Maximum sustained coolant/oil temps?

I'm just curious at what point water (or oil?) temps mean something bad is happening to the metal bits. I'm asking in the context of my Lemons car which would theoretically be running for 7 hours straight, minus fuel/driver changes. It's an all-iron Ford 250 I6 that gets real warm and stays warm.

With a 20psi radiator cap, pure water doesn't boil until like 270 degrees. Assuming I've got the rest of the cooling system able to hold that pressure then I don't need to worry about boil-off until it's way up there.

So then we need to worry about oil temps to make sure the oil's not breaking down. That's like 250-280ish max from what I see.

Assuming we're keeping oil temps in check is it safe to run with water temps in the 240-250 range? Higher? Lower? Why?

I guess my question would be....   why?

It's not hard to have your water temps down at 190 and oil at 220, so why would you run it hotter?

Lots of ducting and you should be golden.

We ran all of our lemons or chumpcars with no fan and the only time it gets hot is if there is actually a problem with the engine or coolant system.

Depends on the engine, some modern engines are made to run very hot with 220-240F operating temps. That said, if it's not one of those (and being a LeMons car, it probably isn't) I'd say 240F for a water temp is definitely too hot, most older engines should not be seeing sustained temps over 220F and 230-240F is where things can really go bad. For oil, I think 250 is a good temperature to become concerned and 270-280 is a good place to set your emergency shutdown if you have a programmable EMS.

Max oil temps really depend on the oil being used and the quantity of it in use. If you have an engine with a 4qt sump that's running near redline a lot, that oil is constantly being recirculated and never gets a chance to wick out the heat nor drop the deposits it picks up. But if that same engine has an 8qt sump, that oil has twice the time to drop out heat and deposits etc. 

Running an oil at max temp can do a lot of things to it and it's hard to make a blanket answer. 

As for water temps, while it's a cast iron block and head and they typically can handle more sustained heat, I wouldn't. Not for an endurance race. There's no need when its fairly easy to add enough cooling capacity to keep it lower.

bobzilla said:

Max oil temps really depend on the oil being used and the quantity of it in use. If you have an engine with a 4qt sump that's running near redline a lot, that oil is constantly being recirculated and never gets a chance to wick out the heat nor drop the deposits it picks up....

And has little time to de-aerate all the bubbles entrained in it. That's a huge advantage of dry sump systems that no one mentions, but I'm off-topic.

wvumtnbkr said:

I guess my question would be....   why?

It's not hard to have your water temps down at 190 and oil at 220, so why would you run it hotter?

We've struggled to keep temps down there, especially when racing in pretty hot weather. If we really stayed on it, it'd get to mid 230s pretty easily.

Especially in the past we ran typical 13-16psi caps, but on driver changes with the engine off it heated up and boiled, losing water. By the end of the day it typically ran hotter because of that.

We've got a giant aluminum radiator, and for this race we'll be adding an even bigger oil cooler as well as a bunch of hood louvers in hopes to improve airflow.

It's shocking how small a radiator can be and still handle all the heat if its inlet and outlet ducting is properly designed. Having radiator inlet ducting is useless if the outlet air is flowing into a high pressure area... it's got to be to an area that's lower than the input side, what's called the "delta P".

kb58 said:

bobzilla said:

Max oil temps really depend on the oil being used and the quantity of it in use. If you have an engine with a 4qt sump that's running near redline a lot, that oil is constantly being recirculated and never gets a chance to wick out the heat nor drop the deposits it picks up....

And has little time to de-aerate all the bubbles entrained in it. That's a huge advantage of dry sump systems that no one mentions, but I'm off-topic.

It's not an issue with modern oils, as long as you're not over the extreme edge. There's lots of advantages of larger sumps and dry sumps for sure. 

kb58 said:

It's shocking how small a radiator can be and still handle all the heat if its inlet and outlet ducting is properly designed. Having radiator inlet ducting is useless if the outlet air is flowing into a high pressure area... it's got to be to an area that's lower than the input side, what's called the "delta P".

