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2002 Impreza RS 2.5
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Discussion Starter #1
I was getting a lift in the shuttle taking me from the Subaru dealer to the metro station to head in to work this morning. (Took my '02 RS2.5 in to get a ding removed by the dealer.) In the car with me was a Volvo guy. He seemed rather knowledgeable in general about vehicle history, Subaru history as well. (I hadn't realized Subaru almost ceased sales in the US due to poor revenue before the Outback came out and "saved them".) Anyway, I was talking to the guy and mentioned my RS 2.5 w/ AWD. He indicated that AWD is great in snow, etc. (I do not race) but that it is why the acceleration is sluggish. Two transmissions, etc. And that seems to make sense. I don't know if I'd call my RS 2.5 (not a WRX) with its 2.5L, 165hp engine "sluggish" but it does seem that two transmissions would sap power.

I wonder what % of power it saps vs. if the car were two-wheel-drive?


blakespot
 

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true, AWD does loose more power through the drivetrain then 2WD (esp. FWD), but its not because of "two transmissions". the guy must have been thinking of conventional 4WD (think Jeep) in which the vehicle is normally RWD, but the transfer case (basically a second tranny) can be employed to send 50% of the engine torque forward. note that this is a constant figure, which is why SUVs/Trucks can't turn for shit while in 4WD. AWD on our cars uses 1 tranny and three diffs. at the very least, the center diff must be some sort of Limited Slip Differencial (LSD) which allows the torque to go to both driveshafts, instead of the one with the least resistance (which would be very bad). not only this, but it can vary the amount of torque going to either driveshaft, which allows the wheels to travel at different speeds, which is necessary for cornering. so the power loss from AWD actually just comes from friction and inertia involved in rotating all those extra diff gears, driveshafts, and wheels ;) this loss (and the increased weight as well) is most offset when traction is at a premium (such as snow, gravel, etc), but can still be quite effective around a racetrack as well. how effective? its hard to say. most high level Touring series prohibit AWD so, while you may see an Impreza or a Skyline, they are RWD. the series that do allow AWD generally through in a huge weight penalty for it, though it can still be very effective if the series employs a standing start, hence the S4s just barely kicking my beloved NSX's ass in the Speed World Challenge.

hope this helps!
 

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If you look at the dyno charts for the GTI-R. Those guys have some big drivetrain loses. Around 33%. Of course even with just over 200hp at the wheels they are running mid to low 12's. The cars only weigh 2700lbs.
 

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2001 Silverthorn Coupe
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Audi in the 80's started a trans-AM team. THey absolutly dominated the season. THey use to race inteh rain and they would be passing cars on the outside of turns. THey outlawed AWD for the next season. In lower HP applications, AWD isn't much of an advantage on a road course (unless there is adverse conditions) as the HP increases traction becomes a premium and AWD begins to become a huge advantage as higher HP RWD and FWD begine to have traction problems out of corners.

cheeRS,

Greg
 

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2.5 RS
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Hey Blank,

These other guys above me are right. The Subaru has a 60/40 split all the time. It is "sluggish" cause it is only 165bhp. How much do you think it acutally going to the wheels. We both are driving around on less than 165. I already have a K and N in there and a larry ganz pipe and noticed a bit of difference. But I also notice pick up when the air filter is brand new, and I put the pipe in, and when it is really cold outside and when the tank is not full. That guy you talked to may know a lot of history but got his facts mixed up. I will notice a big differance when i change my oil watch.:) Maybe the guy is just jealous of the "beauty of all wheel drive". I can't wait till it rains around here, it makes it easier to power slid of empty roads at night. If you see a blue RS going sideways late at night then that is me. ;-)


P.S. I noticed you are in alexandria so am I. If you want to know what you car will be going when it reaches 6000miles let me know.
 

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The Subaru has a 60/40 split all the time
actually, its 50/50 until slip occurs with the manual, and 90/10 until slip occurs with the automatic. speaking of which, the auto is a whole different ball of wax, that uses electronics to vary torque split rather than mechanics, which makes the transfer quicker then the viscous LSD the manual has. also worth noting, some JDM cars that we'll never see ( :curse: )have a driver adjustable center diff, which allows them to vary the normal torque split from something like 50/50 to 30/70. i forget the exact numbers, but you get the idea. I want my Type RA!!!!!
 

