Holy crap, where's all that gas going?

[blitz]

I've kinda been watchin' for a suitable candidate model from another company to compare against a Subaru for fuel economy. Let's try this:

 

 

Volvo V50 T5 AWD / Forester 2.5XT AWD

-------------------------------------------------------------------------

Weight: 3552 lbs. / 3289 lbs.

Displacement: 2521 cc / 2457 cc

Type: Turbocharged I-5 / Turbocharged H-4

Horsepower: 218 hp / 210 hp

Torque: 236 ft. lbs. / 235 ft. lbs.

Transmission: 5 auto / 5 manual

EPA City: 24 mpg / 18 mpg

EPA Highway: 31 mpg / 23 mpg

 

The Volvo weighs 263 lbs. more, has 64 cc greater displacement, has an extra cylinder, makes more horsepower and torque, is saddled with an automatic trans, yet still manages to get 35% better fuel economy. Why?

[mattocs]

Does not make sence...:-\

 

 

Makes my feel better about avg. 24MPG in my OBW(some highway...some city)

[The Dude]

There are many, many cars that are heavier, have more HP, have bigger engines and still get equal or better mileage than a Subaru. Why, is a good question?

Here is my guess. The Subaru H-4 engine generates full HP and torque at VERY, VERY low rpms. So, the car is producing full power (and using more fuel) earlier in the power band. Subarus pack an unusual amount of usable storage space into a relatively small car. So, the car shape is optimized for usable storage, not fuel saving aerodymanics. I could be off, but I think I remember that a Forester has about 2/3 or 3/4 the carrying capacity of an Explorer. Subarus have high clearance, a ground hugging body is best for low fuel consumption. Many Subarus come standard with a roof rack, once again not good for high mpg. Also check the final gears ratios on the cars.

I have some real problems with SOA, but I have found the Forester to be an extremely versatile car. It tows 2000 lbs. (much more really),handles and accelerates well, hauls a ton of stuff for a small car, and I've transported 18' long pieces of lumber on the roof rack. Not too shabby for one little car.

[The Dude]

I've kinda been watchin' for a suitable candidate model from another company to compare against a Subaru for fuel economy. Let's try this:

 

 

Volvo V50 T5 AWD / Forester 2.5XT AWD

-------------------------------------------------------------------------

Weight: 3552 lbs. / 3289 lbs.

Displacement: 2521 cc / 2457 cc

Type: Turbocharged I-5 / Turbocharged H-4

Horsepower: 218 hp / 210 hp

Torque: 236 ft. lbs. / 235 ft. lbs.

Transmission: 5 auto / 5 manual

EPA City: 24 mpg / 18 mpg

EPA Highway: 31 mpg / 23 mpg

 

The Volvo weighs 263 lbs. more, has 64 cc greater displacement, has an extra cylinder, makes more horsepower and torque, is saddled with an automatic trans, yet still manages to get 35% better fuel economy. Why?

BTW, Edmunds.com gives the EPA mileage for the AWD V50 at 19 mpg city,

26 mpg highway. Where does 31 mpg come from?

[blitz]

I pulled the figures for both vehicles from Car & Driver magazine. The Forester; August '03, and the Volvo; September '04.

 

I've seen C&D's figures to be innacurate from time to time, so I hope that's not the case. As far as where the power delivery is, the Volvo's peak torque is @ 1500rpm with zero turbo lag, the Subaru's is @ 3600rpm.

[GeneralDisorder]

You have made the mistake of compareing a computer controlled automatic transmission, with a human controlled manual.....

 

Not only that, but the Volvo's mileage goes in the toilet if you get on that turbo. I had that same engine in my '94 850T - except making 250 HP or so. Very little turbo lag, yes - and the automatic keeps you from spooling it except when you mash it to the floor - but when it spools, damn was it fast! Watch the fuel guage drop like there was a crew of russian cosmonauts under the hood drinking it by the 5th.

 

The manual in the Forester means the turbo will be spooled WAY more often.

 

GD

[Chip]

[Here is my guess. The Subaru H-4 engine generates full HP and torque at VERY, VERY low rpms. So, the car is producing full power (and using more fuel) earlier in the power band.