I was thinking the same thing.  Is it possible to work on aero a little bit to try to create a low pressure zone underneath the engine compartment?  Maybe a bigger air dam.  Also, block off any holes in the core support that would let air go around the radiator.

Edit:  is water wetter allowed in Lemons?  If so, that may be good for a few degrees cooler than running straight water.

Pute water may boil at 270 degrees at 20psi, but:

There will not be 20psi in the engine block, there will be 30-70psi or so, need to measure.

The area around the spark plugs and exhaust ports will be a lot hotter than 270F no matter what the water temp is.

Just a thought experiment... imagine those hot areas are 400F.  I use this figure because I know for a fact that Mazda rotaries get this hot at the spark plugs, per Mazda's own SAE papers.  Anyway, 400F.  The water will boil there so fast that a layer of steam will develop, which will actually insulate the water from the metal.  So the engine will be overheating locally, while the water temperature is just fine.

I suspect this is why Ford uses cylinder head temp sensors and not coolant temp sensors.  They don't care what the coolant is doing, they care about the metal, and rightfully so.

People who have swapped to Evans NPG on hairy edge cooling system engines small block Chevy have noted that when towing up long hills, the coolant temp would hit 280-290, but the engine would NOT be knocking itself to death like with normal coolant and 210-220F temps under the same conditions.  The coolant is hotter because it is actually transferring the heat away from the hot spots, you see, so the engine is actually running cooler.

Endurance racing we regularly see 100C coolant and 115-120C oil for hours on end.

mad_science said:

wvumtnbkr said:

I guess my question would be....   why?

It's not hard to have your water temps down at 190 and oil at 220, so why would you run it hotter?

We've struggled to keep temps down there, especially when racing in pretty hot weather. If we really stayed on it, it'd get to mid 230s pretty easily.

Especially in the past we ran typical 13-16psi caps, but on driver changes with the engine off it heated up and boiled, losing water.

That is the hot spots in the cylinder heads talking.  Either there is film boiling going on that gets uncontrolled once water pump pressure is gone, or the boiling starts when the water pump pressure is gone.

If you want an enlightening experience, run a water pressure gauge in the cylinder head.  Probably smartest to use an electric gauge, to prevent the steam blast effect if a mechanical gauge's line breaks, but a mechanical gauge is more useful because of its reaction time.  Not only is it another neat gauge to look at, but if you see the pressure flutter or nosedive over a certain RPM, that is your water pump cavitating and losing the ability to pump coolant.  Underdriving the water pump so it does not do that at your normal racing engine speeds will get the engine to run cooler overall.

eastsideTim said:

kb58 said:

It's shocking how small a radiator can be and still handle all the heat if its inlet and outlet ducting is properly designed. Having radiator inlet ducting is useless if the outlet air is flowing into a high pressure area... it's got to be to an area that's lower than the input side, what's called the "delta P".

I was thinking the same thing.  Is it possible to work on aero a little bit to try to create a low pressure zone underneath the engine compartment?  Maybe a bigger air dam.  Also, block off any holes in the core support that would let air go around the radiator.

Edit:  is water wetter allowed in Lemons?  If so, that may be good for a few degrees cooler than running straight water.

No water wetter: 100% water only. Makes it easier for cleanup, etc.

The rad support is sealed up pretty good.

It's definitely correlated with the harder/faster we run it. The car runs fine (190-210) when it's not racing (or when our really slow driver is driving), but it creeps up over the course of an hour or so of hard driving. We're geared to run 85% of most tracks in 4th, but if shift more, spending more time at the top of 3rd it heats up more.

I guess this thread makes me optimistic that we have something to gain from airflow management.

How is the ignition/fueling situation? If you're running a lot of advance and are right on the edge, you may not hear or have other signs of knocking, but you will get a lot more heat into the oil and coolant. Fought this issue for quite a while adding cooling to an E30. All the problems were dramatically improved taking a few degrees off the WOT curve.

In reply to mad_science :

That definitely sounds like there is internal boiling if the temp creeps up like that instead of finding an equilibrium.  And the water pump may be cavitating if it runs hotter at higher speeds.

