Just got back from visiting a mechanic who has been developing PP and other racing engines and have some interesting information.

 

It would seem that if you fire the spark plugs simultaneously (as Tracy does) that gives best performance to 6,000 RPM. However if you retard the  trailing spark plug by ( 1 to 3 degrees)  you can gain as much as 30 hp at 9 to 10K - about 10 hp for every degree. How that effects our RPM range I don't know ?

 

It would also seem that if you radiused the following edge of the exhaust outlet, it provides better exhaust flow and less contamination of the inlet charge at idle.

For your digestion!

George ( down under)