A variable geometry turbo (VGT) has movable vanes in the turbine housing that change how exhaust gas hits the turbine wheel. At low engine speed the vanes narrow the passage so the exhaust speeds up and spins the turbo quickly, cutting turbo lag; at high speed they open up for more flow. One turbo can then behave like a small, quick-spooling turbo and a large, free-flowing one. VGTs are common on modern diesels such as the 6.7 Cummins.
What is turbo lag?
Turbo lag is the delay between pressing the accelerator and the turbo building boost. The turbo is driven by exhaust energy, so at low engine speed there is less exhaust to spin it and the wheel takes time to accelerate. Large turbos that make big power at high speed usually lag more at low speed.
How a VGT reduces lag
- Vanes closed: a narrow passage speeds up the exhaust, so the turbo spools quickly at low speed.
- Vanes open: a wider passage lets exhaust flow freely at high speed and keeps back pressure down.
- Electronic control: the engine computer moves the vanes through an actuator to hit its boost target.
VGT exhaust brakes
On some diesels the VGT doubles as an exhaust brake: closing the vanes builds back pressure that slows the truck on descents. The Fleece HE351VE replacement for the 2007.5-2012 6.7 Cummins, for example, is listed as keeping the factory exhaust brake working.
Other ways to cut lag
- Smaller or twin-scroll turbos spool faster.
- Twin turbos: two smaller turbos, or a small and a large one in sequence, give quick response and high-speed power; a single turbo is simpler and lighter.
- Superchargers: driven by the crankshaft, so boost arrives with engine speed rather than exhaust energy; see supercharger vs turbo.
VGT problems
Soot and carbon can make the vanes stick, which shows up as low boost, overboost or erratic power, and actuators can fail. Regular highway driving, clean oil and the correct engine maintenance help. See turbo failure symptoms for the warning signs.
VGT turbos for the 6.7 Cummins
The 6.7 Cummins used the Holset HE351VE from 2007.5 to 2012 and the HE300VG from 2013 to 2018. Fleece Performance makes Cheetah drop-in replacements for both, the HE351VE version and the HE300VG version, each with a 63mm compressor wheel and rated to support up to 700 rwhp. See 6.7 Cummins turbo and 6.7 Cummins specs.
More guides: what does a wastegate do? · what is turbo lag? · 6.7 Cummins turbo actuator.
Trouble codes: P0299 Turbo/Supercharger Underboost · P2563 VGT Position Sensor Range · P0046 Boost Control Circuit Range · P0234 Turbo/Supercharger Overboost · P0045 Boost Control Circuit Open · P2263 Boost System Performance · all OBD2 codes.
Shipping is a flat $80.00 per order to US addresses, with 15-day free returns. Questions? Ask a parts specialist.