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This subi ain't that 'sluggish'..... You wanna feel a sluggish subi just take a cruise in an 85 GL Wagon.... Now that's sluggish! :)

Can't beat the Hi/Lo range though, I miss it.... :(

B
 

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2002 Impreza RS 2.5
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Discussion Starter #8
10th Warrior said:

actually, its 50/50 until slip occurs with the manual, and 90/10 until slip occurs with the automatic. speaking of which, the auto is a whole different ball of wax, that uses electronics to vary torque split rather than mechanics, which makes the transfer quicker then the viscous LSD the manual has.
Can you explain more about the diff between the manual and automatic? I have an automatic. So it's 90% power to the rear unless slip occurs? I heard that it varied based on acceleration, etc. What is this about electronics vs. mechanics varying torq split -- what is involved here. Just curoious. Thanks.



blakespot
 

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well, automatics are black boxes of evil in my mental word, so i don't know them as well as i do manuals, but i will try to explain it, though someone else can probably do a better job.

the auto tranny uses an electronic center diff (ECD). this means that there are several sensors that the ECM uses to determine wheel slipage (not sure, but stuff like ABS wheel speed sensors, TPS, etc). as long as everything is copesthetic, the ECM keeps the ECD sending most of the toque (90%) forward with the remaining 10% driving the rear wheels. now, when the wheel speed starts to vary front to rear, the ECM will change the toque split (to a max of 50/50 IIRC), which will allow the wheels that have more grip to get more power, saving your ass from that nasty deer that jumped out in the road in the middle of a blizzard. since its electronic, it all happens at the speed of light (basically).

now, the manual system is all mechanical. the center diff is a viscous coupling LSD. this means that if a certain ring gear inside the diff starts to speed up (caused by slipping wheels) there is a fluid inside that will be heated up by the extra energy. this heat causes the fluid to become more thick, which acts sort of like a brake on the spinning axle, which means that the engine torque is going to take the path of least resistance, which is now no longer the slipping wheels, but the ones with grip thanks to the viscous fluid. but, as you can imagine, it takes some slip to exist before the torque gets transfered, so its not as quick as the ECD, although modern viscous LSDs are set up with a bit of lock in them, so that they activate more quickly. anyway, you can set what the default torque split is (in my case 50/50), and then as one axle starts to speed up (slip) more torque is transfered to the other. the amount of torque transfer is proportional to the amount of slip, until you reach the physical ability of the diff gears or fluid.

and then there's the STi's, which use a mechanical LSD in the center. its basically the same set up as a manual RS, except that the torque split is varied through a series of gears pushing against one another instead of fluid getting thick. this way is kick ass, and as a bonus, some diffs allow the driver to toggle the static torque split by realigning the diff gears (i think), which is why you can make the 22b completely RWD (as in all the torque goes to the rear wheel).

mitsubishi, on the other hand, uses electronic diffs in the EVO, in a similar engineering concept to your car, but with more intense hardware and a set up designed for Group N rally competion, not keeping people out of trouble on a snowy road ;)

and then there are the WRC cars, which i won't even pretend to understand the level of technology there, but it is all electronic, though bears about as much resemblence to an automatic RS's drivetrain as a Cheetah does to a Pepsi. one cool thing it can do is disengage the rear drivetrain when the handbrake is pulled. Uber cool to say the least.

anyway, i hope this answered your question somewhat.
 

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The auto has 90% of torque driving the front wheels until slip is sensed, then transfers torque. THe auto is a clutch type differential. The TCU controles an actuater that squeezes the clutch pak that transfers torque.

The manual is a viscous type diff that relies on the carictoristics of a viscous fluid that gets thicker (more resistance to flow) when it is warmed. The discs in the diff rub and get hot heating up the fluid and making it more diffacult to have either side turning at a different rate. This type of diff cannot lock. It forces either half to turn at realitivly the same speed, but does not lock.

The old Type R STi's and Type RA's didn't change the torque split with the transmission, they controled the amount of lock between the two axles. You could completly lock the center differential or have it more or less open depending on conditions or driver needs/wants.

You cannot change the defalt torqe split with out using a planetary differential, or different final drives front and rear (with a torqe multiplying gear makiing up the difference, ie the geared section in the tail housing) in the new WRX.

cheeRS,

Greg
 

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95 Impreza L AWD
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You wanna feel a sluggish subi just take a cruise in an 85 GL Wagon
Or a '95 Impreza L AWD with the 1.8!:curse:

though bears about as much resemblence to an automatic RS's drivetrain as a Cheetah does to a Pepsi
:lol:
 

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Is it just me or are the VW guys getting whp very similar to what the manufacturer states? I remember a review of the Golf Gti putting out really close to 180 whp on the dyno (stock). I know some people may not care about straight speeds, but those guys are hitting 7.0 on the 0-60 and 15.3 on the 1/4. Bull?
 
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