Advantages/Disadvantages of Horizontally Opposed Engine - Subaru Forester Owners Forum
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post #1 of 26 (permalink) Old 11-01-2009, 08:19 PM Thread Starter
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Advantages/Disadvantages of Horizontally Opposed Engine

I know the HO engine has a lower centre of gravity and better balance characteristics, but does anyone know if a boxer engine experiences less wear on start-up due to the oil not completely draining into the sump as in a standard in-line vertical configuration? Also, does a HO engine experience greater issues with cylinder ovalization over time?
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post #2 of 26 (permalink) Old 02-18-2010, 10:28 PM
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I think both of those issues are nonexistant in any vehicle with proper maintenance, but subaru does have above average reliability.
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post #3 of 26 (permalink) Old 02-19-2010, 01:44 AM
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I believe that one disadvantage is that coolant will remain in the cylinders/heads when parked. If neglecting coolant change, or using wrong coolant, this will eat the aluminium and head gasket faster than on inline engines.

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post #4 of 26 (permalink) Old 02-19-2010, 03:34 AM
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Quote:
Originally Posted by Erling View Post
I believe that one disadvantage is that coolant will remain in the cylinders/heads when parked. .
Entire coolant system stays filled with both engines.

Be careful what you wish for.
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post #5 of 26 (permalink) Old 02-19-2010, 06:51 AM
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Not to mention inline engines have had many more headgasket failures over the years than boxers. See: gm 2.2l,(88-95); honda 2.3L vtec (98-02), geo metro 3cyl (also valve failures); gm 2.5l (also valve failures).....

Subaru is not alone in this endeavor, and have done way more for their customers than the aformentioned MFGR's. Also if your coolant changes are on time then their the coolant would not be eating at the gasket. When proper mainenance and conditoner treatment is not followed is when the coolant causes problems for the gasket.
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post #6 of 26 (permalink) Old 02-19-2010, 07:05 AM
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Flat engine - Wikipedia, the free encyclopedia
Porsche and BMW motorcycles have been using boxer engines forever. If it is good for them, it should be good for us :-)
Speaking of BMW, here's what the valve train of a S1000RR (inline 4, not boxer) looks like at 14,000 rpms: (The good part starts around 1 minute)

YouTube - BMW S1000RR Technical Video - Valves in Motion High RPM
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post #7 of 26 (permalink) Old 02-19-2010, 08:06 AM
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A boxer motor is innately balanced or rather more balanced making things like counter weights and harmonic balanced less necessary. And as mentioned the "flat" engine design allows them to place the engine lower and further back in the engine bay, giving the car a lower center of gravity and a slightly lower polar moment.

That and it sounds cooler than other motors :)

The biggest disadvantage as an owner is where the spark plugs are, to me anyway. The only thing the L4 motor in my other car has is I can change the plugs in 15 min, in the dark with my eyes closed if necessary. Every other thing though, like oil changes, is a PITA relative to my Subaru.

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post #8 of 26 (permalink) Old 02-19-2010, 08:28 AM
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Quote:
Originally Posted by adc View Post
Entire coolant system stays filled with both engines.
I don't claim to be an expert on this, so I did a quick search. Apparently, there seems to be opposing opinions on the matter.

"On an in-line engine or v engine design, when the vehicle is turned off the fluids such as coolant and oil will drain down to below the head gasket line. On a Subaru with a horizontally opposed engine when you turn the vehicle off the fluids such as the oil and coolant will remain in contact with the head gaskets. If the fluids are not in very good condition, such as outlined above, they will eat away at the head gaskets."
Subaru Head Gasket Problems Explained. - Seattle Subaru Repair

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post #9 of 26 (permalink) Old 02-19-2010, 09:21 AM
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Originally Posted by miko View Post
Porsche and BMW motorcycles have been using boxer engines forever. If it is good for them, it should be good for us :-)

YouTube - BMW S1000RR Technical Video - Valves in Motion High RPM
Nice video. to play devil's advocate: if it's so good, why do only 2 car companies use this config? I know there's higher costs in manufacturing, as you need to make 2 heads, essentially 2 blocks as well. Also if it's all that, why doesn't bmw use it in their cars. I think one reason they use it in the motorcycles is so that the heads are naturally air cooled as they stick out of the chassis.

Oh, another reason why HO works good: transmission layout is essentially a RWD setup because you don't have none of that transverse stuff going on.

But, a disadvantage would be that you can't mount it transversely, and you'll have a longer front overhang because the transmission sits behind the engine not next to it.
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post #10 of 26 (permalink) Old 02-19-2010, 06:26 PM
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BMW still uses it on their motorcycles mostly due to tradition. The K-bikes were introduced in the 1980s to be a replacement for the Air-cooled flat-twin, but customers really voiced an outcry. The decision to abandon a tradition turned out to be too much of a financial gamble, so they nixed it and continued to develop both inline and flat-twin engines simultaneously. If you ride, and have compared different engine layouts in motorcycles, the flat twin is mounted very low, and helps bring center of gravity down in today's modern motorcycles (Honda's Goldwing uses flat-six engines and they ride like couch-seated sportbikes)

The current flat twins also can't really be considered air-cooled. It really is an oil cooled engine, that happens to also have some air-cooling properties, similar to the final few generations of high performance Porsche engines. Oil cooling was how they maintained operable engine temperatures while producing high horsepower numbers.

