This study systematically investigates the effects of various engine operating parameters on the thermal efficiency of a boosted HCCI engine, and the potential of E10 to extend the high-load limit beyond that obtained with conventional gasoline. Understanding how these parameters can be adjusted and the trade-offs involved is critical for optimizing engine operation and for determining the highest efficiencies for a given engine geometry. Data were acquired in a 0.98 liter, single-cylinder HCCI research engine with a compression-ratio of 14:1, and the engine facility was configured to allow precise control over the relevant operating parameters. The study focuses on boosted operation with intake pressures (Pin) ≥ 2 bar, but some data for Pin < 2 bar are also presented. Two fuels are considered: 1) an 87-octane gasoline, and 2) E10 (10% ethanol in this same gasoline) which has a lower autoignition reactivity for boosted operation.This study considers several engine operating parameters, including: intake temperature, fueling rate, engine speed, fuel type, and the effect of various amounts of mixture stratification using three fueling methods: fully premixed, early-DI, and premixed + late-DI (termed partial fuel stratification, PFS). The effects of these operating parameters on the factors affecting thermal efficiency, such as combustion phasing (CA50), amount of EGR required, ringing intensity, combustion efficiency, γ = cp/cv, and heat transfer are also explored and discussed. The study showed that in general, thermal efficiency improves when parameters are adjusted for lower intake temperatures, less CA50 retard, and less EGR, as long as the ringing intensity is ≤ 5 MW/m2 to prevent knock, and combustion efficiency is good (i.e., ≥ about 96%). Additionally, applying a small amount of mixture stratification (using PFS or early-DI fueling) improves efficiency by allowing more CA50 advance when boost levels are sufficient for these fuels to be ϕ-sensitive. E10 gives a small increase in thermal efficiency because EGR requirements are reduced. E10 is also effective for increasing the maximum load for Pin ≥ 2.4 bar, and increasing the high-load limit to IMEPg = 18.1 bar, with no engine knock and ultra-low NOx and soot emissions, compared to IMEPg = 16.3 bar for gasoline. Overall, this study showed that the efficiencies for boosted HCCI can be increased above their already good baseline values. For our engine configuration, improvements of 3 - 5 thermal-efficiency percentage units were achieved corresponding to a reduction in fuel consumption of 7 - 11%.