So far BMS has tested with the stock plugs.
We have been selling aftermarket tuning products for the Kia optima and other Kia and Hyundai vehicles for the past 8 years.
You’re more than welcome to do whatever you think is best with your vehicle, but increasing the boost substantially and running a tune on the factory spark plugs is just asking for trouble.
Over the years, many members have had spark plug failure and even some have had a piece of the failed spark plug brake off into the cylinder, since the factory plugs are porcelain. The aftermarket HKS racing plugs are ceramic and won’t break into the cylinder if the plug fails. The factory plugs are rated for the factory boost and not designed to handle the additional boost made from the tune.
When the JB4 product was first released for the Kia stinger, everyone said the same thing and then multiple people started having issues with their coil packs and other problems associated with running the tune on stock plugs.
It definitely cannot hurt by upgrading to a racing plug designed to handle the additional performance created from running the tune on your vehicle. Therefore, we always recommend upgrading the plugs when installing any aftermarket tune on your vehicle.