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Old 10-05-2012, 12:10 AM   #34
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Join Date: Feb 2008
Location: Philadelphia
Posts: 6,919
My Ride: M3 & ZHP
Originally Posted by dmax View Post
So first, I read that the M3 doesn't have the same set up as non-Ms but I'm not so sure about that. I know there's a hose crossing over to the ET that we don't have, but I think that's just the way it makes the same connection we have to the ET from the upper hose. Just a hose instead of it being incorporated into upper hose fitting.

My car's second ET lasted 9 years. 9 years...and probably a good 200K miles. The first ET was replaced after 2 years.

I do think ET's fail because of overfilling...and made a thread about that...forget the title...maybe 'plastic or human error'?

My window regulator on driver's door was original up until a year or two ago. They're not crappy parts...they're just susceptible to owners who are misinformed, ignorant, dirty, or busy! I think regulators probably fail more because of owners opening windows when they're frozen...or maybe had been closed a long time in the heat and got stuck. Combine that with lack of a preventive and sensual coating of gummi pflege...what do you expect?

Most cars don't have an ET like we do. Jaguar does I know...not sure who else, but not everyone at all.

Our expansion tank is a thermal expansion in your home's thermal expansion tank. In conventional systems, the extra fluid leaves to overflow tank and explained. In ours, the air in ET (maybe 2 cups of it) is what buffers, cushions, the system. Air compresses much easier than water, you know, but when coolant heats, it expands and needs a place to go. Don't give it the air, it will take out an ET.

Also, have to say it doesn't bother me having a system that will blow apart before the engine does. If it was as rock solid as it seems some of you want, you'd then be moving extra pressure to where? The head gasket. No thanks, I'd prefer just replacing ET please.

Don't go hating on BMW guys. Continue on like this and your penance will be severe!
M3's ET/Reservoir has a fluid relief - if the coolant level is too high, it will simply get dumped out. Much lower chance of over pressurizing the system. In a non-M, if you overfill, then you very well can cause the average pressures to increase more than the system was designed for.

Btw the change in density of the coolant with temperature will be pretty small. Not enough to create a major effect on pressures. I'm sure it's something the engineers take into account when designing the system, but it's a tiny effect nonetheless.

Originally Posted by LeMansteve View Post
Intuition is one thing. A meaningful comparison of material properties and material performance is what's really needed to determine the best material for the job.
I think the trend of E46 expansion tanks failing so frequently compared to the reservoirs of other manufacturers' systems is pretty telling on which material has properties better suited for the job at hand.

Originally Posted by lcoleman View Post PV=nRT so temperature and pressure are directly proportional. And pressure is pressure, with or without the air in the system.
Pressure and temperature are only directly proportional if volume (and # of gas molecules) is constant. Since Dmax is talking about the coolant expanding and taking more volume, the pressure will increase at a non-linear rate (it can be calculated if you know how the density of the coolant changes with temperature).

And Dmax is correct in that the air space acts as a buffer. The less the air volume is, the larger the effect of a fractional volume change on pressure.

Last edited by TerraPhantm; 10-05-2012 at 12:24 AM.
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