STi Proportioning valve or not

[subytech]

Ok another questoin concerning my buddies build. We've got front and rear calapers, master cylinder and brake booster out of an 04 STi in a 92 legacy wagon. I'm thinking we should use the proportioning valve out of the legacy since the proportioning rates are for the legacys calculated weight transfer, and the STi's are obviously not. Just wondering if anone had any inputs or has any experience in this aria. Thanks

[BAC5.2]

The stock prop valve isn't compensated for weight transfer (since that will parametrically change with age, driving conditions, and driver input). It IS designed to work with an approximation of the brake torque each end will apply, and it's relationship to peak possible torque.

Personally, I'd use a wagon prop valve if those were my options.

Ideally, I'd swap the entire brake system from the STi, ABS unit and controller included. You can duplicate most signals necessary for the STi ABS to work, though it won't have any control over the center differential.

If you have access to a donor car, that is what I would do, though it may require you to make some brake lines...

[SemperGuard]

USDM STi's don't have prop valves, since they have Supersport ABS.

[subytech]

Prop valves differentiate fluid pressures on a curve that is calculated on how much rear brake torque CAN be applied not WILL be applied, there for creating as close to simultaneous lockup of both front and rear tires as possible, which equals more stability once locked up. The spring rate that predetermines the split point (or curve) of the valve is calculated using the change in the center of gravity when braking when the vehicle is new. The center of gravity changes only because of suspension pitch and weight/inertial change while braking. Exaserbated movement due to a loaded vehicle is the only thing that some valves can adjust for and that adjustment is still based on changes in vehicle hight and weight/inertial changes. Changes in vehicle dynamics such as blown shocks, bad driving conditions, and personal driving style are what ABS and traction control was created for hence the link between the STi ABS module and the center diff. We're not installing the ABS in this car for much the same reason, I’ve driven STi swaps in 2.5rs' the predetermined STi vehicle dynamics are nowhere near the RS' and this causes the STi ABS to wig out.

[subytech]

USDM STi's don't have prop valves, since they have Supersport ABS.

Their mounted on the FR shock tower on the STi and the legacy

[SemperGuard]

Subaru's with Supersport ABS have no need for propvalves. That's the whole point of Supersport ABS. Instead of creating completely new brake lines a two-way connector is placed where the prop valve is on cars without supersport ABS. USDM STi's have Supersport ABS. Go onto STIS and check it out if you don't believe me.

[93forestpearl]

Most proportioning valves don't try to have the wheels lock up at the same time. They make sure the rear wheels lock up first otherwise the car would be dangerous to most people.

[skid542]

^^^ I think you mean the fronts lock up first .

[Buffman]

Yeah I was going to say the prop valve on my RMS is 90% front 10% back.

[subytech]

Yea i realized the whole locking up at the same time thing after i poasted, the text I was referring to was talking about ideal conditions with a driver that new how to drive lol. I can't tell you that the newer STi's have prop valves but i most assuradly can tell you the wrecked 05 in my driveway does.

[SemperGuard]

I was totally having a brain fart. Supersport ABS gives individual control of the rear wheels during ABS instead of just pulsing them both, and EBD replaces the prop valve. Either way no USDM STi has a prop valve. Hell I don't think any Subaru had one after 05 or something. Maybe the baja.

[Legacy777]

Most proportioning valves don't try to have the wheels lock up at the same time. They make sure the rear wheels lock up first otherwise the car would be dangerous to most people.

Usually the fronts lockup first.

[Legacy777]

From my experience the Legacy Prop valves have a more rearward bias then the imprezas, which IMO is better.

I would leave your stock prop valve alone and see how things respond after the swap.

[n2x4]

From my experience the Legacy Prop valves have a more rearward bias then the imprezas, which IMO is better.

I would leave your stock prop valve alone and see how things respond after the swap.

Any way to find out the specs on the valves? I swapped brakes on my Impreza wagon. Went from drums in the back to legacy disk and wrx 2 pot in the front. I'm considering swapping the Legacy turbo valve in for the wagon. Not sure if it's worth it or if it'll make a difference though.

[subytech]

Subaru's with Supersport ABS have no need for propvalves. That's the whole point of Supersport ABS. Instead of creating completely new brake lines a two-way connector is placed where the prop valve is on cars without supersport ABS. USDM STi's have Supersport ABS. Go onto STIS and check it out if you don't believe me.

I did look on STIS and you are right in the diagram they call what looks like the prop valve a "two way connector" but I had the parts guys look it up and subaru calls it an P-C-V assembly and we made double sure it has nothing to do with the engine lol. I'm guessing it stands for Proportioning Combination Valve but with as cryptic as subaru can be i dunno.

[subytech]

From my experience the Legacy Prop valves have a more rearward bias then the imprezas, which IMO is better.

I would leave your stock prop valve alone and see how things respond after the swap.

Thats good to know thanks, we are going to leave the stock one in the car for now, we're just trying to figure out how were going to leave the abs module out of the system. Not to excited about making brake lines right now

[Legacy777]

Any way to find out the specs on the valves? I swapped brakes on my Impreza wagon. Went from drums in the back to legacy disk and wrx 2 pot in the front. I'm considering swapping the Legacy turbo valve in for the wagon. Not sure if it's worth it or if it'll make a difference though.

This specs are in the factory manuals.

Also, in addition to the split points being different, there's also the reducing ratio of the valve, or how fast it tapers off pressure, and there are differences between the first gen legacy and imprezas.