Mikey

I did mention this point in my original post, "these are engine coolant temps, it is reasonable to deduce that underbonnet air temps will also rise considerably, I believe." It seems reasonable to me to draw the conclusion that if coolant temps rise post shutdown, so will under bonnet temps, in an XJS body. What I do believe is that there is no substitute for measurement, which I do not have the equipment to do. But, as we are (essentially) debating and deducing rather than measuring, a couple of points:

The under hood space of the XJS is considerably more full of engine than just about any road car I can think of, and certainly any I have owned of similar (say 4 litres plus) capacity. Any racer who has raced these engines in an XJS body has had far greater problems with heat control than on similar capacity engines on other body shapes. Therefore I do not fully subscribe to the view that other cars are a guide.

Secondly, I cannot see how, if coolant temps rise after shutdown, under bonnet temps will not be expected to. You wrote: "The heat that would normally be shed quickly and efficently into the air via the radiator and unfortunately into the engine compartment while the engine is running is now conducted exclusively through the engine exterior, also heating the engine compartment. The net effect is the same."

This does not seem obviously true to me, as when running, ambient air is entering the engine compartment from other sources besides the radiator outflow, particularly so in the cars of those of us who have dispensed with the rad-surrounding foam. The fact that extra heat is not being generated, does not imply a period of elevated under bonnet air temps will not occur. It must be a question of balance between (i) heat output from the engine, radiator stack and ancilliaries, and (ii) heat loss from the closed engine compartment. My contention is that this balance in an XJS is in favour of elevated under bonnet temps for a period. Perhaps someone will measure it on a 90°F day?

Finally, obviously this is a hot day issue, in the winter the balance between heat loss from the engine compartment and heat entering it from the engine etc, is (quite clearly in my car's case) different; because the heat gradient between the ambient temp and the under bonnet temp is much greater. Even I do not pop the bonnet when there is frost on the ground!

Greg.

 

That's where we disagree. Such an assumption is not taking into consideration many other facts, primarily the tremendous volume of heat being rejected continuously from the radiator squarely into the engine compartment and it's components. The passing airflow HEATS the engine compartment, not COOLS it.

This effect ceases immediately when the engine stops. I often wonder if those who insist upon running their fans after shutdown take this into account.

 

Great stuff folks, thanks!
Yes, valves, wiring, hoses and even the bonnet paint were all thoughts and concerns that prompted my question. I didn't like the idea of all that stuff baking under the hood when it is a simple matter of opening it up. My biggest concern though, was that I may be cooling off the area faster than I should. It didn't seem likely but, as the old saying goes, "I don't know, what I don't know".
So, I guess I'm OK with being ****. Besides, opening her up gives me a chance to have a quick look inside and see if anything is amiss. Also, I think it looks pretty cool.

 

I'd actually love to get some background info on what people on here backgrounds are in and where they derive their information and their general mindset on things. I'll stick to what I do and why I do it and I'll be happy in that. It was once said that the two hottest places were Hell and the engine compartment of a V-12 Jaguar. Opening the bonnet and releasing that heat in no way can be considered a bad thing and NO ONE is going to convince me (especially based on my background/career). That it's not making any difference or has an opposite effect.

 

Of course it is the internet so we can all be whatever we want to be!

 

In my case, over three decades of design, manufacture and customer support with world's largest manufacture of gas turbine (jet) engines. Hot nacelles (engine compartments) are a prime concern during operation and after shutdown. It's considered bad form to melt a customer's aircraft at any time.

 

I dunno what everyones' background is, either.

However, the XJS is a bit quirky. I think some owners are sorta in love with the quirky thing and enjoy being a bit quirky themselves. It all sorta goes along with the XJS experience. I'm not saying that as a criticism, believe me.



That works for me!





Hot, yes. But is it *really* all that hot? Is it really so much hotter than any number of other cars out there? Has anyone every really checked? I dunno. I'm asking!





I don't think anyone has asserted that it's a bad thing




Ok ! For you the matter is settled. Fair enough I hope you don't mind if the rest of us beat it into the ground for a few days, though !

Cheers
DD

 

Further to Doug's point about the rest of us cracking on...

here is a link to a study I have just found about under hood temps and fluid cmbustability.

http://www.mvfri.org/Contracts/Final...atures%201.pdf

They measured air temperatures immediately adjacent to various components, both whilst running and at a standstill after a hot stop. The temps adjacent to exhausts are huge and the spike in temps after shutdown also high. They did measure centre hood temps as well. While these were much lower than by the exhausts, they nevertheless recorded spikes from 50°C while running to 100°C as soon as the cars were stopped. The vehicles were
Ford Focus, Dodge Neon, Chrysler Caravan and GM Silverado. So I think we can all agree an XJS V12 will be generating higher post shutdown temps?
I cannot seem to copy the graphs, but at stationary tickover and at a hot shutdown, these cars centre hood temps basically doubled to about 100°C from around 50. (see graphs 1 and 3 in section 5 of the report.

