Maserati 430 Bi-Turbo cooling

Water radiator modification

My copper/bras 3 layer rad was leaking. In fact both rads I owned were dead.
A new rad costs $900, to rebuild one you need 400 euros in France / $600 in US.
I needed a cheaper and (possibly) better solution.

The rad is a double-pass 3 layer copper/brass design, 46 raw of core tubes, each every 8mm. Every core tube has a 2mm x 13mm section.
Water enterers the top input port, goes left, then down and then rigth to the output port, each port is 38mm external diameter.
I did few calculations and ordered a rebuilt and modified cross-flow 2 layer 46 raw rad, core tubes with a 2x15mm section.
The top input port was moved to the left side, the small hose on the top was moved to the right (closer to the expansion bottle). Of course the internal wall in the right rad tank was removed as well.
The workshop did a great job. Pictures are below.

Rebuilding a two layer rad costs 25% less than a three layer one
the third layer is not necessary to cool in idle because air is cooled enough by two first. A two layer rad will better spread water over the cool first-layer surface (at low RPM water does not go through all raws, it stays on top of the core).
water needs 5-7 seconds to go through the double pass rad, it's too long and thus requires low temperature fan switches which then switch off at a too low temperature.
the double pass rad may limit the flow at high RPMs (especially when the rad starts being clogged). Its Renold number is higher, in theory this should make a better heat exchange. However if the flow is limited the heat release can not grow from some RPM
in a double pass rad the exiting cold water is heated back by the entering hot water as they are too close to each other and share the same tank.
a lot of more powerful European cars of the same period have two layer cross-flow rads of similar sizes

Here is a good paper explaining how to estimate a car radiator:
Smaller Radiator Design with Optimized Heat Transfer Performance, by Mapple

I've also made add a 22x1.5mm thread for another fan switch. I wanted to control fans with two switches, one per fan, in a staged way (low and high temperature) as it's done on most of modern cars. Because one fan should be enough in most of cases to cool down in idle.
As I did not know whether the original low temperature switch (rated 82-69C) was absolutely necessary I kept it in case new switches would not do as expected.
I modified the fan relay wiring to get them controlled each by one switch. It's a very simple operation, you just need an additional wire connecting one of relay inputs to one of your switches.
A lot of existing switches use the same 22x1.5 thread but most of them have different connectors. Mid-80s Alfa and Peugeot cars had switches with the same 6mm male connector pins.

You'll find a large range of temperature rates on Peugeot 205 1.6L cars, still widely available for about 14 euros:

FAE 37800: 88-83/92-87
FAE 37810: 84-79/88-83
FAE 37850: 93-88/97-92

I bought the 88-83/92-87 switch, a bit hotter than the original one. This way I can make any mix of switch temperatures.
I think adding another thread was unnecessary because a Peugeot 205 1.6L 84-79/88-83 double switch would do the job.
All these mods costed me less than just rebuilding the rad to the original.

Only one cooling hose needs to be modified to install my rad. You'll find pictures below.
The rad works well, the engine temperature stays as before the mods. I'll wait for the next summer to know if the rad works well during long lasting idle phases at hot days.

Update one year later
It's been one year I did the mod. Since then I drove the car many times in many conditions including very hot days. As expected everything works fine. I did not notice difference in behavior at idle, except that the second rad never switches on (which is a good thing) even after a hard driving. So I'm confident in my mod.
The first fan is controled by the original fan switch, the second by the 88-83 half of the additional switch.
This means the additional thread is not necessary, the original temp switch can be replaced with the Peugeot 205 1.6L FAE 37810: 84-79/88-83 switch, whose first stage is close to the original switch rated as 82-69.