Stephen,

Larger diameter pistons in and of themselves will do nothing to change the compression ratio. They will increase the displacement, but that's all. This is, given that there is no change to the stock configuration (you maintain the stock ratio of 'unswept' volume to total volume). However, if you change things like the dish profile, you've changed that volumetric ratio.

For example, w/ VW's, they made an 8.5:1 1.8 and a 10:1 1.8. Both are the same bore/stroke, It's just that the 8.5:1 pistons had a much larger 'dish' in the top.

So, what you get out of going 0.040" over is more displacemnt, which lets you move more air/fuel mixture through the car on a given stroke, thereby making more power. All else being equal, a motor that is 0.040" over-bored should make more hp than an engine w/ stock bore.

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MARRS #25 ITB Rabbit GTI