Just remember it won't matter one bit how big your exhaust valve is if you can't get the fuel/air mix in.. You will just be wasting space with an over sized Ex valve if your In valve (or whole intake system in reality) isn't capable of getting it in.

Or putting it very simply, why have a 110l/m exhaust hose when your intake hose can only do 90l/m, you are better off balancing the flow so you get equal amounts in and out..

Yes I was saying if you have the exhaust 1mm smaller than the inlet there is nothing to be gained going that way due to the exhaust only needing to be round 70-80% the size of the inlet. If you go larger exhaust you risk overlap or losing inlet charge through the exhaust if valves are too close or cam has more overlap.

Since ex. valve on H89 is already 31mm & 4X or more pressure is avail. from exhaust spend money on a 40mm inlet valve & suitable cam for your needs. 40mm x 77.5% / 100 = 31mm or just try stock 37 x 31 H89 presto. Nissan engineers are clever cookies & gave us lots of good stuff.

since the h89 is 31 using the ratio I gave you and the fact 4 or more times more pressure is available for the exhaust you spend your money on a bigger inlet and suitable cam for your needs.

Nah, I was just contemplating that instead of making both bigger, use the available space and bring the exhaust valve size up to par and spend the rest of your cash on having the perfect camshaft cut.

There is no gain to have an exhaust 1mm smaller if thats what you mean?
The exhaust has 4 times more pressure behind it than the inlet charge
so best use a ratio of inlet vs exhaust valve size around 100/80.
eg.
40mm inlet x 0.8 = 32mm exhaust
40mm inlet / 32mm exhaust

Still a stock H89 head has perfect valves and ports from stock for turbo

Ok so what if the intake valve was still a bit bigger but not as significantly and the space utilized for a bit larger exhaust valve? I.E. - custom camshaft to be cut - what if instead of 40/35 you size them more like 38/37 and rather increase the ex. duration just a little more? I mean as D said, if you already have an adequate intake valve, why not bring the exhaust valve up to par and just manipulate the cam timing?

I learned quite a bit from that link!

I'm still curious though: intake air is either pushed in via Forced Induction, or sucked by the downward motion of the piston. Either case requires that the flow is simply improved. That coupled with the fact that exhaust gas pressure and temperature is so much higher than intake pressure and temp already, shouldn't the exhaust gasses flow more quickly from an identically sized valve even before the piston pushes the rest out? That being said, once again shouldn't the exhaust then be the one requiring the improved flow?

CA18DXX already has 34.15mm inlet valves from factory
these are quite large valves well capable of 400+hp.
Just need longer duration inlet and exhaust camshaft.
This is done simply using factory exhaust cam on the inlet
side and getting a custom grind for the exhaust.
Simple & less money spent for Lil gains on already big inlets.

The factory Ex. cams have longer duration so put them on the
inlet side then get a custom exhaust camshaft for the exhaust
side for more duration and ready to go focus on other areas
of the package to support the hp required.

The inferior size but famous Honda D16 1.6 sohc 16v turbo
engine makes over 450+hp with stock 26mm exhaust valves
and 30mm inlets the most is 31mm inlets for these 76mm bore
engines and using cast suzuki vitara pistons with 24+ psi.

A CA18Dxx head is already a great as not many hydraulic
lifter heads of this size can spin to 8500rpm before needing
to go solid lifters for more rpm.

This also helps me as my engine builder said that he will put bigger intake valves in my CA and was wondering why not the exhaust too.

There's a lot to it but the basic principal is that for induction you have at most 100kPa to push the air in (atmospheric pressure). For exhaust there's typically around 400kPa above atmospheric pressure when the valve opens and this pressure is dumped/ wasted. That means to get the air moving you have roughly 4 times as much pressure available at exhaust so you can use a smaller exhaust valve to make room for a bigger intake.

There's also some fluid mechanics going on there, the fact that sudden expansions cause more pressure drop (losses) than sudden contractions, again this means you want a bigger intake valve than exhaust.

In a 16v head the exhaust valves would be bigger because there's room to fit them and get a better exhaust flow. For each engine size, valve timing and rpm there's an optimal port/ valve size.