Sequential vs. compound: why they are not the same thing
People lump every multi-turbo setup together. Compound and sequential are different animals, and the difference decides how the car actually drives.
Compound: stacked for pressure
A compound system runs a small turbo that feeds its compressed air into a larger one, so the two multiply boost pressure. That is how diesel builds reach very high boost. The catch: piping and heat management get complex, low-RPM lag stays because of system inertia, boost delivery is not always smooth, and getting both turbos to agree is a tuning headache.
The STM: true sequential
The Hlava STM runs two different-size turbos on separate flow circuits. They do not feed each other. At low RPM exhaust goes to the small turbo for fast spool. As load rises, internal valving and a three-inch bridgepipe progressively bring the large turbo online. At the top, both contribute, with the large turbo handling bulk airflow.
Side by side
| Feature | Compound | Hlava STM |
|---|---|---|
| Turbo layout | Small feeds large | Independent circuits |
| Boost behavior | Multiplied, often abrupt | Additive, smooth, linear |
| Low-RPM response | Lag from inertia | Primary sized for low load |
| Tuning | Pressure ratios must match | Simpler routing and staging |
| Install | Complex piping and heat | Integrated design |
| Best for | High-boost diesel, drag | Street, drag, track, rally |
Compound has its place, especially in heavy diesel and drag builds chasing maximum pressure. For a car you also want to drive, the STM gives flow control and transition smoothness a compound setup cannot match.