###### Abstract  
In axial-flow combine harvesters, guide vanes direct crop material through the threshing and separation unit. The research object has the standard configuration of guide vane assembly; the six rear vanes are adjustable, while the two front vanes--located in the threshing zone--are fixed, which limits material flow control. In European conditions, where crop biomass is typically higher, improved control in the threshing area is essential to reduce losses and maintain grain quality. This study introduces a guide vane angle evaluation to combine performance and a modified guide vane system that enables all eight vanes to be adjusted simultaneously between 10-35\({}^{\circ}\). Field tests were conducted using two identical combines (A and B) in the same winter wheat field, under identical operating conditions. Combine A was equipped with the modified system, while Combine B retained the original manufacturer configuration. Both machines operated at a rotor speed of 980 rpm and a concave clearance of 15 mm. Results showed that Combine A achieved higher throughput (23.78 kg s\({}^{-1}\)), lower broken grain (0.18%), and lower fuel consumption (0.84 L t\({}^{-1}\)) compared to Combine B (20.6 kg s\({}^{-1}\), 0.61%, 0.99 L t\({}^{-1}\), respectively); the separation and sieve losses were also reduced in Combine A. Analysis of the results demonstrated that full-range guide vane adjustability--including in the threshing zone--can improve crop flow, grain separation, and harvesting efficiency in high-yield conditions. Keywords:axial-flow combine harvester; guide vane adjustment; longitudinal axial threshing; separation system; crop flow control 202  
have become more common in Lithuania and other European countries. Their share of the national fleet has grown from 12% in 2015 to 35% in 2023 [4]. These machines use a rotor-based threshing and separation system consisting of a rotor, a concave, and a top cover with internal helical guide vanes. Grain is separated from straw through a combination of impact, rubbing, and centrifugal force as the material moves along the rotor [5]. Material flow speed through the system is regulated by four key adjustments: rotor speed, concave clearance, rotor configuration, and guide vane inclination. While rotor speed and concave settings have been extensively studied, guide vane inclination has gained increased attention for its strong effect on performance. Guide vanes help direct the crop axially through the rotor housing. Smaller vane angles slow the crop down, increasing residence time and improving separation, but potentially increasing grain damage. Larger angles accelerate crop flow, reducing damage but raising the risk of unseparated grain loss. Several studies have confirmed the importance of guide vane inclination. [PERSON] et al. [6] observed that rotor speed, guide vane angle, feed rate, and grain moisture all significantly influenced grain losses. In a related study, they reported that guide vane inclination accounted for 40.66% of threshing losses [7]. [PERSON] and [PERSON] [8] showed that different vane inclination patterns affected both energy use and separation losses, although they had little effect on grain breakage. [PERSON] et al. [9] reported that increasing vane inclination in a corn shelling unit reduced losses, but also led to a linear increase in power consumption. Despite evidence, most harvesters still use fixed guide vane settings, particularly in the threshing area. This can become a major limitation in field conditions, where the feed rate and biomass vary throughout the field. In high-yielding European wheat fields, high biomass volumes are common, which puts additional load on the rotor system. Variability in crop density, lodging, and moisture leads to uneven feeding and inconsistent threshing-separation performance [10; 11]. This can reduce grain quality and increase loss rates [12; 13; 14].