Low Pressure Rad Cap
05-14-2007, 08:32 AM
#1
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Low Pressure Rad Cap
After replacing the coolant expansion tank recently, I decided to protect my investment with a low(er) pressure cap. Capt'n Carl quickly sent a 7 lb (0.5 bar) cap my way 
.... which lowers the relief pressure to 1/2 that of a normal cap.
All the hoses, heater core, tank, rad should last longer due to lowered operating pressure even though the thermal load is unchanged. The coolant boiling point will be depressed ( I'm too lazy to calculate how much) - but for a system in good condition, should be undetectable: the 928 has a very capable cooling system.
I only use the 83 C thermostats, and with the cap change, there is zero change in the coolant operating temp during our warmest days since .... at 27C ( ~80F).
Something to consider ..... a pocket change investment that may prolong the life of components - and at a DIY level that pre-schoolers can do, once they master the peanut butter jar.
.... which lowers the relief pressure to 1/2 that of a normal cap.All the hoses, heater core, tank, rad should last longer due to lowered operating pressure even though the thermal load is unchanged. The coolant boiling point will be depressed ( I'm too lazy to calculate how much) - but for a system in good condition, should be undetectable: the 928 has a very capable cooling system.
I only use the 83 C thermostats, and with the cap change, there is zero change in the coolant operating temp during our warmest days since .... at 27C ( ~80F).
Something to consider ..... a pocket change investment that may prolong the life of components - and at a DIY level that pre-schoolers can do, once they master the peanut butter jar.
05-14-2007, 12:10 PM
#2
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Sounds like a good idea. But why is the system equipped with the 15 lbs cap? I bought one of those ionizer caps and haven't put it on b/c the pressure is 12 IIRC???
05-14-2007, 12:32 PM
#4
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Originally Posted by Garth S
Something to consider ..... a pocket change investment that may prolong the life of components - and at a DIY level that pre-schoolers can do, once they master the peanut butter jar.
05-14-2007, 03:50 PM
#5
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Maybe I'm missing the point of this.
The programmed weak spot in the cooling system is the coolant bottle cap. After that, the water pump seal. Or maybe it's that little hose between the head and the heater valve. So making the cap a weaker spot should help 'preserve' the integrity of the other less-weak spots, right. Kindergarden stuff.
Meanwhile, back at Thermodynamics U., there may be good reasons to use the correct cap.
In its mission to carry heat from hot spots in the engine to the rest of the engine and the radiator, coolant depends on continuous contact with the transfer surfaces. This maximises transfer of heat, since transfer through steam or air bubbles is much slower due to different densities of the fractions. Remember then that allowing a higher coolant system pressure will allow better heat transfer from the block. There's a balance that's achieved in the system, where the system pressure rises as heat input rises compared to heat dissipated so long as the heat is actuall transferred to the coolant. In a system with a lower pressure limitation, there can easily be so much localized boiling at engine hot spots that the coolant temp doesn'r rise high enough to get you attention even when temperatures at certain metal parts of the enine may be way too hot. Tht localized boiling is a somewhat self-reinforcing problem too, since the hotter that spot gets the more likely it is have more boiling and therefore continue increasing some in temperature.
Many may remember how old-timers would warn against taking the thermostat out of a car when trying to 'cure' an overheating problem (in a "normal" car this was). The thermostat is a flow restriction that causes the water pump to build pressure in the block and therefore improve heat trasnefr from the block to the coolant. Many speculated that the problem was that the coolant is rushing through the system 'too fast to pick up the heat' but in fact it was probably doing a little bubbling and boiling in the critical contact areas.
SO--
If you have weak spots in your cooling system that you think need the "protection" of a lower-pressure cap, find them and fix them. At best a lower-pressure cap is a temporary band-aid fix for a system problem. Using the lower pressure cap may mask those problem, but can also lead to false low temperature readings, excessive loss of coolant, but most importantly it will lead to excessive thermal stress and possible failures of head gaskets and such.
All for a $6 coolant cap...
