Mr. Bill is right, You are limited to the factory coil.
I once experiment with an 11" DD coil made by Eric Foster on my Whites Pi, From past experiments on other Pi's I knew if I did get the 10uS DD coil working on the Whites pi it would lose sensitivity on smaller size conductors but may gain it back in stability in harsher mineralized soil. This DD coil is really designed for the GoldQuest which operates at 10uS, But from past experiments it will work on some of Eric's 15uS Pi's.
It was a long shot and my Whites Pi is not setup to accept the DD mod as found on Eric's pi's, On an Eric Foster Pi or at least the ones I have owned it is possible to make a minor adjustment so the Pi will accept a DD coil. My Whites Pi didn't have that feature so I wired it different and the DD coil did work and as I knew from the get go it did lose some sensitivity on smaller size targets which is normal when switching to a 10uS DD. However, This may be gained back with better stability in harsh soil.
I didn't really take the time to test out the Whites pi with the DD coil out in the field, Too many things going on at the time and have since remounted the stock coil back to the Whites Pi.....Maybe this summer I'll remount the DD coil back to the Whites pi, See how it does at the ocean.
HH, Paul (Ca)