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You can't... For the system to work people must gain SP so the developers made the lowest amount you get as SP from the post %50, and you can change it to %100 if you need.

SP is what makes SBD and STEEM valuable, the more people have SP and don't power down, the more value the other two currencies should have. (I say should because now, the prices move according to market rather than the quality of the blockchain.)

Oh man, I'm sorry. I have always been using 100% Power Up and didn't notice there was no option for 100% SBD. Thanks for sharing your advice, from now on I'm going for 50% SBD/Power split.

Glad I helped.