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Whilst transferring the heat of faster-warming coolant to oil during the warm up phase will help against general engine wear while it is "cold", it does not make sense to me to hold back the radiator's ability to cool down the cylinders to the best it could if warmer oil flows through it once oil has reached its normal operating temperature, which should be higher than the 85°C/185°F the cooling system is designed to work at.
In a set up using both the radiator and the external oil cooler I would ideally try to modify the circuit so that oil does not flow anymore through the radiator once it is up to temperature. I doubt anyone driving in bump start traffic at minimal loads may see oil temperature climbing any higher than say 110°C/230°F even in very hot climate, and at the corresponding engine speeds close to idle I strongly doubt that such an oil temperature may cause any damage to the various engine bearings.
I like Roger have a GTS and operating in Phoenix in the summertime (~120F) - can easily cause the oil to get so hot that you see oil pressure warnings. Typical scenario with the stock setup, hot day (very hot day) AC max, high speed run - then you hit stop & go traffic... The existing heat soak in the engine causes temps to climb as soon as you loose all the extra airflow cooling - and suddenly the oil cooling also basically stops - oil temps climb quickly and oil pressure drops - eventually things settle out - but this is many minutes later... in the mean time <<1bar reported pressure. The pressure may not really be so much a problem - there is probably lots of flow of very low viscosity oil - but cooking the oil isn't good for its long term behavior...
Better oil does help (AMSoil 20W50) but the switch to dual air/oil stock + radiator side tank really does help. Engine coolant temp is not an issue for me - so the radiator has avail capacity (I have a non-stock deeper Aluminum radiator).
Its a real issue on the GTS at least - in any really hot operating environment. I believe a suitably sized dedicated air/oil cooler with on demand (temp switch) fan assistance for 0 airflow cases would be a better solution, however on a stock 928 - where to get adequate passive airflow?
Would it be feasible to put a fan that blows are across/through the oil cooler?
(be it the stock oil cooler or the pretty version Carl F is making)
Perhaps routed in a sorta-mirror image of the alternator cooling hose, but with a fan to pull air from the right side wheel well and blow it across the oil cooler?
(I assume that would be a LOT of cooling hose and maybe not flow much...)
Or perhaps a couple of small electric fans, thermostatically activated, hooked to the back side of the oil cooler?
You guys in the hot parts if the country need to think about the benefit of loosing 100 degrees of front A/C condenser heat.
On a 120 degree day, with the A/C running, the air that the radiator "sees" is ~220 degrees. Pretty tough to keep anything cool, with that "incoming" temperature.
Even if the precision of the oil pressure gauge may vary from a car to another it may still be useful to establish even some loose correspondence between the readings of the oil pressure gauge and some external oil temperature gauge.
I usually wince when the oil pressure at hot idle dips below an indicated 3 bar on my 951 and that corresponds to an oil temp of around 110°C/230°F in the sump with ambient temperatures around 35°C/95°F, but your feedback goes in line with what I observed recently spending time with a stock '87 S4, even running it in far lower ambient temperatures than you both, around 10°C/50°F. Even if both engines share similar design features the V8 obviously generates a lot more heat than the I4, but a V8 engine generally seems happy enough to generally run at lower oil pressures at idle, as I recently observed on a fresh LS7 running 1.5 bar at idle with an oil temperature of 80°C/175°F, and this is an engine where the dry sump and the massive oil cooler placed in front of the radiator should keep oil temperature way below what is usually seen on a stock-ish 928.
Knowing the actual oil temperature, and at idle it should be the hottest in the sump, should tell if oil gets cooked. However, I assume everyone on here use superior quality oil that is arguaby better than the oils available back when the engine was designed so I doubt any oil cooking could take place when the engine is running at speeds close to idle with insignificant load.
Alternatively, some 951 folks have successfully used a 964/993 oil cooler as a significant upgrade over the standard undersized unit. It does take a lot of room though, but once fitted it can make good use of the inlets in the front bumper cover. I am unsure if such a unit, coupled with its dedicated cooling fan as used on the 964/993, would ever fit behind a 928 bumper somewhere ahead of the front wheels, and as the lower openings do not go beyond the line of the side members like they do on a 951 additional openings may be needed (something along the lines of intercooler feeds on Kuhn's twin turbo set up).
You guys in the hot parts if the country need to think about the benefit of loosing 100 degrees of front A/C condenser heat.
I have no problem keeping my engine (GTS) cool in the hottest situation. So possible solutions to the hot oil and low pressure are ----
1) Add fans to the below radiator cooler
2) Move the oil cooler to the front lower part of the radiator so air is being sucked through it when stationary.
3) Consider the GB additional AC condenser in the fender.
Roger & Alan - do you monitor oil temperature?
I do not but may consider it. However a friend here in DFW with a 93 GTS did and I will ask him what temp he was seeing in the worst case scenario. He also was having the low oil pressure issues.
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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."
You guys in the hot parts if the country need to think about the benefit of loosing 100 degrees of front A/C condenser heat.
On a 120 degree day, with the A/C running, the air that the radiator "sees" is ~220 degrees. Pretty tough to keep anything cool, with that "incoming" temperature.
I work on the basis that 44C [110F] is the practical limit for running the 928 with the a/c operational and even then it needs to be moving some [as in highway speeds] for the car to be comfortable. In traffic at those temps the a/c is non too clever and my idle oil pressure starts to dip just below an indicated 2 barg. I have run the car at 49C [120F] and felt uncomfortable doing so.
