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R33 gets hot only with Ac on

Blakeo
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Blakeo Rising Star

Blakeo

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I've done everything, koyorad, oem pump, oem fan. Pressure test coolant system, vacuum bled cooling system, new headgasket, head crack tested, new radiator shroud, new oem fan clutch. 

Literally the only thing left is the electric fan on the front. It may be lazy so I'll look for a replacement. 

Any other suggestions? On a 40 degree day or can reach 95 degree with aircon on if waiting in traffic for long enough. Then I turn ac off it comes back down. 

Anyone else's r33 does this? I think it always has but since I've got gauges I'm more aware of it. 

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KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted
2 hours ago, joshuaho96 said:

Is this why all the oil cooler kits on the market have laughably low thermostat crack temps like 70C? 

They work the same way that water thermostats do - i.e. they do not limit maximum temps they are designed to regulate minimum temps so that the engine is not operating below its efficiency temperature. Some are set at 70/ 80 deg C some are adjustable.

joshuaho96 Veteran

joshuaho96

Posted
Posted
19 hours ago, KiwiRS4T said:

They work the same way that water thermostats do - i.e. they do not limit maximum temps they are designed to regulate minimum temps so that the engine is not operating below its efficiency temperature. Some are set at 70/ 80 deg C some are adjustable.

I was under the impression that you want to hold engine oil temps at 100C and coolant around 90C, so a 90C thermostat seems more reasonable? Pretty much every oil cooler kit for RBs has a thermostat crack temp of 70C which is kind of ridiculous.

GTSBoy Grand Master

GTSBoy

Posted
Posted

Crack temp and hold temp are not the same thing. There's a whole mass and energy balance thing going in between the engine dumping heat into the oil and the cooler dropping it out. Just because the TS cracks at 70 does not mean that the cooler has any hope of holding the oil at that temperature. The factors at play are the "effective HTUs" of the core, which is comprised of the oil temperature and the cooling power of the ambient air flowing across it (more oil T drives HTUs up, less oil T drives HTUs down, likewise, more and colder ambient air flow drives HTUs up and vice versa). Against an ambient air T of, say 35°C, you only get ~half the HTUs out of the core at 70°C oil T as you would with 100+°C oil T. So even though you may see the 70°C crack T as being horribly low, it actually gives quite a soft start to the cooling power, which ramps up as the oil gets hotter (both because the TS opens more and as the hotter oil drives more heat transfer out of the core).

joshuaho96 Veteran

joshuaho96

Posted
Posted
On 6/14/2020 at 11:23 PM, GTSBoy said:

Crack temp and hold temp are not the same thing. There's a whole mass and energy balance thing going in between the engine dumping heat into the oil and the cooler dropping it out. Just because the TS cracks at 70 does not mean that the cooler has any hope of holding the oil at that temperature. The factors at play are the "effective HTUs" of the core, which is comprised of the oil temperature and the cooling power of the ambient air flowing across it (more oil T drives HTUs up, less oil T drives HTUs down, likewise, more and colder ambient air flow drives HTUs up and vice versa). Against an ambient air T of, say 35°C, you only get ~half the HTUs out of the core at 70°C oil T as you would with 100+°C oil T. So even though you may see the 70°C crack T as being horribly low, it actually gives quite a soft start to the cooling power, which ramps up as the oil gets hotter (both because the TS opens more and as the hotter oil drives more heat transfer out of the core).

That makes sense, and explains the trend of electrically heated thermostats. But the OEM coolant thermostat cracks at 76.5C or so and under normal driving with good cooling it shouldn't greatly exceed 85-90C. In comparable conditions I would imagine a 70C thermostat on the oil cooler to lead to oil that never gets past 100C unless you exceed the cooling capacity of the system. Oil should be hotter than coolant so wouldn't you want to run an 80C or 85C thermostat?

GTSBoy Grand Master

GTSBoy

Posted
Posted
22 minutes ago, joshuaho96 said:

But the OEM coolant thermostat cracks at 76.5C or so and under normal driving with good cooling it shouldn't greatly exceed 85-90C. In comparable conditions I would imagine a 70C thermostat on the oil cooler to lead to oil that never gets past 100C unless you exceed the cooling capacity of the system. Oil should be hotter than coolant so wouldn't you want to run an 80C or 85C thermostat?

