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I never seen one that he has done in person but I know beveling the bottom is one thing but I have seen him talk about unshrouding the impeller. it where they cut the top of the liner to match the top of the impeller blades. I have not done this but planned on trying it on a old liner just to see what happens. The RPM gains he talks about comes from doing the impeller and shoe at the same time.
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I imagine if he is cutting the liner to unshroud the impeller this is how he is getting rpm up. Kind of like how now the new 6 7/8" stainless 3 blade impellers which mercury/quicksilver makes the new ones are cut back on the top to help the 4 chokes spin up better as they couldn't spin the old style ones meant for 2s which made more power.
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The only real way to increase rpm's, just from working the pump, are to either cutback the impeller, induce slip, lighten the impeller or handwork the foot to "scoop" or increase the ram effect of water on plane. The first two ways, ultimately reduce performance.