The programmed weak spot in the cooling system is the coolant bottle cap. After that, the water pump seal. Or maybe it's that little hose between the head and the heater valve. So making the cap a weaker spot should help 'preserve' the integrity of the other less-weak spots, right. Kindergarden stuff.
Meanwhile, back at Thermodynamics U., there may be good reasons to use the correct cap.
In its mission to carry heat from hot spots in the engine to the rest of the engine and the radiator, coolant depends on continuous contact with the transfer surfaces. This maximises transfer of heat, since transfer through steam or air bubbles is much slower due to different densities of the fractions. Remember then that allowing a higher coolant system pressure will allow better heat transfer from the block. There's a balance that's achieved in the system, where the system pressure rises as heat input rises compared to heat dissipated so long as the heat is actuall transferred to the coolant. In a system with a lower pressure limitation, there can easily be so much localized boiling at engine hot spots that the coolant temp doesn'r rise high enough to get you attention even when temperatures at certain metal parts of the enine may be way too hot. Tht localized boiling is a somewhat self-reinforcing problem too, since the hotter that spot gets the more likely it is have more boiling and therefore continue increasing some in temperature.
Many may remember how old-timers would warn against taking the thermostat out of a car when trying to 'cure' an overheating problem (in a "normal" car this was). The thermostat is a flow restriction that causes the water pump to build pressure in the block and therefore improve heat trasnefr from the block to the coolant. Many speculated that the problem was that the coolant is rushing through the system 'too fast to pick up the heat' but in fact it was probably doing a little bubbling and boiling in the critical contact areas.
SO--
If you have weak spots in your cooling system that you think need the "protection" of a lower-pressure cap, find them and fix them. At best a lower-pressure cap is a temporary band-aid fix for a system problem. Using the lower pressure cap may mask those problem, but can also lead to false low temperature readings, excessive loss of coolant, but most importantly it will lead to excessive thermal stress and possible failures of head gaskets and such.
All for a $6 coolant cap...
05-14-2007, 04:50 PM
#6
Drifting
The cap acts as a pressure release valve. Stick with the manufactures recommendations.
Coolant doesn't circulate perfectly, so temperatures in the cooling jackets will be hotter than the temperatures the thermostat sees.
This link provides a table of pressures and temperatures one can expect with anti-freeze...
http://www.are.com.au/techtalk/coolants.htm
Coolant doesn't circulate perfectly, so temperatures in the cooling jackets will be hotter than the temperatures the thermostat sees.
This link provides a table of pressures and temperatures one can expect with anti-freeze...
http://www.are.com.au/techtalk/coolants.htm
05-14-2007, 05:22 PM
#7
Under the Lift
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I don't think this is for everyone. The 928 cooling system is capacity limited in the later higher HP models, like my S4 (supercharger or not). Running in the desert at 95-100F I need all the cooling capacity I can get and keeping the pressure normal (still only 15 PSI), raises the boiling point (along with a 50% glycol mix). You may not think the coolant would get all that hot and boil off at 1/2 bar with a good coolant mix, but when you stop the motor, a tremendous amount of heat comes out of the block and is no longer cooled by the radiator and could cause a lot of boil-off through the low pressure cap.
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05-14-2007, 11:52 PM
#8
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Some interesting views .... 
It is clear that the coolant system operates at some positive pressure after attainment of operating core temperature, and especially when discharging the thermal load of 'heat soak' when idling .... any doubt of that is erased by trying to spin off the pressure cap too rapidly ..... pssssscht. However, that positive pressure is generally far less than the pressure relief setting of the cap - or otherwise the cap would always be venting as a relief valve, blowing coolant out the discharge pipe. Consquently, the question is more along the lines of how much relief pressure is actually required to protect from boiling ( two phase mixture = defficient heat exchange = bad ) ..... knowing that the cap pressure is rarely, if ever exceeded.
