MM

I've not used any Nova Rossi Plugs. I find the O.S. easier to get in my area. The C5TF appears to have a similar heat range (HOT) as the O.S. P3. The P3 is an Ultra Hot, which is the hottest O.S. plug you can get.  I find for most of my applications the O.S. RB6 or RB7 works a bit better with 10% fuel in hot weather. I still use the P3 on occasion (when I can find them), but only when the Air temp scoots below 60 degrees F.

Les

MM

No, I don't know the comp ratio. I use some factory heads, it depends on the event.

For my Moki and Nelson .15's, I'm using heads I've machined from (Nelson plug) Blanks, or modified O.S. 18TZ head buttons (Turbo Plugs), or some made from Turbo plug blanks manufactured by Greg Settles.
I set the plug height first, then adjust the combustion chamber bowl area as necessary, and finally the squish band. I don't do anything radical, just trying to get the comp and plug height optimized.

Les

When it comes to engines, I'm not much of a "Math Guy" per se, but more of a cut and try fellow.

However, I have found that numbers that I've measured on my pressure fed Rossi 15's (ie. GP Rossi's) that were optimized, worked very well when used on my suction .15's. If anything, I could probably shrink the bowl area diameter a bit, but the plug height #'s seem pretty much spot on.
That said, I haven't completely optimized each head button for all of my suction applications, but I do believe they are fairly close. The numbers for the .15's also work pretty well on the .21's, especially the Plug Height.

Tim Gillott had an article in an old issue of "Speed Times" where he relayed some numbers relative to plug heights for the various common engine sizes we use. These weren't optimal numbers, but would get you close.

Les