FWD vs. MPT vs. VTD driving impressions

[NiftySVX]

I've driven about 40,000 miles on FWD, 45,000 on MPT/ACT-4 and now about 3,000 on VTD in the same exact car.

Since I have been getting a few questions about the real world dynamics and driving experience of the VTD transaxle, I thought I would share my impressions of each stock drive system as I would assume I am the only person to ever have experienced each in the same car. I’m going to try and be as thorough as possible so that the post is useful to everyone who may come across it without having to reference other posts to understand. Keep in mind all three transmissions are identical except for ancillary parts relating to AWD.

FWD:

How it works:

Transmission output flows from the reduction drive gear to the reduction driven gear, which is splined to the front differential pinion shaft. Final drive ratio is 3.70.

What this means:

Front wheel drive only.

The rarest of the SVX drivetrains and for good reason. As a review or two put it, the SVX is “compromised”. The negative effects of having a heavy, 230 hp engine with a relatively deep first gear means that unintentional tire spinning is annoyingly present in first gear. Good tires and using manual 2nd mitigate this, but this increases heat produced in the transmission. I would use 2nd start whenever it was wet or whenever I needed to accelerate quickly (such as a left turn in front of traffic, etc.) Torque steer and bump steer are pretty common. Hard acceleration from a stop requires a firm hand on the wheel to keep it in line. The whole experience of a 2WD SVX seems to illustrate why the car was designed to be AWD to begin with.

MPT/ACT full time AWD

How it works:

Transmission output flows from the reduction drive gear to the reduction driven gear, which is splined to the front differential pinion shaft. A clutch hub is welded onto the reduction driven gear. A piston and special friction discs that are designed to slip are inside a clutch drum which is mated to the rear output shaft. 3.545 FDR.

What this means:

Power is sapped from the reduction drive gear and sent to the rear differential, which is then allocated to each rear wheel via a viscous type LSD. The pressure applied to this clutch is varied via a duty cycle solenoid as some percentage of main line pressure. This process is dynamic and is based on the principle of predictive modeling. Its primary goal is to prevent wheelspin and to mitigate it when it occurs. It is important to note that the front pinion shaft is still directly connected to the reduction driven gear, which is in mesh with the reduction drive gear. It is mechanically impossible for there to be more than 50% power to the rear wheels, nor is it possible for the rear shaft to rotate faster than the front.

Basically a power take off of the front wheel drive unit. In stock and unmodified form, the control unit is conservative with the rear apply pressure due to the fact that binding can and will occur if the rear drive torque gets close to the front torque and the two shafts are turning at different speeds. The bold text above should be kept in mind when thinking of the system, and it is painfully obvious when driving the vehicle. Due to limitations in design and compromises to ensure reliability, this system is almost never “locked” into a full 50/50. This provides for a sure footed vehicle for the most part as the tendency is towards understeer and the open front differential usually causes the torque to the front wheels to be directed to a spinning wheel in a low traction situation. It is quite possible to encounter wheelspin in the front, especially when the engine speed is low. Driving in very low traction situations can be unpredictable due to the fact that one front wheel may be receiving the entire allotted amount of power directed to the front while the rear LSD forces a split. This can cause the tail to swing out unpredictably but this is rare due to the fact that 99% of the time this situation is encountered the transfer clutch is not applying enough torque to the rears to make them both slip. The SVX still torque steers and bump steers due to the front bias.

VTD

How it works:

The reduction drive gear is driven by the forward planetary gears, which in turn drives the reduction driven gear and the front output. The rear planetary gears drive the clutch hub, which drive the clutch plates, which in turn drive the clutch drum that is mated to the rear output shaft along with the rear sun gear. The two sets of planetaries are of different sizes which provide for a rear bias of about 60%. FDR: 3.70

What this means:

Quite simply, it means that the car feels whole. The track problems of the other two systems are gone as there is no imaginable situation where the front wheel torque would be higher than the rear. Torque ratio between front and rear is naturally balanced to where the rears receive more than the front, which is nice as the front wheels are unloaded by weight transfer when accelerating. This gives the car a RWD feel and makes wheelspin very unlikely, even without a clutch. The clutch itself has thinner and less robust clutch plates as it does not have to bear the full brunt of the torque split, instead it must only suppress the differential action. This requires less friction in the clutches and is therefore much, much better at equalizing the two output shaft speeds than a MPT clutch alone could ever hope to be.

In summary, I would like to point out the main advantages of the VTD system

• Mechanical rear bias, power must travel through the planetary gearset to reach the front wheels.
• No more “keeping up with the front shaft speed” objective.
• Additional rear torque is made useful via the rear LSD instead of additional front torque being misused by the open front diff.
• Less force is required to equalize shaft speeds resulting in better operation of the clutch pack.
• SVX suspension seems designed for this rear bias
• 3.70 final drive ratio
• JDM tcu is vastly superior to US tcu.
• More reliable system (no broken transfer clutch hubs here)
• The SVX drives like a proper sports coupe instead of a front wheel biased high power mid 90s legacy.

A final note. The VTD setup used in the SVX is unique to the SVX. There is no other Subaru produced that used identical parts, so sourcing VTD parts from a non-svx donor would require a completely custom TCU using the donor vehicle’s VTD apply maps to prevent binding and provide proper operation.

As usual sorry for the lengthy post, comments, questions, and corrections are always welcome.