You might be able to get by with jetting a little richer to help the exhaust run a little cooler.  Airflow management definitely cannot hurt, either, and is easier to work with inside the framework of the rules you have. 

Since you can only run water, so you cannot play with different cooling mediums, I would be curious if it would run cooler with a restrictor in place of the thermostat smaller than the current thermostat's opening.  The idea is that it would increase the water pressure in the block, cutting down on the boiling in the head.  An old circle track rule of thumb is to shoot for 50psi in the heads, but this is for small block Chevys, but it is as good a starting point as any.

I have found on a 385 series engine that 75psi in the engine will push a thermostat shut, causing pressure to spike over 200psi and blow core plugs and heater hoses apart

gearheadE30 said:

How is the ignition/fueling situation? If you're running a lot of advance and are right on the edge, you may not hear or have other signs of knocking, but you will get a lot more heat into the oil and coolant. Fought this issue for quite a while adding cooling to an E30. All the problems were dramatically improved taking a few degrees off the WOT curve.

Yep it's easy to run too much advance without getting noticeable/detectable pinging or losing power, but producing more heat. Some people like to advance the ignition until they get knock or lose power, but it's better to go the other way - if you think you're close to the sweet spot, retard the ignition until you start losing power.

Pete. (l33t FS) said:

In reply to mad_science :

That definitely sounds like there is internal boiling if the temp creeps up like that instead of finding an equilibrium.  And the water pump may be cavitating if it runs hotter at higher speeds.

You might be able to get by with jetting a little richer to help the exhaust run a little cooler.  Airflow management definitely cannot hurt, either, and is easier to work with inside the framework of the rules you have. 

Since you can only run water, so you cannot play with different cooling mediums, I would be curious if it would run cooler with a restrictor in place of the thermostat smaller than the current thermostat's opening.  The idea is that it would increase the water pressure in the block, cutting down on the boiling in the head.  An old circle track rule of thumb is to shoot for 50psi in the heads, but this is for small block Chevys, but it is as good a starting point as any.

 

I have found on a 385 series engine that 75psi in the engine will push a thermostat shut, causing pressure to spike over 200psi and blow core plugs and heater hoses apart

We have a restrictor washer in place of the thermostat, which seems to work better than running a thermostat or no thermostat.

Running Holley Sniper EFI (super cheaty, but we suck so no one cares), running too rich is a different problem I'm trying to work on...

Ignition is typically ~10-12 BTDC idle, but need to check what total is.

What's a good way to know if the water pump is cavitating? We run this Flow Kooler high flow unit...maybe better off to run a stock one for such high RPMs? Now I'm off down a rabbit hole about water pump cavitation...

Oooh, nice pump!  That shrouded impeller and close tolerances should cavitate less than stock.  Not that stock is necessarily all that great.

The easy way to tell if you are cavitating is with a pressure gauge as noted above  

My direct experience with cavitation issues is with rotaries.  I noted a long time ago at track days (Nelson Ledges) that if I ran 3rd gear out to 8500 (~90mph) then my coolant temps would skyrocket.  If I short shifted to 4th and kept the revs down then it would be much happier temp-wise, albeit about 10mph slower on the back straight before the Kink   Fast forward ten years, I was fighting overheating while at rallycross with the revs kept in the 6500-9000rpm range.  Was also eating belts at an alarming rate.  I happened to find a chart by Racing Beat that actually listed the cavitation RPM for various pumps and pulley sets.  Turns out that the pump I had would stop moving coolant over 6000rpm.  Underdrove it and it was fine, could run high RPM all day long without a whimper, and belt life went from one per event to essentially immortal.  It wasn't getting a staccato load on it anymore, you see.  I actually changed the belt last month because it was cracking due to age, I think it was about ten or twelve years old.

My direct experience with Fords is that they had water pumps that worked really, really well in the sub 3000rpm range because that is where the engines lived 99% of the time, and the 1% was short enough bursts that they could rely on thermal mass to absorb heat and not coolant flow.   I never checked one of any flavor for cavitation, my problems were more related to having too much flow for the amount of outlet area.  But again, this is also for street rod type applications where it is mostly low RPM with occasional SHORT bursts upwards.  The thing about running on tracks is you go far off the pathological end of the stress curve of every system

Pics of current radiator ducting will tell the tale I believe.

