+2mm ST Exh Valve VVT 1.8, w/8.6:1: must ct relief?
 

Am investigating fitment of +2mm Exhaust Valves in 1.8 VVT motor.
949Racing website says that the +2mm valves require claying and cutting custom pockets into Pistons.

I am on 8.6:1 Supertech pistons, and I presume 949Racing typically refers to high compression 11:1+ NA motor: does anybody know if +2mm valves clear 8.6:1 pistons, or not?

Pretty sure it has to do with the reliefs cut into the pistons, not just how tall the pistons are. If you look at 8.6:1 pistons they still have the relief pockets cut into them.

Yes, I can see that.
Was hoping low compression pistons have generous reliefs.
Perhaps not.

Turns out Supertech seems to make these valves for Duratec, not for NB Miata: therefore they know nothing about it.

You will need to cut larger diameter reliefs.

So if I am following this correctly, reliefs are made to clear adequately +1mm valves, but for +2mm exhaust relief should be increased +1mm in diameter compared to what is currently on the 8.6 Supertech piston?
Since lift is not changed, only diameter, does this make perfect sense, or am I missing something?

If cutting relief I would like to do it right away, and assemble. I am not chasing every bit of power, but trying to make it right the first time. Going per recommendation of Emilio who pioneered this +2mm this is the appropriate solution to optimize our heads.

Let me ask Mike Keegan, he is the engine builder who builds most of 949's engines.

pretty sure it will not clear. I think JasonC experimented with even stock valves and they touched at full advance. (search vvt = interference or something along those lines)
but looks like cyontani will get you the final answer

Miata2fast on here has conflicted +1 valves with 12 to 1 pistons, but at the outside edge where they weren't clearanced far enough when cast. If you are +2 you will need to get them flycut to match.

BTW, that's a big valve.

Keegan said that you need to open up the valve relief in the piston slightly for proper clearance.

Thank you Cyotani,

By open up, you/Keegan mean open up diameter of relief +1mm perhaps?
Or also deepen the relief (increase the depth of cut)?

yes, increase diameter of the relief to the diameter of the valve + whatever clearance you think is sufficient (~1.2 mm?). You should match the current depth of the relief of the super tech pistons

Make sure you leave a final .1-.12" clearance between the valve face and the piston and valve edge and reliefs to allow for thermal expansion.

If you plan to go aftermarket cams (more lift/duration) at all in the future, make sure to calculate that in the final measurements.

Instead of starting a new thread, I'll ask the question here regarding the 30mm (+2) exhaust valves.

Is there enough meat in the stock valve seat for a 30mm valve or will a new uncut (or possibly larger) seat need to be installed? It looks like the stock seat has a ~32mm OD which should be sufficient, but I would love to hear from someone that has installed 30mm valves.