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Premium Member
2004 Forester XT
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923 Posts
Discussion Starter #1
I have been doing a bit of reading and notice that the VTD is superior to the MPT because it allows 50/50 transfer at launch as opposed to the MPT which only allows 90/10 transfer....which basically puts it in FWD mode during a launch.

I also hear that this will not be an issue unless I am making serious power. I really don't get how my MTD never squeals off the line if I am Stage 2 and floor it though if the vehicle is really launching at 90/10.

I am not saying that stage 2 is a whole lot of power, I am saying that if a FWD vehicle had this much torque off of the line it would definitely be buring rubber through 1st gear!

I also found a few threads on possibly improving the MPT transmission electronically, but I am not enough of a transmission guy to understand.

Could you explain what all of this stuff means to a newbie, and if they help a MPT 4EAT? Could you explain exactly what about a 04-06 MPT that makes it garbage?

Thanks!

http://www.subaruforester.org/vbulletin/f89/tc-lockup-mod-06-fxt-38281/

http://www.subaruforester.org/vbulletin/f89/valve-body-upgrades-4eat-38896/

http://www.subaruforester.org/vbulletin/f89/4wd-demand-4eat-3091/
 

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Premium Member
2009 Outback XT-B 5MT
Joined
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10,409 Posts
Computers are fast, right?

The TCU monitors front and rear output speed sensors, compares the values, and reduces the solenoid duty cycle accordingly.

The front output sensor is going to register a variation in speed from the rear sensor WAY (relative to computer processing speed) before the tires will receive the torque required to overcome friction (and other modes of energy dissipation, like tire deflection and losses through bearings and such).

Before your pedal touches the floor, the TCU has already begun locking up the plates.

As for electronic improvement to the MPT transmission, you could do one of two things. Replace the MPT pack with the VTD differential. Or... that's really it.

The only thing that directly controls lockup is the dutyC solenoid. The TCU monitors that, and until someone hacks the TCU, all you can do is interrupt the signal (RE: third link you posted).

If you want to see the front tires spin, "powerbrake" in Drive in the rain or snow. Dump the brake, and the front tires will sing the glory song breifly. That, or make a BUNCH more power. Then you can smoke the front tires all day long if you want!

The 90/10 condition really only exists once cruising. At take-off, the TCU locks up the clutches pretty readily.

Some people say that it also registers TPS, and provides lockup based on that. I don't agree with that, however. If you unplug the TPS on a cable-throttle car, the TCU doesn't throw a code. Logic IS changed through ECU->TCU communication, but that logic affects shift point and harshness and not necessarily DutyC lockup. While the TCU may implicitly utilize TPS values for lockup calculation, lockup is not a simple direct result of TPS variation.
 

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Premium Member
2004 Forester XT
Joined
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923 Posts
Discussion Starter #3
Computers are fast, right?

The TCU monitors front and rear output speed sensors, compares the values, and reduces the solenoid duty cycle accordingly.

The front output sensor is going to register a variation in speed from the rear sensor WAY (relative to computer processing speed) before the tires will receive the torque required to overcome friction (and other modes of energy dissipation, like tire deflection and losses through bearings and such).

Before your pedal touches the floor, the TCU has already begun locking up the plates.

As for electronic improvement to the MPT transmission, you could do one of two things. Replace the MPT pack with the VTD differential. Or... that's really it.

The only thing that directly controls lockup is the dutyC solenoid. The TCU monitors that, and until someone hacks the TCU, all you can do is interrupt the signal (RE: third link you posted).

If you want to see the front tires spin, "powerbrake" in Drive in the rain or snow. Dump the brake, and the front tires will sing the glory song breifly. That, or make a BUNCH more power. Then you can smoke the front tires all day long if you want!

The 90/10 condition really only exists once cruising. At take-off, the TCU locks up the clutches pretty readily.

Some people say that it also registers TPS, and provides lockup based on that. I don't agree with that, however. If you unplug the TPS on a cable-throttle car, the TCU doesn't throw a code. Logic IS changed through ECU->TCU communication, but that logic affects shift point and harshness and not necessarily DutyC lockup. While the TCU may implicitly utilize TPS values for lockup calculation, lockup is not a simple direct result of TPS variation.
Very intelligent and informative post, thank you for the details. One of the things that I gathered by skimming through it (correct me if am wrong), is that there really is no issue with the MPT at launch because the computer will already ensure a 50/50 launch. Why would the VTD be that much better at launching and 60' times then. (All I care about is launching by the way).

Sorry if my knowledge is very basic.

Thank you!
 

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Premium Member
2009 Outback XT-B 5MT
Joined
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10,409 Posts
The computer doesn't ensure a 50/50 launch. The TCU is reactive to sensory input. It doesn't know you are going to launch before you do.

VTD is better because there is a static, mechanical, torque split. In low-traction situations, like a high-power launch or in the snow or something, the plates do less work, which means they can work more quickly. Lockup on the MPT clutch can be sluggish, whereas the VTD clutches can lockup almost instantly (considering that the majority of their duty is to lock and only lock).

