[Cooling System Upgrade] 1-BAR Radiator Cap

Originally Posted by joegr

So, all this theory is interesting, but I wonder if something is being missed in practice?

I have three other cars that have 15 PSI relief caps. Once heated up, I can squeeze on a radiator hose and tell that the system is pressurized, but not how much. On the BMW, when I squeeze on the hose (once heated up), there is noticeably more resistance to squeezing than the other cars. To me, this implies that it is indeed pressurized to a higher pressure than the other cars I have.

We can speculate about this, but I suggest that it is easy to find out for certain. Just tap a small pressure gauge in and find out what pressure it goes to. It seems like the bleed screw on the upper radiator hose would be a good place to do it. It would be a sacrifice of a $25 hose, so not too bad.

The hose on our BMW may just be more stiff than other cars too. Also, I have no doubt that the output side of the pump (that hose) will be at a higher pressure than the rest of the system. This is where a flow sim would help, but I don't have enough data. Tapping a place for a pressure sensor/gauge is hard, because you will likely weaken the connection. You could cause a part to crack open.

I am willing to donate a good upper radiator hose for this. Just did my cooling system a couple of weeks ago, and don't want take things apart again. If anybody wants it for the purpose of measuring the pressure, let me know and I'll send it to you.

Great forum members -

We have:
WDE46 = Automotive Engineer
SeanC = Ph.D Theoretical Physics

Now if we can find a Materials Engineer to chime in... we'll be rockin'.

Originally Posted by WDE46

The hose on our BMW may just be more stiff than other cars too. Also, I have no doubt that the output side of the pump (that hose) will be at a higher pressure than the rest of the system. This is where a flow sim would help, but I don't have enough data. Tapping a place for a pressure sensor/gauge is hard, because you will likely weaken the connection. You could cause a part to crack open.

Actually our upper and lower rad. hoses are quite soft when brand new.

Also, "tapping" into our cooling system at the bleeder screw (and at the auxiliary sensor) locations wouldn't be a bad idea without too much concerns of weakening or causing a part to fail.

Originally Posted by WDE46

The hose on our BMW may just be more stiff than other cars too. Also, I have no doubt that the output side of the pump (that hose) will be at a higher pressure than the rest of the system. This is where a flow sim would help, but I don't have enough data. Tapping a place for a pressure sensor/gauge is hard, because you will likely weaken the connection. You could cause a part to crack open.

No, the hose itself is not stiffer. It is the same resistance to squeezing when the engine is cold, as the hoses on my other cars when they are cold.
No, it is not due to the flow from the water pump, because I am squeezing after shutting the engine down, not while it is running.

I don't present this test as proof of pressure, just as reasonable doubt to it not being higher pressure. If you want proof, install a pressure gauge. If this discussion is not about actually finding out what is going on, then carry on as is.

Originally Posted by joegr

No, the hose itself is not stiffer. It is the same resistance to squeezing when the engine is cold, as the hoses on my other cars when they are cold.
No, it is not due to the flow from the water pump, because I am squeezing after shutting the engine down, not while it is running.

I don't present this test as proof of pressure, just as reasonable doubt to it not being higher pressure. If you want proof, install a pressure gauge. If this discussion is not about actually finding out what is going on, then carry on as is.

Interesting. Thanks for the insight. Your methods are pretty much valid, you just can't put a number on it. Do keep in mind though, that the change in pressure we calculated (about 0.3 bar) is probably enough to feel, as it is a 30% increase. Imagine feeling a bike tire at 30 psi then you bump it up to 40 psi. You can easily tell the difference. This may account enough for the difference you feel in the hose, but I can't be sure. Honestly, given the operating temperature, there is no way for the system to be over 1.3 atm when static, unless it is solid (which it is not).

Also, i need another piece of information. Where is the car's coolant temperature sensor located? Is it on the outflow from the block or is it after going through the radiator? This is a pretty big thing to know.

Great forum members -

We have:
WDE46 = Automotive Engineer
SeanC = Ph.D Theoretical Physics

Now if we can find a Materials Engineer to chime in... we'll be rockin'.

I'm actually a mechanical engineer who happened to learn a lot about engines and vehicle dynamics and I work in the automotive industry. I also know a good bit about material properties, though mostly metals. We need a Polymer/Fiber engineer or a Materials Scientist for the ET material. I can apply my knowledge of fatigue and failure theories, but I can't break it down to plastic thermal wear and thingso n the molecular level.

