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Wasn't the an ATF cooler mounted there on earlier models?
Still is on 87 to 95 cars including manual 92 to 95 GTS cars. Hence my question in post #27.
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission? George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
In my scenario the additional heat dissipation brought about by the radiator end tank cooler is the difference of 7 psi to 15 psi (0.5 to 1.0 bar) in oil pressure. When at idle in GTS cars that is the difference between the red oil pressure light coming on or not.
In such running conditions what are the readings of your oil pressure gauge, for both set ups?
Nobody has spoken to the disadvantage of an oil cooler in the radiator - so I will. The temperature delta is much less. The Temperature Delta: when cooling 270 degree oil with 180 degree water, the temperature difference is only 90 degrees. When the oil cooler is in the air, even at 100 degrees (all temps in F) the difference between the two temps is now 170 degrees. The temperature delta between the two mediums is much greater, and much more heat transfer will occur. This is likely why Porsche themselves moved the oil cooler out of the radiator and into the air, IMO.
I will take mild exception to this statement. A liquid to liquid heat exchanger(oil cooler in rad) is many times more efficient than a liquid to air. Adiabatic thermal expansion in air is a common problem not often accounted for in engineering because many eng(including me, at various times) do not adequately take into the account the 1st law of thermodynamics. The most glaring example is the P-51 Mustang which uses an liquid to air radiator. The designers had serious trouble getting the engine to cool properly until the right solution was found in the air EXIT plenum, rather than messing with the air entry. I have not, and will not be doing the Carnot heat engine process to determine the limits of the 928 cooling system woes.
I will opine that Porsche moved the oil cooling radiator out of the water cooling radiator to simply offer greater cooling area/volume. As the 928 aged, the HP increased from around 180 up to 345. As we know, HP is also a function of BTU generation, expressed in work rather than heat. I will again guess that removing the oil cooler from the rad, allowed greater water jacket cooling and the added small air radiator was about the only means avail to add frontal area to the overall cooling system to dispose of the greater heat of later high HP engines. Porsche did change from a belt driven single fan to a dual electric high speed fan, and that helped some, but the stone-cold fact is that the 87-95 engines just don't have sufficient volume or area to cool the lump, and it affects both oil pressure, and engine temps. I can't recall if the early rads have larger volume, or more tubes than the early ones.
I'm sorry I don't have an answer for the challenge of heat dissipation in the 928. I once had an RV that had a serious issue with overheating, and I did a cheap but effective misting system in front of the radiator as I was carrying water around with me in the RV tank. A simple washer pump, and a couple of misting nozzles with a dash switch allowed me to mist the rad, and improve the Q of the radiator somewhat, but this is impractical for 928s which don't carry a 60 Gal water jug around under the bed.
In such running conditions what are the readings of your oil pressure gauge, for both set ups?
1 bar approx. on the gauge on my 93 GTS. About 1.5 bar on the 95 GTS. The 93 gauge is slightly different to the 95. 93 pegs at 4.5 bar while the 95 pegs at 5 bar. Inaccuracy in the gauges.
Stationary traffic after a fast run (extra hot engine) 100+ F
Still is on 87 to 95 cars including manual 92 to 95 GTS cars. Hence my question in post #27.
Originally Posted by Carl Fausett
Thanks, Fred. My "initiative" as it were, was to make the external oil cooler available when they weren't any more. That was my primary goal. The secondary goal was a while-you-are-at-it, could we make it better. The science is solid, and the new two-pass cooler will remove more heat than the old one. How much is what remains to be seen.
Nobody has spoken to the disadvantage of an oil cooler in the radiator - so I will. The temperature delta is much less. The Temperature Delta: when cooling 270 degree oil with 180 degree water, the temperature difference is only 90 degrees. When the oil cooler is in the air, even at 100 degrees (all temps in F) the difference between the two temps is now 170 degrees. The temperature delta between the two mediums is much greater, and much more heat transfer will occur. This is likely why Porsche themselves moved the oil cooler out of the radiator and into the air, IMO.
Food for thought.
I also always assumed that Porsche moved the oil cooler from the radiator to the "air only" position because they felt that they had reached or were close to the limits of the radiator maintaining temperature control.
Cooling ability of the stock radiator was a major concern when I built my first few high horsepower street engines. However, I quickly found out that the cooling ability of the stock radiator, when the horsepower went up 50% (or more) was a non issue.
I "fretted" again, when I added, back into the radiator, oil cooling on a big engined GTS, which obviously was going to increase the heat load, especially at idle, with the A/C running in So California 100 heat.
Turned out to be a non issue....no overheating, no additional high speed fan operation. The stock radiator worked fine.
My assumption about why Porsche had moved the oil cooler below the radiator was obviously wrong.
The biggest thermal change, for the radiator, I've found is my "supplementary" A/C condenser system for increased A/C function (which is very "marginal" on any stock 928.) On a 90 degree day, the temperature of the air that "hits" the radiator is 100 degrees lower......at idle!
That makes temperature gauge drop and the fans get quite a break!
I'm thinking along Thom's lines, that an engine at idle makes little heat. There is no load on the motor, and the throttle is shut. I agree with Thom that little heat soak can occur under such conditions, and that's the only time that there is no air flow over the external oil cooler. Now, I will temper that with environmental-specific concerns: the 928 owner in Texas has a different set of cooling issues than we do here in the north.