 

 

Just because a motor is turning at it's maximum torque or HP doesn't mean it's using more fuel, (often, the exact opposite is true).For the sake of argument, let's assume that a motor develops it's maximum power at 1500 rpms,(a L6 Jeep motor for example). On the highway at 60mph in 5th you'd be turning around 1500 rpms. According to your theory, if you were to go faster, or at a higher RPM, you'd use LESS fuel ?....highly unlikely.

[Ranger83]

Consumer Reports tests cars on a 150 mile loop. The Forester got 25 mpg on their test.

 

Their don't seem to be any actual tests of the Volvo, yet.

 

A Ford Explorer is EPA rated for 15 mpg city, so they must get 25 mpg in the RW as well?

[blitz]

I just looked again at the Volvo stats and the mileage figures I listed of 24 city, 31 highway are manufacturers PROJECTED ESTIMATE. Doh! Looks like a red herring, sorry 'bout that.

 

OK, even so... plugging-in the less optimistic Edmunds figures given by Dude, the same dilemma exists; the heavier vehicle with the bigger engine and more cylinders, is making more power and still using less fuel. Now the highway figures I can understand on account of the Volvo having less aero drag and a taller top gear. It's the city figures that are more difficult to explain away with the torque converter wasting energy as heat and a heavier vehicle to move away from stoplights, and most of all... this fuel economy thing is a pattern. So I'm not starting a gripe thread, I'm just trying to figure out the one thing that is consistently unique about the Subaru which would account for the tendency to consistently use more fuel, all else being equal. I have my own theory (which I've partially mentioned before), but I'll hold off until everyone's had a chance to chime in.

 

I'll try to come up with a few more comparos. I seem to recall recall some AWD mini - utes using a normally aspirated V6 in the displacement range of 2.7-3.0 liters, weighing 500 lbs. more, equalling or bettering fuel economy of my 2.5 OBS.

[later, Peter]

I bought a '99 OBW a few weeks ago. I expected the mileage to be roughly, 20+ city 25+ Hwy. I got about 30 highway. My wife, however, got +/- 35 hwy on the return trip... she drives the speed limit & doesn't often pass other vehicles. gotta teach to drive....

Later,

Peter

[Commuter]

There was a mention of gearing. That can have a "lot" to do with it.

 

I don't know specifically between these two models, but I can give you a real life example. I have a 97 OB with the H4 2.5L 165 hp engine. I routinely get about 24 to 25 mpg with mostly highway driving. My wife has a 99 Honda Odyssey with a V6 3.5L 210 hp engine. While I haven't done extensive logging, I know that it gets about the same milage on the highway! hmmmm

 

At 70 mph, my OB is turning over at 2700 rpm.

At 70 mph, the Honda is turning over at 1900 rpm.

 

Just thought I'd throw in my 2 cents.

 

Commuter

[cookie]

culprit. My Forester is about like a barn door for areo shape also.If the car had a sixth overdrive gear it would make a huge difference. Most modern autos have a built in overdrive.

On old Volvos I used to fit the overdrive from the P1800 S and pick up a few MPG but there is no practical way to do this to these cars. A very common mod many years ago was to fit Jeeps with an overdrive for the same reason.

My BMW roadster has a five speed and weighs less than my friend Doug's new Corvette. I only get about 18 MPG and Doug can hit 20 with a bigger engine and car. His six speed overdrive is the main factor.

I just had a Hyundai rental car for a couple of weeks with an auto tranny with overdrive. It consistantly cruised at about 1500 RPM giving it pretty good mileage. My Forester would have been about 3,500 RPM at the same speed.

If your torque peak is kept low and you keep the RPMs low you are going to get good around town mileage too.

[blitz]

OK, here's my thesis: The lower fuel economy is due to mechanical and thermal inefficiencies regarding the engine. Not the boxer layout in itself, but rather the specific implementation.

 

The width constraints that Subaru must contend with in stuffing the boxer into a front-engined topography, forces the use of a big-bore, short stroke (oversquare) cylinder dimension as well as short connecting rods which results in a somewhat low rod/ stroke ratio of 1.65.

