Guys
This is a follow-up to my first underbonnet temp post of a few weeks ago. My last remaining test was to cut some holes in my scrap bonnet to simulate louvres, and to see what effect this had on underbonnet and coolant temperatures, if any. Previously, this was what I posted with a standard bonnet and 25 degree C ambient air temp:


On the move above 40 MPH the front temp is about 56 C; the rear is about 52 C
On the move below 40 MPH front temp rises to about 60 C and rear to about 56 C
Once stopped, engine off, bonnet closed, both temps rise to 80 to 82 C immediately. These temperatures do NOT fall significantly after ONE HOUR if the bonnet stays closed. An open bonnet immediately reduces air temps to about 50 C. Even with the bonnet left open for an hour, air temps at the sensor are still 46 C
On the move again temperatures drop to under 60 within a mile or two. If the main electric fan is manually switched on (it does not trigger on my car until a 90 C water pump inlet temp), temperatures drop slightly faster.
At rest, aircon on, engine on, auxiliary and main electric fan on: At rest after a run, simulating a traffic hold up, front air temp climbs very fast to 70 - 72 C. Rear 4 C lower. Water temps remain stable, possibly dropping a touch.


Well, yesterday I cut two holes in my scrap bonnet and fitted it. Pics attached. The holes are about 8.5 inches long and 6 inches wide, and are above the fore and aft centre of the inlet manifolds and air boxes.


Ambient was 35 degrees C during the test run, ie 10 degrees C higher than the previous tests. These are the comparative results (aircon always on):


Speeds above 60 MPH (100 KPH): air temps 57 front 54 rear
Speeds below 40 MPH (60 KPH): air temps 58 front 55 rear
Stopped, engine off, bonnet closed: MAX temp 72 degrees C however long bonnet left closed, max temp starts to drop after 10 minutes


During a hot stop, to simulate a traffic hold up, engine on, main fan on, aircon on, temps: front 68 degrees C, rear 66 C.
On the move after a hot stop, temps drop very fast, faster than before to running temps shown above


Coolant temps: needle at least half an N's depth lower on the gauge, possibly more. Yesterday I had a long hot run in the car with the standard bonnet and the coolant temp needle was at least an N's depth higher than today (both days ambient 35 C).


Aircon performance: The aircon which is very effective in my car anyway, was clearly performing better, and the air existing the vents felt noticeably cooler than yesterday, and the auto setting did not once cut up to the higher fan speeds to maintain the selected cabin temp.


So from all this I conclude louvres will cause:

• a VERY big drop in underbonnet air temps;
• a very large drop in coolant temps as a result of better airflow out of the engine bay and thus into it through the radiator stack;
• better aircon condenser performance and thus more efficient aircon performance and a less stressed system;
• far lower engine bay temps after switchoff, thus substantially reducing the heat load on rubber and loom components after shutdown.

So I will start saving up to have them done! It has a HUGE and very beneficial effect, that is for sure. I estimate that, given equivalent ambient air temps, the louvres reduce underbonnet air temps by more that 7 degrees and coolant temps by 5 or 6 C. Tomorrow I shall prove it by closing off the holes with cardboard!


Greg

 

WELL DONE.

Glad you used the scrap bonnet, damn, that would upset her indoors if it was the real deal.

The proof is in the printing, nuff said.

 

Greg What price are you thinking for permanent louvers

 

I found a guy that will punch louvers for $2 each including layout. He's in Kelowna BC

 

Rex
There is a guy in England called Mike Beakhouse at Sportcar Metalworks, I have only spoken to him on the phone, but he seems a decent bloke. He will charge about 120 or so quid, and make an appointment, drive to car to him, whip off the bonnet and it takes him a couple of hours depending upon how many you want. Pretty good price if you ask me. Of course, then you have to give the bonnet a flashover! This is the pic from his site:

Link here:
Sports Car Metal Works | Metal works specialising in MGA, T-Type, Triple M, MGB parts
The next time I am in the UK, I am in there, that is for sure.
Greg

 

There was someone on an episode of Wheel Dealers who louvered bonnets in the UK. Apologies but I can't recall the car or episode. Might even be the same guy.

 

If the louvers are done right, it might be worth it on just esthetic grounds. It's a great look, but it can go wrong if proportions aren't right. I've been planning on doing this when I eventually get around to respraying the car.

