Without higher compression, a turbo, or 2.4 swap, you will not need one that large. A 55mm will be more than enough. And yes, the ATX has a 52mm stock. If you do plan on doing one of the above, then a 60mm is better. Or if you plan on running a motor like mine you will need a 72mm T/B. That's needed for a 2.4 with any comp. over 12:1, or 2.4 with turbo.