Sorry not going to agree here .gosurfun wrote: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.
You have a few freedoms with a flat or V bank engine that you don't get with inline ones . Primarily they shorter for the same number of cylinders (along the crankshaft axis) and lower in vertical height which suits cars with low bonnet lines .
When engines are short its sometimes easier to house large bore sizes and actuall a bit essential to get the capacity into the crank case/s .
Two litre fours have been made in all manner of bores and strokes and a square one from memory is 86/86mm bore and stroke .
When it comes to making high performance engines you learn imediately that if it doesn't breath it won't perform , it it can't fill its cylinders it won't make good torque or horsepower because it can get enough oxygen to burn . It should have reasonable cylinder filling to a point and then inlet restriction will strangle it . I expect it will have a low torque peak because it loses breathing ability early in its rev range .
The thing is you cannot have much in the way of valve area in small bore engines because they either don't fit in the chambers or hit the blocks deck if they do . Having four valves per cylinder type heads is all about fitting more valve area into compact chambers and the compact chambers being better for squish/quench and spark placement - in a true high performance engine anyway .
It really isn't as simple as a bore size and stroke length because too many other design facets have a say in the sum of and engines parts .
Things like bore/stroke ratio and rod length to stroke ratio and port priorities etc etc etc .
One Nissan engine I liked in the past was the FJ20 DOHC 16V turbo two litre . 89mm bore 80mm stroke 140mm centre centre rods bigish ports and 256 deg cams std . It made real good torque and revved like a turbine as well .
The cars it came in were RWD and 194Kw was thought to be pretty good in 1984 .
Subaru 5spd gearboxes don't like big torque and big torque at low revs is very hard to reliably withstand for the driveline . Have a look at a truck prime mover and note the sizes of the gearbox drive shafts and differential assemblies . They are huge and it because these vehicles make huge torque from big low revving high CR turbo diesel engines .
Subaru didn't want the cost/bulk/weight in the drivelines of their pre EJ25 turbo or flat 6 powered cars . Std Rexs if not abused didn't often break gearboxes especially if they had the factory spec clutch in them .
One you powered them up and beefed up the clutch the std trans didn't cope too well if pushed hard .
Subaru knew full well the limitations of their drivelines and not surprisingly refused to warrant modified cars .
They , like all manufacturers , also know that if they torque limit their engines in certain load/speed conditions it saves them the expense of higher torque rated drive line components .
Like Mitsubishi Subaru had to find ways to make a lot of torque from a two litre turbo four that couldn't rev very high with turbo restrictors . I can't quote what Subaru cranked out of their 99/2000 spec WRC cars but Mitsy was getting 640 nm from a 2L 4G63T at 5000 odd revs and they weren't much faster than the rally Rexs of the day .
So it can be done with a big bore short stroke short rod EJ20 but the transmission budget would buy outright a house in a nice part of Sydney today .
The other apect thats crept in since the early 80's is emissions and consumption legislation . Best mean torque ignition timing and air fuel ratios often don't give acceptable , to the EPA , exhaust emissions and if you can't meet the standards you don't sell the product .
So clean green standards mean hot lean engines and same old same old - need more cubes or forced induction to haul around heavier and heavier "cars" . Boosted petrol engines in heavy cars aren't a recipe for low down torque particularly if the engine is a little small . They can make torque from higher in the rev range but doing it down low costs more for twin scroll turbos intercooling oil cooling etc .
Trust me , Subaru could have had more torque from their donks but if their cars sold well what have they to gain by spending more to produce them .
The thing they always did struggle with was fuel consumption comparedc to their opposition . I reckon it had more to do with flat engines packaging and limited scavanging abilities from having the heads exhaust ports so far apart .
A .