Is this normal on a hot day?

…if it’s not losing ….

 

Well, that's a little apples and oranges, because a pushrod, 5.7 litre L-98 and an overhead cam 3.0 F106/105 are very different animals, but it does well illustrate that different motors have different critical temperatures. With the Corvette motor, the main concern with overheating was likely bearing surfaces--which remains a concern for LS motors to this day--not cylinder head temps. Also--as you point out--achieving 50-state legal emissions was also a factor, and catalytic converters do their best work at higher temps. Because of notoriously inadequate cooling in many C3 vettes, I suspect GM leaned heavily into giving the C4 extra cooling capacity, so they were more comfortable tuning the system for a stasis temp that was closer to critical. Third, and finally, if you're talking about indicated temps, it's almost impossible to compare two completely different engines, because the location at which temps are measures makes an enormous difference. 240 F indicated on an 87 Corvette doesn't necessarily equate to 240F indicated on a 85 Ferrari V8. Calibration and sensor placement vary. I mean, given that period Ferrari speedometers read 8-10% high, and our tachometers are usually about 100-150 rpm off actual, I don't expect the coolant and oil temp gauges to be faultlessly aligned with international standards either.

 

...And WHERE exactly is that extra energy going? Since you too have an engineering degree (undergrad? masters? phd?) I don't have to remind you about the first law of thermodynamics. That heat (energy) needs to go somewhere. Boiling reduces heat by phase transition from a more dense state (liquid) to a less dense state (gas). However, when confined in space, that same transition produces pressure (the gas desperately wants to expand). What happens when the coolant mix expands in your car's cooling circuit? Best case scenario, your pressure cap vents the excess, but retains it's max rated pressure and no hoses or gaskets blow. The boiling point remains elevated to spec, and a small amount of steam is released by the cap--taking some heat energy with it. That won't be enough to bring your coolant down to acceptable temps, but it will give you a few more moments to reduce engine temps by other means (more airflow, or less combustion) before something more serious happens. By more serious, I mean a hose or gasket rupturing, and the system pressure suddenly equalizing to atmospheric. At that point, it's like a deep sea diver returning to the surface too quickly and getting the bends. Incipient phase transition gives way to instant and wide-spread sublimation. The results can be explosive.

I totally respect the work you've done to set and automate system monitors for you engine--sounds like great stuff and much more sophisticated than the measures I've taken to enhance the effectiveness of my cooling system. BUT, I am not convinced about your cause-and-effect arguments with regard to automotive cooling system design.

QUOTE="mk e, post: 149689687, member: 253"] Engineering school followed by a stint doing cooling system design. [/QUOTE]

What *exactly* was your concentration in engineering? (Mine was aerospace control systems and digital avionics, from Naval Post Graduate School, Monterey, (not directly applicable to old cars, but I've been in the old-car fixing hobby since before I even had a driver's license.) What sort of cooling systems did you help design?

QUOTE="mk e, post: 149689687, member: 253"] Boiling [is] pretty obvious when it happens so you know its time to shut down. [/QUOTE]

It is NOT always obvious, unfortunately. Especially in a mid-engine car with a front mounted radiator and about 40 feet of plumbing. Early on, I had mild boil-over (with the old stock .8 bar cap) and I only noticed because a saw a puff of steam in my side view mirror where coolant was puking out the overboard dump pipe. The indicated temp was high, (~220) but far from pegged.

Now if a hose or fitting lets go during a boil over, you might not get any warning at all. I've seen guys drive halfway around a road course laying a line of coolant and ignoring the black flags pointing at them, and afterwards they tell the track marshall "but my temp needle never even pegged out." I tend to believe them, again, because I've seen similar things myself.

QUOTE="mk e, post: 149689687, member: 253"] I'm not saying you want to boil the system, but it is better than letting temps out of spec which tends to ruin everything. [/QUOTE]

Let's just agree they are both bad and should be avoided. Incipient boiling is one thing, a full-one runaway transition of the water in your coolant into high-pressure gas is another. The former is not going to tear your engine up, unless it was already on its last legs. The latter can total your engine very, very quickly. At that point, you won't care how quickly heat energy was dispersed into the atmosphere, because you'll be shopping for new cylinder heads.

