Two simultaneous strategies were used to reduce diesel engine emissions. Optimized manifold designs were used with gas-to-liquid (GTL) fuel and its blend with diesel fuel. Six new spiral-helical manifolds were tested, which could be divided into two groups. The first group is with the same inner diameter (2.6 cm) and outlet angle (30 deg), but the different number of spiral turns (1t, 2t, etc.). The second group is with different inner diameters. The results showed that the highest pressure and heat release were achieved by m(2.6,30,1t) with the diesel–GTL blend. In addition, the heat release rate decreases with the increase in the number of turns. The same combination also reduced the pressure rise rate (dP/dθ) by about 24% compared to the normal manifold. For the emissions, the maximum reduction in CO emissions was achieved by using m(2.6,30,3t) and GTL with about 34%. In addition, the maximum hydrocarbon (HC) reduction was achieved by m(2.1,30,3t) and GTL, which is about 99% lower than that of the normal manifold. NO emissions were reduced by about 25% when m(2.6,30,4t) and GTL are used. The total particulate matters (PM) were the lowest for m(2.6,30,1t) and normal manifold in the case of diesel. Generally, it was found that the combination of m(2.6,30,1t) with GTL and its blend gave the optimum performance and low emissions among all manifolds.