alpinaturbo

iTrader: (1)

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?

aidandj

iTrader: (5)

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.

alpinaturbo

iTrader: (1)

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.

sixshooter

iTrader: (12)

You will need to cut larger diameter reliefs.

alpinaturbo

iTrader: (1)

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.

cyotani

iTrader: (8)

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

18psi

iTrader: (76)

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

sixshooter

iTrader: (12)

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.

cyotani

iTrader: (8)

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

alpinaturbo

iTrader: (1)

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)?

cyotani

iTrader: (8)

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

Der_Idiot

iTrader: (2)

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.

fourwhls

iTrader: (2)

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.