wira Se 4g15 stock piping mendrrel bent is 1.75" (inner diameter)
putra stock piping only 1.75" (inner diameter)
wira 4g13 and 4g15 normally using 1.6" (inner diameter)
wira 4g92 sohc also same 1.75" (inner diameter)
okey...but once your engine toach up slightly...normally you can increase by 0.5" inner diameter
but once you have done a heavy mod like PNP or cam (depend on cam duration or lift)
4g93 will increase up to 2.3"
4g15 will increase up to 1.8"
4g92 will increase up to 2"
4g13 will increase up to 1.7"
some how cam design also will effect the output of the engine...even if you play on the adjustable cam pulley..
and also measurement always done by using inner diameter....alot shop out side..they only give you the outside diameter....coz some material are thin...or niether way..
also if you are using menderl bent piping..size also different abit..
*above is my exsperience on change my car piping, if you found out not useful can ignore it