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Message: [QUOTE="TARM, post: 368188, member: 578"] Do you already have the 4708? If not looking solely at the maps the 4718 looks like a much better atmosphere turbo. What you want to do is look at the maps and using those consider where you want them to be with also taking into account the ratio on the turbine side. First you need to decide on what your total boost goal. With that you get your total PR. To do PR for compounds the two are multiplied together so going from a total PR you divide one to get the other. Start with the atmosphere as you can easily get the boost and PR from it directly. So say a 70psi boost gauge pressure. [70+14.7(atmosphere)]/14.7= 5.76 From here looking at maps you can play with some numbers. As an example take a look at almost evenly spreading the work between the two turbos to start. Of course the 4708 is going to be taking in a significant more air volume or air. Let say you want the 4708 to be pushing 20 psi boost or a PR of 2.36. total PR 5.78/2.36=2.44 PR for the 38r or 21 psi boost. See where that gets you with turbine drive pressure and ratio to boost. Then you can play with it from there based on your power goal, what the drive to boost ratios total and each turbo are etc.. Now to be accurate you would need to account for the actual pressure at your average altitude for where you will be driving as well as account for the filters resistance. But it seems easy if you just go with standard level for comparison. Again this is just paper numbers for real world exp help you need to talk to the BTDT guys. [/QUOTE]