After Market Bigvalve Head

I noticed that the after market BIGVALVE HEAD head having the same diameter/circumference to a stockhead at the exaust side where our elbow connected or header., and even at intake side. I bought a 22/20 SPS bigvale head and i noticed they are the same in stock head exaust and intake. Im planning to port the exaust but i dont know how big to be removed. Please help me how to calculate how many mm to be removed at the exaust side. The FORMULA if you have. Im using 62mm block ang 28mm pe carb....



After Market Bigvalve Head



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