 

The thermal losses are accrued two ways:

 

1. The large bore creates much aluminum surface area for thermal energy to be lost through; a. the combustion chamber roof into the cooling system and; b. through the piston crown into the oil.

 

2. The low rod/ stroke ratio results in comparatively less dwell time of the piston near TDC during the early part of the power stroke. Since the piston is moved away from TDC quicker, the combustion event has less time to impinge upon the piston, causing less transfer of force to the piston, which slightly deteriorates the engine's thermal efficiency.

 

 

The mechanical losses are friction related and are likewise accrued in two ways:

 

1. The low rod/ stroke ratio (short rods) results in proportionately higher angularity of the connecting rods during certain parts of the stroke, yielding greater side loads between the piston and cylinder wall, causing friction.

 

2. Piston ring frictional losses in general tend to be greatest at TDC and BDC by way of a phenomenon known a "stick-slip". The oversquare bore/ stroke dimension means a large ring with a large stick-slip friction area. The oversquare bore/ stroke dimension means reduced piston speeds at all points in the stroke including the stick-slip zones in the vicinity of TDC & BDC. The increased dwell time in the stick-slip zones gives rise to much stick-slip losses.

 

I believe I'm correct, but I know there's folks on this board that know a lot more about engines than I do, so I'm open to rebuttal and whatnot (mostly the whatnot).

[slo5oh]

[Here is my guess. The Subaru H-4 engine generates full HP and torque at VERY, VERY low rpms. So, the car is producing full power (and using more fuel) earlier in the power band.

Just because a motor is turning at it's maximum torque or HP doesn't mean it's using more fuel, (often, the exact opposite is true).For the sake of argument, let's assume that a motor develops it's maximum power at 1500 rpms,(a L6 Jeep motor for example). On the highway at 60mph in 5th you'd be turning around 1500 rpms. According to your theory, if you were to go faster, or at a higher RPM, you'd use LESS fuel ?....highly unlikely.

Engines will use the most fuel at peak TQ. Obviously under full throttle. You are dead on with your jeep example. The only flaw is that nothing cruises on the freeway at 1600rpm. Even the stock gears in my mustang kept me at about 1800rpm at 70mph in 5th. Our cars tend to run too short of gears. I know my soob pushes 3500 rpm on the freeway (of course I'm doing 80). I would probably get 30mpg if I were doing 40mph everywhere. The taller (smaller number) you can get your gears and still maintain freeway speeds the better mileage you get. Of course the reverse is that the lower your gears (bigger number) the better the car "feels" driving around. It will feel like you are making more HP.

[slo5oh]

There was a mention of gearing. That can have a "lot" to do with it.

 

I don't know specifically between these two models, but I can give you a real life example. I have a 97 OB with the H4 2.5L 165 hp engine. I routinely get about 24 to 25 mpg with mostly highway driving. My wife has a 99 Honda Odyssey with a V6 3.5L 210 hp engine. While I haven't done extensive logging, I know that it gets about the same milage on the highway! hmmmm

 

At 70 mph, my OB is turning over at 2700 rpm.

At 70 mph, the Honda is turning over at 1900 rpm.

 

Just thought I'd throw in my 2 cents.

 

Commuter

My parents have a 99 honda with the 4cyl and while I don't know the RPM, I do know my father with a lead foot usually gets between 30 and 38mpg... his honest average at about 35.

[Chip]

Engines will use the most fuel at peak TQ. Obviously under full throttle. You are dead on with your jeep example. The only flaw is that nothing cruises on the freeway at 1600rpm.

No flaw....

@ 60mph in 5th gear my '00 TJ turns at around 1600-1800 rpms. It's geared 3.07,(stock).

I just took a picture at 100kph,(62mph)...rpms = 1600. Tell me how to post it and you'll see for yourself.

[earthworm]

And I have read where the big 40 ton rigs can hit 22 mpg when cruising in 22nd ! I'll bet that is downhill with a tail wind ! Hard to believe, of course; in real life they average 4 to 6 mpg, or 200 ton miles per gallon and more !

