Effect of Throttle Body Bore Diameter on Torque and Power Output of a 150 cc Fuel-Injected Motorcycle Engine: An Experimental Chassis-Dynamometer Study

Abstract

This study examines the effect of throttle body bore-diameter variation on the power and torque output of a Honda Vario 150 cc motorcycle under factory-standard engine and Electronic Control Unit (ECU) conditions. A quantitative experimental design was employed, in which three throttle body diameters – 26 mm (standard), 28 mm, and 30 mm – were each tested three times on a chassis dynamometer across an engine-speed range of 6,000–9,000 rpm. Peak power and torque values were analysed using one-way Analysis of Variance (ANOVA) with Tukey HSD post-hoc comparison. The 30 mm throttle body produced the highest mean peak power (11.32 HP), while the 26 mm standard throttle body produced the highest mean peak torque (11.12 Nm) with the smallest standard deviation, indicating the most repeatable performance. However, ANOVA results showed that throttle body diameter had no statistically significant effect on either power (F(2,6) = 4.731, p = 0.058) or torque (F(2,6) = 2.489, p = 0.163) at α = 0.05, although the associated effect sizes (η² = 0.612 and 0.453, respectively) were large. The 30 mm throttle body tended to sustain higher power at elevated engine speeds (7,500–8,500 rpm), whereas the 26 mm unit delivered the most consistent torque in the mid-range. These findings indicate that enlarging the throttle body alone, without a corresponding recalibration of the ECU fuel map, does not translate into a statistically reliable performance gain, and that any bore enlargement should be accompanied by engine-management adjustment to fully exploit the additional airflow