As described In CP 30, page 5, the 6" prop extension has continued to provide trouble free operations. We have sufficient time on these extensions on two Long-EZs here at RAF to feel confident in recommending the 6" extension as well as the 3" extension. Note that we have only tested these on Lycoming engines. Due to the smaller crankshaft diameter of the Continental engines and the lack of data, we cannot recommend anything but the thoroughly tested 3" extensions for Continental engines. The 6" extension does reduce the noise level in the pilots seat by as much as 3 decibels (OBA scale).
However it also increases cylinder head and oil temperatures slightly. The worst case is a new or recently overhauled engine, in a new airplane. It is possible that engine temperatures could go out of acceptable limits during the first few hours of operations, especially during ground testing. We have noticed on an engine with hundreds of hours, that if we are forced to run the engines standing still on the ground for extended periods of time, in excess of 30 minutes or so, the cylinder head temperature can climb right to the red line. Using a 3" extension this will not occur. To summarize: We recommend both the 3" and 6" prop extension for the Lycoming engines and only the 3" extensions for Continentals. You, the builder must decide which to use in your application.