I noticed a slight lack of full power while I was on my last flight so I returned to the hanger for some trouble shooting. I got a lot of stumble on one of my mag checks. One mag has less than 50 hours TTSN and the other about 12 hours since a 500 hour inspection and repair so I de-cowled the plane and removed all the plugs. All are Champion's: 6 are 38E's and the other 6 are 40E's with a little over 500 hours on them. That's what came on the engine when I purchased it used. I have been rotating them upper to lower whenever I clean them. One is completely dead on the ohm test so I have decided to just go ahead and replace all of them with the new Tempest plugs. I have no idea why the previous owner had the mix of plugs and the application guide from Tempest has both the 38E's and 40E's as applicable plugs for my IO-540V4A5 engine. Any guidance help for me which to order would be appreciated. Thanks! Dave