 

It's leaving the system as escaping steam.

Draw a circle around the car. At first the only thing crossing the boundary is cold air in and hot air and exhaust out, all the heat( thermal energy to be correct) is carried out in the hot air and exhaust. Once it boils over steam is also leaving, so a lot more thermal energy exiting.

Boiling is definitely not what you want, but it does maintain the temperature in your engine exactly like it does on you stove top where the water heats to 212F then begins to boil and remains 212F no matter how hot the burner is....the steam carries the energy away.

 

sorry, at the track today answering when I can.

on race cars ip's pretty standard practice to add a coolant pressure sensor and tie it to a temps pressure table to flash a warning light. I don't do track events with my car so never bothered but did have them on the all the race cars I built/work with.

On the street overheating in traffic, it's usually obvious. Sad, but obvious.

Edit: newer cars all have level switches which solve the problem of not knowing, pretty easy to add. I don't like them on race cars because of soshing, but fine for street use.

The talk of high pressure explosions I'm a little confused about. The radiator/cooling system cap is a pressure blow-off valve design to prevent any severe over pressure. I guess in theory if you could somehow boil all the water at once it would be very bad, but that's not possible and the size of the cap is designed to be addaqute to prevent system over pressue.

I think you might be making way harder than it is. This stuff is all thermo1 type math. The hard stuff is getting uniform cooling across surfaces with unequal heat flux and around all the corners and flow obstacles and preventing localized boiling in a system that is within normal ranges....it can be diabolical hard. Just doing the math on getting the heat out and sizing the blow-off though is pretty quick and easy.

The simple answer is the 308 radiator is about 30% too small and the fans move about 30% too little air to have a nice stable system....so fan and radiator upgrades make a lit of sense on a car that gets driven. I personally would start with the radiator but I get trying fans first because they are easier/cheaper. Once the cooling system is stable, a higher pressure cap should reduce false alarm weeping which is why I run one.

 

To the OP. I had an 84 308 QV years ago which ran slightly hot on a summer day. Two things helped - I opened the brass knob on the top right side of the radiator periodically when the car was warm to bleed air out if the radiator and my shop installed a strip of dense foam on the top of the radiator to close the gap and force air through the radiator and not over it. Helped a noticeable amount. Good luck with the car.

 

Bleeding it periodically ( car parked up a slight hill so the rad cock is highest ) I understood was pretty std maintenance with these.
Also the integrity of the foam fillers , top and sides is a must .

Makes you wonder how many owners don’t do this kinda stuff ?
3#8 s are hands on cars .
With boat ownership someone told me 50 % of the “pleasure “ is actually maintaining it and the other 50 sailing the thing .
3#8 s are not quite that ratio 50/50 but unlike a modern car that’s 99.9 % driving and 0.5 % some sort of owner intervention ( maintenance) they need a certain amount of “hands on” effort .

What you say is an example of that .

 

And foam each side of the rad. I replaced mine only for it to blow out again!

 

I found some peel and stick stuff at the hardware store, like 1" wide x 0 5" thick that worked pretty well.....it was a rubbery foam.... I don't recall what is was actually for, camper to pickup bed I think? It stuck, sealed, and survived well though

 

Actually works better if the car is level as long as the coolant level in the expansion tank is higher than the top of the radiator. In my car that means higher than the seam point in the tank.

 

I never bleed mine. Once its filled correctly no need. And if you use one of the vacuum systems to fill it no need to bleed even then. Have not bled either of mine in 15 years maybe.

 

Well, yea, BUT, for every cc of water you boil (at 1bar, 250*F) you get 14 ccs of steam which is why you don't want to boil the coolant.

FWIW, it's 92 in my areas today. I just drover my 85 QV about 20 miles and then for kicks I let it idle for 5 minutes is the driveway. Every part of the cooling system is original, radiator, water pump, most hoses but the system has been flushed every 5 years. This what the water temp reached. Been that way since the car was new. If it gets above 95*F it will hit the first mark past 195. Never seen it higher.

