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I can't remember exactly off hand, but the Liberty 2.2 and the OB 2.5 I have seem to run at around 2700 rpms at about 100k's where as my commodore, my wifes daewoo, and the 2010 falcon at work sit around 1800 rpms at 100ks' (all above are auto's)
Why is this?
The boxer engine design?
The gearing in the auto boxes?
Does high revs mean more fuel consumption?
What are the advantages of higher revs?
Disadvantages I would have thought would have been more wear and tear, but these engines seem to outlast other brand 4 cylinders.

Gear ratios.

apg39 wrote:Gear ratios.

+1
Don't forget Subie Auto's are only 4speed. With the 5speed in the 2.5 outback at 110km/hr it runs at 3000rpm with stock gearing.
With my 2.0 in the Foz (5speed manual, stock gearing) It runs at 3300rpm when at 110km/hr.

Yes higher revs will mean they use more fuel but if the revs were say at 1800rpm while on the highway in a 4cylinder N/A Subie then you would have f**k all power to overtake at all. Thats why they rev higher, well with my understanding.
4cylinder Subies get pretty good fuel economy on the highway though so you can't really complain. But if you do then get a foz/outback diesel.

Its all about Specific Fuel Consumption. This is the RPM where the engine makes enough power to maintain a specific speed, (100km/h) but at the lowest possible fuel consumption. The gearing ratios are then chosen to make the most of this SFC sweet spot in the rev range.


This graph gives you an idea of SFC, but isn't typical of Subaru


Subaru's, having relatively small engines (2.5L), need to rev more than larger engines of falcons and commodores (3.8L) to create enough power to keep the car moving.
You will find that they all use the same amount of fuel, its just that the larger engine uses its fuel over more cylinders at low rpm, the smaller engines use the same amount of fuel with less cylinders, but more rpm

Its just the way the engines are built. Boxer engines like high revs. I think youd find if they had put ratios to run the 4 cylinder at 1800 RPM at 110km the fuel economy would be worse because the engine is straining a lot harder, i.e your have to pump a lot more fuel into the engine to maintain it.

He majority of power is found in the 4 cylinder at 3000+ RPM. I know with my outback if you put your foot down yeah decent but when it gets to ~3500 RPM it just absolutely takes off as if a turbo kicks in.

EDIT^ Gannon beat me but we sorta said the same thing. He just did it with more detail.

Flat engines for various reasons are generally not stump pullers torque wise without a having a few revs aboard .
With Subaru engines generally the bores are reasonably big but the rods and the stroke are shortish compared to many inline fours or sixes .
The other issue with Subaru and I think most flat fours is the firing order and the fact that the exhaust ports are so far away from each other in the opposed cylinder heads . Its makes the exhaust timing almost impossible to get right and therfore scavanging is difficult to get working properly .
When an engine does do it properly the low pressure generated downstream of the exhaust valves helps draw all/most of the burnt gasses into the exhaust at the end of the exhaust stroke so the cylinders have the best chance of filling with clean cool air .
The ONLY Subaru flat four exhaust header that is done properly is the twin scroll turbo one and in modified form may function well on an NA engine .

Six cylinder engines are different , inline ones anyway , in that their crank pin phasing is different to inline fours which have a flat plane crankshafts ie 180 degree phasing . An I6 is sort of like two I3s joined at the crank and the phasing is 120 degrees . Generally the firing order is 153624 so looking at our "joined" pair of I3s the exhaust events are alternatively from each group of three cylinders ie the 123 set or the 456 set . In order from the front set 1 then the back set 5 etc etc . It is very easy to get an I6 to scavange when the manifold/header is split into the two groups of 3 .
Subaru flat 6s have some of the same issues as flat fours and some say they lack torque for their capacity .
The cure all for low torque engines is to use shorter diff ratios and gain from the increased mechanical advantage of the shorter ratio . The engine will cruise at higher revs and if the manufacturer designs NVH and quiet exhausts I think its hoped no one will notice .
The Ford and until recently Holden engines are/were not small at 4 litres and 3.8 so its easier to make low rev torque from the capacity . Fowlcans and Commode Doors are quite heavy cars currently and they need good torque to shift their lard . The current Commode V6 is a torqueless dog IMO and as soon as you touch the gas they want to kick down and pull lots more revs to get going than the older Eco Tech engine did .
Subaru have known for a long time that their Lizard engines don't win class leading economy awards but they persist with them anyway .

My take anyway , A .

compare a six to a four in this regard will hurt your brain. Surely most fours through history have been revvers, and sixes and eights not so. The Daiwoo must b an exciting car to drive

People have often bought fours for fuel economy and that economy is usually comparitively only better in traffic for the four to a six.

How much more efficient is a 2.0 litre four gong to be on the highway doin' 3200 at 100kph when a 4.0l six, with twice capacity doing half the revs?

steptoe wrote:compare a six to a four in this regard will hurt your brain. Surely most fours through history have been revvers, and sixes and eights not so. The Daiwoo must b an exciting car to drive

People have often bought fours for fuel economy and that economy is usually comparitively only better in traffic for the four to a six.

How much more efficient is a 2.0 litre four gong to be on the highway doin' 3200 at 100kph when a 4.0l six, with twice capacity doing half the revs?

Well my 2.0l seems to get around 7.5l/100km if you sit at 110km/hr with nothing on the roof. Revving away at ~3250rpm. Goes upto about 8.5l/100km if you sit on 120km/hr then if you sit on 140km/hr at 4000rpm you get about 9.5-10l/100km. But around town I have managed to get 700km from 58l so can't really complain. Although its not even close to anything quick with a bigger engine.

