[modeleng] Blaastpipe calculations

Hi All
What calculations do you use to calculate the blast pipe diameter? The 
cylinders are 1.6875" diameter by 2.4375" stroke. This scales up very closely 
with the full size engine dimensions of 19" diameter  by 28" stroke. Someone 
suggested that I scaled the full size blast pipe which at 5.125" would give me 
a dimension of 0.454" diameter however this seems a little large. What does 
everyone think? The model design calls for 0.25" but that gives a fair bit of 
back pressure and a very strong blast. How about using  the 0.25" designed 
diameter with a multiple jet nozzle? Would that offer any advantage?
Andy

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