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Guest Message by DevFuse

Pressurised vs. Non Pressurised Water Cooling

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4 replies to this topic

#1 Roger 912

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Posted 09 December 2011 - 12:32 PM

I have just been having an argument with some guy on the Benz website about this subject. About 10 years ago the header tank on my old Merc E220 decided to crack and being polyurathane that does not glue and also the fact that I could not find a replacement, I put some epoxy over the crack and fitted a non pressure cap to the tank. The overflow pipe I took into a reservoir bottle half filled with coolant making sure that the pipe is right at the base of this bottle. Now, when the water heats up the coolant expands into the reservoir and when the coolant cools down it sucks the fluid from the reservoir back into the header tank. The header tank is therefor always full. This system has worked faultlessly for the last 10 years even when driving on days with 40 degrees Centigrade plus.

The dude on the Mercedes web site who says he is a Mercedes trained mechanic (he calls himself a technician), says it cannot work. He says that an unpressurised system will run hotter than pressurised.

I explained to him that the reason they run a pressurised system is that with a 5 p.s.i. pressure, water boils at 112 degrees C and the greater the difference between the coolant temperature and the mean air temperature, the smaller the radiator has to be and is therefor cheaper to manufacture.

Any thoughts from knowledgeable people on this site?

Edited by Roger 912, 09 December 2011 - 12:33 PM.

#2 jonwat



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Posted 09 December 2011 - 01:40 PM

I agree with you Posted Image

#3 cjtpb13



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Posted 09 December 2011 - 02:20 PM


There are numerous reference books on this subject. One of the better ones I have read is Engine Cooling Systems by Ray T. Bohacz . He covers theory, design, and performance.

A pressurised system allows the coolant an additional 3 degrees Fahrenheit for each pound of pressure above the boiling point of 212 degrees Fahrenheit ( sorry for the lack of conversion today...long work day already ) . A nonpressurised system will not necessarily run hotter than a pressurised system ; however a nonpressurised system will overheat faster than a pressurised coolant system. Especially when in high altitude driving, as the pressure cap compensates for the reduction of atmospheric pressure. Also the pressurised system at the coolant pump inlet reduces the possibility of cavitation damage to the water pump.

As per reference to the radiator. The radiator has no concept of pressurised or nonpressurised coolant. Its only function is to remove heat from the coolant. Though you are correct in the fact that a pressurised system allows higher temperatures so the radiator will transfer the heat to the air quicker because of the temperature differences. ( which rely upon the First and Second Laws of Thermodynamics ) The design size of the radiator has more to do with the engineered engine power, anticipated road speeds, loads, and additional tasks ( whether it be near an aircon condenser emitting its own heat or has transmission or oil cooling capacity ) rather than the pressure. Which does plays a minor roll in the size of the radiator but the increased pressure forces the design to have to be stronger.

A bit off subject however, in reference to allowing less expensive manufacturing, in the 1970's the Germans and the French began to use an aluminum core radiator with plastic header and collector tanks. This allowed lower weight less expensive materials, and allowed more complex designs to fit in the ever changing face of the automobile.

Bit of automotive trivia. The first pressurised radiator cap was used on a 1939 Buick by John Karmazin from GM's Harrison Radiator Division.

I happened to do quite a bit of research on this subject because when my Esprit is back I will be using Evans NPG+ coolant in both the engine coolant system and the chargecooler system.

Hope this helps...probably will create a large discussion though

Edited by cjtpb13, 09 December 2011 - 02:25 PM.

#4 Roger 912

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Posted 09 December 2011 - 06:31 PM

One of the problems with living up in Johannesburg (not that I do) is that at 6000 feet above sea level, their water boils at 93 degrees Centigrade. Obviously a need for thermostats to operate at a slightly lower temperature as well.

#5 surferphil



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Posted 10 December 2011 - 01:04 AM

I thought it was pressurised to raise the temperature at which the coolant will boil and evaporate.
It works if you say it does, who can argue with that? Typical Technician - trained to follow procedure, my Grampy who ran a ford garage in the 70's and 80's was an Engineer, says it all really.

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