Keep in mind that our closed reservoir system cap faces moist compressed air as opposed to the typical cap in an expansion tank system. In this latter case, the pressure cap ( a true 'rad' cap) directly faces hydrostatic pressure from expanding coolant .... and routinely opens and closes to allow passage of coolant to the tank. In the latter case, relatively warm liquid can sufficiently expand to open a 1.0 bar rad cap ..... but at a temp far below the saturation temp of 129 C .... which would be required to lift the relief valve on our expansion tanks.
Anyway, at a mean core temp. of ~ 88C, the coolant temperature in an effectively functioning 928 system remains below the atmospheric bp of 100C for water or 108 C for 50/50 water/antifreeze .... and below the 120 C bp @ 0.5 bar and/or 129 C bp @ 1.0 bar. A well maintained system is generally designed to run at atmospheric pressure, as noted in other sources ....
Local ambient conditions, stalled city traffic, etc all have a bearing of what must be done to protect the cooling system when it is stressed: everything else being equal, the killer is lack of air flow ..... and one of the cures is a higher relief setting on the pressure cap.
When faced with the ~ 100F environment that Bill quoted, I readily accept that a lower pressure relief valve may not provide the cooling protection required. I will more typically see 75 F (24 C) with peaks of 90 F .... so this makes a suitable experiment .... which is cooling along quite nicely so far.
In a way, much of this is academic if one accepts the point that the cooling system, while pressurized, typically operates at considerably less pressure that the relief valve set point.
It is clear that the coolant system operates at some positive pressure after attainment of operating core temperature, and especially when discharging the thermal load of 'heat soak' when idling .... any doubt of that is erased by trying to spin off the pressure cap too rapidly ..... pssssscht. However, that positive pressure is generally far less than the pressure relief setting of the cap - or otherwise the cap would always be venting as a relief valve, blowing coolant out the discharge pipe. Consquently, the question is more along the lines of how much relief pressure is actually required to protect from boiling ( two phase mixture = defficient heat exchange = bad
) ..... knowing that the cap pressure is rarely, if ever exceeded. Keep in mind that our closed reservoir system cap faces moist compressed air as opposed to the typical cap in an expansion tank system. In this latter case, the pressure cap ( a true 'rad' cap) directly faces hydrostatic pressure from expanding coolant .... and routinely opens and closes to allow passage of coolant to the tank. In the latter case, relatively warm liquid can sufficiently expand to open a 1.0 bar rad cap ..... but at a temp far below the saturation temp of 129 C .... which would be required to lift the relief valve on our expansion tanks.
Anyway, at a mean core temp. of ~ 88C, the coolant temperature in an effectively functioning 928 system remains below the atmospheric bp of 100C for water or 108 C for 50/50 water/antifreeze .... and below the 120 C bp @ 0.5 bar and/or 129 C bp @ 1.0 bar. A well maintained system is generally designed to run at atmospheric pressure, as noted in other sources ....
The pressure type (Normally Closed) radiator cap is the preferred design used by most automakers. Cooling system engineers prefer to have cooling system operating at atmospheric pressure as much as possible to prevent constant strain on the radiator, hoses, and water pump seals.
When faced with the ~ 100F environment that Bill quoted, I readily accept that a lower pressure relief valve may not provide the cooling protection required. I will more typically see 75 F (24 C) with peaks of 90 F .... so this makes a suitable experiment .... which is cooling along quite nicely so far.
In a way, much of this is academic if one accepts the point that the cooling system, while pressurized, typically operates at considerably less pressure that the relief valve set point.
05-15-2007, 02:43 AM
#9
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Its a very bad idea. when you start pushing the temperature envelope, it will boil and then boil over. ever run your car with out a cap, or have a leak in the system?
we are talking 14psi here !!!! heck its a plastic tank. ever feel 14psi in a bicycle tire. heck, a plastic sandwich bag can almost handle this. Im sure there is NO measureable wear differnce between a 7psi cap and a 14psi cap, however, if you go into boil over, you will end up with no coolant, try and drive to the nearest gas station trying to "just make it" and probably blow a headgasket or warp a head. our systems are incredibly unforgiving for air in the system.
put on the standard cap , on your nice new expansion tank and think to yourself with confidence that it will probably last longer than the car will .
