The 16% has a smaller chamber.

Is there much to be gained in changing to the 16% nitro head over reducing the shims on the stock 25% head?

If we have two engines that have the same compression ratio.then the one that has bigger gap between piston and button,will have more power in medium-high rpms.
the one that has smaller gap between piston and button will be more powerful in low-medium rpms.
Always with the same compression ratio at overall!

That's not entirely true, up to the point of too high squish velocity the engine will be able to produce more power over the entire rpm range and use less fuel with a smaller clearance. But you might need to offset it with different glowplug and/or manifold.
There is usually a couple of sweet spots as well where it's pretty happy even if you change the nitro slightly, but the design needs to be good.

So you mean it will make more power overall but you have to make something for the too high squish velocity thay is created by so small squish gap?

Basically, the gas trapped in the squish area doesn't burn soon enough to contribute to any work, so the less volume you've got in that area the more gas is available to make power and isn't wasted.
But you can get several problems with too high velocity, on these engines it's usually that it will drop the over rev of the engine.
It's not always the best overall idea to run it super close as you take away some other functions in the process, if you aren't willing to redesign the head.

If the piston is as close as possible to the head at TDC all the mixture will be compressed in the combustion chamber. It will be compressed a lot and will give a faster and more powerfull combustion. The larger gap is set because at high RPM the flex in all parts will move the piston closer to the head. And to prevent it will touch the head when there is some play on the bearings and rod al larger gap is choosen.

Redesigning the head is already done by Somoder Singh with his so called Singh grooves.
https://somender-singh.com/articles/...comesfree.html

Yes, I thought that was understod without mentioning it, the piston moves further up at high rpm.

Those singh groves has been debated to death, I don't believe there has ever been a reliable test done to prove that they work. And it wasn't what I talked about. :)

So then if your motor comes with one button head set for 16%, and you want to use 25% nitro and you want the full effect of 25%, then you have to buy the 25% head because if the the squish area is smaller on the 16% then just removing a shim still won't be the same as using a 25% button head...
do we all agree this to be true???

If you want to use 25% with a 16% button you have to add a shim. And yes, the head clearance will be bigger with a less effect.

Is it noticable? I have played a lot with head shims and buttons, I can not tell hat there is a huge difference. Noticable maybe but real effective...? Maybe in the hands of a Kurzbuch running for his next WC title

Sorry i ment to say add a shim, but yes ok what i figured we need to have the other button head ....

It will most likely be best to run the 25% head with 25% fuel as you can run tighter clearance (less shims).

With head clearance you basically have two ways to go (even though on these engines it isn't super critical) really big or tight, in the middle you can have issues.
Some head designs isn't extremely sensitive, so if it's right you might just need to change glow plug instead of shims when changing nitro percentage.

And yes, there is plenty of power and engine characteristic hidden there, though hopefully most of them is pretty good now days.

Hey from sydney , i just reserved a vir-us 21A on s good deal , what is the front bearing size and rear that are ceramic , im thinking of replacing the standard after run in possibly.. Thoughts? , tips on tuning appreciated.

In the manual all is listed. If it is the same design as a few years ago then you can only buy original Novarossi bearings and to be honest, getting ceramics is a waste of mony if you think they will perform better.

okay thanks for the feedback.