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The steam will blow out the cap which is why there is a spring in the cap, it is a pressure relief safely valve. It makes a mess and means you need to shut down until it cools and can be topped up but it's not nearly as big a problem as letting the system go over-temperature, at least not in the short term.

Coolant running at 220 is suboptimal for the engine. Pretty normal for a stock 308 i guess, but definitely suboptimal. For sure the high end of where you want it to be.....whichbid why thats about where the original setup would boil.

I think your regular system flushes are the answer to system long-term life and performance. The corrosion inhibitions break down over time so whether the car is driven or not the coolant needs to be replaced and is often neglected. I know I'm certainly guilty of neglecting it

 

It's not that simple.

 

I promise it is, at the macro level.

The general system is simple, it has an air flow either from the fans or the motion, it has a heat exchanger of a fixed surface area, it has a boiling point, it has a blow-off valve (aka cap) controlling system pressure. Easy.

At the micro level, well not so much. The system has always steam in it that is mostly what causes the pressure to go up as it heats up. Below boiling its a balance of evaporation and condensation and above boiling evaporation far exceeds condensation. There is always micro bubbling/boiling around the combustion chamber and exhaust ports, just like a pan on the stove, you see small steam bubbles even with cold water and more and more as it gets closer to actual boiling. The system pressure varies dramatically with the cap basically setting the lowest pressure, and right after the pump could be over 30psi higher. The temperature is anything but uniform, as someone else pointed out earlier the engine sensor can read 10-20F above the radiator temp and the sensor is the mixed return water, inside the engine it can be boiling around the exhaust ports or combustion chamber which is cooled and condensed by the water flowing past the cylinders.

Very complicated at the micro level....but most of this is baked into the design. We can kind of control some of it. Going to a higher pressure cap does reduce the localized boiling and as long as you WATCH the gage and don't let it get above the upper hash mark it's probably is better. If you are likely to not likely to watch the gage then letting it boil, which is a cooling process, is probably better.

I know the factory fan switch is 195.and that puts the engine at 210-220. That is really hot. It will survive like that but that doesn't make it good. Better radiator ( there are drop in aluminum options that work much better than stock) better fan(s) and dropping the fan temp switch setting all make things better. Stock works when in good condition, better parts work better though. I run a LOT more than stock hp and a LOT better radiator. With the 500+hp V8 I ran a 180F thermostat and fan switch and a 15"? psi cap. With the 800+ engine I'm running an 18psi cap (to be honest, because that is what the local autoparts store had that would fit) and because I've had head gasket issues I've dropped to a 160F thermostat and gone from a 50/50 coolant mix to a 80/20 mix and the temp is stable between 160-170F driving or on the fan. So running cool and not giving it a second thought is absolutely possible.

 

I think johnk's picture sums it up. If the fans keep it stable at that point, sitting still on a hot day, it's working and that's with stock components. It's that simple folks. If it doesn't remain stable but continues to rise and overheats then there is something wrong. But it doesn't mean the system needs to be re-engineered or modified, it just means you need to correct the faulty component that is causing the issue.

 

You think so? I've got one, got it from Nick's Forza. You know they have to be about twice as big to equal the efficiency of the stock radiator right? The only justification for an aluminum radiator is to save weight. I like the looks of mine but it doesn't perform any better than my stock radiator did so where are you getting your data from?

 

A couple things. You said Aluminum needs to be twice as big but then said you have a (drop in?) aluminum that that doesn't cool any better, or I assume any worse? Those 2 statements don't really match right?

Copper conducts maybe 80% better than aluminum, but a radiator is more than a single material property. For 1 the fins need to be attached to the tubes, on older copper cores those fins are soldered with lead based solder that is a terrible conductor and then painted which insulates them from the air flow making them relatively poor preforming compared to aluminum where the fins are aluminum brazed and left raw. The tube shape maters as well, alum is much stronger than copper so the tubes can be made wider which again improves fin contact and also air and water flow somewhat. The result is that brand new, 2 row aluminum which is the same thickness as a 4 row copper, like 2.25-2.5 thick? cools better than a 5 row 3" thick copper.