Mk
we are talking 14psi here !!!! heck its a plastic tank. ever feel 14psi in a bicycle tire. heck, a plastic sandwich bag can almost handle this. Im sure there is NO measureable wear differnce between a 7psi cap and a 14psi cap, however, if you go into boil over, you will end up with no coolant, try and drive to the nearest gas station trying to "just make it" and probably blow a headgasket or warp a head. our systems are incredibly unforgiving for air in the system.
put on the standard cap , on your nice new expansion tank and think to yourself with confidence that it will probably last longer than the car will .
Mk
05-15-2007, 02:57 AM
#10
Rennlist Member
no, most systems do not operate at ambient pressure. its almost impossible for this to occure. you need the cap to prevent boiling. at the new pressure level with a 7psi cap, you can reach that pressure and it will start to vent its a one way valve by the way, air cant go back in as it is a closed system, with a one way valve. the air pocket is designed to be a pressure container if it starts to vent, you run the risk of thermal run away.
There is no logical reason to use a 7psi cap in our cars. any use of it, unless you never see traffic or hot days, would be a mistake. any advice for others to follow this idea, based on "saving the life" of the expansion take, is irresponsible.
I had a leaking tank before in my old race car, it would be fine around town, but in traffic or on the track, it couldnt keep the pressure above 5psi or so. again, it would work fine on the hyway, but once you stopped in traffic , it would start to vent and boil over. you could tell, the pressure in the lines were not as pressurized by feel, as they are with a proper cap where they feel like a properly pressurized football, not a balloon doggie.
mk
There is no logical reason to use a 7psi cap in our cars. any use of it, unless you never see traffic or hot days, would be a mistake. any advice for others to follow this idea, based on "saving the life" of the expansion take, is irresponsible.
I had a leaking tank before in my old race car, it would be fine around town, but in traffic or on the track, it couldnt keep the pressure above 5psi or so. again, it would work fine on the hyway, but once you stopped in traffic , it would start to vent and boil over. you could tell, the pressure in the lines were not as pressurized by feel, as they are with a proper cap where they feel like a properly pressurized football, not a balloon doggie.
mk
Originally Posted by Garth S
Some interesting views ....
is rarely, if ever exceeded.
Keep in mind that our closed reservoir system cap faces moist compressed air as opposed to the typical cap in an expansion tank system. In this latter case, the pressure cap ( a true 'rad' cap) directly faces hydrostatic pressure from expanding coolant .... and routinely opens and closes to allow passage of coolant to the tank. In the latter case, relatively warm liquid can sufficiently expand to open a 1.0 bar rad cap ..... but at a temp far below the saturation temp of 129 C .... which would be required to lift the relief valve on our expansion tanks.
Anyway, at a mean core temp. of ~ 88C, the coolant temperature in an effectively functioning 928 system remains below the atmospheric bp of 100C for water or 108 C for 50/50 water/antifreeze .... and below the 120 C bp @ 0.5 bar and/or 129 C bp @ 1.0 bar. A well maintained system is generally designed to run at atmospheric pressure, as noted in other sources ....
Local ambient conditions, stalled city traffic, etc all have a bearing of what must be done to protect the cooling system when it is stressed: everything else being equal, the killer is lack of air flow ..... and one of the cures is a higher relief setting on the pressure cap.
When faced with the ~ 100F environment that Bill quoted, I readily accept that a lower pressure relief valve may not provide the cooling protection required. I will more typically see 75 F (24 C) with peaks of 90 F .... so this makes a suitable experiment .... which is cooling along quite nicely so far.
In a way, much of this is academic if one accepts the point that the cooling system, while pressurized, typically operates at considerably less pressure that the relief valve set point.
is rarely, if ever exceeded.
