The terms 'straight cut' and 'helical cut' refer to the way the teeth are cut on the gears. In a straight cut gear set the teeth are cut straight across the gear, like in figure 1.
Figure 1 Straight Cut Gears
Helical cut gears on the other hand have the teeth cut at an angle across the gear like in figure 2.
Now the problem with straight cut gears is that they are very noisy due to the teeth hitting each other as the gears turn. Helical cut gears reduce the noise because the teeth effectively slide across each other as the gears turn. However the trade off with helical cut gears is that not all of the force transmitted goes to turning the gears. Some of the force goes to pushing the gears sideways against the thrust bearings, resulting in lost power.
Figure 2 Helical Cut Gears
So in summary, helical gears are better for road cars because they are quieter and straight cut gears are better in race cars because they are more efficient.
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