2004 Subaru 2.5 Question

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So, the 2.5L in my 2004 Outback is a 4, it's flat, and it's an opposed arrangement. It's a 16-valve (my 2004 is unless I'm reading wrong) but it's a SOHC. What are the advantages to this arrangement? This dopey question coming from a guy with DOHC 16-valve fours in an upright configurations all his life, including (especially, my C-14). Engineers are smarter than I, but looking under the hoods of these things, I can't fathom the extra complexity and especially, the "why" of it. What is gained? Something HAS to be gained, I'm sure there is a body of thought behind it, but comparisons aren't to be found on the web, or not easily.
 

hpb

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I think Subaru do the flat 4 just to be different - their Unique Selling Point if you will. Because you're right, that layout is more complex, and it has to be more expensive to produce. But, it's their "thing" that makes them stand out from the crowd. A bit like your C14 in a way - it doesn't really need VVT, for example, but Kawasaki put it on anyway, to show their engineering prowess and to have something that most of the other manufacturers don't have. Or I could be completely wrong on both counts...
 
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Originally Posted By: Concours14
This dopey question coming from a guy with DOHC 16-valve fours in an upright configurations all his life, including (especially, my C-14). Engineers are smarter than I, but looking under the hoods of these things, I can't fathom the extra complexity and especially, the "why" of it. What is gained? Something HAS to be gained, I'm sure there is a body of thought behind it, but comparisons aren't to be found on the web, or not easily.
There are pros and cons to all engine architectures. One of the biggest advantages, at least in my opinion, to the flat-4 is the lower center of gravity and the inherently good balance the engine has (balance of internal moving parts). Cons include more difficult packaging due to the wider nature of the engine and possibly more difficulty in servicing (harder to get to spark plugs, cylinder head cover gaskets, etc).
 

JTK

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I think you'll find it to be smoother than an inline 4cyl. It's center of gravity is lower for the boxer engine as is it's overall height, allowing for a lower hoodline, etc. Plus they allow for old school north-south (rear wheel drive style) drivetrain layout. IMO, the only thing that makes them more complex, is the 'two of everything', given the opposing jugs. *EDIT: Sorry Hokes! You beat me to it!
 
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The crank bearings are more stable as they are sandwiched between the two block halves. What I cant fathom is why Subaru uses a split crankpin instead of one shared crankpin for better crank rigidity. All this make a stronger rotating assembly for boosted engines with over 2+ hp/ cu-in. Our Ej253 sohc 16V had a variation of Honda's VTEC that provided good fuel mileage and torque around town at "lugging" revs with the 5 speed manual trans. Engine Mainly ran on 1 intake valve per cyl with a tumble flapper at low revs. then engaged a hotter profoile on the second valve above 4K rpm to provide good "passing power" smile Same HP as my 83 4bbl 302 Mustang V8!
 
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ARCO is the man when you need to know Subaru powertrain tech info. My $0.02: The native AWD they build all their vehicles around requires a longitudinally mounted engine which avoids right angles in the powertrain gearing thus minimizing the % of power lost in the drivetrain. Added benefit is the lower CG already mentioned. Yields the high efficiency native AWD system and good handling that Subaru is noted for.
 
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Flat-4 is a historical thing with Subaru going back to their first cars. Subaru is all about balance in vehicles and into drivetrain. The arrangement allows for front-aft placement of motor that is perfectly centered in vehicle. Also the obvious handling benefit of low center of gravity of engine and also the motor in hard collision will go under the passenger compartment instead of into it. Historically crash ratings have been extremely good. One thing about Subaru historically is you paying for an engineers car and drivetrain more then rest of vehicle. The downside is some extra complexity and difficulty of repairs to head. I experienced this with a burnt valve in my turbo Subaru motor. Instead of normal head removal in-line four the entire motor was pulled adding significant amount in labor. At least in the process all maintenance is set for 100k since everything exposed. I am just changing oil and air filter moving forward.
 
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Originally Posted By: ARCOgraphite
The crank bearings are more stable as they are sandwiched between the two block halves.
This was probably more significant years ago, when most engines used individual crankshaft bearings. Almost all modern inline-4s (and indeed many V engines as well) use a full crankshaft girdle that promotes extremely good rigidity in the crankshaft area. I suppose it could be argued that two block halves are theoretically stronger than a block-and-girdle pair, but I think the difference between the two is likely too small to be able to measure.
 
