1.4 bar vs. 2.0 bar radiator cap

[JHZR2]

Hi,

Since Im doing all the coolant replacement work on my 91 BMW 318i, I figured it good to get a new radiator cap. My new cap is stamped 200 - 200 kPa=2bar=30 psi... The old one is 140 - 140kPa=1.4bar=20 PSI.

I double checked and my car currently has the lower pressure cap, but the higher pressure cap is technically correct and spec.

Am I hurting myself running the 1.4 bar cap? Am I hampering cooling?

I do know for a fact that I get high NOX. If Im not overheating or venting steam, can I assume that the coolant inside is the right phase, and not an issue at the current 1.4bar? Will I get more/better cooling at 2bar?

My car does not let off good heat into the cabin - higher pressure could theoretically be hotter - but would this really help much?

What is the weakest link in the system to go if I upped the pressure??? Hose? heater core? radiator? head gasket? While I dont want premature failures, if the higher pressure cap buys me something good, and still is the 'weakest link', then all is well - my main concern is a ruptured heater core or headgasket.

Any insight would be appreciated.

Thanks,

JMH

 

[FowVay]

If it specifies a 2 bar cap then use a 2 bar cap. Higher pressure will reduce the bubbles in the coolant at the surface of the hot metal.

Surface bubbles (cavitation) inside the engine cause the engine to run hotter because the heat from the metal is not transferred to the coolant as efficiently. The air in the bubbles acts as a insulator instead of a conductor which insulates the coolant from the hot metal surface.

Reducing the amount of surface bubbles in the coolant is how products like Redline's Water Wetter work. (that's a tongue tickler)

 

[JHZR2]

aha, this may be why I am borderline failing for my NOX emissions on the dyno test!!!

The car has had the 1.4bar cap since I bought it... rock solid stable temperature, even in the worst traffic/heat... but that doesnt mean that it is doing the cooling system justice!

Thanks!

JMH

P.S. Ill be buying schaeffer clean & cool which is like the redline stuff, from BOB, to help out too!

 

[Shannow]

Quote:

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Surface bubbles (cavitation)

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Nope, not cavitation.
Maybe nucleate boiling, but not cavitation.

 

[Steve S]

how bout raises the boiling point.

 

[JHZR2]

right, I wasnt sure about cavitation (that is generaly an effect around impellers where you have a surface vacuum and a gaseous phase at the boundary layer), and how it pertains to the water jacket of a block... however localized boiling does make sense. There really is not a lot of info out there about the pros/cons of changing something like this (though OE IS apparently the 200kPa cap), though I did find at:
http://www.stewartcomponents.com/tech_tips/Tech_Tips_2.htm
Which was the only non-ricer "some guy giving non technical information" site that I could find ina routine search. The comment is:
"In a cooling system, a higher pressure equates to a higher boiling point for the coolant. Higher coolant pressures also transfer heat from the cylinder heads more efficiently. We recommend using a radiator cap with the highest pressure rating that the radiator is designed to accept."

OK, the higher boiling point makes sense - back to basic chemistry for that one... but the heat transfer efficiency from the heads is something that I dod not consider. If this is because of localized boiling, it actually does not reall make sense to me... evaporation is a very efficient method of moving heat, as the phase change can suck a lot of enthalpy from the surface, effectively cooling it. Heck, evaporative chilling was used by the Egyptians years ago, and is still used in process cooling towers. I guess the proble is that in developed flow, the dual-phase region stays in the boundary layer of the cooling surface, effectively killing the heat transfer coefficient further down the line.

Anyway, my biggest concern is that a higher pressure could make a head gasket weep. Is that the case? I dont know... if the radiator fails, it doesnt owe me a thing with 16 years... if a hose springs a leak, theyre original. Im concerned about majors like headgasket weep putting coolant in the oil, etc.

THanks!

JMH

 

[punisher]

Just stuff I picked up from an old engine builder who worked in Winston Cup and several other racing series over the years. He retired to Austin and I was fortunate enough to be able to pick his brain for 5 or so years before I quit the automotive field.

Cooling systems (as far as heads are concerned) are for the most part marginal designs. Water jackets are the last thing of concern after intake flow, exhaust flow and external dimensions/engine packaging are finalized. You always have areas of high coolant flow and areas that receive very little flow, eddies and stagnation points as well. Problems occur in a system when due to a lack of flow, either due to low coolant flow or stagnation/eddies, you develop a local hotspot. At this point is where you have a vapor pocket form. Vapor cannot absorb heat nearly as well as a liquid (after the phase change of course) so you have a jacket area that continues to produce heat but there is no way for that heat to make it to the coolant efficiently as it is insulated by the vapor pocket. The area grows as the heat has nowhere to go so a small problem becomes a big problem. On a cup motor with 16:1 or more compression (in the old days) this lead to almost immediate engine death due to detonation. He also said it was possible, though not probable, to move coolant fast enough to form low pressure areas and create a lot of small bubbles that attach themselves to the interior of a coolant jacket and begin the formation of vapor pockets.

As far as cavitation he said you could look at water pump impellers and it would look like someone took a beadblaster with coarse media, and blasted the back sides and edges of the impeller until they were rounded off. Usually since the bubbles were created in a very low pressure area (impeller) they collapsed in the pump area itself. He said, as far as he knew, cooling systems were running 30+ PSI of pressure, likely higher.

 

[Steve S]

Over the years I have noticed the bead blasted look and wondered slight corrosion. Makes sense .Thanks for explaining to us . I have to think autobahn when thinking BMW not German built ricer type car in U.S.A.