A turbo is supposed to add useful air without turning the exhaust side into a restriction. When turbine drive pressure rises above compressor outlet boost, that balance is backwards. Now the engine is corked up on the exhaust side, heat stays in the cylinders, cylinder pressure stays elevated, and the pistons have to push against turbine backpressure during the exhaust stroke. That costs crankshaft power and tells you the turbo has run past what it can support efficiently. On this Duramax L5P, that pressure relationship was the clear sign the stock variable geometry turbo had reached its limit and needed an upgrade.
The video centers on diagnosing why a particular turbocharger is performing poorly. The discussion is framed around a basic but important question: why this turbo is, in Gale's words, "out to lunch." Rather than relying on impressions or general assumptions, the evaluation is based on direct measurement of temperature and pressure throughout the turbo system.
On the compressor side, the team measures both temperature and pressure at the inlet and at the outlet of the compressor. These readings are used to show what the compressor is doing to the intake charge and whether the boost being produced is coming efficiently or at too high a thermal cost. Measuring both pressure and temperature across the compressor provides the basis for judging whether the turbo is contributing useful air density or simply adding heat while demanding excessive turbine work.
The same approach is applied to the turbine side. Temperature and pressure are measured into and out of the turbine so the team can compare the exhaust-side energy available to the work being extracted by the turbocharger. This is critical because the turbine does not operate in isolation; its restriction directly affects the engine's ability to clear exhaust gases from the cylinders.
A key diagnostic comparison is turbine drive pressure versus compressor outlet pressure, or boost. When turbine drive pressure is higher than the boost outlet pressure from the compressor, the system has entered a negative operating condition. In that situation, the turbo is demanding too much exhaust pressure to produce the boost it is delivering. Instead of representing an efficient exchange of exhaust energy for intake pressure, the turbo becomes a restriction that works against the engine.
The problem with excessive turbine drive pressure is that it effectively corks up the engine. Exhaust heat and energy cannot leave the engine efficiently, so heat is retained where it is least desirable. That trapped heat remains in the cylinders and in the exhaust side of the engine rather than being converted cleanly into useful turbine work and expelled downstream. The result is not just a thermal issue but a broader efficiency problem.
High back pressure at the turbine also keeps pressure in the cylinders during the exhaust stroke. That means the pistons must push exhaust gases out against the resistance created by the turbine. In practical terms, the engine is pumping against turbine back pressure, and that pumping work consumes power. Instead of the crankshaft benefiting fully from combustion, some of its output is spent overcoming the restriction imposed by the turbocharger.
This is why the condition is described as being hard on power output. The compressor may be adding boost, but if the turbine requires excessive drive pressure to make that happen, the engine gives up crankshaft power during the exhaust stroke. The net effect can feel upside down: power is effectively being put in with the compressor side while being taken back out through the turbine-side restriction. That imbalance is the core reason the turbo is considered fundamentally mismatched or inefficient in this operating condition.
