Oil pressure drop when oil cooler is hooked up

I just rebuilt the engine in my '85 XJ6. I noticed that if I use an oil filter housing that does not require an oil cooler my cold oil pressure is 60+ pounds, then decreases to 45 pounds at hot idle. If I use the same spin-on filter on an oil filter housing with the oil cooler attached, the pressure will be about 10 pounds lower at cold start up, and hot idle. I suspect there is enough oil flow resistance through the cooler system to decrease total flow downstream through the block/bearings, that the ā€˜bleed offā€™ of oil pressure at all the bearings is going to result in the 10 pound loss difference. Not to start any big controversy, but would you leave the oil cooler in the system and suck-up the 10 pound loss in pressure, or bypass the cooler? I donā€™t drive in any really hot temperatures, and I donā€™t race the car. I have cleaned and checked the oil pressure relief valve in both filter blocks, and they are clean, and I donā€™t think it is the source for the different readings.

Your pressures are great, of course you leave the cooler installed, 45 psi is plenty in all situations, and at idle you donā€™t need half that.

It could also be a relief valve as you change that with the housing. As long as youā€™re over 15 psi at idle and at 40 when you drive nothing will ever go wrong. Jag spec is 40@3000 for a healthy engine.

Pressure is measured at the filter, not sure if before or after the cooler, but Iā€˜d say it will be the pressure drop after the cooler youā€˜re seeing at the gauge. No concern.

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I am getting 45 pounds pressure for a hot idle without the cooler, but 35 for a hot idle with the cooler. I am using a direct reading gauge on my dash panel, with a copper line tapping off the block at the ā€˜plugā€™ that is the first aft point behind the filter housing. The cam oil feed line comes off the top of the filter housing. So you are right, I am reading pressure after the cooler. My Idle rpm is: 750.

Sounds good. I read 35 hot, mine was down to 20 hot, you have excellent oil pressure even with the cooler.

The oil cooler was only really needed for towing or high speed runs, more of a caravan- and Autobahn addition that increased reliability. I would think that it can keep hot oil pressure up when coming off the highway so you have more reserves now. As you know the earlier cars had nothing, it became an option for police cars first IIRC, and it was introduced really late so it is not that important unless you really use the car.
Theoretically the oil cooler will mean the oil takes longer to get hot, but who knows if that is true. It was built with the cooler so I wouldnā€™t question it and keep it original. Your pressures are good.

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While I donā€™t tow much or for about 95% of the driving I do with the car, I have a hitch on it and have towed a 5 x 10 trailer from here down to the Lower-48 and back a couple times, and a 14 foot Zodiac out to go fishing. I guess Iā€™ll keep the cooler in the system, if only for those rare times. Thanks!

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The oil cooler was fitted standard to SIII cars for good reasons. Due to their higher output more heat is produced in the head and benefits from better cooling. So, yes, Iā€™d install the cooler for sure and wouldnā€™t care about the OP loss. Looking at Maxā€™ recent bad experience Iā€™d probably replace the oil cooler hoses (which might restore a few pound of pressure as well) before putting the thing back in.

That being said, an oil cooling circuit open under all circumstances is not an ideal solution. Engine oil should reach operating temps as soon as possible and maintain it. Hence, it would be better to have a t-stat open the cooling circuit only once the oil temp has reached some 160Ā°F. But this isnā€™t standard, Iā€™m afraid, on SIII cars.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Everything is fine. I would certainly be interested in the role cooler oil plays in cooling the head, which is supplied by a tiny oil feed line. The splishy splash of a little oil, cooler by 10Ā°, will have little effect on the head that produces many thousands of watts in heat under load. This goes to the coolant.

I tried to estimate but find the following hard to believe: The XK has 150 kW of crankshaft output at full throttle/5000 rpm. ā€žTheyā€œ say about 70% is wasted as heat, so 350 kW of heat is produced while 150 kW are converted into work. Half of this 350 kW allegedly escapes via the exhaust and the other half needs to be radiated to the atmosphere. That means 170 kW need to be shed by the cooling system.

A normal home heater or hair dryer produces about 2 kW.

If 1l of petrol contains 8.7 kWh, 500 kWh total tell us a full power consumption of about 60 l / h. That checks out (30l / 100 km going uphill while doing 200).

The good thing is that this amount of power requirement is very rare :slightly_smiling_face:

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I should probably use gross bhp, letā€™s say 280 hp? But itā€™s enough as it is. Also, the 70% is probably a driving estimate, but is it more or less efficient at full power?

I found:

At a steady 80 m.p.h. nearly 16 miles are covered on a gallon, but at 100 m.p.h. one of the 10-gallon tanks would be drained in the hour.ā€ (These figures are using
imperial gallons)

Thatā€™s 40l for normal driving at 160. 60 is certainly realistic although I feel it should be closer to 70l/100km.

Realistically, using 15l/h, the petrol is good for 130 kW and thus 70%, 91 kW of heat, 45 via the radiator, and the larger part of this comes from the head which makes up half of the combustion chamber and is in contact with the hot exhaust gasses.

David,

there is some uncertainty about interdependency by water cooling capacity being reduced by the air flow restrictions caused by the oil cooler.

Leaving this aside for a moment Iā€™d strongly believe that 8 litres of engine oil kept, say, 20Ā°C cooler by an oil cooler, make a big difference to the cooling of the entire engine as they are taken through the gallery and cool the engine block while at the same time allowing the head to disperse more heat to the block and from there to the ambient air in the engine bay. The simple amount of oil is almost half of the amount of coolant in an XK engine.