This seems to me to be pretty convincing evidence that after stopping under hood temps shoot up and that popping the hood will greatly reduce under hood temps after stopping;

However, more seriously, ALL the under hood fluids, oil, glycol, ATF, PS fluid, petrol would spontaneously ignite as a result of the high temps round (eg) the exhaust manifolds, even without a flame or spark being present!

The tests were done to find out the temps that would be present in the event of a VERY sudden stop, AKA a crash...
Greg

 

Here is some stuff from a Cobra forum, the entire post is longer, and eals quite clearly with running, tickover and standstill temps. This part deals with heat at standstill, engine off.

"I also took time to measure another temperature, hood vent air temp. I clipped the sensor right to the middle of my right hood vent grill. Initial idle temperature was 136 F. After 2 minutes driving a 30 mph temp was 85. At a stoplight for 30 seconds temp shot up to 103F, rolled another two blocks temp down to 85F. Then another stoplight 45 seconds, temp went to 146.5F. This pattern was consistent, while driving the air coming out of the hood vents was 85 within 1 F up or down. At each light or intentional idle it would rocket up to over 150F. I stopped at a gas station, engine off for 2 minutes and hood vent air temp was 167.8F. That is a lot of heat escaping from the engine bay.

This demonstrates precisely what an SVT engineer told me personally. Opening the hood vents helps with "heat degas" (removal of heat)"

Link:
'03 COBRA UNDERHOOD TEMPERATURE MEASUREMENTS WORTHWHILE

So I say, pop the bonnet / USA pop the hood. And Ronbros' louvres got to be good.
Greg

 

Whoa, cool it down boys (pun most definitely intended)!

Let's face it, less heat = goodness! but I wouldn't go cooling my XJS with liquid nitrogen post-run! I'm sure opening the bonnet wouldn't hurt, and it might help, but that's just a "might".

The only way you'll prove it either way is to get two brand new cars (impossible, unless you get that flux capacitor fired up), drive them both identically, service them both identically, and do bonnet-up for one, and bonnet-down for another.

My personal opinion, is that with manufacturing differences, plus between 40 and 20 years of unkown conditions (maintenance, operating parameters etc) that bonnet up or down will make a statistically small difference in failure rates of any kind, be they serious (dropped seats) or minor (cooked wires) problems.

Regardless of that, if you feel good opening the bonnet, do it. As I said, I'm sure it wouldn't hurt!

Dave

 

No-one is anything but interested, Dave, not heated, I am sure. None of my points concern valve seat failure, as I posted earlier. Where I do disagree with you is that the benefit of reducing heat build up after a stop by raising the hood is unproveable at this age of our cars.

All the major manufacturers take very careful measurements of under hood temperatures and I believe the actual evidence I mention above proves my contention that there is a serious increase in temps after a hot stop. After that, there is so much published data on the deterioration of polymers etc etc as heat rises, that it is not credible to contend that a 100°C heat bath will not seriously affect wiring insulation , rubber components etc etc. This is accepted by all manufacturers, and does not need a 25 year XJS specific test to establish its truth.

Now obviously, one hot stop per year is not material, but a regularly driven XJS with several such stops per week will experience far faster degradation of these sort of items than one that does the same stuff but has an open hood and thus no 100°C heat soak. In this regard the XJS is no different from a modern car for which such evidence abounds.

After all, the degradation of the injector loom is just one example of the sort of failure i am thinking of, and there is lots of evidence that this item fails because it is cooked. Add to that the fact that the evidence I mention above shows under hood temps are in the 50°C range while the cars are in motion, and shoot to double that when stopped, there does not seem to me to be much question about the benefit of bonnet popping. And while I am aware that the injection loom was deep in the valley (as factory) there are lots of other loom bits that get equally cooked that are nowhere near the valley. No loom got cooked at 50°C ! Of course I accept there is radiant heat and IR emissions that cause damage, and I have no idea of the balance of damage-causing between them all.

My point is only that I cannot see how anyone, faced with the evidence, can with a straight face maintain that (a) the under bonnet temps do not go up after a hot stop, and (b) this somehow has no deteriorating effect on the items we are discussing and that (c) opening the hood will not affect such temps beneficially.

Greg

 

What we need now, Greg, are the charts and graphs showing that the life of the hoses and wires is increased so significantly that the open-the-hood ritual is just too irresistibly beneficial to pass up.

I think what we have here is a situation where being either 'right' or 'wrong' really doesn't amount to very much real world difference in the grand scheme of things.