As far as installing a pressure gauge, I want 2 on there. I want one in the top of the ET and one on the output side of the water pump at least. A dial gauge would work well enough. We don't need one of those $500 high temp pressure transducers haha (they are incredibly accurate and log great data though).

I think we need a couple liquid filled gauges. I'm not sure if the non filled ones will read well with the engine vibration or not.

http://www.omega.com/ppt/pptsc.asp?r...ies&Nav=preg05

Only $20 plus an NPT tap and new parts for the ones we **** up haha. The 0-30 psi gauge should do it. I would do this myself if I had a second car, but I don't so I can't really spare it to do this.

Originally Posted by DHK

Causing cavitation in a liquid is very difficult. It requires a rapid change in pressure that just wont be experienced in a car's cooling system.

Having a 1 bar cap will have some negative effects. You will no be able to keep as much coolant in your car, and it boil at a lower temp.

These could both lead to problems, however weather either of these issues rise to a level where they could cause damage... I dont know.

Try spinning an impeller at 7000 RPM in water that is already close to its boiling point. You get cavitation. I've read of a couple of totally stock engines that would have this problem, none were BMW engines, but proof it happens without much difficulty.

Also, your statement that a different pressure cap will not allow you to keep more coolant in your car is not true. The cap has no effect on the volume of the system, so not sure what you mean by that.

For those of you who said the n54 cars have 1 bar caps... What's your source? I just checked my dad's 535xi, and it has a 200KPa (2 bar) cap.

So taking into the fact that for approximately every 1 PSI in pressure increase the boiling point of water is increased by approximately 3 degrees and the fact that most coolant increases the boiling point of the cooling system.

Why in gods name, even at altitude would you need to pressurize the cooling system to close to 30 PSI and increasing the boiling point of the coolant approximately 90F!

The E46 cooling system really operates on a cushion of air inside the expansion tank anyway.

Something just does not make much sense regarding a 2 Bar coolant cap.

I sure would not want my cooling system with all the plastic and rubber parts operating at 30 PSI and over 200F!

Do all the math you want, put a pressure gauge on any E46 and drive it during the Summer months and I would bet you would never be much above 10-12 PSI at its hottest operation point.

Originally Posted by WDE46

Interesting. Thanks for the insight. Your methods are pretty much valid, you just can't put a number on it. Do keep in mind though, that the change in pressure we calculated (about 0.3 bar) is probably enough to feel, as it is a 30% increase. Imagine feeling a bike tire at 30 psi then you bump it up to 40 psi. You can easily tell the difference. This may account enough for the difference you feel in the hose, but I can't be sure. Honestly, given the operating temperature, there is no way for the system to be over 1.3 atm when static, unless it is solid (which it is not).

Also, i need another piece of information. Where is the car's coolant temperature sensor located? Is it on the outflow from the block or is it after going through the radiator? This is a pretty big thing to know.

It's located on the lower radiator hose. Not sure which way the fluid flows - I've always imagined that the water flows into the radiator from that side, but I have no basis for that thought.

Originally Posted by TerraPhantm

For those of you who said the n54 cars have 1 bar caps... What's your source? I just checked my dad's 535xi, and it has a 200KPa (2 bar) cap.

I was just quoting other people in this thread. No other source lol...

Originally Posted by jfoj

So taking into the fact that for approximately every 1 PSI in pressure increase the boiling point of water is increased by approximately 3 degrees and the fact that most coolant increases the boiling point of the cooling system.

Why in gods name, even at altitude would you need to pressurize the cooling system to close to 30 PSI and increasing the boiling point of the coolant approximately 90F!

The E46 cooling system really operates on a cushion of air inside the expansion tank anyway.

Something just does not make much sense regarding a 2 Bar coolant cap.

I sure would not want my cooling system with all the plastic and rubber parts operating at 30 PSI and over 200F!

Do all the math you want, put a pressure gauge on any E46 and drive it during the Summer months and I would bet you would never be much above 10-12 PSI at its hottest operation point.

That's already what we have been saying all along



Sent from my SGH-I997 using Bimmer App

You may have been saying all of what I stated, but the length of this thread and all the school teachers here, I chose to skip to the point.

Not worth debating, would love to talk to the foolish German that came up with the 2 Bar cap. Maybe there was a reason, maybe they never spent anytime outside of the classroom and decided more is better, who knows?

