Extra oilcooler - in series or parallell?
03-12-2004, 05:15 PM
#16
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having spent the better part of my youth in the convective heat transfer arena, i can tell you there is no short answer.....and the real answer you are looking for can be had from the installation of a couple of sensors and guages on both sides of the system.... the other aspect of this is that in parrallel you could have more flow thru one cooler depending on the design of the lines (you could prove that to yourself with guages on both sides of both coolers).....regardless of your choice, to get the best effect from each cooler, all the lines should be connected to the top of both coolers thereby eliminating any air pockets/short circuit....
03-12-2004, 05:42 PM
#19
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This is an interesting thread, and although I'm sure there is an answer, I'm also sure that I don't know it. But one other aspect that should be considered is that heat is transferred most efficiently when the difference between the oil and the air is greatest. This means that if the oil is partially cooled by the first cooler, and then goes to the second cooler, and the flow rates are the same, the first cooler will cool the oil more efficiently than the second will. So, this factor would favor the parallel set up, in which the hottest oil is passing through the coolers.
When I get my Cargraphics cooler (soon, I hope), I'll put them in series anyway!
Chuck
When I get my Cargraphics cooler (soon, I hope), I'll put them in series anyway!
Chuck
03-12-2004, 07:09 PM
#20
Originally posted by Chuck Price
But one other aspect that should be considered is that heat is transferred most efficiently when the difference between the oil and the air is greatest.
But one other aspect that should be considered is that heat is transferred most efficiently when the difference between the oil and the air is greatest.
The question is, do I dare!?
/Erik
03-12-2004, 08:43 PM
#21
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Originally posted by Mike_A
the other aspect of this is that in parrallel you could have more flow thru one cooler depending on the design of the lines
the other aspect of this is that in parrallel you could have more flow thru one cooler depending on the design of the lines
03-12-2004, 09:12 PM
#22
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Bear with me as I think this one through...
It seems to me that one cannot assume the flow rates through both coolers in a parallel setup will be identical. How oil flows though each of the coolers After it has split off the main line depends largely on pipe lengths, turns, elbows, and the differences of such fittings between the two coolers. If one can guarantee that the plumbing after the split to the two coolers will be of identical fittings and lengths, only then can one reasonably assume flow though the two coolers will be similar. The worst-case scenario is if one cooler has flow that is considerably less than the other (say a few extra elbows in the plumbing constricting flow relative to the other). Since fluid travels to the path of least resistance, oil will struggle to get through this cooler and may even be quasi-stagnant as it flows through the other one freely, thus almost entirely defeating the purpose of having two cooling mediums.
Also, if the two coolers are pancaked atop one another, then even if they are plumbed in parallel, the front cooler will probably always bear the lion's share of the cooling since it gets cool fresh air (thereby offering the greatest temperature differential) but the oil cooler behind is getting the warmer air that the front cooler has exhausted (thus a narrower temperature differential). If this is, indeed, the case, then parallel coolers aren't really any "more efficient" since the second cooler will ALWAYS get warmer air and cooler oil than the front cooler.
OK, so while I am no engineer, these are my speculations based on my limited understanding of mechanical things. Ultimately, the answer is in actual testing, as Mike_A had stated. One obvious point, though, is that best cooling will be obtained though two coolers mounted independently of each other, and each with their own dedicated airflow...not possible in a basic 993 setup. And even if one were able to do such a thing, whether series or parallel is more efficient is still a question that can be answered by testing out the systems...all else is just speculation. It's an interesting thread, though...too bad no beer accompanied it.
Edward
It seems to me that one cannot assume the flow rates through both coolers in a parallel setup will be identical. How oil flows though each of the coolers After it has split off the main line depends largely on pipe lengths, turns, elbows, and the differences of such fittings between the two coolers. If one can guarantee that the plumbing after the split to the two coolers will be of identical fittings and lengths, only then can one reasonably assume flow though the two coolers will be similar. The worst-case scenario is if one cooler has flow that is considerably less than the other (say a few extra elbows in the plumbing constricting flow relative to the other). Since fluid travels to the path of least resistance, oil will struggle to get through this cooler and may even be quasi-stagnant as it flows through the other one freely, thus almost entirely defeating the purpose of having two cooling mediums.
Also, if the two coolers are pancaked atop one another, then even if they are plumbed in parallel, the front cooler will probably always bear the lion's share of the cooling since it gets cool fresh air (thereby offering the greatest temperature differential) but the oil cooler behind is getting the warmer air that the front cooler has exhausted (thus a narrower temperature differential). If this is, indeed, the case, then parallel coolers aren't really any "more efficient" since the second cooler will ALWAYS get warmer air and cooler oil than the front cooler.
