I have to admit I write some answers with some craziness to keep my self from getting bored or if any reads them.

You are doing one even with me....your asking the Questions. This is like Mutt and JEFF here!

First of all the difference between the two radiator in coolant capacity might be a pint or two.

The amount of water is not the objective there. It is exposing air to the water with the extra rows of surface area. Moving away a larger quantity of heat.
Like spreading your arms to cool off. You still have the same amount of blood!

They just have to fill the space up with something.

The coolant bottle is reservoir. The coolant level in it is at the highest level of the entire system. Because air is thinner and therefore lighter. It will rise like hot air balloon out of the coolant.

The purpose of this bottle is to allow air to come there. That way the air is not in the engine or anywhere else. Where ever there is air there can be a air bubble. Where there is a air bubble. There is a loss in the conduction of heat away from the heat source. i.e. head & cylinders.

Now you ask and I know you are! Why have air in the bottle? On big ole answer. EXPANSION. Air compresses and water does NOT! When water freezes. It even expands too!

All hot water systems. Pressurized or not, have expansion compensation and safeties designed in. That space is there to help prevent damage if the pressure cap does not release enough capacity fast enough and to limit coolant loss under extreme circumstances.

Most drivers heed proper maintenance or they write the rest in!

A system is design to exceed the heat removal of its given heat source. Plus a safety factor under specific range of operating conditions.

You changing the amount of water above the design will effect nothing. Because you should be dissipating the excess heat as it is created above a designed in proper operating temperatures. Your thermostat that's recommended.

You adding more coolant is like carrying it as extra weight. Around in you trunk!

Changing the pressure cap means there is more or less heat in that given quantity of coolant. There is more heat in water under more pressure. Catch is the system has to be stronger and maintained to withstand the stress!

If the heat source exceeds the transfer medium"s" capabilities, it overheats. Solids in the coolant. i.e. scale and rust. Radiator with same and debris on it outside fins. Slipping belts. You get the picture!

If you want to think on something along this line. Think on this.

With all the effort that has been spent to make the internal combustion engine only 30% efficient at actually producing mechanical work.

What could we have done with the other 60% that left radiators, engine blocks and tail pipes?

Consider heat is energy straight up.

Since the steam days we have so much more knowledge in thermal dynamics. The whole field is so immense that it boggles my mind and I'm no nothing.

Its seems to me that with all I've seen change in my life time we could have blown Henry Fords little toy away by the 80's. Should have by now, for sure!

Think on what we've done and the know how we have now to do it.

Then think on. What if out youngsters can be motivated differently.

Next 100 years could be a real ripper!

Good Day

Phil