But as I have said many times, most expansion tanks explode when a new owner gets their car, starts poking around under the hood and decides to top off the expansion tank like a domestic car.

Then shortly after they top off their expansion tank, they are then ready to replace it!

Originally Posted by jfoj

You may have been saying all of what I stated, but the length of this thread and all the school teachers here, I chose to skip to the point.

Not worth debating, would love to talk to the foolish German that came up with the 2 Bar cap. Maybe there was a reason, maybe they never spent anytime outside of the classroom and decided more is better, who knows?

But as I have said many times, most expansion tanks explode when a new owner gets their car, starts poking around under the hood and decides to top off the expansion tank like a domestic car.

Then shortly after they top off their expansion tank, they are then ready to replace it!

OK then, just use a 1 bar cap and carry on. And let us school teachers do the debating. I don't understand your attitude...

Sent from my SGH-I997 using Bimmer App

Originally Posted by jfoj

So taking into the fact that for approximately every 1 PSI in pressure increase the boiling point of water is increased by approximately 3 degrees and the fact that most coolant increases the boiling point of the cooling system.

Why in gods name, even at altitude would you need to pressurize the cooling system to close to 30 PSI and increasing the boiling point of the coolant approximately 90F!

The E46 cooling system really operates on a cushion of air inside the expansion tank anyway.

Something just does not make much sense regarding a 2 Bar coolant cap.

I sure would not want my cooling system with all the plastic and rubber parts operating at 30 PSI and over 200F!

Do all the math you want, put a pressure gauge on any E46 and drive it during the Summer months and I would bet you would never be much above 10-12 PSI at its hottest operation point.

I trust my math more than the intuition here. The math says that the system shouldn't experience any more than about 1.4 bar absolute pressure. That means the ET will experience a 0.4 bar maximum of pressure exerted on its inner surface. That is 1.6 bar below the cap's crack pressure. This never approaches 10-12 psi. Unless we are severely miscalculating something, I don't see why we have the 2 bar caps.

The only thing we haven't taken into account with the pressure calculation is water vapor in the ET air and the coolant expansion. The water expands by 3.8% (about 1/3 of a quart) when heated from 20C to 100C, so that could conceivably almost fill the ET depending on its size. It could be a very major factor. I need its dimensions, specifically air volume at min and max. We could find this, by topping off the ET with distilled water, then removing it into a measuring device until the max level, take a reading, then continue to the min. That is the easiest and most accurate way. If it so happens that the air volume is cut in half by the expansion of water, then we will find the system actually runs at around 2 bar. If this is the case, then I would recommend not filling the ET to maximum to reduce system pressure and extend ET life.

The most complex part of all this is the mixtures that are present. We have 50/50 water and antifreeze for our fluid mixture and a mix of water vapor/air/antifreeze vapor in the ET buffer volume.

Originally Posted by jfoj

You may have been saying all of what I stated, but the length of this thread and all the school teachers here, I chose to skip to the point.

I'm certainly no school teacher. Sorry I use proven math to reason through things. That's engineering. I want to find a more exact answer, so I'm going to try by any means possible. Right now I am low on data so I can only determine so much.

Originally Posted by WDE46

I trust my math more than the intuition here. The math says that the system shouldn't experience any more than about 1.4 bar absolute pressure. That means the ET will experience a 0.4 bar maximum of pressure exerted on its inner surface. That is 1.6 bar below the cap's crack pressure. This never approaches 10-12 psi. Unless we are severely miscalculating something, I don't see why we have the 2 bar caps.

The only thing we haven't taken into account with the pressure calculation is water vapor in the ET air and the coolant expansion. The water expands by 3.8% (about 1/3 of a quart) when heated from 20C to 100C, so that could conceivably almost fill the ET depending on its size. It could be a very major factor. I need its dimensions, specifically air volume at min and max. We could find this, by topping off the ET with distilled water, then removing it into a measuring device until the max level, take a reading, then continue to the min. That is the easiest and most accurate way. If it so happens that the air volume is cut in half by the expansion of water, then we will find the system actually runs at around 2 bar. If this is the case, then I would recommend not filling the ET to maximum to reduce system pressure and extend ET life.

The most complex part of all this is the mixtures that are present. We have 50/50 water and antifreeze for our fluid mixture and a mix of water vapor/air/antifreeze vapor in the ET buffer volume.

