BMW N55 STX 78R vs Stage 1

The SteamSpeed STX 78R Turbo for BMW N55 Install

 BMW N55 Stage 2.5 Turbo Install

Here are some action shots of the new SteamSpeed STX 78R Turbocharger being installed into a N55 equipped BMW 335i.  This car has successfully been running our STX 67 (aka stage 1) turbo for about a year now.  The owner was able to put down 400 whp with the 67 on pump gas + meth.

We’ll see what he can do with the STX 78R.  As you can see it is quite a bit larger than the STX 67, about +11mm on the exducer side.  Based strictly on the comp wheel, this turbo should be able to make between 500-600 whp, but the OEM turbine housing should limit that to some degree.  Initial dyno testing will show to what degree.

Base purely on inductive reasoning, this turbo should perform somewhat better than the currently popular Pure Stage 2 turbo: their turbo has wheels somewhat larger than a GTX3076R, our 78R is somewhat larger than a GTX3076R, they remachine the OEM housings, so do we, but the main difference is that our turbo has a BB center section, and Pure reuses the OEM JB CHRA.  Given that the only main difference that impacts performance is that our has a BB CHRA, what does that impact?

BB CHRAs have some standard advantages over JB: it is more responsive especially with transient boost, it is more efficient, it is more durable, etc.  In this case, the biggest difference, I think, will be the fact that BB CHRAs can be stay efficient in spite of backpressure much better that a JB CHRA.  Normally JB is fine up to 2:1, but BB can still be efficient at much higher ratios.  This matters a lot in this case, because the OEM manifold/turbine housing is going to generate a lot of backpressure when you try to make over 500 whp.

The BMW N55 STX 78R turbo going in.

BMW N55 STX 78R turbo Install
BMW N55 STX 78R turbo Install

This is what SteamSpeed STX 67 “stage 1” looks like next to the STX 78R “stage 2.5.”

BMW N55 STX 78R vs Stage 1
BMW N55 STX 78R vs Stage 1
BMW N55 STX 78R vs Stage 1
BMW N55 STX 78R vs Stage 1

SteamSpeed STX 67 “Stage 1” for BMW N55 installed.

SteamSpeed STX 67 "Stage 1" for BMW N55 installed
SteamSpeed STX 67 “Stage 1” for BMW N55 installed
Stock vs PS2 vs SS2.5
Stock vs PS2 vs SS2.5

SteamSpeed STX 78R “Stage 2.5” for BMW N55 installed.

SteamSpeed STX 78R "Stage 2.5" for BMW N55 installed
SteamSpeed STX 78R “Stage 2.5” for BMW N55 installed
SteamSpeed STX 67 "Stage 1" for BMW N55 installed
SteamSpeed STX 67 “Stage 1” for BMW N55 installed

Here are some initial results:
v3 – 16 psi
v4 – 18 psi
v5 – 19 psi

BMW N55 STX 78R v3,4,5 Tunes
BMW N55 STX 78R v3,4,5 Tunes

Update 1

The car has had some more tuning.  The red lines are where we left off last time. Looks like it is hitting peak torque around 4k fairly consistently. It looks like we were able to pick up another 50 ft*lbs and 50 whp comparatively. Version 9 has new colder plugs, so the tune is backed off.

BMW N55 STX 78R tuning in 3rd 2011 335i
BMW N55 STX 78R tuning in 3rd 2011 335i

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.