Tag Archives: 2015 WRX

SteamSpeed STX 71R BB Prototype Tested: 470 WHP

Hey guys,

Sam here from SteamSpeed.  We super happy to get back to you with the results of our SteamSpeed STX 71R ball bearing turbo for FA20 applications, ie. the 2015+ WRX.  The basic specs are:

  • Stock frame meaning it just bolts up in place of the OEM turbo
  • Utilizes a Garrett GT ball bearing CHRA (center housing rotating assymbly) sourced from Garrett Japan
  • It is GTX2971R spec meaning the we make our own compressor wheel of that size, and reuse the Garrett GTX29 turbine wheel.

[STX 71R for FA20 Prototype Pre-Test Recap]

Several months ago, I wrote a longish article discussing challenges with the JB CHRA and how the new BB CHRA.  It is a great read, and I highly recommend that you read it, but it lays out why we think the BB version of the 71 will be a superior turbo to our JB version.  The next section outlines, the basic points.

[STX 71R for FA20 Performance Performance Hypothesis [Pre-Test]]

  • The OEM style twin scroll turbine housing is restrictive for turbos larger than the OEM unit and our STX 67 JB turbo.
  • This means for our big turbos, exhaust back pressure ratios can get well beyond 2:1 that is efficient for a JB CHRA.
  • If you are going 5:1 or 7:1 makes it hard to build power and it wears out the journal bearings and especially the thrust bearing.
  • Tuners that have a good strategy to manage this backpressure can make good power with the larger STX 67+ and 71 turbos, but if they didn’t, sometimes our customers would be disappointed.
  • The main point of that post was that a BB CHRA actually solves all of these problems:
    • The CHRA can stay efficient even if the pressure ratios are 5+:1
    • This means, it will be a lot easier for all tuners to build power with the BB version of our turbo.
    • The turbo will just make more power under the curve in general
    • The the thrust bearing is more durable, so the CHRA will stand up to more punishment.

[STX 71R for FA20 Test Results]

We set out to actually prove if the BB version of the STX 71 would perform as well as we had hypothesized, and solve the issues we had with the JB CHRA on the larger turbos.  The short version is that yes, the SteamSpeed STX 71R BB Turbo for FA20 did exactly what we thought it would.  It was a lot more efficient that the JB version of a similar size; therefore, it made more power everywhere.  I suspect that tuners all over will be having an easier time getting results their customers want.

Here is the dyno result.  470 whp on E50 and 410 on 91 octane pump gas, and not measured on this chart, a ton more response everywhere.  Note: this is at high altitude in Utah.

SteamSpeed STX 71R for FA20 Dyno Plot. Green – E50, Black – 91 pump gas

[Technical Notes From Jessie at FNP]

Jessie: “Hey, First let’s go over what we saw, liked and disliked with the unit.

Dislikes:

It’s big, if the new unit has a clearance for the oil pan and obviously ships with hardware we are good.  Obviously as the prototype it is going to have clearance issues, and fitment issues.  There were literally no other issues noted from Luke on install.”

SteamSpeed: This will actually be a non-issue for retail units.  We have actually already resolved all fitment issues on our production 71R.  The production model has a modified turbine housing with a cutout to clear the OEM oil sump without modifying it.  We also designed and manufactured custom studs that we include with the 71R install kit.  This is how the retail unit will work:

Notice we modified the turbine housing to work with the OEM oil pan without modification. We also include custom studs with the install kit.
Notice we modified the turbine housing to work with the OEM oil pan without modification. We also include custom studs with the install kit.

Jessie:

Likes:

Sounds epic.  Do yourself a favor a crack open the boost nipple when running.  The ball bearing turbo sounds incredibly mean at idle.  With a catless exhaust it should sound great out the tail pipe.  Think diesel turbo, screaming at idle.

Response, Response, Response.  This turbo is incredibly responsive compared to the previous version.  Transitions in and out of boost are much quicker.

More linear boost curve via WGDC input.  What do I mean?  Check out this boost profile compared to WGDC on the old vs new turbo.  The new unit is much MUCH more linear with interrupt cycle.  This tells us the effects of back-pressure are far less of an issue with this upgraded unit.  You can also see the old turbo have more “Creep” under the curve.  The new unit doesn’t not do this.  The compare for RPM isn’t valid, as the previous tests were done in 4th, the current in 3rd.

67+ JB WGDC

71R BB WGDC

Makes more boost in the upper RPM’s.  This also is a direct causation from the upgraded cartridge.  It seems to be able to operate at higher levels of back-pressure with ease.

Red JB vs Yellow BB

Less oscillation of MRP than the outgoing cartridge.  Just one of those anecdotal observations, normally we see much greater fluctuations in boost on the FA20 with our incredibly fast sample rates.  This unit fluctuated much less, the average was 18% realized lower fluctuations.  This is a great indication of how much more air is being delivered.

 

Much more efficient flow from the turbo.  Check out the new vs old charge air temps!

New unit held much better boost.  From 1.9bar avg on the old unit at redline on 100% interrupt, to 2.3bar avg.

Output: Was increased by 12.35% over the older unit.  This was also impressive as the turbo could have easily generated around 8% additional output, but the owner of the test vehicle was very specific to “Take it easy”.  Based on the airflow averages and their changes, I’d say this observation is fairly precise.

Check out these airflow differences:  Old Turbo 229 average max, New over 300!  (it was 309 average when extrapolated up).  That is an increase of 35%.  Same intake, and injector scalings were used on both turbos.  Compared to the stock turbo this is over 56% increase in flow!

Well, hope this helps you guys.  I poured over all the data and these were the things that popped out at me.

Jesse @ FNP Tuned”