 

Thus, the most important factor in mpg is the ultra strong Diesel working hard; The six spped, etc, will help a good deal. These are available in Europe, but not third world nations such as the good old USA !!

 

Lets face it, the Subaru is a gas hog ! As was the old VW beetle under some circumstances..

 

And I put very little creedence in the EPA figures, I think the European din numbers may be more accurate.

[slo5oh]

No flaw....

@ 60mph in 5th gear my '00 TJ turns at around 1600-1800 rpms. It's geared 3.07,(stock).

I just took a picture at 100kph,(62mph)...rpms = 1600. Tell me how to post it and you'll see for yourself.

And you can maintain that speed? Jeeps have the aerodynamics of a brick. I had a 89 with the 4.2 carbed engine. It made gobs of TQ, but I could only run 5th on the freeway for downhills and flat surfaces.. any sort of hill and I would have to put it in 4th. I was also running oversized tires, so maybe that was part of it. Mine too had 3.07s. And the best mileage i ever got was around 14mpg.

[cookie]

than the old carbed ones.I just sold a 79 CJ7 with a 350 rear gear and a Turbo 400 automatic mated to a 258. It would not get better than 15 MPG on the freeway.

The Subaru Forester has replaced it and hauls more and can go nearly as far off road. At least in my use, I don't climb rocks.

I picked up 10 more MPG with the Forester and a lot better driveability.

But as far as Blitz's engine theories they are interesting.

Porsche has had much the same answer to its boxer problems and has added gears and a higher level of sophistication in both power delivery and engine management. I might suggest that Blitz take a look at what they have been doing as he will find it interesting.

In the old days everyone went to oversquare bores for power and long strokes for torque, except the British who were limited by a strange bore tax law.

In many ways Subaru has done exactly what many of the other manufactuers have done. The quick fix for slightly better mileage is a six speed, but as long as we have AWD we will have to pay for the additional rotating mass and weight. Superior fuel injection would help I think as I am not aware of any other FI engine that carbons up like a Subaru.

[kingbobdole]

Hi,

A few things, fisrt my theroy is gear ratios. Autos usually have a better highway ratio than manuals. My extreme example. My XT-6 FWD came factory with an auto. 70 mph and like 2500 rpms, the car used to average about 28 MPG and I drove it hard. Got the super close ratio turbo tranny now. 70 Mph in 5th like 3200 RPMs the damn thing is really hard pressed to make 22 MPG but with the way I drive... its like 20. Two last notes. My fathers Ford F-150 is running like 1500 or so RPM at highway speeds, Fored autos have very high gears. It has a 5.4 triton V8 and gets 15 MPG. We once drove it to Iowa pulling a trailer loaded with wood and the bed loaded. It got 15 MPG. we unloaded and got rid of the trailer coming back to CO. It got 15 MPG. Last note, my little 97 horse work car, the super duper Honduh Civic gets about 38 MPG on the highway. I have to downshift to get it over hills. I drive the hell out of it. It has over 240K miles on it and the check engine light is on. 38 MPG... did I say that? The damn thing is screeming down the highway. It has no tach but its gotta be way up in the rpms. It almost gets 400 miles..... on like 10 gallons. Did I tell you it will blow the doors off an automatic saturn on a flat surface?

I don't know where I was going with that,

Phil

[sduford]

Let's face it guys, Subarus are not fuel efficient, they never have been, and never will be. I don't think it has anything to do with aerodynamics or AWD. I've been evaluating Subarus against others for a few years and they are always the worst on fuel eceonomy. The boxer engines just seem to need a lot of fuel.

 

Here's a couple of examples:

(EPA Ratings from MSN Autos)

 

Forester XT, 19/23 (auto)

Highlander V6 AWD, 18/24 (230HP 3.3L V6, Auto, much larger vehicle)

 

Imprezza 2.5 RS, 22/28(wagon, auto)

Highlander 4Cyl AWD, 21/25 (similar engine size and power to Imprezza's)

 

So the much larger and taller Highlander with a V6 230HP engine gets the same gas mileage as a Forester XT (or an Outback XT for that matter).

