MR cox I have been doing my own stuff for awhile now, learned and still learning from guy at the field. I can get with in 300 or so rpm of his work now. Just by raising exhaust doesnt gain much in some engines and may even kill power/torque down low. The key is the blow down measurement. Some engines have the transfer and boost ports so high you the blow down is way to much. Some engines you can fix this my lowering the sleeve in the engine but again can only go so far as you will run into sub porting. Sub port and tuned mullers(as some people call them) or tuned pipes do not mix well. The irvine has low transfers and boost ports allowing the exhaust to raised up to a good number while at the same time keeping the blow down in the good range.
By the way the Jett 1/4 wave tuned mufflers are about 200-300 down from a tuned pipe for most applications.
take the back cover off an engine shine a light into the carb and then rotate the crank and you normally see light coming in when crank pin is at the lower left corner screw and closes around the upper right corner screw