Ed_Moorman

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You are correct about the Ultra Thrust. Here is a photo of one I cut apart after I broke the mounting ear in a crash.

The Tower muffler is a large, expansion chamber that gives a good bit of tuning. I have gained 800-1000 rpm on .46-.52 siezed engines. You can gain 1500 rpm or more with an Ultra Thrust provided you use the correct size prop. The Ultra Thrust, and the Jett, which is similar, are cut to length for a fairly high rpm range. You need to use a prop that lets your engine run at that rpm. Generally, they need to turn a minimum of 13,500-14,000. You can definitely hear when they stage on the pipe.