Well... Assuming you start at 20ºC and end up at 95ºC the final volume would have to be ~42% of the starting volume to reach 3 bar. So if the coolant expanding by about 300mL causes the system to reach that point, that would mean the air space when cold is about 515mL. Which I suppose sounds reasonable enough.

So assuming the system is capable of reaching 3 bar - the question would now be why BMW wants such high pressures. Maybe it's due to a potential for cavitation, maybe it's due to a possibility of boiling coolant at localized hot spots. Or maybe it's something I can't think of.

But assuming any of those risks are legitimate, a better solution might be to use a lower temperature thermostat in conjunction with a 1-bar or 1.4 bar cap. From what I've read, the higher operating temperatures were chosen for emissions purposes.

The tags on this thread make me lol.

Originally Posted by TerraPhantm

Well... Assuming you start at 20ºC and end up at 95ºC the final volume would have to be ~42% of the starting volume to reach 3 bar. So if the coolant expanding by about 300mL causes the system to reach that point, that would mean the air space when cold is about 515mL. Which I suppose sounds reasonable enough.

So assuming the system is capable of reaching 3 bar - the question would now be why BMW wants such high pressures. Maybe it's due to a potential for cavitation, maybe it's due to a possibility of boiling coolant at localized hot spots. Or maybe it's something I can't think of.

But assuming any of those risks are legitimate, a better solution might be to use a lower temperature thermostat in conjunction with a 1-bar or 1.4 bar cap. From what I've read, the higher operating temperatures were chosen for emissions purposes.

Yep, you pretty much answered all your questions. BMW, and just about any current car manufacturer will run as high a coolant pressure as they can, because less chance of cavitation, ability to run hotter engine temperatures, a greater margin of error in case of an overheating situation, and so on. You get a lot of benefits, with no downside other than the cost of slightly higher pressure rated cooling system parts.

Running a hotter coolant temperature does a lot of things as well. Better emissions, better gas mileage, and also increases the efficiency of the radiator as you have a greater temperature difference so heat transfer is faster, which reduce emissions and a gains little bit of power from not having the fans go on as often.

Originally Posted by mkodama

Yep, you pretty much answered all your questions. BMW, and just about any current car manufacturer will run as high a coolant pressure as they can, because less chance of cavitation, ability to run hotter engine temperatures, a greater margin of error in case of an overheating situation, and so on. You get a lot of benefits, with no downside other than the cost of slightly higher pressure rated cooling system parts.

Running a hotter coolant temperature does a lot of things as well. Better emissions, better gas mileage, and also increases the efficiency of the radiator as you have a greater temperature difference so heat transfer is faster, which reduce emissions and a gains little bit of power from not having the fans go on as often.

I wouldn't worry about radiator efficiency - S54 has an 82 degree thermostat and cools just fine. In fact, the Z4M has the same radiator as the automatic 330i. The hit to gas mileage could be there, but my gut feeling is it'd be a pretty small hit. I don't really care about emissions.

And I do think there is a downside to higher pressures - the exploding expansion tanks. That's not really an issue on the M3, so I'm not going to consider running lower pressures. But if I still had a 330, I think I'd consider going down to 1.4 bar and a lower temperature thermostat

Originally Posted by TerraPhantm

I wouldn't worry about radiator efficiency - S54 has an 82 degree thermostat and cools just fine. In fact, the Z4M has the same radiator as the automatic 330i. The hit to gas mileage could be there, but my gut feeling is it'd be a pretty small hit. I don't really care about emissions.

And I do think there is a downside to higher pressures - the exploding expansion tanks. That's not really an issue on the M3, so I'm not going to consider running lower pressures. But if I still had a 330, I think I'd consider going down to 1.4 bar and a lower temperature thermostat

You may not need the extra cooling, or emissions, or gas mileage, but you can't use that as an argument to discount that BMW needed to do this for essentially a world market car. People are going to look at the gas mileage in the dealer parking lot, some cars are going to be stuck idling in traffic in 110°F weather with AC on, and BMW did aim for a pretty high standard, and that's why the US market non-Ms at least are classified as Ultra Low Emission Vehicles.

The exploding expansion tank isn't a problem of the pressure, it's the problem of a poor expansion tank design. Doesn't the M3 have the exact same pressure, but almost non-existent expansion tank failures? Notice the fittings aren't failing, nor the hoses, nor radiator, nor any part of the block. JUST the expansion tank.

Originally Posted by lcoleman

The tags on this thread make me lol.

I put the criclejerk one. Callin myself out haha