Compound turbo setups use two turbos in series: a small high-pressure turbo that spools quickly for low-RPM response, feeding into a large low-pressure turbo (the atmospheric stage) that provides the airflow needed for big horsepower. Done right, you get the low-end torque of a small turbo and the top-end power of a much larger one.
How the pressure ratio splits
In a compound setup, total boost is the product of each stage's pressure ratio (not the sum). For 60 psi total boost (PR ≈ 5.1):
HP turbo at PR 2.0 × Atm turbo at PR 2.55 = Total PR 5.1 (60 psi)
The HP turbo handles most of the heat work; the atmospheric turbo does most of the volume work. Match each to its job — don't oversize the HP turbo or it never spools.
Common compound combinations
- S300/S400: Classic Cummins compound. Small S300 (62–64mm) feeds large S400 (75–80mm). Good for 700–1000 HP street/strip.
- S400/T6: Race-only compound. Huge atmospheric turbo for 1200+ HP.
- HX35/HX55: 12V Cummins favorite. Stock HX35 feeds a 55mm. Tight-package, ~500–650 HP.
Drive pressure stacks too
Just like boost, drive pressure compounds. The HP turbo sees the highest drive pressure of any setup — sometimes 2× boost. That stresses the small turbo's shaft and bearings.
This is why race compound builds use ceramic ball bearings and oil-only cooling. Cheap small turbos won't survive sustained compound use.
Lift pump and fueling must keep up
A compound turbo build that breathes 1000+ HP worth of air needs the fuel to match. CP3 alone caps around 700 HP; beyond that you need dual CP3, larger pistons, or a Stage 2 CP3 modification. Also size the lift pump for 0.5–0.6 gph per HP at the rail.