MPT clutches lock as needed, and based on feedback. They lock when the speed sensor readings vary. Once they are the same, they begin to unlock, if they vary again, they begin to lock.

That's well and good, but doesn't account for plate slip. If the clutches are locking, and you are slipping the plates, it will keep trying to lock. The trouble is, the coefficient of sliding friction is much lower than that of static friction. Slip the plates, and you have to lock up MUCH further than you would normally have to in order to couple the plates. You could also reach the bounds of the plates. They slip, the dutyC goes full-lock, and the plates keep slipping because there is insufficient force on them to get them to stop.

That's the advantage of VTD. You can very quickly clamp the plates, and the torque variation between the plates is relatively small. So you can clamp them with almost no slip. That reduces wear and increases efficiency, which makes for better and more consistent launches.

Think of it like an MT clutch. If you launch, and it's slipping, and you don't let off, the clutch can keep slipping even when fully engaged. But if you launch HARD and don't allow the clutch to slip significantly, then you get a much more positive engagement and a much more efficient transfer of power.

As an aside, I believe the clutches in the VTD diff are about the same size and of only a slightly fewer number than the clutches in the MPT pack. I don't know a whole lot about the 4EAT, so I can't say for sure. I know A6n6d6y probably knows the mechanical ins-and-outs of both diffs better than most technicians at Subaru do.

Now, this isn't to say the MPT is a bad arrangement. I mean, they used the same design for almost 20 years, and year after year Subaru was heralded as being one of the best AWD systems in the world.
 

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Premium Member
2004 Forester XT
Joined
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923 Posts
Discussion Starter #5
The computer doesn't ensure a 50/50 launch. The TCU is reactive to sensory input. It doesn't know you are going to launch before you do.

VTD is better because there is a static, mechanical, torque split. In low-traction situations, like a high-power launch or in the snow or something, the plates do less work, which means they can work more quickly. Lockup on the MPT clutch can be sluggish, whereas the VTD clutches can lockup almost instantly (considering that the majority of their duty is to lock and only lock).

MPT clutches lock as needed, and based on feedback. They lock when the speed sensor readings vary. Once they are the same, they begin to unlock, if they vary again, they begin to lock.

That's well and good, but doesn't account for plate slip. If the clutches are locking, and you are slipping the plates, it will keep trying to lock. The trouble is, the coefficient of sliding friction is much lower than that of static friction. Slip the plates, and you have to lock up MUCH further than you would normally have to in order to couple the plates. You could also reach the bounds of the plates. They slip, the dutyC goes full-lock, and the plates keep slipping because there is insufficient force on them to get them to stop.

That's the advantage of VTD. You can very quickly clamp the plates, and the torque variation between the plates is relatively small. So you can clamp them with almost no slip. That reduces wear and increases efficiency, which makes for better and more consistent launches.

Think of it like an MT clutch. If you launch, and it's slipping, and you don't let off, the clutch can keep slipping even when fully engaged. But if you launch HARD and don't allow the clutch to slip significantly, then you get a much more positive engagement and a much more efficient transfer of power.

As an aside, I believe the clutches in the VTD diff are about the same size and of only a slightly fewer number than the clutches in the MPT pack. I don't know a whole lot about the 4EAT, so I can't say for sure. I know A6n6d6y probably knows the mechanical ins-and-outs of both diffs better than most technicians at Subaru do.

Now, this isn't to say the MPT is a bad arrangement. I mean, they used the same design for almost 20 years, and year after year Subaru was heralded as being one of the best AWD systems in the world.
Your knowledge in this area far supercedes mine, so I have attempted to dissect what you are saying. So the MPT doesn't ensure any particular F/R ratio it varies due to the feedback it receives. The VTD has less of a mechanical limitation so it is less apt to slip.

You say that the MPT will not guarantee a 50/50 split. I am guessing that the
MPT will attempt to provide a 50/50 split during a launch unless it becomes overwhelmed for the reasons that you stated above?

Also, what is the advantage of the 3rd link that I posted? Does that ensure (or attempt to ensure) more consistency of some sort?

Thanks
 

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I'm pretty sure the tcu looks at throttle position for lockup on all the direct control 4eats (04+ XT, the models that have the squiggley gate shifter) but not on the older style 4eat (the one with the button and straight gate). It definitely sends power forward when you let off the gas (which sucks and is the main reason I'd rather have vtd than mpt, lift off oversteer on an awd car?!)

If I brake torque it and launch it, it spins all the wheels if i'm on snow or just bogs down on the street (my TC only stalls to about 2700).

Otherwise everything thats been said here is correct. VTD is a huge improvement over the MPT setup, especially if you're taking it to a track where you want it to be completely predictable all the time. However there isn't really any easy way to swap it in as the MPT tcu can't control the clutch packs in the VTD properly (I believe a6n6d6y had to keep the fwd fuse in and was just using the mechanical effect of the diff without the clutches, I don't think he ever got further than that point before he parted it out).

