STI AWD question

[dashton]

Took my first ride in a 2005 STI last night. Wow - what a car! I could sure get in a lot of trouble driving one of those. Good thing I just have my old legacy. But as my friend was showing me all the features of the car, I had a few questions he couldn't answer. There is a switch and dial on the center console to control the power distribution of the AWD system. He can switch it to "auto" to engage the AWD as needed, or he can put it into a manual mode to lock it up or send more power front or back. This is on the six-speed manual tranny. I was under the impression that all subaru manual tranny's have a viscous coupling to the rear wheels. The switch is his STI seems to be activating something more like what's in my legacy's automatic - a rear clutch pack. Is this true? Does the STI's manual transmission have a rear clutch pack like the automatics? And if it does - how does it engage? Electrically? Hydraulically? I didn't think manaul tranny's had oil pumps in them to engage a hydraulic clutch pack. Sorry for the silly question, but I'm an engineer and I have to know these things (you other engineers will understand). Thanks. Dave

[cannonball]

Took my first ride in a 2005 STI last night. Wow - what a car! I could sure get in a lot of trouble driving one of those. Good thing I just have my old legacy. But as my friend was showing me all the features of the car, I had a few questions he couldn't answer. There is a switch and dial on the center console to control the power distribution of the AWD system. He can switch it to "auto" to engage the AWD as needed, or he can put it into a manual mode to lock it up or send more power front or back. This is on the six-speed manual tranny. I was under the impression that all subaru manual tranny's have a viscous coupling to the rear wheels. The switch is his STI seems to be activating something more like what's in my legacy's automatic - a rear clutch pack. Is this true? Does the STI's manual transmission have a rear clutch pack like the automatics? And if it does - how does it engage? Electrically? Hydraulically? I didn't think manaul tranny's had oil pumps in them to engage a hydraulic clutch pack. Sorry for the silly question, but I'm an engineer and I have to know these things (you other engineers will understand). Thanks. Dave

I saw somewhere a few days ago a animated link to how all the Subie AWD systems works. I thought it was on Subaru.com, but I can't find it now. Anyways I think there are 5 AWD systems. VC,Active,VDC, VDT, and what the STi has CDDC IIRC. I think it's close to VDC with the enhancements you listed. It's all electronic as far as I know, but that ain't much. Someone with better knowledge can correct me if I'm wrong.

 

Charles

[rweddy]

Took my first ride in a 2005 STI last night. Wow - what a car! I could sure get in a lot of trouble driving one of those. Good thing I just have my old legacy. But as my friend was showing me all the features of the car, I had a few questions he couldn't answer. There is a switch and dial on the center console to control the power distribution of the AWD system. He can switch it to "auto" to engage the AWD as needed, or he can put it into a manual mode to lock it up or send more power front or back. This is on the six-speed manual tranny. I was under the impression that all subaru manual tranny's have a viscous coupling to the rear wheels. The switch is his STI seems to be activating something more like what's in my legacy's automatic - a rear clutch pack. Is this true? Does the STI's manual transmission have a rear clutch pack like the automatics? And if it does - how does it engage? Electrically? Hydraulically? I didn't think manaul tranny's had oil pumps in them to engage a hydraulic clutch pack. Sorry for the silly question, but I'm an engineer and I have to know these things (you other engineers will understand). Thanks. Dave

http://auto.howstuffworks.com/differential9.htm

 

The switch on new sti will allow the vehicle to front or rear wheel biased for handling proposes.

[DerFahrer]

Using the latest Drive Performance Magazine as a resource, here's what I know about the DCCD (Driver Controlled Center Differential) on the STi.

 

It does indeed appear that it has a multi-plate transfer clutch like the Subaru automatics, which would be the first time this technology has ever been used in a Subaru manual. They have always used a simple viscous coupling in Subaru manuals.

 

The system depends on a myriad of sensors: from wheel speed sensors, lateral G sensor with yaw control, and rear differential oil temperature switch, to tapping into the sensors the car already has; the ABS sensors, the throttle position sensor, the brake light switch, etc.

