docmirror

Couple of us have been chatting about that setup already. The holdup is mostly in finding a exchanger that will fit in the fender well. It's easy to understand why exchangers want to be thin, but very large frontal/surface area. A box, or rectangle echanger is just not as efficient because the coils in trail of the front set of coil tubes are not carrying away as much heat, due to the adiabatic and heat soak from the front. Lets just for theory sake that we have a differential of 100F(100F ambient air, and 200F oil). The front set of coils of a box exchanger might be 85% efficient in cooling, but the next row behind will fall to some lesser value, because of the heat already drawn out from the front coil. So the second set of coils will only be 62%(theoretical example), and the set behind that only 46%, and so on for as many stacked coils to make up the rectangle box. Drawing air through with a suction fan will help, but the fact is, without frontal area, it's an ever decreasing efficiency.

I've been chatting with a company that makes custom stuff, and they do aviation coolers so they are very high quality. I don't have any costs yet, because I want to get the dimensions for the coils, and the plenum chambers yet. I'm sure it is not going to be lo-cost. If there were a boxed exchanger that was rated at 300F and 10Bar, I haven't found it. On that note, small aircraft use a sort of boxed cooler for oil, which meets the temp and pressure ratings, but the price of them is oh - my - god expensive. I might go over to Air Salvage and see if I can get a couple used ones to stack together and see what happens.

V2Rocket

doc, could you show a photo of what you mean by boxed cooler?
i'm way more interested into oil coolers and heat exchangers that one probably should be...but haven't heard that term before.

docmirror

https://www.aircraftspruce.com/catal...CABEgLCH_D_BwE

5 pass(row)

https://www.qaa.com/products/aircraf...ote-oil-cooler

Just google 'airplane oil cooler', you'll see plenty of examples. They are not tall, wide and thin they are just like a loaf of short bread.

V2Rocket

ah...
hint, 70s-80s era GM evaporator cores are the same form factor and probably size range, and are dirt cheap...and have long been used by budget racers as oil coolers.

http://forums.pelicanparts.com/porsc...il-cooler.html

docmirror

From what I've seen of the fender inner well, those are too big. It might go in there in some kind of angled, or sideways fitment, but it's going to be tight. I'll wait until I measure some thing before going forward. I know the aircraft ones will fit in there.

polecat702

What's really needed is a Shell & Tube heat exchanger. They're small & compact. Run the oil thru the tubes, and coolant water thru the shell. It could be plumbed and sit it anywhere there is space. We use them in the oil refineries, all the time. Except they're generally plumbed to the chill water system, where production equipment needs it's lube oil cooled, so it doesn't trip out the breakers, from overheating. Trust me they come in all sizes. This is the best heat transfer method. It'll work just like an extra in radiator cooler. I'm bout half in the bag, so forgive me for being slow tonight.

There's one on Ebay, 85,000 BTU, for $139.00. It'll cool the hell outta the oil!
I could probably fab up a custom one, that would fit perfect under the radiator like the air coil coolers. I'd just need an oil cooler outta one of the radiator tanks. Or I could get real creative, and make one like the fuel cooler, where it uses the Freon, from the AC. I figure a 45K BTU exchanger would drop the oil temp by 30 degrees. Our radiators are so efficient, it's possible.

FredR

Whereas an S&T cooler will cool the oil nicely it does not solve the basic problem because the cooling water then needs to be cooled and that can only be done with an air cooler which in turn is more problematic because the cooling water being at a lower temperature has less driving force so in effect it is simply kicking the can down the road as it were. That or use the cooler in the radiator [that is an S&T cooler of sorts] but then the concern about coolant system heat transfer capacity re-emerges. The stock system can be made to work with a few minor mods up to 40C ambient temperatures after that the system starts to struggle until it reaches 44C and then it goes downhill rapidly.

The only way the oil temperature can be controlled with a separate air cooler is with a fan mounted system. I manage this problem in my 928 by avoiding rush hour traffic and extremely hot ambient temps but then I can do that easily. In big metropolitan areas like LA or Houston such cannot realistically be avoided if one lives there and has to run the car as a daily driver. If I were running a 928 in such conditions for sure I would use a 20W50 synthetic oil as that can better withstand the higher oil temperatures. In a perfect world Porsche should have fitted a coolant temperature indicator to monitor the temperature of the coolant as it returns to the engine- ironic given that the measurement exists within the car just that it is not displayed anywhere. I have often wondered about fitting indication of both the oil temperature and the coolant return temperature as that will tell us a lot more about what is really going on when driving on those extreme days and thus better informed to understand what needs to be improved. I have a couple of analogue gauges to fit if I could get round to it one of these days.

