At this point I think it's safe to say Z1 doesn't just use the RCT4000SA because it's an "upgrade" and that their kits require it to function properly. It seems this is more of a supply/availability decision. I think I'll just try to source the OEM sized RCT40SA3 to eliminate any questions/issues. The weather is starting to get nice here... I want to be driving, not dropping transmissions! Thank you for the personal experience share...