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[Cooling System Upgrade] 1-BAR Radiator Cap

80K views 219 replies 52 participants last post by  mr_y82 
#1 ·
this thread will document my experience in the trial of a 1-bar radiator cap for the cooling system on my e46. i will document coolant temperatures, any noticable benefits, any noticable differences, etc.

the cooling system is presurized to 2-bar from the factory from BMW. a 1-bar cooling system will theoretically provide more reliable cooling system performance.

i will update this thread periodically. for those of you that have any questions, you can PM me or post in the thread. i work a lot and travel frequently for business, but i will update this thread as often as i can.

thank you,

bryce
 
#54 ·
Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?

Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.

Long story short: waste of time.
 
#55 · (Edited)
It's not a waste of time and there is a very simple justification for why ideal gas law should work here.

First of all, we are only interested in the pressure that the gas exerts onto the cap in the expansion tank. This gas doesn't circulate the system. Coolant does. So it doesn't have anything to do with the system being static.

Also, volume of air is fixed if it is contained, as in the case of ET (since the coolant expands a little, the volume of the gas actually decreases by the same amount, but this is negligible). In a system with a fixed volume, increasing temperature increases the Pressure only. Directly seen from the ideal gas law.

Second, while the coolant reaches temperatures above the 96 degree celcius in the engine block, it is around 96 degree celcius in your lower radiator hose. It is likely to be higher by 10-20 degrees in the ET, but that doesn't affect the conclusion (10-20 degrees more will increase the pressure by a fraction of a bar).

At thermal equilibrium (e.g. at operating temperature), the coolant and the gas in the ET have the same temperature.

That's pretty much it.
 
#56 ·
I will agree, but only to the point that I can't refute your logic. Just so I am clear, you believe the overall pressure in the system is somehow determined or limited by the radiator cap itself? You mention that a "2 bar cap would raise the boiling point xx degrees."
 
#57 ·
Overall pressure is determined not by the cap, it is determined only by temperature of the gas (in other words, temperature of the coolant in the ET at thermal equilibrium). Since fluids are incompressible, the gas pressure is transferred throughout the system. There will, of course, be pressure differences at different parts of the system, but the average pressure is the pressure of the gas.

Cap pressure rating important, say, when you're tracking the car. In situations where you are driving above 4k rpm in 2nd-3rd-4th gears all the time, your radiator cannot keep up with the cooling requirement of your engine alone. So temperature of your engine, hence that of the coolant increases (your gauge doesn't show this, but this is what happens). Temperature increase results in pressure increases. Gas molecules start beating on the cap more and more. But since the cap can take it, overall effect is just an increase in pressure. It is not the cap that increases the boiling point per se, but the pressure of the gas that does so.

A simple example is the following: Pressure cookers cook the food faster by allowing pressure build up inside. This increases the boiling point of the water. Result is your beans cook faster at a temperature higher than 100 degrees.
 
#61 · (Edited)
^Ok, I will rephrase that the radiator will not be able to keep up with the cooling requirement alone in a high-demand situation as much as it does under normal driving conditions. I stand behind my logic for the rest of my statements, and I do believe the M54 operates below 1 bar unless you push it too much to its limits.
 
#62 ·
Well, I do agree with your facts even if I disagree with your conclusions. But, help me understand: why would a 1 bar cap help anything at all, even if the system does stay <1 bar under normal conditions? By your own logic, since neither a 1 bar or 2 bar cap would vent, nothing has changed overall. I just think that a 1 bar cap will vent, or BMW probably would've put a 1 bar cap on in the first place.

To further compound your thoughts on the radiator in high-demand situations, high rpms means more rpms at the pump, and thus more flow. But, higher rpms over a sustained period almost always means more air flow through the radiator, and thus more cooling/delta T/whatever (and lower system pressure, lower thermal capacity of the coolant, etc). Not particularly relevant with regard to cap pressure, but that's what the thermostat is for--to hold temperature as close to constant as possible to prevent venting from overtemperature or inefficiency from undertemperature.
 