You need to funnel air into the radiator and back out, not just seal it where it mounts.

As long as the coolant and cooling system can shed more heat than it absorbs, the temperature is only dictated by the boiling point of the system.  Combustion events happen around 2700F or higher, so the engine doesn't care if the coolant is 200 or 300 degrees.  Our mindset of "250 is too hot" is because we're stuck with aqueous cooling systems that will boil if they get too hot.  Where you have boiling in a water jacket, you don't have heat transfer.  Everywhere there is a steam bubble instead of liquid, the heat transfer sucks hardcore.  When you hear about a head warping from overheating, it isn't because the head reached 250 degrees, it's because parts of the head had steam pockets and were allowed to get screaming hot.

Aluminum melts at 1200F.  250 degree coolant isn't what warps it, it warps because some of the head is 250 and the parts where it's boiling it gets to three times that number.  Think of it this way.  Take a cast iron pan and put it in a 500 degree oven.  It will be perfectly fine.  Take that same pan and focus an acetylene torch on one corner of it until it's 500 degrees.  You'll ruin the pan because the rest of it is still way cooler.  Boiling doesn't cause damage because the coolant reached the boiling point, it causes damage because the spots near the boiling had momentary absence of heat transfer and got super hot.

If you're running a cooling system that boils at 250, obviously keep it under 250, but the damage caused by exceeding 250F has nothing to do with a magical 250-degree number, it has to do with the spikes in temps that parts of the engine will see when it starts boiling.

Highest temps I saw when running Evans coolant (caddy 500, all iron) was 330.  Oil temps stayed well within spec thanks to the pushrod V8 layout only sending about 1 quart to the heads.  It was a weird feeling towing 10,000 lbs up a mountain with my foot to the floor watching the temp gauge climb and peg, but there was never a ping, and it was then I realized how aqueo-centric my thinking was.  I could have gone another 75 degrees with my 5psi cap and not hurt a thing.

Long story short.... worrying about a few degrees difference between 230 and 250 when you're dealing with metals that don't melt until 1200-2700 degrees isn't the issue.  Keeping you A) safely away from boiling which causes uneven heating and B) unsafe oil temps which break down lubrication... is what you need to focus on.

As long as:

1. the coolant doesn't boil
2. the oil stays within a safe range
3. the cooling system can shed more heat than it absorbs

then you kinda can't fail regardless of the farenheit number associated with the temp

I anyone's curious (over a year later) I did eventually find a major contributor to the temp problem:

My brake light sensor (DIY bracket/switch) wasn't allowing the pedal to come all the way up. Not enough to actually drag the brakes meaningfully, but just enough that the master wouldn't release pressure as they system got hot.

So for any quick drive the brakes never get hot enough to build up pressure in the system so it's undetectable. Then after a track session the front brakes would be nearly locked up. Then the next morning they'd spin free again.

Just moved the switch bracket a bit to let the pedal come all the way up and the problem's been gone for 2 races now.

(also ditched the I-6 for a V8, but that's a separate discussion)

You might be thinking of another issue with the same car, what about the coolant/oil temps?

Oil and coolant temps came way down after fixing the dragging brakes. Basically we were just pushing against the brakes the whole time, which not surprisingly was driving temps up.

Same car, same grille, same radiator with 80 more cubic inches and 100-150 more horsepower is running like 215 max water and 230-240 oil.

That actually is surprising.  Dragging brakes should just limit your speed.  It shouldn't actually put any more load on the engine unless you weren't driving full throttle.

In reply to wvumtnbkr :

Not true. Any extra drag in the system be it mechanical/aero/etc will require more power from the engine to over come the extra drag. You will also lose top speed and take longer to accelerate to a given speed. Take the idea to the extreme and image the undriven brakes are completely locked. It would take a lot of power from the driven wheels to get the car to move even 1mph while overcoming the tire grip on the ground.