 

It controls the traction situations automatically based on input from all these sensors, and it's probably the best idea to leave it this way if you're not driving competitively. But if you do choose to control it yourself, here's how it works: By turning the Manual switch on, you can adjust the center diff as having 35% power to the front and 65% power to the back when you turn the wheel all the way back, and as you move it forward, you send more power to the front wheels until you put it in L for Lock, in which the center diff is completely locked at 50/50. Subaru and I both recommend that you not lock the center diff on pavement, or you could damage anything from the transmission to the axles.

 

Hope that clears up any questions.

[rweddy]

put it in L for Lock, in which the center diff is completely locked at 50/50.

I did not know it had a diff lock.

Has there been any other US models with a diff. lock since the RX's of the 80's.

[dashton]

Using the latest Drive Performance Magazine as a resource, here's what I know about the DCCD (Driver Controlled Center Differential) on the STi.

 

It does indeed appear that it has a multi-plate transfer clutch like the Subaru automatics, which would be the first time this technology has ever been used in a Subaru manual. They have always used a simple viscous coupling in Subaru manuals.

 

The system depends on a myriad of sensors: from wheel speed sensors, lateral G sensor with yaw control, and rear differential oil temperature switch, to tapping into the sensors the car already has; the ABS sensors, the throttle position sensor, the brake light switch, etc.

 

It controls the traction situations automatically based on input from all these sensors, and it's probably the best idea to leave it this way if you're not driving competitively. But if you do choose to control it yourself, here's how it works: By turning the Manual switch on, you can adjust the center diff as having 35% power to the front and 65% power to the back when you turn the wheel all the way back, and as you move it forward, you send more power to the front wheels until you put it in L for Lock, in which the center diff is completely locked at 50/50. Subaru and I both recommend that you not lock the center diff on pavement, or you could damage anything from the transmission to the axles.

 

Hope that clears up any questions.

Thank you for the explanation. It makes perfect sense to me to have a clutch pack back there to adjust the power distribution. I've put a switch in my legacy that locks that center differential when I need 4WD (my AWD is broken). I've been wondering lately if I can put in some type of variable potentiometer to manually adjust the voltage to my Duty C solenoid and get the same results as I saw is his STI. Might have to get more serious about it now. By the way, I was absolutely impressed by the STI in every way. Never had a car impress me like that. Thanks for the replies. Dave.

[RallyKeith]

Using the latest Drive Performance Magazine as a resource, here's what I know about the DCCD (Driver Controlled Center Differential) on the STi.

 

It does indeed appear that it has a multi-plate transfer clutch like the Subaru automatics, which would be the first time this technology has ever been used in a Subaru manual. They have always used a simple viscous coupling in Subaru manuals.

 

The system depends on a myriad of sensors: from wheel speed sensors, lateral G sensor with yaw control, and rear differential oil temperature switch, to tapping into the sensors the car already has; the ABS sensors, the throttle position sensor, the brake light switch, etc.

 

It controls the traction situations automatically based on input from all these sensors, and it's probably the best idea to leave it this way if you're not driving competitively. But if you do choose to control it yourself, here's how it works: By turning the Manual switch on, you can adjust the center diff as having 35% power to the front and 65% power to the back when you turn the wheel all the way back, and as you move it forward, you send more power to the front wheels until you put it in L for Lock, in which the center diff is completely locked at 50/50. Subaru and I both recommend that you not lock the center diff on pavement, or you could damage anything from the transmission to the axles.

 

Hope that clears up any questions.

 

Well you got part of it right.

 

The center diff in the STi is an open diff with an electronically actuated lock-up clutch pack and a geared torque (power) distribution of 35%front/65%rear. When placing the diff into Manual control mode you are doing nothing more than setting the amount of lock the clutch pack is providing. In diff free mode the center diff acts just like an open diff, in diff lock it is locked solid. (The later year GLs/Loyale/XTs had a fulltime 4wd 5speed that had an open center diff) Anyhow, in the middle of the dial range you are controlling the amount of lock and in no way putting more power to the front or rear.

 

Keith