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.
This is what SteamSpeed STX 67 “stage 1” looks like next to the STX 78R “stage 2.5.”
SteamSpeed STX 67 “Stage 1” for BMW N55 installed.
SteamSpeed STX 78R “Stage 2.5” for BMW N55 installed.
Here are some initial results:
v3 – 16 psi
v4 – 18 psi
v5 – 19 psi
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.