Hi guys how much performance gain can be expected with 1mm over size intake valves on a ca18.

How much power do you have at the moment? What size cams do you have? what are the rest of the engine specs? Have you match ported the inlet manifolds? how long is a piece of string?
Seriously 1mm over inlets will give you sfa if you have a small turbo maybe if you are pushing 600 atw you might get another 30 up top. Exhausts are where its at with a ca...

hks, nismo, greddy or other hydra camshafts will do much more as valves are already "HUGE" best leave head as is get cams, manis and quality turbo

EDIT: from what I remember McDat you wanted a very punchy, no lag 280-300hp atw so apart from the above you could also get some mild port work, clean up of the casting slags is the most you need. No need to do the short turn radius as some interesting testing on a friends suzuki 16v (like a smaller ca18de 4 port with same wide angle heads and proportioned ports and port angles) showed no improvement once head had short turn radius massagedd, conclusion so far seems that there is less reversion of pulses on the inlet when this is left untouched and fuel mix stays tighter flowing within the tumble pattern instead of spreading onto the bore walls due to the short turn radius promoting opposing eddies or turbulence on the main inlet charge that is more prominent on the top of the valve.

edit 2: I stress again get a good BB turbo

No disrespect but perhaps put some more consideration into the question itself before asking. It's like asking how much further you'll travel with an additional 100ml of fuel.........without stating any details or objectives.

What exactly are you trying to achieve?

I'd even expand on that - going to bigger valves by themselves can in a lot of cases COST you some flow. You really have to tailor the bowl area and throat area (the region in the ports just above the valves/valve seats and the valve seats all to work properly with each other. This isn't particularly hard to do, but it is certainly more work than just fitting the valves and cutting the seats so they technically seal well.

As a non datsun example, with ford cleveland 2v heads, the 'standard' option used to be to fit 4v sized valves. But 9 times out of 10, esp if aiming for a broad power band, the clever porting guys could get more flow (and more ideal velocity) from 2v sized valves.

A similar thing is also the case with early holden v8 heads, for most streeters, running stock, or near stock valve sizes gets a better overall powerband and a quicker car. larger valves have their place but a fair bit has to be done to make it work.

The type of work I'm discussing, teh short turn/bowl/throat/valve seat area - is often called 'pocket porting'. It's not horrendously expensive and probably 80% of the total gains you can make as far as flow goes will come from this. And to get that last 10-20% will require far more extensive work (and whilst most porters can do pocket porting, not all of them can truly get the most out of any cylinder head - it takes a lot of time and trial and error, which also means money - to do this) anyhoo that last 10-20% of gains can easily cost 3-4 times as much as pocket porting. I'd make a reasonable case for anything short of a 'no expensive spared' racing effort - that pocket porting is as far as one should go.

Having said all that, presumably this is a turbo ca18 yes? In which case, whilst it might not sound as cool, you can make up for various shortcomings with just a little bit more boost. Certainly if you do headwork (and combine it all to work as a package and specifically to work in a boosted application) the results will be even better, but if the power/performance goal is not astronomically high, you will be able to achieve it with a stock head, if you catch my drift.

There are exceptions - holden 6s come to mind. Their heads flow pathetically - and you could practically take a poo in the ports and they'd flow better. On them you have to do a fair bit of work, even with a lot of boost, to make halfway decent power. For a streeter sure, you can just get an off the shelf YT head (if you can find one in good condition) which will do the trick, but for sub 10s in a relatively light lc/j torana, you have to spend a heck of a lot.