Are the stock ls pistons strong enough?
i have scat hbeam rods b1 block ls crank, gsr head with crower springs and retainers and stock gsr cams. 81mm bore

 

why low compression piston on ls vtec?

Unless you boosting then it’s understandable.
If your doin N/A then you’ll basically have a stock jdm ls b18b compression with vtec not worth using ls pistons unless boosting. Otherwise no boost then I would suggest to upgrade pistons to type r.