Another 928'r has just PM'd me to get one of our coolers at a discount so he can test it. He has a GTS and is in Atlanta - so this will be a good test. Love it.
Another way to improve air flow through both the radiator and the oil cooler is by using a chin splitter. This chin splitter extends back past the oil cooler, making air pass through the oil cooler because it cannot just go under it like it does now. In addition, chin splitters create a higher-pressure area in front of the radiator, improving radiator cooling too.
Love the ideas Carl but the issue is no air flow in my case so will not help me.
Currently the weather in TX is really cold at circa 30F and no problem at all maintaining 3 bar in stationary traffic 8>)
I've never had a problem maintaining oil pressure at idle, either. And we do not idle long around here. Not that many traffic lights or other reasons to.
We need to give a nod to the Porsche engineers for our oil pump. It's just terrific. It flows great volumes of oil at damn good pressures.
data point..
My car probably runs in the hottest environments here. Combined with the supercharger and my power output, cooling is an issue for me in the summer. id like to think my cooling system is up to par. (may be not). I basically baby my car in the summer...if i drive around with a heavy foot it wont take long until the red coolant light comes on. The engine just cant get rid of the heat. However, I never see the oil pressure trigger the red light. Yeah, it drops, but is at a "normal" point at those temps at idle. At highway speeds(55-70) it wont peg the upper end of the scale like it does in the cooler months either.
anywhoooo... My thinking with a cooler like this or any external oil cooler is to avoid heat transfer INTO the radiator fluid and thus increasing the coolant temps. I think I would just rather have a dedicated external cooler myself??. let the air cool my oil and the radiator cool the engine only. but..Gregs words above, with the out put of his engines, is making my think something is perhaps amiss in my cooling system?
My thinking with a cooler like this or any external oil cooler is to avoid heat transfer INTO the radiator fluid and thus increasing the coolant temps.
+1 That's good thinking. I always thought that transferring the heat from the oil into the water (and then pumping that water back into the motor) was a little silly. The idea being to get rid of the heat, not to capture some of it and pump it back in. Given the low hood profile of the 928, our radiators are small enough and fully tasked already.
Wanna test one, Tony? We'd all love to hear back from you whether it helps your issue. Just let me know.
+1 That's good thinking. I always thought that transferring the heat from the oil into the water (and then pumping that water back into the motor) was a little silly. The idea being to get rid of the heat, not to capture some of it and pump it back in.
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Again, I'm sorry Carl but you proceed from a false beginning assumption. Consider the entire heat load of the engine. All that heat load must be dissipated somewhere. A part of it is turned into mech energy to move the car(maybe 30%?), the rest is waste heat. In a perfect Carnot engine, the differential in temperatures between the heat generation, and dissipation would be extreme, and that can be modeled by operating the engine in cold climates, where the water pump, and the cooling fans have very little(no) work to do, and the parasitic drag from the alternator and water pump are minimized. However, we are operating at the opposite end of the cycle, and we still have the same, or greater dissipation needs. The oil heat in the radiator cooled setup is transferred to the liquid cooling fluid. That heat is then dissipated by forward motion, or by the fans drawing moving air through the radiator, of a combination of both. That heat load, whether it's in the radiator, or in the bottom air radiator is dissipated just the same.
The change over from liquid cooled, to air cooled was done to expand the total frontal area and increase the ability to dissipate the heat load. It is clear that the radiator is the same basic size, volume and shape, but the added oil radiator increases the overall ability to dissipate the greater heat load generated by higher HP. And still, it is insufficient in practice as anyone who has driven a 928 in anger, or Tony's example who has driven a high HP car even modestly aggressive on a hot day proves that the 928 will not be able to fully dissipate the heat load.
The only solution to this is greater thermal media, in a greater frontal area, or a much greater air flow, or a combo of all those. Roger asked if moving the air/oil radiator in front of the car would make a difference. The answer is both yes and no. At idle, where there is no airflow, the heat will be minimal, but the airflow provided by the fans will help reduce the temp. However, at speed, the total heat will be greater, but the overall frontal area of cooling avail has been reduced. Some of the waste heat from the air/oil cooler will now be transferred to the radiator behind it, thus increasing the dissipation load of the liquid cooling system. It's my opinion that this will reduce the overall efficiency of the engine, but will help in the corner case of a stopped car, with the fans pulling air. Note that in this condition, the engine BTU generation is at its minimum, and the HP needed to simply maintain the engine turning requires less overall heat dissipation.
So in closing, the heat from the oil cooler in the radiator is not captured. It is transferred from the oil, through the liquid to liquid heat exchanger in the tank, then to the cooling fluid, and finally drawn away by the movement of the car, or the fans, or both. Later versions of the 928 with higher HP increased the total area to dissipate ALL the heat by adding frontal area, and removing one heat exchange in the process.
So, by adding the remote external oil cooler, we have increased the surface area of the cooling system, and in so doing, we have increased the dissipative capacity of the entire system. Correct? I understand that the whole system has the same heat load, but now, technically, the water radiator now has a lower heat load than it did before. Do I have that right?
^^ Yes. And yes. ^^ I think the dual pass air/oil cooler is a fine looking product, and will help reduce oil temps when the car is in motion. It will have little to no effect on oil temps when stationary, unless provided with a draw or blow through fan(s).
01-24-2019, 11:54 AM
George Layton March 2014
