13/12/2025
Many clients asks why we stopped using bullet bolts in our hybrid turbochargers?
We conducted many tests, and that’s why we stopped using them a long time ago. We’ve received many questions asking why we no longer use them - so here’s a post explaining our decision.
What does the compressor actually want?
At the inducer (compressor inlet), the goals are:
Uniform axial velocity
Minimal pre-swirl
Maximum static pressure recovery
Stable, attached flow into the blade leading edges
Anything that disrupts this hurts:
Compressor efficiency
Surge margin
Why is the bullet-shaped nut worse?
It creates a stronger stagnation region.
A pointed nose forces the flow to decelerate earlier and more abruptly.
This creates a larger stagnation pressure zone upstream.
The flow then accelerates rapidly around the nut.
Rapid acceleration plus curvature increases boundary-layer separation risk.
A flat or short nut:
Creates a smaller, more compact stagnation zone that reattaches faster.
The bullet nut also:
Increases wake length and turbulence
Produces a long, low-pressure wake
Sends this wake directly into the inducer eye
Causes velocity non-uniformity entering the blades
The bullet shape:
Acts like a flow director
Converts axial flow into unwanted radial and tangential components
Increases pre-whirl, which compressors do not want at the inducer
A blunt nut disturbs flow less directionally.
Reduced effective flow area
Even if the bullet has a lower drag coefficient:
Local acceleration around it increases Mach number
Higher local Mach → higher losses (especially near choke)
The inducer “sees” a smaller effective inlet
This results in:
Slightly worse choke flow
Earlier efficiency drop at high mass flow
Why OEMs use the “ugly” nut
OEM turbo designers optimize for:
Total pressure loss
Stability across the entire compressor map
Manufacturing repeatability
CFD and test data consistently show:
Short, blunt, axisymmetric hub features perform better
Especially in high-Reynolds-number, low-Mach internal flows
If the bullet nut were better:
OEMs would already be using it everywhere
They don’t—because it doesn’t help.
Final thoughts
At our ETK headquarters, we have balanced many turbochargers, and bullet-shaped bolts are significantly harder to manufacture compared to stock bolts. They are also much heavier.
Internal flow is not governed by drag coefficient alone. In a turbocharger inlet, pressure recovery, wake behavior, and flow uniformity dominate. A bullet-shaped nut increases wake length, induces swirl, and worsens inlet flow quality—reducing compressor efficiency instead of improving it.
Basically: it looks fast, but performs worse.
We apologize for the grammar, as it is not our native language.