#63 ·
Well, I do agree with your facts even if I disagree with your conclusions. But, help me understand: why would a 1 bar cap help anything at all, even if the system does stay <1 bar under normal conditions? By your own logic, since neither a 1 bar or 2 bar cap would vent, nothing has changed overall. I just think that a 1 bar cap will vent, or BMW probably would've put a 1 bar cap on in the first place.

To further compound your thoughts on the radiator in high-demand situations, high rpms means more rpms at the pump, and thus more flow. But, higher rpms over a sustained period almost always means more air flow through the radiator, and thus more cooling/delta T/whatever (and lower system pressure, lower thermal capacity of the coolant, etc). Not particularly relevant with regard to cap pressure, but that's what the thermostat is for--to hold temperature as close to constant as possible to prevent venting from overtemperature or inefficiency from undertemperature.
Please pay attention to the bolded statements below:

Just did a quick calculation. Apparently 1 bar cap is good enough. Take it for what it's worth:

Assuming 96 degree Celcius coolant temperature, which is pretty much the max, the pressure of an ideal gas (e.g. air) is 1.3516 atms. That means you'd need a cap capable of holding 1.3516 bars at the minimum. This would be called a 0.3516 bar cap. This would, however, increase the boiling point of the coolant by a mere 8.5 degrees (Celcius):

So %100 water as your coolant would boil at 108.5 degrees Celcius instead of 100 degrees.
%50-%50 mix would boil at 114.5 degrees Celcius instead of 106.

These numbers are ok theoretically, but they are too close for comfort in real life. Locally, temperatures could be higher than these (e.g. engine block), and your coolant will turn into gas whenever this occurs. Since gas occupies more space than liquid, you'd be replacing your hoses quite often. I am assuming the engine block can take a beating.

On the other hand, 1 bar cap increases the boiling point by about 24 degrees Celcius. So for 100% water, you'll get 124 degrees Celcius as your new boiling point. For 50-50 mix, it will be 130 degrees. Similarly, 2 bar cap raises it by 48 degrees, so your new boiling points for 100% water and 50-50 mix will be 148 and 154 degrees, respectively.

Since the operating temperature of your engine doesn't change, using a 1 bar cap will lower your chances of blowing the head gasket. You'll probably see your car smoking before the temp gauge hits the maximum. But there is more chance of introducing excess air (gas) into the system, which could require you to use heavy duty hoses in lieu of what comes from the factory.
 
#67 ·
Sounds like a bad idea. Decreasing cooling system pressure increases the chance of localized boiling and cavitation. Localized heating leads to detonation and warping. Cavitation leads to reduced cooling and increased coolant pump impeller wear.

That's a lot of pretty bad negatives just for a cheap fix for sporadic expansion tank failures.
 
#69 ·
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.
 
#86 ·
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.
 
#70 ·
I am only going to say this once!

As everyone knows raising the pressure on a liquid increase its boiling point, that is the main purpose of a pressurized cooling system.

The thing with the E46 the cooling system uses an expansion tank so the coolant is pressurized against an air pocket long before it even gets close to the 2 bar pressure cap.

The pressure release in any cap is more of a safety valve or mechanism, just look at any water heater pressure release valve.

1 Bar is approximately 15 psi, which is about where most domestic cars run that do not use an expansion tank system.

2 Bar is approximately 30 psi, which is totally absurd for any car using an expansion tank system.

Just look at all the expansion tanks that burst the minute new BMW owners check their coolant and top off the expansion tank like it is a domestic car with a radiator. This is when the pressure cap is pushed towards its limit and just looks what happens, BOOM. There is no way most of the parts in the E46 cooling system could survive 2 Bar on a daily basis. Also I would hate to have the heater core blow under 2 Bar and dump scalding hot water on the occupants feet.

You can also squeeze the upper radiator hose on the car and feel there is nowhere close to 2 Bar of pressure in the cooling system, matter of fact there is probably nowhere close to 1 Bar in the cooling system under most situations. Almost anytime the pressure cap is removed from the expansion tank there is very little pressure built up.