Not really. Remember, the thermostat does nothing to cool the fluid. It just lets it through. Logically, if you covered all except for 10% of your radiator with cardboard, the fact that the thermostat cracks at 76° won't do anything to stop it from overheating. It is the cooling capacity of the cooler that determines that.

And so, the size of the coolant radiator and the size of the oil cooler core and the relative amounts of heat that need to be dumped out through these and the relative capacity to dump heat are somewhat unrelated. You put a little oil cooler on and it will do wonders, up to a point. Then it won't be able to cope. Bigger cooler will keep oil T under control until some higher load. Then it too won't be big enough.

A little perspective. I have a Neo. Stock oil/water HX under the filter, but with a sandwich plate added for oil T sensor. Aftermarket radiator. No oil cooler otherwise.

  • Neo thermostat runs about 10° hotter than vanilla 25's TS. So, just in completely normal operation, almost** regardless of the season, my oil temp shows ~90°C, which, given the oil-water HX, is pretty much the expected water temperature too. The target coolant T for a Neo is in the order of 90-92°, or something like it.
  • **In summer though, when it is really hot, my oil T will often be up at about 100°C. The oil-water HX can't move any more heat into the coolant, and the coolant is probably hotter too.
  • And when it is really really hot weather, my oil T will be up at 110°C after everything is settled down to a steady state. And I would presume that my coolant is quite a lot higher than 100°C too.
  • And when it is really really really f**king hot weather, my oil T will often peak at ~120°C as I get home after a ~30km drive from work in the afternoon. But that's fine. I'm not thrashing it. I'm just driving it.

All of the above ignores use of air-con. If I have the AC running then the worst case temps in the above list will happen at lower ambient temps.

Another mental note is that all of the above was with a different radiator than I have now. So we will have to wait until summer to find out if the new radiator is worse/same/as good.

joshuaho96 Veteran

joshuaho96

Posted
Posted
1 minute ago, GTSBoy said:

Not really. Remember, the thermostat does nothing to cool the fluid. It just lets it through. Logically, if you covered all except for 10% of your radiator with cardboard, the fact that the thermostat cracks at 76° won't do anything to stop it from overheating. It is the cooling capacity of the cooler that determines that.

And so, the size of the coolant radiator and the size of the oil cooler core and the relative amounts of heat that need to be dumped out through these and the relative capacity to dump heat are somewhat unrelated. You put a little oil cooler on and it will do wonders, up to a point. Then it won't be able to cope. Bigger cooler will keep oil T under control until some higher load. Then it too won't be big enough.

A little perspective. I have a Neo. Stock oil/water HX under the filter, but with a sandwich plate added for oil T sensor. Aftermarket radiator. No oil cooler otherwise.

  • Neo thermostat runs about 10° hotter than vanilla 25's TS. So, just in completely normal operation, almost** regardless of the season, my oil temp shows ~90°C, which, given the oil-water HX, is pretty much the expected water temperature too. The target coolant T for a Neo is in the order of 90-92°, or something like it.
  • **In summer though, when it is really hot, my oil T will often be up at about 100°C. The oil-water HX can't move any more heat into the coolant, and the coolant is probably hotter too.
  • And when it is really really hot weather, my oil T will be up at 110°C after everything is settled down to a steady state. And I would presume that my coolant is quite a lot higher than 100°C too.
  • And when it is really really really f**king hot weather, my oil T will often peak at ~120°C as I get home after a ~30km drive from work in the afternoon. But that's fine. I'm not thrashing it. I'm just driving it.

All of the above ignores use of air-con. If I have the AC running then the worst case temps in the above list will happen at lower ambient temps.

Another mental note is that all of the above was with a different radiator than I have now. So we will have to wait until summer to find out if the new radiator is worse/same/as good.

Right, if the cooler is insufficient then the thermostat crack/full flow temp spec won't be what determines the oil temperature. But for street use when ambient is like 4C I would worry about the oil never getting up to temp on the highway and just turning to milkshake.

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