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I don't think they require balance shafts as they are better balanced than a flat 4. I think the biggest production four was a 3.0 flat four in a Porche. Bigger inline fours vibrate a lot, although old airplane engines were sometimes flat fours. I don't think they revved very high, less than 2000 rpm.
 
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On top of what's already been said, I think their biggest need for the flat 4 is the drivetrain layout. The way Subaru's transmission/front diff is set, an inline 4 would be an awkward arrangement. The flat 4 is very short in both height (low CG) and length.
 
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My old 70's Audi 100LS had the slant 4 engine and trans in front of the front wheels ala current Subaru. Fuji boxer is just a backwards Beetle smile
 
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Originally Posted By: LoneRanger
ARCO is the man when you need to know Subaru powertrain tech info.
Thanks LR! But I will always concede to BlueSubie and sometimes Rand's Boolean logic smile smile Even though weve owned possibly 8 Subarus from Justy to SVX - I'm getting too old to recall and count smile
 
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Even though Im a big subie fan - our (short lived) 2005 Rav 4 was a better driver all round than the Subaru - regardless of this symmetrical AWD bologna which I don't think holds weight - though I am a longitudinal mounted engine fan. Subaru requires TWO efficiency scrubbing ring and pinion sets whereas a transverse engine/transaxle arrangement would not require a lossy 90deg. power transfer up front (or, if rear powered - out back)
 
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Don't forget about the Honda Goldwings! I'm on my second flat-6 powered Honda Goldwing 1800. The Goldwing definitely takes advantage of what a flat-6 engine offers. Low center of gravity, torque, and balance.
 
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Originally Posted By: mrsilv04
Don't forget about the Honda Goldwings! I'm on my second flat-6 powered Honda Goldwing 1800. The Goldwing definitely takes advantage of what a flat-6 engine offers. Low center of gravity, torque, and balance.
Same with my Valkerie. What a great design. And these things last virtually forever if taken care of. Too bad Subaru doesn't hold up as well...
 
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Originally Posted By: SteveSRT8
Originally Posted By: mrsilv04
Don't forget about the Honda Goldwings! I'm on my second flat-6 powered Honda Goldwing 1800. The Goldwing definitely takes advantage of what a flat-6 engine offers. Low center of gravity, torque, and balance.
Same with my Valkerie. What a great design. And these things last virtually forever if taken care of. Too bad Subaru doesn't hold up as well...
Motorcycles are perceived to last a long time. However their usage is quite limited and in perfect weather typically vs the harsh life of a car with far more mileage/stop and go/and temperature extremes.
 
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Originally Posted By: rjundi
The downside is some extra complexity and difficulty of repairs to head. I experienced this with a burnt valve in my turbo Subaru motor. Instead of normal head removal in-line four the entire motor was pulled adding significant amount in labor.
I just did this in my FXT! cry -Dennis
 
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Originally Posted By: ARCOgraphite
Even though Im a big subie fan - our (short lived) 2005 Rav 4 was a better driver all round than the Subaru - regardless of this symmetrical AWD bologna which I don't think holds weight - though I am a longitudinal mounted engine fan. Subaru requires TWO efficiency scrubbing ring and pinion sets whereas a transverse engine/transaxle arrangement would not require a lossy 90deg. power transfer up front (or, if rear powered - out back)
Wha?!?!? You are still going to have a power take off somewhere? It will be a 90deg to send power to the back(or front). It may not be a ring & pinion the size of Subaru but it will be there.
 
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Originally Posted By: Thermo1223
Originally Posted By: ARCOgraphite
Even though Im a big subie fan - our (short lived) 2005 Rav 4 was a better driver all round than the Subaru - regardless of this symmetrical AWD bologna which I don't think holds weight - though I am a longitudinal mounted engine fan. Subaru requires TWO efficiency scrubbing ring and pinion sets whereas a transverse engine/transaxle arrangement would not require a lossy 90deg. power transfer up front (or, if rear powered - out back)
Wha?!?!? You are still going to have a power take off somewhere? It will be a 90deg to send power to the back(or front). It may not be a ring & pinion the size of Subaru but it will be there.
Yes, there is a PTO. Both of our FWD-based AWD vehicles have two 90-degree gear sets.
 
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