 

The same hunking Highlander with a 2.4L 160HP/165ft-lbs engine gets only slightly less gas mileage than the tiny Imprezza with a similar sized engine.

 

The Highlander has full-time AWD and it is a tall/wide no so aerodynamic car. So those excuses do not apply here.

[Chip]

And you can maintain that speed? Jeeps have the aerodynamics of a brick. I had a 89 with the 4.2 carbed engine. It made gobs of TQ, but I could only run 5th on the freeway for downhills and flat surfaces.. any sort of hill and I would have to put it in 4th. I was also running oversized tires, so maybe that was part of it. Mine too had 3.07s. And the best mileage i ever got was around 14mpg.

 

I can maintain 60 mph on a flat road or on a slight hill in 5th. I've got the H.O. 190HP 4.0L but I still wish it was geared a bit higher...3.73 or 4.10. Downshifting into 4th only brings the revs up about 500 rpms but helps alot.

If you look at almost any vehicle, you'll notice that it's designed to cruise at or near it's maximum torque figure. It's at that speed that it's most efficient. You could re-gear a Subaru to cruise way outside of it's torque curve but your mpg's would suck,(literally).

[blitz]

Thanks Cookie. Actually the first three points that I made are fairly well documented priciples of internal combustion engine design that had all been worked out by 1950 or thereabouts.

 

One can only go so far in either direction with regards to undersquare or oversquare bore geometry and selection of connecting rod length before "falling off the curve" so to speak. The tradeoffs in either direction are fairly linear up to a point beyond which certain penalties rapidly become disproportionately large and Subaru is hugging the edge of that curve.

 

The last point that I made regarding the "stick/ slip" friction is, less well understood (due to insufficent research), and therefore open to rebuttal. Essentially it implies that the rings glide freely through the stroke on a hydrodynamic oil film (thin lubes yielding lower friction losses), but get stuck as they come to a stop, and need to be yanked free (thicker lubes yielding less stick /slip losses). This whole analysis came from an article on choosing the proper oil viscosity for minimum friction, and implied that a given viscosity would show the lowest total friction loss (best economy). Going thicker than optimal was met with a linear increase in (fluid) friction losses, while going thinner was met with a rapid rise in (stick/ slip) friction losses.

 

The feisty, punchy, midrange character of the Sube motor is attributable to the combination of the oversquare bore geomety and short rod/ stroke ratio (and the intake manifold). The short stroke combined with lots of valve area makes for poor cylinder filling at low revs, while the short rods tend to make the power fall off rapidy at the top.

 

A couple things I've noticed about getting efficient operation out of a Sube motor are counterintuitive in that it seems happier and more efficient with a slightly thicker oil (12-15 cst.), and that it likewise seems happier and more efficient at 3000-4000 rpm. 12-15 cst. is p*ss-water to most Ozzies.

[cookie]

around WW2 for aero engines if I recall correctly. As far as I know we are still not using two stage superchargeing in cars yet.

For the size of the engine I agree with you that a Subaru is a torquey little engine. This was one of the reasons I bought one as they felt more peppy to me than the Honda.

Blitz I think you really got us all thinking about this one!

It mystifies me why the engine would be happier with a thicker oil as it seems counterintuitive.

I wonder if the smaller Subaru engines are actually more efficient than the 2.5s?

The main thing that I think we will soon see for a general change is in smaller engines and hybrid drivetrains.

The electric motor should give the engine a chance to operate in a more efficient range and make transmissions much more flexible.

I suspect that Subaru will use the Toyota system GM is licenseing and this will give Subaru another chance to get the boxer right or move on. I do love that boxer smoothness, but it must be more expensive to build two heads.

[kingbobdole]

I disagree that subaru will ever get away from the boxer design. Not only are they know for it, but it greatly lowers the hoodline and..... Compinies these days are evil. So what if its a little more to do two heads, they manage to charge you for it. Ever notice the bargan basement 2.5 impreza starts around $17K? That ain't cheap when you can get a honda or focus or neon or whatever under $12K. Also, when you break this thing head gasket wise the kits alone are twice as much as a honda or what and if you have it done they charge you there too. For a 4 cylinder this this is a literal cash cow.

Phil