Hi guys...Need some expert help on this. I'm driving a 1991 FC3S Turbo.
Previously I was using standard radiator with 0.9bar cap. Read some where said that this is the correct pressure cap to use for standard radiator.

I recently changed to a Koyo aluminium radiator and it comes with 1.3 bar cap. So which pressure cap should I use?

 

you should never use anything more than a .9 bar unless you have upgraded the coolant seals in the engine as they are designed for a .9 bar system. You can use the 1.3 cap, but eventually you will develop a coolant leak or blown water seal

 

what if your coolant system has 2 caps? one on the neck and one on the rad? would it matter if there was a .9 and a 1.3? does it matter if they are different? i am assuming no...

 

If you have a S4 with two caps, the one without the overflow can have any cap you want on it. You could put a 10 bar cap and it would not make a difference.

 

i have a s4 t2 with a koyo and an overflow nipple under the cap on the coolant neck. i have a 1.3 bar cap and a stocker. which should go where?

 

The stock cap (.9 bar) goes on the bung with the overflow nipple.
If they both have overflow provision, it doesn't matter which cap goes where.

 

This Is Interesting. ICEMARK,so you are saying that even If I get a Koyo Rad as an Upgrade For My Cooling system(s4 n/a),That I should still use an OEM Cap,Correct?..and that the OEM is .9 Bar?.Any Higher Range would be Detrimental as the stock Coolant Seals are only Rated for that Range?...Also,Question..When someone has to Use a Coolant System Pressure Tester,on thier engine,What is the Maixumum Allowable PSI to "take" the Cooling system up to,To pressure test it?(Thanks, In advance, Mark!,and hopefully I am not Breaking Forum Protocol).Apologies to the Original Poster of this thread.Hopefully this is Useful Info for you as well!

 

Aluminium radiator is cooler than stock radiator. So by using 0.9 cap, would the Alu radiator have enough pressure to circulate the coolant from radiator to reserve tank and back to radiator? Just a assumption

 

Yeah, you didn't change the water pump or the engine's coolant seals, correct? Then they are still designed for a less than 1 bar system.

You are not driving the motor or loading it during a pressure test, so you get a little extra room, but with used housings I wouldn't go too far past 1 bar.

Alum has better heat rejection than brass, so by using a alum radiator you are gaining better heat rejection... pressure has absolutely nothing to do with it functioning better or worse. BTW, most FC's use a alum core with plastic end cap radiator anyway, so using some aftermarket alum core radiator is not that big of a jump.

Pressure is simply to prevent the coolant from boiling too early or to reduce the coolant from cravatting going past the water pump fins. If you have the proper mix of coolant to water, there is absolutely no need to get that extra lowering of the boiling point.

Some cooling system basics:
Pressure Cap
The radiator cap actually increases the boiling point of your coolant by about 45 F (25 C). How does this simple cap do this? The same way a pressure cooker increases the boiling temperature of water. The cap is actually a pressure release valve, and on cars it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure.

When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled.

 

Now I get.... 0.9 it is....
thanks Icemark..

 

i've been running a greddy 1.3 bar rad cap for about 1 1/2 years now on the stock system rad, water pump ect... i have had no problems thus far!! i do have a 87 gxl with 125xxx on the clock! but while we are on the subject of koyo radiators does any one know what the two out let holes on the end tank on the (if you are looking at the rad out of the car and with the rad inlet and outlet facing you) right?? they are threaded and was wondering if you had to plug them some how or is it just fine as is?? sorry to the op for the thread jack but i am hoping this will help too!!

 

they both have an overflow...so use .9 bar on both? damn i just got a mazdaspeed 1.3 bar cap

 

They both have a overflow? So you have two coolant lines going to two different overflow tanks? A coolant line for each overflow?

 

it goes from the coolant neck to one side of the stock bottle, then from the other side of the bottle to the rad. how else would it go? i have an extra plastic top piece for the coolant neck without the nipple on it

 

So again you have two coolant lines feeding the overflow bottle? Normally there is just one line... between either the radiator and overflow or the coolant neck and overflow.

Anyway in that case if you really have two lines, it wouldn't matter which one got the 1.3 bar. You would still have a .9 bar on the other making the maximum pressure the .9 bar cap anyway.

 

yes i have 2 coolant lines. thats what i was thinking. thanks a lot