Towing a trailer along the highway too with my same 2.0l Foz it does alright. I used 1/3 of the tank to do 170km.

My mothers 4l 6cyl jeep gets better economy then all but 1 of my fours

mud_king91 wrote:My mothers 4l 6cyl jeep gets better economy then all but 1 of my fours

Really? My work mates 04 Jeep Wrangler (4l straight 6) gets 15l/100km on the highway and worse around town but he does have 31" tyres and redone the ratios so they are lower for offroading.

my pootrol barely hit 1000rpm at 100km/hr in 5th

still used inxs of 20l/100km.

mind you she redlined at 4000rpm, 6cylinder 4.2l petrol running 33in tyres.

i like driving a revvy car. Way more responsive. Maybe i should buy a rotary haha

alex

Yea hers is factory she gets about 600 to a tank

.

Bore/stroke ratio fellas, Subaru motors generally are short stroke and big bore = lack of low end torque but good Kw with revs.

Torque is twisting force, so the further out from the centreline of the crank the big end bearing is the more torque/twisting force it has, but this makes it throw a larger radius causing other problems. You can't get anything for nothing without trading something else off.

Want a good offroad motor then go for longer stroke than bore measurement. Want a good street motor then go for shorter stroke than bore measurement. Obviously someone will say that their something car does not follow this rule but there will be substantial reasons if it doesn't. Tractors and lugging vehicles are long stroke ratio and racing cars and performance street bikes are short stroke ratio. The gear ratios are just a means of making the motor fit the application.

Bore/stroke ratio fellas, Subaru motors generally are short stroke and big bore = lack of low end torque but good Kw with revs.

Torque is twisting force, so the further out from the centreline of the crank the big end bearing is the more torque/twisting force it has, but this makes it throw a larger radius causing other problems. You can't get anything for nothing without trading something else off.

Want a good offroad motor then go for longer stroke than bore measurement. Want a good street motor then go for shorter stroke than bore measurement. Obviously someone will say that their something car does not follow this rule but there will be substantial reasons if it doesn't. Tractors and lugging vehicles are long stroke ratio and racing cars and performance street bikes are short stroke ratio. The gear ratios are just a means of making the motor fit the application.

Why do some of these posts work and not others ???? quick guide to where I'm going wrong.

You have posted basically the same thing 3 times. What are you trying to do?

steptoe wrote: The Daiwoo must b an exciting car to drive
?

Yeah I'm a bit of a thrill seeker :-D:-D:-D

Good answers guys, thanks for the info.

steptoe wrote:compare a six to a four in this regard will hurt your brain. Surely most fours through history have been revvers, and sixes and eights not so. The Daiwoo must b an exciting car to drive

People have often bought fours for fuel economy and that economy is usually comparitively only better in traffic for the four to a six.

How much more efficient is a 2.0 litre four gong to be on the highway doin' 3200 at 100kph when a 4.0l six, with twice capacity doing half the revs?

Its not really as simple as this . Some manufactureres have made I6 2L engines and others like Porsche have made 3L fours .
The easiest lazyest part throttle torque comes from larger capacity engines - meaning in relation to a given vehicles gross mass .
Generally how it works is that the largest engine options for a given vehicle often come with taller gearing purely because the larger engines low rev torque can pull taller gearing .

As for big bore short stroke engines not being capable of making good torque , that doesn't hold water either .
The more piston crown area you have the more area you have for the expanding gasses to exert the pushing forces on .
Also into larger bores you can fit and open larger valves meaning potentially better cylinder filling and higher dynamic or effective compression ratios .

Like all the others flat engines have issues unless they are designed from the outset to be a high performance one . The fact that as mentioned the exhaust ports are so far apart doesn't help here .

BORE AND STROKE RATIO !!!!!!

To fit the boxer Subaru motor between the chassis rails the bore is enlarged and the stroke shortened to get the required motor capacity. Short stroke large bore motors are poor on torque, it comes down to how far the big end bearing is from the crank centre. Imagine winching up a boat onto a trailer, a short crank handle needs lower gearing and lots of rotations, a long crank handle has more lever or torque and needs less rotations but is harder to spin fast to do the same job. Torque is twisting force, it is what snaps the gearbox and diffs.

Subarus tend to be poor on torque unfortunately, but they have many other good points. The Tribeca motor now has asymetric con rods to get as much motor capacity as possible (by increasing the stroke) between the chassis rails as they have already added two cylinders to it length. Its a boxer motor problem the same as BMW R series boxer motor bikes, increases in stroke tend to make the motor even wider.

Tractors are long stroke for torque and high performance cars and bikes are short stroke for HP/KW power (2 and 4 strokes). Probably why the rotaries are poor on torque but good on HP/KW as well.

93 L Series 4wd with 185/70r-14's

At 105 km/h in 5th,
dropped into 4th to see if it would help me accelerate up to say 115 km/h...
It still took about 3-5 seconds to gain 10km/h.
It feels like it would make no difference just leaving it in 5th.

So my engine will rev high,
but it feels the high revs aren't very useful
(aside from having a greater spread of wheel speeds in one gear for sand driving)

Soon I'll be installing a tachometer
so I'll actually know when my engine is pulling and when it's bogging.


*My 08 Civic with the ordinary 1.8L was though.
It would come alive at 2000rpm and pull hard from 2800rpm to 4500rpm.
After 4500rpm it would get louder, but not perform impressively considering the extra raucous.
Easily twice the torque at 3600rpm vs. 1800rpm