OK, so while I am no engineer, these are my speculations based on my limited understanding of mechanical things. Ultimately, the answer is in actual testing, as Mike_A had stated. One obvious point, though, is that best cooling will be obtained though two coolers mounted independently of each other, and each with their own dedicated airflow...not possible in a basic 993 setup. And even if one were able to do such a thing, whether series or parallel is more efficient is still a question that can be answered by testing out the systems...all else is just speculation. It's an interesting thread, though...too bad no beer accompanied it.
Edward
03-12-2004, 10:12 PM
#23
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If it were me I would put them in series for the reasons stated by Edward. The oil flow rates are not predictable in the parallel setup.
At least with the series setup you are sure to be getting the benefit of both coolers, even if the second one is less efficient.
At least with the series setup you are sure to be getting the benefit of both coolers, even if the second one is less efficient.
03-12-2004, 10:28 PM
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kim....sorry, didn't mean to confuse the issue....think of it this way.....if you have , say, 4 gallons/min going in to and out of the parallel system, one branch could be at 1.5 and the other at 2.5 gpm.... all things being equal (they are not) oil out of the low flow branch would have lower temp... how much lower is hard to compute.....also, in this application it may not be that critical.......note that it is the temperature of the COMBINED flow back to the engine that is in question, parallel vs series......in theory, if you could insure that the hydrodynamic head losses in the 2 branches are exactly the same , you can achieve greater heat loss across the 2-cooler system....as a practical matter that is difficult to attain in a production car...... but with care you may be able to get close enough to make a noticeable difference in the resultant temperatures......if you really need that last bit of delta T, you just gotta plumb it and test.....i would opt for the series installation for simplicity and, if i needed more temp reduction , go to larger coolers
03-12-2004, 10:33 PM
#25
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Many correct points have been made:
Since the highest temperature delta nets the greatest heat loss, parallel is the way to go.
All racing air cooled 911's, except the GT2, are in series.
You guessed it, the GT2 is in parallel, and has larger coolers.
Unequal flow probably is not an issue. The design of the oil system put the lines on the right side of the car. The left cooler is guaranteed to have more flow restriction due to the extra line length. I'll bet all these effects cancel out when you consider the viscosity of the oil which changes with temperature.
The total flow restriction is not important (within reason), since the oil cooler flow is handled by the scavenge part of the oil pump. The pressure pump never sees restriction from the lines or coolers.
One interesting and nasty habit that 993's have:
If you have dual coolers plumbed in series, and the oil thermostat happens to open when the engine is at 7000rpms, it is very likely that the pressure surge from the pump suddenly trying to pump cold oil out of the coolers will blow the oil lines out of the housing below the oil tank.
Chris Cervelli
Premier Motorsports
Since the highest temperature delta nets the greatest heat loss, parallel is the way to go.
All racing air cooled 911's, except the GT2, are in series.
You guessed it, the GT2 is in parallel, and has larger coolers.
Unequal flow probably is not an issue. The design of the oil system put the lines on the right side of the car. The left cooler is guaranteed to have more flow restriction due to the extra line length. I'll bet all these effects cancel out when you consider the viscosity of the oil which changes with temperature.
The total flow restriction is not important (within reason), since the oil cooler flow is handled by the scavenge part of the oil pump. The pressure pump never sees restriction from the lines or coolers.
One interesting and nasty habit that 993's have:
If you have dual coolers plumbed in series, and the oil thermostat happens to open when the engine is at 7000rpms, it is very likely that the pressure surge from the pump suddenly trying to pump cold oil out of the coolers will blow the oil lines out of the housing below the oil tank.
Chris Cervelli
Premier Motorsports
03-12-2004, 10:41 PM
#26
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Originally posted by Premier Motorsp
One interesting and nasty habit that 993's have:
If you have dual coolers plumbed in series, and the oil thermostat happens to open when the engine is at 7000rpms, it is very likely that the pressure surge from the pump suddenly trying to pump cold oil out of the coolers will blow the oil lines out of the housing below the oil tank.
Chris Cervelli
Premier Motorsports
One interesting and nasty habit that 993's have:
If you have dual coolers plumbed in series, and the oil thermostat happens to open when the engine is at 7000rpms, it is very likely that the pressure surge from the pump suddenly trying to pump cold oil out of the coolers will blow the oil lines out of the housing below the oil tank.
Chris Cervelli
Premier Motorsports
My mechanic has warned me about this several times, never happened to me or anyone else I know thankfully. I just keep a watch on it and shift at 5k rpm or so until the thermostat opens.
03-12-2004, 10:51 PM
#27
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So, the Cargraphics cooler, which DOES allow for a cooler on each side of the car, and thus with separate cool air sources, should be used carefully, and the engine should be warmed before really getting on it. This should be normal engine conservation management, I think. I never take my engine above 4000 rpm until the oil is warm. But the warning is especially important if oil coolers are in series. And I also agree with the stipulation that the left sided cooler will have more resistance due to the extra several feet of hose. So for it to receive much oil, it should be in series.
Chuck
Chuck