The mpt does not ever have a set ratio, it does it all based on inputs, but when cruising it tries to send as much power forward without creating a difference in f/r speed in order to improve coasting, gas mileage, etc.

Also, in 1st and 2nd (at least in non vdc 4eats) it seems to lock it 50/50, although its possible in 2nd it just sends power rearward more aggressively than in 3rd and D. If you go out in snow you can really tell what its doing if you do some figure 8's in each gear, really the best way to get a feel for the transmission.

The mpt isn't bad, but when you start getting to higher tq and hp, it really does start to be the weak link of the 4eat (aside from the weak front diff casing).
 

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Also, what is the advantage of the 3rd link that I posted? Does that ensure (or attempt to ensure) more consistency of some sort?

Thanks
That mod basically forces the solenoid to lockup the MPT clutches. This will give better predictability and traction, however it will put MUCH more stress on the transmission and the clutch packs especially if you're cornering hard or spinning tires (basically if you overwhelm it you're just slipping the clutches and it will wear out just like a clutch in an MT).

I'm pretty sure my MPT is wearing out, it doesn't feel quite as nice as it used to, and I could definitely smell a clutchy burnt oily smell after drifting in the snow for multiple hours (although that could have had something to do with my bright red hot turbo lol).
 

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Administrator
2004 Forester XT Premium 4EAT
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30,105 Posts
You might want to look into this: http://www.subaruforester.org/vbulletin/f89/mpt-vtd-conversion-37441/

Might not be warranted quite yet at stage 2, but if you ever upgrade your turbo you will experience the shortcomings of the MPT pretty quickly.

Also, the 4WD conversion you posted in your third link isn't meant so much for launching as for those who do some serious offroading and need can benefit from locking differentials.
 

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You might want to look into this: http://www.subaruforester.org/vbulletin/f89/mpt-vtd-conversion-37441/

Might not be warranted quite yet at stage 2, but if you ever upgrade your turbo you will experience the shortcomings of the MPT pretty quickly.

Also, the 4WD conversion you posted in your third link isn't meant so much for launching as for those who do some serious offroading and need can benefit from locking differentials.
He never posted back about getting the actual clutch action to work, just the physical diff to fit and work. I have a feeling the programming is different for the vtd and mpt 4eat tcu as they have very different amounts of f/r control.

My goal at some point is to swap in a baja 4eat (or maybe just the tcu?) with the sportshift and get vtd working (unless the baja already had vtd?)
 

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Premium Member
2004 Forester XT
Joined
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923 Posts
Discussion Starter #10
Thanks for the load of information! I am beginning to develop an understanding of what you are saying.

My last question is this:

The two of you seem to have a difference of opinion as to whether or not the switch mod would be useful for launching.

You both seem to agree on it destroying the tranny quicker.

Again, I plan to use it only when I need to take off in 1st and only if I get a bigger turbo.

This will be seldom or never.

Could you all conclude whether or not it would help launching?

From my research on the internet in general it should be the hot ticket for launching the MPT if you are brave enough.

Also is the mod good for driving in snow? I am also interested in safety during the winter when driving carefully.
 

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2007 FSTI and X 6 MT
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22,330 Posts
I'm pretty sure the tcu looks at throttle position for lockup on all the direct control 4eats (04+ XT, the models that have the squiggley gate shifter) but not on the older style 4eat (the one with the button and straight gate). It definitely sends power forward when you let off the gas (which sucks and is the main reason I'd rather have vtd than mpt, lift off oversteer on an awd car?!)

If I brake torque it and launch it, it spins all the wheels if i'm on snow or just bogs down on the street (my TC only stalls to about 2700).

Otherwise everything thats been said here is correct. VTD is a huge improvement over the MPT setup, especially if you're taking it to a track where you want it to be completely predictable all the time. However there isn't really any easy way to swap it in as the MPT tcu can't control the clutch packs in the VTD properly (I believe a6n6d6y had to keep the fwd fuse in and was just using the mechanical effect of the diff without the clutches, I don't think he ever got further than that point before he parted it out).

The mpt does not ever have a set ratio, it does it all based on inputs, but when cruising it tries to send as much power forward without creating a difference in f/r speed in order to improve coasting, gas mileage, etc.

Also, in 1st and 2nd (at least in non vdc 4eats) it seems to lock it 50/50, although its possible in 2nd it just sends power rearward more aggressively than in 3rd and D. If you go out in snow you can really tell what its doing if you do some figure 8's in each gear, really the best way to get a feel for the transmission.

The mpt isn't bad, but when you start getting to higher tq and hp, it really does start to be the weak link of the 4eat (aside from the weak front diff casing).
The only thing I know is that I can stall my VDC up to 3500 before the brakes are overpowered. As far as actual stall speed, I have no idea what the factory rating is. Im sure its low.
 
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