My experience after owning a couple V12s and working on a couple more is that those hoses and wires in the "vee" and/or otherwise located on or very close to the engine are the ones that take the real beating. All the others, not so much. If anyone has a V12 with original fuel hoses, injector wiring, vac hoses etc then it's a fair bet that they're all cooked. No amount of engine bay ventilating will make them new again.

Most anyone reading this thread, though, has already replaced these items. They've almost certainly used higher temp wiring and better sheathing, and relocated it up and out of the vee.....all of which reduces heat soak damage significantly, I'd imagine, even without ventilating the engine bay.

The fuel hoses have already been changed and will be closely monitored for condition.....and perhaps even be summarily replaced every 5-7 years or so just on principles. We'd naturally replace any iffy vacuum hoses at the same time. Ventilating the engine bay....or not ventilating it.....is unlikely to change our behavior in this regard.

I'm sure you can see where I'm going with this


Cheers
DD

 

Agreed Doug, Agreed!
Greg

 

BOY must be full moon again!!

i just happen to have some temp measurements(digital in 2* steps) both coolant & underhood air temps,on my V12 XJS,78yr.

when i got my car 22yrs ago, i was all aware of the dreaded overheat thing.

so added a standalone temp measure system, coolant measured at the rear of the left bank, air measured just inside of inlet mani. near TB, right bank.

couple days ago 100F ambient, came back from a 3/4hr cruise, checked temps , coolant 185F at shutdown, air 175F at shut down.
waited 3-5minutes, opened hood,engine off, coolant had risen to 196F, air at 180F hood open.

here is where it gets interesting, using a infrared hand held gun , temps were very close to my in car readings, BUT the top of the cam cover measured 210F, must be from the oil splash inside cover!

my personal evaluation is i need a much larger engine oil cooler!

guys kick this subject around and see if anyone has a usefull answer.

altho it has been this way for many yrs, and car runs great, but you all know how **** we are about our cars!!

 

All I can say is that cracking the hood(I do not fully open it) can only help and not hurt anything. I only do it when I return home and park my baby in the garage. LOL. I have owned a good amount of original classic/muscle cars and have always cracked or opened the hoods when returning home. The XJS is not the only vehicle that gets hot under the hood. I know some collectors/enthusiast's that still have original wiring and hoses on there cars that are 75 years old and they still run fantastic. I usually take their advice on most automotive repairs/conditioning that I do. Of course, I go my own way on some things and can be a little stubborn, but it has cost me a few bucks. LOL. Doing the little things can sometimes make many items/components last longer. Your car, your choice.

 

Interesting and surprising data. Thanks for digging it up.

My own experiments showed some similarities but some critical differences. The component surface temperatures and their patterns of increasing of temp after shutdown make perfect sense- this is the soakback discussed earlier.

My own data differs in that it consistently shows much higher engine compartment temps while the engine is operating, both at a stand still and while in motion. These temps in fact were very similar to what your study shows as the after shutdown spikes.

The primary car in used for tests was an early 70s Corvette with a modified small block engine producing 400 gross HP. The purpose of the test was to establish the effectiveness of the factory-installed cold air intake supposedly taking advantage of high pressure air found at the base of the windshield. Despite test speeds of up to 160 km/hr, the best temp reduction achieved inside the air cleaner housing was a paltry 2*C. The engine bay temps at that time were approx 80*C.

After testing and while stopped at in traffic, engine bay temps hovered around 90*C. After shutdown, this number decreased slowly until ambient temp was reached some time later. A friend with a '67 big block Corvette recoded similar temps on his car and again, no upward spike after shutdown.

It appears then that the type of car used for testing influences the results.

I found the numbers you quoted in an other post from the 'Cobra' owner rather low all round- till I realized that it's not a real Cobra, just a fancy Mustang. Not really a performance car.

 

Ron
This is with louvres in the hood, right?
Greg

 

sorry greg,my actual test was with my 78 xjs,, never tested the red 86-6L car ,but owner requested Grp44 shop to do the louvers!(he had connections).

he was aware of the overheat V12s,when having the conversion done!
Grp44 raced many V12s, and 95% of the races they lost was because of cooling problems.

thinking about it, a V12 has a lot more mass of heat than a modern 4cyl car, wife has a japnese 4 banger, it cools much faster after shutdown than the garage heating V12(great for winter tho).

 

If it seems right, and it feels good.....just do it!

 

I love the OBC thing. Never heard that before. I think a V-12 owner will use any excuse to look at that beautiful engine. Like bikers will use any excuse to go for a ride, no matter how short. And similar to old H-D bikes, the XJS ride should be as short as possible or you might not get back. lol
RagJag