So small advantage to aluminum when new. Over time the copper corrodes and oxide builds up inside the tubes and owners tend to repaint to make it look nice and both degrade the performance. This is why I said a couple times the OEM stuff in like new condition does work, but over time most have issues. Aluminum doesn't get the oxide layer inside and isn't painted outside so it trends to be much more constant over time....but sticking with stock and just replacing the old copper radiator with a new copper when temps get out of spec/control should get it back to where it started.

As far as getting the car to run cooler, under what conditions? I'm believing in my head that they come with a 195F t'stat, I don't know why I believe that, could be wrong...but confirming what you have and installing a cooler t'stat should reduce the temp when you're driving. Then reducing the temp the fans turn on at should reduce the sitting temp....if the fans and radiator are up to the task. Just replacing the radiator should return it to stock like performance, so 195ish on the highway and 210-220ish sitting? Its a cooling system, all the bits do something and changing the performance of the system usually means changing more than 1 item. My car runs about 40F cooler than stock, the radiator has a 21"x18" 2 row cross flow aluminum core, 160 t'stat, single 18" fan

 

Maybe it's not "twice" as big but it's about an inch and a half or two inches thicker and there are notches in the lower header so it will fit in the stock saddles. The reason it's bigger is because an equivalent sized one wouldn't match the cooling capacity of the stock radiator. So in the end it performs about the same as the stock rad but It weighs a whole lot less if that's your goal.

 

I don't think that's right, it should do as well at the same size and while I didn't design it, I suspect its thicker because its being billed as an upgrade/improvement?...and it probably is an upgrade. The classic answer for bolt-in is make it thicker, which helps some, going from "4 core" 2.25-2.50 thick to "5 core" 3" get you like 5% more and its about the same with aluminum. It only helps a little because the air is already mostly hot in that last 1/2-3/4" of thickness so there just isn't much left to gain. Making the surface area bigger (height and width) is WAY more effective but then it won't fit the stock location...I had to pull out the spare tire fiberglass stuff to fit the radiator I use.

But again, that fact that the temps didn't drop with the radiator change doesn't mean the new radiator isn't working better, it means the old radiator was meeting the system specs and it was the t'stat and fan/fan switch that were controlling the temp not the radiator. You can buy lower temp t'stats and fan switches (and maybe better fans) and when paired with the new radiator should lower the operating temp if that is your goal. At some point the radiator will be the limiting factor but until you've try changing the parts that set the temps I guess there is no way to know.

 

Stock QV T-stat. The fan switch is set to come on at 184*F. That on the exit side of the radiator. If you assume the stock radiator gives about 20 degrees of cooling that means 204*F coming in.

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Thanks John! I don't know where I got the 195 from but clearly it was nonsense, thanks for the correct numbers!

But sadly this means any QV at least running over about 170 on the highway or about 200-210 parked has radiator and/or fan issues.

 

What it means is the cooling system was inadequate from the factory and they were surprised by it. The cars never received durability testing in a hot climate. When they came out those of us in dealers in hot climates did a great deal of experimentation on the factories nickel trying to get them cooler. Ferrari finally redefined overheating as boiling over. If it wasn't puking up coolant, it wasn't overheating. To this day if one is not in a cold climate it gets straight water and water wetter.
One of the reasons I have a 328 and not a QV. A 328 is happy in a traffic jam In Riyadh in the summer with the A/C going.

 

I'll throw this out, idle rpm has a pretty big impact on coolant temp when your sitting...drop rpm 10%, drop the heat produced by 10% kind of thing. Low idle rpm can/does cause other concerns like low oil pressure or the alternator cutting out and for sure 600 it too low but 800-900 generally seems to work ok.

Losing the cat if you have one also helps as does euro ignition timing...but they were OK new so everything stock with a like new radiator should be fine, the little changes just make it more robust.

 

Reducing engine RPM will also reduce coolant flow rate and and reduce the heat transfer coefficient.