[shark]

Jeff, I experimented with all of the above and found that the "open" spacer (birch wood - to help alleviate heat soak) worked best on my dual plane manifold. I tried alloy, phenolic?? & wood. Also, make sure that whatever configuration you use clears the butterflies - full open-closed. Not all butterflies and hole spacer sizes are equal. Ask me how I know....

Regards,... sharkey