I personally believe that the increased mechanical complication and more difficult (wider engine) packaging is the downfall of most horizontally opposed engines. Manufacturers can get the same performance numbers and weight bias numbers out of smaller, lighter, and more mechanically simple engines (all of which help keep costs lower). Notice, Porsche has stayed true to the Boxer and H-6 engines since the first cars, and they still win in all manner of sports car comparisons and world championship series...and their cars are all more expensive.

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post #11 of 26 (permalink) Old 02-19-2010, 08:09 PM
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Quote:
Originally Posted by gdawrx View Post
Nice video. to play devil's advocate: if it's so good, why do only 2 car companies use this config? I know there's higher costs in manufacturing, as you need to make 2 heads, essentially 2 blocks as well. Also if it's all that, why doesn't bmw use it in their cars. I think one reason they use it in the motorcycles is so that the heads are naturally air cooled as they stick out of the chassis.

Oh, another reason why HO works good: transmission layout is essentially a RWD setup because you don't have none of that transverse stuff going on.

But, a disadvantage would be that you can't mount it transversely, and you'll have a longer front overhang because the transmission sits behind the engine not next to it.
The primary reason Subaru and Porsche use it? Packaging. The motor is "shorter" if you will. W/ the motor being longitudinal and needing to shove a tranny in front or behind it, you'd have a monster hood (or trunk in Porsche's case) and a crazy moment arm (again, polar moment of interia) in terms of handling. Makes sense in this regard.

Ummm...didn't Ferrari use flat motors for a bit as well? Dusting off some memories there.
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post #12 of 26 (permalink) Old 02-19-2010, 08:18 PM
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Advantages:
- Extremely easy to work on
- Outstanding reliability (mainly Subaru, not necessarily the boxer design itself)
- Lower Center of Gravity
- ...and the "cool" factor.

Disadvantages:
- Spark plug rather a pain to change, but not impossible.
- Replacement parts are not usually not in stock, and have to be ordered.

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post #13 of 26 (permalink) Old 02-19-2010, 10:55 PM
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Quote:
Originally Posted by Erling View Post
I don't claim to be an expert on this, so I did a quick search. Apparently, there seems to be opposing opinions on the matter.

"On an in-line engine or v engine design, when the vehicle is turned off the fluids such as coolant and oil will drain down to below the head gasket line. On a Subaru with a horizontally opposed engine when you turn the vehicle off the fluids such as the oil and coolant will remain in contact with the head gaskets. If the fluids are not in very good condition, such as outlined above, they will eat away at the head gaskets."
Subaru Head Gasket Problems Explained. - Seattle Subaru Repair
Oil yes Coolant no, remember there's no air in the cooling system so actually it should be full on all the time on all cars.

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post #14 of 26 (permalink) Old 02-20-2010, 10:48 AM
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precisely, but even before the system sealed the engine will get filled above the head gasket line. Once during one of my rebuilds on my Nova's V8, i had my dad refilling the rad while i was working on something else, and didn't have my t-stat and housing installed yet, and coolant came gushing out of the t-stat opening. And this is pretty much the highest point on the engine, front top of the manifold, ABOVE the heads.

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post #15 of 26 (permalink) Old 02-22-2010, 07:08 AM
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Wonder if the Subaru ties with Toyota will eventually produce some sort of in-line engine in a Subaru car? The boxer engine certainly costs more to manufacture as previously mentioned when compared to an in-line engine.

Most "general aviation" aircraft engine are boxer types, with four, six, and a few eight cylinder versions out there. Lycoming and Continental are the big names in these air cooled boxers. Rotax, the Austrian (I think) company makes among many things, a water cooled boxer engine, both turbo and non turbo, that are used in some homebuilt aircraft, light sport aircraft, and aircraft with standard airworthiness certificates. These relatively small displacement engines make around 80 H.P. for the non-turbo to about 115 H.P. for the turbo version (or somewhere around there...)

Moto-Guzzi's motorcycle engine design isn't a boxer, but it's not an in-line design either. It's a two cylinder Vee design. Sort of like a BMW motorcycle, but with the two cylinders at some somewhat small angle separating the two which result in a "knee-knocker/knee-heater" for the motorcyclist.

I wonder if the real reason Subaru keeps the boxer is because surely the R&D costs have been recouped, they have all the necessary tooling to continue building it, and it's "traditional" for them.

Oh, and didn't some small German company build a few of these boxer engines? Some obscure little cottage industry type? Now what was their name? Folks Wagons? Something like that, I think.


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