Keep in mind that our closed reservoir system cap faces moist compressed air as opposed to the typical cap in an expansion tank system. In this latter case, the pressure cap ( a true 'rad' cap) directly faces hydrostatic pressure from expanding coolant .... and routinely opens and closes to allow passage of coolant to the tank. In the latter case, relatively warm liquid can sufficiently expand to open a 1.0 bar rad cap ..... but at a temp far below the saturation temp of 129 C .... which would be required to lift the relief valve on our expansion tanks.
Anyway, at a mean core temp. of ~ 88C, the coolant temperature in an effectively functioning 928 system remains below the atmospheric bp of 100C for water or 108 C for 50/50 water/antifreeze .... and below the 120 C bp @ 0.5 bar and/or 129 C bp @ 1.0 bar. A well maintained system is generally designed to run at atmospheric pressure, as noted in other sources ....
Local ambient conditions, stalled city traffic, etc all have a bearing of what must be done to protect the cooling system when it is stressed: everything else being equal, the killer is lack of air flow ..... and one of the cures is a higher relief setting on the pressure cap.
When faced with the ~ 100F environment that Bill quoted, I readily accept that a lower pressure relief valve may not provide the cooling protection required. I will more typically see 75 F (24 C) with peaks of 90 F .... so this makes a suitable experiment .... which is cooling along quite nicely so far.
In a way, much of this is academic if one accepts the point that the cooling system, while pressurized, typically operates at considerably less pressure that the relief valve set point.
05-15-2007, 04:38 AM
#11
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Try driving around for 30 minutes on a warm day (doesn't have to be all that hot), then stop the motor and immediately pull off the radiator cap. Within 30 seconds you will have a nice geyser coming out of the overflow tank. If you keep the engine running or the cap on, this won't occur. Maybe a 1/2 bar cap is enough to prevent this, but somewhere between 1/2 bar and zero there will be a problem.
05-15-2007, 03:16 PM
#12
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Bills right. and dont forget the boiling that can occur and create voids in the cooling system. again, our systems are very unforgiving with regards do this. as the problem starts, the highest point is the thermostat area. by the way, thats where the temp sensor is as well. all you need is some incorrect temp values showing the engine is cooler than it is when there is an air pocket in that region of the system
mk
mk
05-15-2007, 04:47 PM
#13
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Ah .... "the highest point is the thermostat area. by the way, thats where the temp sensor is as well. all you need is some incorrect temp values showing the engine is cooler than it is when there is an air pocket in that region of the system..." Which is why there is a breather hose on that housing to vent air into the coolant tank just as there is a breather on the top of the radiator plumbed to the tank. I have had the old very brown 1980 boil over after a run group on a hot day if I shut it off too soon coming of the track hot . I vote for the stock radiator cap.
05-15-2007, 08:36 PM
#14
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Just for fun, I ran the car today until at core temp, then made a very hard and long uphill run to heat it as much as possible: I stopped and quickly shot a series of temps with a laser pyrometer. The series was repeated after 10 min of idle, and again after 5 min of shutdown and heat soak. At 10 min of S/D, everything was cooling down so much as to be meaningless.
The max temps are consistent with the 83C thermostat - and the absolute max temp was the backside of the water bridge at 90C. If the table below remains unscrambled after posting, it is clear that the thermostat is doing the only thing it can do - controlling the minimum temperature: otherwise, it is wide open** and temps may rise if cooling deficiencies exist, ambient conditions are severe, etc .
** wide open is somewhat misleading, for another primary function of the thermostat is to act as a flow constricting orfice .... one of several contributors to the net backpressure seen by the centrifrugal pump. Heat transfer from the block to the coolant is fundamentally controlled directly by flow and by the square root of pressure: back pressure is good, and when all the pressure drops across rads, hoses, head gasket orfices, etc are added up - generally a healthy 15psi is seen _-_-_- ..... the operating backpressure does not come from the pressure relief cap, for coolant operating temps are generally well below vapour saturation required to create that pressure .... the cap sits there doing its job as a safety valve .... just like the hot water tank in the basement ... hardly ever called upon like the Maytag repair man.
