"Plunge cut, what is it and why the debate?"
It is what ALL head builders (OE and aftermarket, which can be further broken down) NEED to do after they install the valve seats.
For the OEs it's a cost equation. Cost to produce really consistent castings v cost to machine them better.
For our head builders it's about getting the most power possible, and that is not a bad thing. In fact, I would argue that with transparency, it's the best way to get us where we want to be, on even footing.
For the cost of a new motor from Mazda, you can make your used motor even better than the crate. We went down the crate path. I got 110HP and my buddy got 113. That's what you guys who are telling John to not allow STR modifications want?????
The plunge cut itself gets you nothing, in fact, making the valve seat diameter bigger is taking power away. The best plunge cut will ONLY remove as much of the NEW seat material as required to smoothly transition to the previously machined plunge cut.
Plunge cutting a head that has not had new valve seats installed is not necessary and probably counter productive.
Blending the STR on the other hand is easy and cheap (broken record) and if you go too far, you will give up the gains the first little bit of smoothing gave you. You don't need any measuring tools here guys. Allow the 1600 and NA 1800 builders to go 12 mm into the ports and the NB builders to go 9 mm into the ports.
If you do any more than remove a little from the STR and take ANYTHING away from the LTR you will be going backward. "He went crazy with the ports...." elicits fear for most reading, but finish that up with "....and now he can't keep up!" probably brings a chuckle.
Ignorance is the enemy, so let's keep talking and see if we can't overcome it.