So until someone has connected up a pressure tester and logged the cooling system pressure under daily and track conditions and can prove these engines even get close to 1 BAR, I think all this concern over not using a 2 Bar cap is a waste of air and mental capacity, assuming there is much mental capacity to even waste!
 
#74 · (Edited)
Let's try that again. That equation is not "simplified". You cannot combine two gas equations that "easily". If you do, you'll end up with more unknowns (4) than you have equations (1). The way you wrote it applies to systems where you have 3 of those 4 unknowns beforehand. See my reply to Terra above. I hope it's clear now.
Thanks for explaining basic algebra to me like I'm a 5 year old.

No we have 1 unknown, and that is P2 in my equation. We have P1,T1, and T2 through the use of our coolant sensors and barometric pressure. There is also the assumption of constant volume, which may or may not be poor, depending on how much the water actually expands. This is where you use steam tables to solve for everything (empirically determined numbers with interpolation). I'm not writing a paper here, I thought I could skip that part of showing my work...

Good thoughts, but you need to quantify how much fluid velocity will affect it's temperature, hence its boiling point, in order to convince me. I never said my calculation accounted for everything in this system. It's not comprehensive by any means. I am well aware of a lot of other things that might affect, but I am not an automotive engineer. This is the best I can do with my current knowledge of the matter
I am one.

You're clearly a smart guy, and we both know what the other is talking about. Let's stop arguing semantics. I also see that we both enjoy math which is hilarious. I always find myself making things a bit more complex than they are, just because I want to use my math.
 
#75 ·
Thanks for explaining basic algebra to me like I'm a 5 year old.

No we have 1 unknown, and that is P2 in my equation. We have P1,T1, and T2 through the use of our coolant sensors and barometric pressure. There is also the assumption of constant volume, which may or may not be poor, depending on how much the water actually expands. This is where you use steam tables to solve for everything (empirically determined numbers with interpolation). I'm not writing a paper here, I thought I could skip that part of showing my work...
Why do you need two sets of P and T? One set is not only redundant, but will also bias the results. For example, barometric pressure P1 is not the same everywhere, and what value of T1 you choose? It is arbitrary.
I am one.

You're clearly a smart guy, and we both know what the other is talking about. Let's stop arguing semantics. I also see that we both enjoy math which is hilarious. I always find myself making things a bit more complex than they are, just because I want to use my math.
That's what I am trying to convey to you. The situation is simpler than you think it is. You only need a single temperature value to calculate the pressure in this case.
 
#77 ·
^Yep, we both got surprisingly low value for pressure at operating temperature. Makes the use of a 2-bar cap even more of a mystery in the e46. Even more so given that the e90's use only a 1-bar cap. Wonder what exactly is different between two systems.
 
#78 · (Edited)
I'm glad we have come to the same conclusion. I can think of no engineering justification for the 2 bar cap :hmm:. I also don't think a 1 bar cap will help with the longevity of the ET. I think it WILL help prevent a headgasket failure in the event of an overheat, but in day to day operation, the system never runs over 1.4 bar and that is generous. I will continue to try to justify a 2 bar cap, but I don't think I ever will.

EDIT: I may try to calculate the temperature necessary to generate 3 bar pressure to trigger the 2 bar cap. I just got over 500C for an ideal gas, but at that temperature there will be lots of steam in the ET and the water will have expanded, reducing the air/steam volume. I'd have to reference one of my books for this, though I might have a matlab program for it.
 
#82 ·
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.
 
#90 ·
I was just quoting other people in this thread. No other source lol...





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 :hmm:



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#88 ·
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.
 
#93 ·
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.
 
#91 ·
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!
 
#92 ·
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... :rolleyes:

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#112 ·
Yes, you may not allow enough expansion space for the coolant. ET will blow.

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#125 ·
Jesus, just get a pressure gauge and connect it to the bleeder port or modify an expansion tank cap. Then measure the max pressure.

All the math in the world will not answer the problem as there are a number of assumptions being made and many times either the assumptions are incorrect, inaccurate or you just fail to take something else into consideration.

This also assumes you do not make any basic mathematical errors.
 