Bottom line Porsche stuck the oil cooler under the radiator on the later models for a reason and clearly it was not to save $$$'s. One presumes they felt that radiator cooling capacity was the limiting factor and that an external air cooler was the lesser of two evils as it were. I know for a fact that the earlier S4's ran to the red line on hot days here. My 90 S4 would run a tad over the last white line suggesting a temperature out of the engine of 101C. Redline water wetter enabled the 90S4 to run a tad cooler and nowadays sits to the left of the last white line under most situations suggesting a coolant temperature out of the block of about 97C. The other thing i noticed was that the motor feels best at such temperature.

Finally, if the oil pressure drops too low we know this is because the viscosity of the oil has dropped and with it the resistance to flow. What we do know is that the oil flow rate has not changed but what we do not know is how that flow is distributed. My presumption is that more oil flows through the mains and big ends and if correct this means that less oil is spraying onto the cams and thus GB's comment about cam lobe wear being more notable on later models should be rather worrisome noting that those models with higher lift and/or stiffer springs logically will suffer more than those with say S4 cams. .

polecat702

Fred, I'm in Las Vegas, my 89 is supercharged, and it never overheats in traffic. I did replace my radiator with a factory replacement 4 or 5 years ago. Mine also has the AC unit in the rear. I did some welding and fabrication work, for a company that builds custom refrigerated vans, for food distribution. These vans use a refrigeration unit that can pull the cargo box down to 15 degrees in the Vegas summer heat over 100 degrees in 12minutes. And that's taking the temp off the aluminum floor, not air temp in the box.

The owner is a wizard at refrigeration, he designed and builds these systems in house, he changed the refrigerant in my car, and it blows ice cubes, freeze you're butt outta the car, and mine is black. He told me he could design a thermosyphon oil cooling system, to run off the AC. I'd just have to fab it up. It's a simple system, sorta like the oil cooling system used on screw compressors.

FredR

If everything is working as it should you should not have any overheating issues at 100F. The original issue was whether the external cooler fitted to later models [90 onwards?] works adequately which I believe is not the case at 40C temps in slow moving traffic.

I envy your "ice cubes"- my a/c system does ok but I have no choice but to run with R134a and I feel that costs a little a/c performance- still it is generally acceptable but again slow moving traffic and hot temps it most certainly struggles somewhat. What refrigerant did your friend fill with?

Difficult to visualise how a thermosyphon oil cooling system integrated into the a/c system can possibly work in a 928 given there is no headroom available worth talking about.

docmirror

This is not heat removal from the circuit, just heat transfer load from the oil to the engine coolant. We already have that with the earlier cars which have the oil cooler coils in the tank of the radiator. Not to mention the added plumbing to route coolant through the shell.

Lotus has a intercooled intake which is air - to - liquid, but does not use coolant. The 'charge-cooler' has its own pump, radiator, and plumbing to cool the incoming intake charge. For a liquid to liquid, we would still need air discharge somewhere.

docmirror

Once again, this is only moving the heat load from one existing thermal mass to another pre-existing thermal mass. If the AC system were to be used to cool the oil, that heat load will still be present in the existing AC condenser. Either the AC evap interior cooling will suffer greatly, or the heat of the condenser will rise to the point where it will no longer condense the fluid back to a liquid. The AC condenser blows hot air through the radiator, no added cooling capacity is gained.

If your car is working very well in these conditions, I'm guessing you drive conservatively on hot days, and not stress the engine with long idle times, or high HP runs. But whatever you're doing to keep it cool, tell us more.

polecat702

I'll talk to Richard, over the weekend. Didn't realize today is Saturday. Lost a day somewhere. It's a bitch getting old.

drnick

jpitman2

Strikes me that when you need max cooling the exactly worst place to put a cooler is close to the bitumen that is radiating heat at a great rate - black rough surface soaking it up when exposed to the sun, then giving it back out again. Just watch for track temps vs ambient temp next time you watch a race event. Surely it would work better inside the grille area, say 1/2 to 1/3 the way up the radiator core?
jp 83 Euro S AT 57k

Thom

I did not say it was going to heat up, but the heat radiating from the core will not allow oil to flow out as "cool" as with a single pass core.