual advantages and disadvantages of each engine layout. We have a 3D printed EJ20 Subaru boxer engine as well as a 22RE Toyota inline four-cylinder engine we’ll start fairly basic and work our way through several different topics for each layout including the four strokes the firing interval and order the vibrations of each layout the packaging differences the cylinder head differences and finally a bit about the sound of each engine. Looking at either layout, they’re both based on the same four strokes intake, compression, power, and exhaust. Both engines fire one cylinder for every 180 degrees of crankshaft rotation but they have slightly different firing orders on each engine. We can see cylinders 1, 2, 3, and 4 for the boxer engine the firing interval is 1, 3, 2, 4 while on the straight four it’s 1, 3, 4, 2. So the order of the last two cylinders firing is switched. Now these layouts matched with the firing interval make for some interesting differences as far as how the engines are balanced on the boxer engine you’ll notice the pair of pistons move in and out together this means that the primary forces when the pistons reach the top of the cylinder as well as when they reach the bottom of the cylinder are canceled out on the inline four-cylinder engine it’s the same story the primary forces cancel out as the pairs of pistons reach the top and bottom at the same time when we get into secondary forces however the engines begin to differ. Secondary forces are created due to the piston traveling faster at the top half of the cylinder then at the bottom half something i’ll include a link to in the video description that breaks it down in great detail what you need to know though is that? when the piston reaches the very top of the cylinder or the very bottom the secondary forces point up we’re out from the piston now with the boxer engine since the pistons point opposite each other these forces are balanced out resulting in a very smooth running engine for the inline 4 all of the forces point in the same direction and thus do not cancel each other out causing the engine to vibrate unless balancing shafts are used the boxer engine isn’t perfect however because the pistons do not perfectly align with each other it creates a rocking moment which makes the engine want to rotate back and forth along the vertical axis. What’s fascinating though, is that if you add two cylinders to either of these designs whether it’s a boxer six or an inline six you can perfectly eliminate all first and second order forces and moments you might think the boxer six would have a rocking motion from the cylinder banks of three but each bank of three cancels out the rocking motion of the other unlike in a V6 configuration looking at the two engines both of these 3D printed engines are at 35% scales you can actually compare them size-wise to one another because of the inherent vibrations of inline four-cylinder engines you won’t tend to see engine size as much larger than about three liters so historically they have existed at much larger sizes one of the largest gasoline four cylinders currently made is in fact a Toyota engine a 2.7 inline 4 used in the Toyota Tacoma only porsche and Subaru currently used flat engines in their vehicles so the size doesn’t tend to be any larger than typical inline 4 cylinders even though it does have better vibration characteristics the other biggest advantage of the boxer engine is the low profile which keeps the center of gravity low and thus reduces the amount of load transfer you have in the car during braking cornering or accelerating which improves grip with a lower center of gravity you can also reduce body roll and choose to use softer springs additionally in the event of a collision it’s easier to position the engine so that it goes underneath the passenger compartment rather than into the passenger compartment for improved safety. Now that’s not to say that the inline four doesn’t have its own size advantages generally, it’s a bit more compact with only one cylinder head and it’s not quite as wide as the boxer engine this leaves more room for suspension geometry and can also allow for a better steering angle since the tires won’t have as much of an interference at full lock moving on to the valvetrain although this particular inline 4 has a single overhead cam you’ll much more commonly see dual overhead like the boxer engine on modern vehicles the big advantage with the inline 4 is that there’s only one cylinder head meaning only one intake and one exhaust camshaft less moving parts and some weight saved. It’s also far far easier to access the cylinder head for service, whether it’s adjusting the valves or replacing spark plugs the I-4 design makes it a much easier task finally we get to the topic of sound and here’s where most people will agree the box rumble is a better sound but it’s not really a true advantage The boxer rumble is a result of unequal length headers and because Subaru is moving away from this exhaust design New ones will generally sound pretty similar to other four-cylinder engines it would in fact be possible to design an inline four-cylinder engine with different length exhaust pipes to create a unique rumble generally associated with Subarus. But it’s not ideal for exhaust scavenging to have unequal pulses it’s not the smartest way to route an exhaust or four-cylinder engine however packaging does not the unequal header length seem appealing when paired with a boxer engine. And a big thank you to Eric Harrell for providing the 3D printed models you can find links in the video description. If you have any questions or comments feel free to leave them below, thanks for watching!