As for the maths, I guess youā€™re pretty close. Cars are hogs on energy. Iā€™ve read some place that the heating or cooling capacity of a modern sedan is about as high (and needs to be!) as a one-family house built to modern insulation standards and in winter times is happy about the heat produced by the combustion engine.The bulk of the heat though is lost through the exhaust or just wafted off the engine compartment. The radiatorā€™s cooling capacity is necessary, but taking on just a part of the job.

Anyway, what on earth are you doing at this time of the year, David? You must be bored as hell to think about such questions;-)

Best wishes

Jochen

75 XJ6L 4.2 auto (UK spec)

In regular use the radiator will not care about the oil cooler, this will only begin to matter when the thermostat is fully open. Rarely is. Nothing matters until the engine gets really really hot and then the improved oil quality matters most. Sure, oil takes away some heat but in the grand scheme I canā€™t see a slight oil temperature reduction be noticeable for the head.

Besides, oil is pumped by the little oil pump at half the crank speed, coolant by a massive impeller running faster than the engine.

Merry Christmas, Tom! I know those relief valves arenā€™t very precise, so even if both are functioning, one would expect some difference between them. It would be interesting to swap and see if some of the OP difference goes with the relief valve alone.

Thereā€™s another, similar valve in the later filter block with provision for the cooler. Itā€™s also spring-loaded and bypasses the cooler if too much pressure is required to pump the oil through it. Because of that if functions as a thermostatā€“cold oil is so viscous that it bypasses cooling until it warms up. To my knowledge, no one knows whether, or if, this cooler bypass valve is partially bypassing the cooler even with warm oilā€“still serving as a thermostat. Same for the relief valve, for that matterā€“I wonder if a small amount of oil is dumped directly into the sump even under normal operating conditions.

The bypass valve is internal, but thereā€™s an external threaded plug (almost a disk) that can be removed. Then, access for a screwdriver is provided to unscrew and remove the bypass valve assembly. That provides a large internal passage between the two cooler ports. Removing the valve would allow you to directly measure the effect of the cooler and its hoses on OP.

It does sound like you have excellent OP, but remember that excessive OP takes energy and itself generates heat. Maybe the relief valve spring is too strong in your non-cooler block. Iā€™m not so lucky with my tired Series 1. I have good cold OP, but after long drives in very hot weather, the OP drops at tickover quite a bit. I have an oil temp gauge, and have noted that the OP drop is only a concern when the oil is heated upā€“which requires about an hour, depending on how you are driving. The oil gets pretty hot. I bought a Series 3 oil cooler for hot weather about 20 years ago. I will get to it. I tend to buy bits and keep them for a long time without fitting them. :slight_smile:

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Way more, by the way. And IC generators are made for use in heating/electricity for buildings.

I think it is important to understand how awesome cars are, and how much you get in such a seemingly simple package. So many people take them for granted or donā€™t appreciate them nearly enough. Theyā€™re worth every cent.

I forgot about that second valve, very cool indeed!
My engines donā€™t go over 45 psi hot, which is reached very quickly so a lot has to be dumped into the sump - no need to use/waste power for the pump, but itā€™s necessary to produce more flow so thereā€™s enough at idle.
My S1 doesnā€™t go down to less than 20 when really hot, with more than 10 I wouldnā€™t bother, itā€™s plentyā€¦?

Hi Robā€™t, Good to hear from you! I donā€™t make it down to STL much at all, or Iā€™d get together with you for lunch. I have two of the SIII oil filter heads with the oil cooler ports. One is on the engine currently, with the oil cooler system hooked up and internal bypass unmolested. I took the other head and ā€˜cappedā€™ the two cooler hose ports with a piece of heavy iron with some spacers and a gasket. I unscrewed the access cap on the bottom of the housing and removed the internal cooler system bypass valve. That way, I have one ā€˜normalā€™ SIII filter head, and one filter head that does not have any cooler restrictions at all. As an aside, I use the same SIII type of filter block for both of my '69 E-types so that I can use a spin-on filter (with the internal bypass removed and cooler ports covered over. I am also using the block heater bushing I got from you on my XJ6 if it is going to be driven in cold enough weather.

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My first reaction was that 45 psi at hot idle (750 rpms) is far too high. It implies that pressure at driving speeds is very close to, or above, relief valve opening.

Which then means that ā€˜someā€™ oil is returned to the sump without going through the engine - which is counerproductive. The bearings are supposed to bleed off oil to ensure circulation through them, maintaining the oil film and cooling the bearings. Actual oil pressure at the bearings is immaterial - it is the volume/flow that is importantā€¦

If the rebuild included new bearings; even with proper clearances they are fairly tight - which means high pressure. I suggest you bring the pressure down by using thinner oil. Jaguar specs indicate ā€˜40 psi at 3000 rpmsā€™, and excessive oil pressure is counterpoductive - which is why the relief valve valve is there!

As an aside; the oil warning lamp is triggered at some 5 psi - which means that this oil pressure is adequate for a hot idling engineā€¦:slight_smile:

The difference with and without the oil cooler is neither here nor there; if the pressure is within spec, thatā€™s the essential point anywayā€¦

Frank
xj6 85 Sov Europe (UK/NZ)

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Hi Frank, I am using 10w-40 oil. I suspect the oil pressure will come down a few pounds once I do some driving with the car to bring the oil and engine up to a little higher temp than the hot idle I get after running the engine for about 15 minutes without loading it by driving.

Classic overthinking. Great oil pressure.

I consider hot after at least 15-20 minutes of at least highway speed driving. Thatā€™s when the idle pressure on my s3 was down noticeably.

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