I wonder if real louvers might actually perform better than your experiment. In motion, the cut holes probably create some less than ideal air flow effects. I'm not certain of the dynamics and I'm no expert, but it's possible that the air flow over the louver raised surface area may create pressure differences that will help to pull hot air out.

 

Are you guys figuring in repainting the hood ? I believe getting the louvers punched in are the cheap part.
Lawrence

 

Lawrence, a "flashover" is cartrade slang for a quickie respray. 100 quid should do it!
Greg

 

Painting a bonnet isn-t that expensive...

 

When I worked,about 20 years ago, I drove through hooley on the A23 past costa and I saw in their car park an immac xjs with E Type louvers in the bonnet. It was an immaculate car and i always wondered who owned it. But after reading your post regarding louvers I found a guy in Crawley adverting pressed louvers on ebay. I will get the link tomorrow

 

this is an old pic, 1986 XJS with a 6.0L conversion, it was deemed nessary for getting heat out ,recommended by Bob Tullius racing, GRP 44

 

I personally like the louvers on the e-type. But I also think they're primitive in design and style. And I don't really care for any of those I have seen on the XJ-S.

I feel like they really should parallel the windshield / hood junction and the sides should be parallel to each side of the hood shape.

What i'd like is a chrome flanged set of drop ins that have the shape above and are vertical rather than horizontal and replicate the bars in the plenum grille at the windshield. That's what I'd like. Maybe even chrome the bars and the plenum too.

Crazy talk. I know.

 

It may be even more involved, but I wonder what it would look like to have the hood (bonnet) modified to look like the 1969 Camaro hood? The bulge would mimic the shape of the hood as it is, but would align with the windshield wiper grill. If I had a clue as to how, would like to do a photoshop? of the thought.

 

 

I haven't done any measurements but I can feel a tremendous amount of heat leaving the engine bay after I've parked my car. I can only imagine the effects of having that heat being contained.

 

Guys, yesterday I wrote as my conclusion: "I estimate that, given equivalent ambient air temps, the louvres reduce underbonnet air temps by more that 7 degrees and coolant temps by 5 or 6 C. Tomorrow I shall prove it by closing off the holes with cardboard."

Turns out this was a bit optimistic, as today in 35 c temps at midday, I taped up the holes with cardboard and did a test at various speeds: simulated traffic stops, shutdown test etc. Mother in Law wrote down the numbers. Then I tore off the cardboard taped-on hole covers and repeated the tests; so as good a comparison as is possible to do, I think.

Well, holes uncovered made an average of 3 C air temp reductions across the range of test speeds I used, which were 50 KPH, 70 KPH, 90 KPH, and 110 KPH. I was pretty surprised it was not more, but that was it.

So a big question whether it would be worth the bother and expense of louvres and painting the bonnet for such a reduction.

Just shows to take nothing for granted. I am sure the aircon benefitted though, and also that the temp gauge needle was definitely half an N lower with holes open; even so, nothing the car cannot handle without the holes. Top hose temps and cam cover temps immediately after shutdown were 86C and 90 C respectively, so well within the normal temp range.

So a definite benefit, but not as great as I had thought and hoped.
Greg

 

Maybe like this?




XJ-S with '69 camaro style power bulge.<br/>photochopped.

 

@ Greg

Cheers for your second post! That really was my thought.

In my head and in my opinion, IF Jaguar had seen the need to fit some louvers, like on the E-Type, they would have done so. They did it to the XKR, they fitted side exits on other models. So IF there was any need, Jaguar wouldn't have gone through all the effort to model an engine bay and bonnet, with such sleek lines and good air flow.

Certain parts can be modified, but I doubt to be really honest, that the louvers do anything, than "bling" and, when raining, let more water into the engine bay and therefore a higher liklyhood of getting some electrical gubbins to go wrong. They seem to work when parked, but that is an expensive way of just popping the bonnet for a little while...

 

Well, it's better to know now than to spend money and be disappointed. And there's still the substantial cool-down benefit.

I wonder if an engine tray from the radiator right back to the crossmember would be beneficial? Theres a lot of low pressure behind that spoiler. I suspect airflow from the fans is largely pulled down and out right after the radiator creating a fairly stagnant rear and top engine bay at speed. It's possible that louvers in the hood altering that airflow to allow circulation at the top and rear of the engine is as significant as the venting.