When I studied the 928 cooling system way back when, the interesting thing I concluded was that it does not make any difference what power the motor is making, the amount of heat it needs to remove from the motor to keep control of the situation is pretty much constant even though that seems counter intuitive. This conclusion probably holds well given some your motors can and do make 500 BHP and as I am aware you do not have any additional cooling capability. For sure the a/c system will benefit from additional surface area- I reckon the condenser is OK up to 38C [100F] - beyond that it starts to struggle.
Run the car flat out as on a race track and as long as the a/c is non functional and preferably the condenser is removed I reckon they should be ok even at 44C [110F] ambient temps. Mark's car has done that albeit on one occasion he found the limit of what the cooling system could take.
On cars like Tony's that are supercharged I have a feeling that the additional heat load added by the increased temperature of the inlet air will take its toll assuming any intercooler fitted will not get rid of all the heat that is added due to compression and thus limits its working envelope some, whereas a naturally aspirated motor pumping a lot more ambient air is getting a cooling benefit from that.
I was wondering that myself. When I got my car, the clock area had been cut out for some sort of stereo setup (not a double DIN, but a second component). Rather than trying to put it back to original, I just put a couple gauges in. One is an oil temp. I see "normal' temps around 190F. Pushing it near 90 mph for any extended time will see temps more like 220 or so. 85 Euro, 16v. So I don't have the issues you guys do with the cooler.
Originally Posted by FredR
Apologies that numb-nuts here does not understand how to use the multi quote feature- I have tried and failed miserably! If this is explained anywhere much appreciate a link to it...
Fred - Click on the 'multi-quote' button at the bottom of each post. A check mark will appear in the box inside the button.
Do this in the order you wish the quotes to appear. It does not have to be in the order they appear in the forum.
On the last post you wish to quote, simply hit the 'quote' button.
That should bring up the 'post' page, just like normal quotes, but all the posts selected for multi-quote should appear too.
We consider 200 to 220 deg F pretty normal 928 oil temp, depending on the car (supercharged, NA, etc). Our race car routinely runs 240 deg F and holds steady there.
I have an oil temp gauge. Wasn't really sure where to place the sender so it is basically tapped into the oil drain plug. I figured it would measure what the oil temp is near the pick up . Is that a reasonable location? . .
This pretty typical out here. I have never seen the oil above 240 the coolant will always be like this when its hot. As you can see, this was when i was having issues with the stock set up.
I have an oil temp gauge. Wasn't really sure where to place the sender so it is basically tapped into the oil drain plug. I figured it would measure what the oil temp is near the pick up . Is that a reasonable location? . .
I hope so. that's where I put mine. Drilled & tapped a hole in the drain plug itself.
Given how quickly the oil circulates, I would think anywhere in the 'big puddle' in the oil pan would be ok.
Aftermarket part for a Mazda rotary, fitted to the 928 and sitting in open air in front of the radiator. Looks like it’s a dual pass cooler.. I’m not sure I buy into the logic of dual pass being inferior due to the oil heating back up on the return run. Not much driving in anger yet to compare but at least it’s not leaking
Aftermarket part for a Mazda rotary, fitted to the 928 and sitting in open air in front of the radiator. Looks like it’s a dual pass cooler.. I’m not sure I buy into the logic of dual pass being inferior due to the oil heating back up on the return run. Not much driving in anger yet to compare but at least it’s not leaking
Nick,
The oil is not going to heat up traversing the second pass- just not possible.
What can happen with a two pass cooler in this system is that there is a finite amount of differential pressure available across the thermostatic valve and if the specification of the cooler is not correct then the pressure drop can limit flow through the double pass cooler and should such happen then the amount of heat transferred could be less than in a single pass cooler.
Good thread. A couple of thoughts:
- Maximum engine power isn’t a general concern for the ‘problematic’ scenario (heat soak in hot conditions with no airflow) because - for NA engine - the 300 hp engine and 500 hp engine need to produce virtually the same hp output to propel the car down the highway at the same speeds. Heat exchange requirements are the same except for additional inefficiency from higher pumping loss with the presumably bigger engine. Sure, if you’ve been cruising at 195 mph in an ORR with your 600hp engine and then have to come to a complete stop with no cool-down you’ve got a bigger problem but that’s not the common problematic scenario.
- Belt-driven positive displacement superchanged motors will need more heat exchange than an NA motor of the same power due to far less efficiency at low loads.
- All the new mid and ***-engined Porsches have big fan driven heat exchangers in front of the front wheels that vent to the wells. Perhaps that’s where an auxillary HEX and fan can be located. Obviously it’s not a perhaps since GB’s add-on condenser lives there. Why can’t an oil HEX with a sucker fan live on the other side (assuming there’s already not an intercooler there?)
Aftermarket part for a Mazda rotary, fitted to the 928 and sitting in open air in front of the radiator. Looks like it’s a dual pass cooler.. I’m not sure I buy into the logic of dual pass being inferior due to the oil heating back up on the return run. Not much driving in anger yet to compare but at least it’s not leaking
did you verify that it is in fact "dual pass" not just in one fitting, down 2 inches and out the other?
have seen these for a while but no visible "divider" welds or coloration on the outside from internal welding gives me pause.
that coupled to the fact that i actually bought an aftermarket "dual pass" radiator recently, and there was no flow divider whatsoever - coolant made a straight shot from upper to lower ports without needing to pass through the core.
01-24-2019, 06:09 PM
George Layton March 2014
