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If you aren't in too big of a hurry just keep talking about it. Like at dinner, on the road, while watching tv. Keep it fresh in her mind until she just wants you to get it over with so she doesn't have to listen to the advantages of a twin screw or centri or which tuner will work best or whatever. Bore her into accepting the fact you need a supercharger just to shut you up.
Bruce is on the right track. I talked about supercharging for about A year. She saw me looking at Whipple, KB and Prochargers over and over. I would tell her about the different opinions people had on this forum and others. I would always comment on how expensive Supercharging was. Then all of a suden I come across A paxton HO tuner kit
that seems to be HALF the cost of all the other kits, Show it to her, And she says you should get that one. Got my Harley that way.