#126 ·
No ****. So do you have a pressure gauge and some sort of adapter handy to do this? No? Do you even know what size threads the bleeder screw has (honestly that would be useful)? I can't just hook up any old pressure gauge to it. It has to be set up right and I don't really have the time or extra money to do it.
 
#131 ·
Thanks Terra, I am familiar with the ideal gas law, I was just trying to figure out what SeanC was values he was using to get his answers. Here's what I get for being mr. streetsmart without calculations. :rofl:


Volumetric expansions of coolant:

dV = V0 B (t1 - t0)

B Water = 0.000214 1/°C
B Ethylene Glycol = 0.00057 1/°C
V0 of E46 Cooling System = ~8.4L
t1 = 100°C
t0 = 0°C

Solving for 50/50 coolant mix

dV = 8.4*.5*(0.000214+0.00057)*(100-0)
dV = .329 Liters


Required Change In Volume to get a 2 bar increase in pressure:

Using a modified ideal gas law on the remaining air in the expansion tank(as so clearly explained by TerraPhantm) to find the ratio reduction in expansion tank volume:
V2/V1 = P1T2/P2T1

P1 = 1.013 Bar
P2 = 3.013 Bar
T1 = 273.15 K
T2 = 373.15 K
V1 = Volume of coolant at 0°C and atmospheric pressure
V2 = Volume of coolant at 100°C and atmospheric + 2 bar for expansion tank cap

V2/V1 = (1.013*373.15)/(3.013*273.15)
V2/V1 = 0.459 or 45.9%


So, only factoring the volumetric expansion of coolant with temperature, and the ratio of air before and after coolant expansion to reach the expansion tank venting pressure, the minimum volume of air required to prevent venting through the cap is:

V2 = V1 - dV = V1-.329
V2/V1 = .459

Solve for V1

V1 = .608 Liters minimum of air in expansion tank to prevent reaching vent pressure.
V2 = .279

checking work, .279+.329=.608, .279/.608=.459; good

There will be some expansion of the block and hoses which will decrease the volumed required a small amount, my guess would be something in the range of .1L or less.


Interpretation of results:

Thinking about this value and looking at the expansion tank, I'd guess the minimum volume is very close to to amount of air that is in the expansion tank when it's properly filled. If you go to a lower pressure radiator cap, you are going to be venting pressure every time the car heats up, and then pulling a little bit of air back in every time the car cools down, and you will lose a small amount of coolant every time through evaporation. With BMW's setup, it seems like the pressure gets very close to the the limit of the expansion tank but doesn't surpass it to prevent very slow coolant losses over time.

Also, with keeping the pressure higher, and having the minimum amount of air volume required, when your engine starts to overheat, the system pressure will increase faster to reduce boiling, and the higher pressure gives a greater safety margin before there is some serious boiling and localized heating, which creates warping. Everything could be modified for a lower pressure, but you just decrease your engine's temperature safety margins, and you will need to increase the expansion tank volume prevent losing however small amount of coolant every warmup and cooldown cycle.

Damn these BMW engineers, I think they are actually pretty darn crafty, even when it comes to something as small as an expansion tank! It's a shame they didn't make the expansion tank just a little bit stronger.
 
#135 ·
Hey guys im doing an experiment on my car where i drive around with no expansion cap on, no oil cap on, a cheeseburger in the passenger seat and the fuel filler cap not screwed in as well. I will be driving under hard and soft circumstances. I will let you guys know how it turns out.
 
#137 ·
Looks more or less right to me. The expansion of water will be slightly different since the rate of expansion varies with the absolute temperature, but those numbers are close enough for our purposes.

Not sure I agree with your conclusion though. Higher overall temperatures will only increase the risk of warping. If the boiling point was lower, the overall temperature will not exceed the boiling point until the coolant is converted to a gas.
 
#143 ·
thread is hilarious. you guys have chased OP out of here and convinced him that his thermostat was stuck open when he's trying to get some test results on an aftermarket expansion tank cap. Hope he listens!!! So much for his results holding any validity if he's got the thermostat stuck open.
 
#144 ·
:rofl: yeah if you look at it from that perspective...

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