100 thoughts on “The Differences Between Inline Four & Boxer Four Engines

  1. I own both a 2.5i(2018 outback) and a 22re (92 pickup) and I gotta say I love both of these motors. The 2.5i is a nice and very smooth motor. The 22re can be put through a world of torture though (wheeling and offroading). I dont think ide put my 2.5i through what my 22re has gone through. lol.

  2. I WANT THAT 3D PRINTED ENGINE! NOW I HAVE TO BUY A 3D PRINTER. LOOK WHAT YOU HAVE DONE. ARE YOU PROUD?

  3. Differences of Boxer-4 and inline-4 engine sounds:

    Boxer-4: Brrrrrrruuuuuuuuuuummmmmmmmm

    Inline-4:
    ReeeeeeeeeeEEEEEEEEEEEEEEEEEEEE

  4. Back in the 60s and 70s VW engines were the easiest to remove and rebuild on a chopping block. Today I own a Subaru outback legacy…07….head gasket regular maintenance…127.thousand ..garbage kept. Unless it dies on the road I'm keeping it till I die.

  5. Question: On the boxer engine, why can't 2 pistons kinda the same cylinder so as one side fires, it compresses the other side?

  6. 22r & re were great engines for their time. 200,000 or more miles was a good engine back in the 80s. But let’s face the facts. They didn’t make any power, and still got less than 20mpg. If I’m driving around in a underpowered truck it better be getting at least 25mpg or better. Also there iron blocks and aluminum heads made for head gasket issues.

  7. One large disadvantage of the boxer is where the oil lies. It really needs to be a dry sump motor like most Porsche motors (air cooled and the 996 turbo) to get that oil from smashing into the cylinders and blowing past the rings. Cornering, overfilling or anything but level parking is not good for a wet sump. Boy do they feel good.

  8. That lovely sound you hear coming from a Subaru is the sound of your head gaskets being eaten alive.

  9. You should make a 3D printed version of a woman's brain, then maybe we'll understand them better

  10. There is NO such word as "anyways", it's an adverb, there is no plural! It's "anyway"! I love your videos, you are a talented young man with great knowledge.

  11. why do people call opposed engines boxer's?
    they have been around a long time you know. the Hawker Typhoon had an odd engine, the Napier Sabre was 2 opposed 12 cyclinder engines on top of each other to make a 24 cyclinder opposed 2400 hp engine lol. though it was an odd sleeve valve design, pretty compact

  12. All I could think of is how cool 3d printers are. I mean seriously I remember programing in basic now you have this stuff …. incredible

  13. Gr8 job..im running a 83 subi ea80 n find it being used in many an application from ultralights .helos and more I'm new to these little guys old school hot rodder that can't afford the gas like in the old days..so this does help thanks

  14. Why does no one use a v4 engine in a car anymore? It would help with subaru’s constant oil leak issues, and they’re super cool. My motorcycle has one it’s 1.2L with 170hp and 91ftlb torque, without a turbocharger

  15. My S2000 is for sale for anyone interested! No reserve, 20% to charity. Happy bidding!
    https://bringatrailer.com/listing/2002-honda-s2000-48/

  16. You made a mistake with the displacement if the 22RE. It's 2.4 liters, not 2.0. Still an informative video.

  17. No, they both stink and sound awful. Engines should be clean and silent. Rolls Royce made silent cars over 70 years ago. Exhaust emissions are a different challenge – even for VW – and only electric motors are an affordable solution to that one today. Unless someone can develop a cheap and efficient engine which emits only steam. The technology exists, but the oil industry pressures people into using their products, and delights in encouraging (brainwashing) people to like noisy engines.

  18. Boxer is a CAR name. The ENGINE type is called "horizontal opposed". Basic "engineering" terms, douchebag. Duuhhh.

  19. Por que mierdas ponen el titulo en español si el video esta en inglés? Vaya mierda de manera de ganar visitas en la comunidad hispana

  20. В России Субару очень сильно обманывает людей. Мы перестали их покупать. Дилеры в России полное говно.

  21. So whats the deal with the boxer? I know of 2 people who had to have their engines overhauled after only 60 000 miles

  22. 知っているぞ。
    高さ以外、エンジニアの感じるメリット位しか優位がないっって事を。
    横幅が倍、奥行きは同等。オイル滲みのハズレは漏れに発展し易い。
    メンテが苦行。
    等など。

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