There was a very modest backpressure on the cap when opened - hardly noticeable when opened after 5 min of heat soak when back in the garage: a calibrated mercury thermometer dipped into the reservoir confirmed 75 C. - the equalization of heat in the system after sitting, for the running condition under the pressure cap was 44 C
Hardly any risk of boilover at these temps .... the slight pressure that developed in the reservoir was due to air space compression by the expansion of the coolant liquid - and the actual operating temperatures are below atmospheric ( normal) boiling points: one could argue that a pressure cap was not needed .... but that is not a recommendation
top hose...bottom hose....T-stat...heads...reservoir
hard run----81-----------44-------------44------50--------42
idle----------85-----------80-------------80-----76---------70
heat soak/idle-84-----------78-------------77-----80---------78
If this were repeated in Death Valley ( or anywhere in California), the results may be predictably different .... but not all of us live there ..
On a side note, I went through the race logs of my friends 550hp stock car: these guys really battle with heat managment ...expansion tanks are not allowed and the vent line exits onto the base of the windshield for obvious reasons: the max. recorded was 200 F ( ~93 C). They use a 30psi cap along with a calibrated air void in the rad .... not to control boilover, but to control coolant expansion against a compressible gas ( for coolant loss is frowned upon at the track), and to enhance heat transfer at higher pressure.
The max temps are consistent with the 83C thermostat - and the absolute max temp was the backside of the water bridge at 90C. If the table below remains unscrambled after posting, it is clear that the thermostat is doing the only thing it can do - controlling the minimum temperature: otherwise, it is wide open** and temps may rise if cooling deficiencies exist, ambient conditions are severe, etc .
** wide open is somewhat misleading, for another primary function of the thermostat is to act as a flow constricting orfice .... one of several contributors to the net backpressure seen by the centrifrugal pump. Heat transfer from the block to the coolant is fundamentally controlled directly by flow and by the square root of pressure: back pressure is good, and when all the pressure drops across rads, hoses, head gasket orfices, etc are added up - generally a healthy 15psi is seen _-_-_- ..... the operating backpressure does not come from the pressure relief cap, for coolant operating temps are generally well below vapour saturation required to create that pressure .... the cap sits there doing its job as a safety valve .... just like the hot water tank in the basement ... hardly ever called upon like the Maytag repair man.
There was a very modest backpressure on the cap when opened - hardly noticeable when opened after 5 min of heat soak when back in the garage: a calibrated mercury thermometer dipped into the reservoir confirmed 75 C. - the equalization of heat in the system after sitting, for the running condition under the pressure cap was 44 C
Hardly any risk of boilover at these temps .... the slight pressure that developed in the reservoir was due to air space compression by the expansion of the coolant liquid - and the actual operating temperatures are below atmospheric ( normal) boiling points: one could argue that a pressure cap was not needed .... but that is not a recommendation
top hose...bottom hose....T-stat...heads...reservoir
hard run----81-----------44-------------44------50--------42
idle----------85-----------80-------------80-----76---------70
heat soak/idle-84-----------78-------------77-----80---------78
If this were repeated in Death Valley ( or anywhere in California), the results may be predictably different .... but not all of us live there ..
On a side note, I went through the race logs of my friends 550hp stock car: these guys really battle with heat managment ...expansion tanks are not allowed and the vent line exits onto the base of the windshield for obvious reasons: the max. recorded was 200 F ( ~93 C). They use a 30psi cap along with a calibrated air void in the rad .... not to control boilover, but to control coolant expansion against a compressible gas ( for coolant loss is frowned upon at the track), and to enhance heat transfer at higher pressure.
05-16-2007, 01:26 AM
#15
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go drive your car with the cap on , but not screwed down to the pressure sealing level. you will have a volcano in no time. maybe longer than 10 mins, but it will evacuate the entire cooling system and over heat. if not, you have some freak 928 that probably doesnt need a radiator!
mk
mk