Greg, where did you duct the cold air for your intakes ? Behind the headlights seems the best on US Jag's. If you did get your cold air from there, where did you relocate the Lucas FI box that's mounted right where I want to drill my 3" hole ?
Lawrence

 

You might be confused from my post. I have better airflow INTO the engine bay, I did not say into the air intakes. This is because I was concerned that in standard spec the coolant temps after a traffic stop did NOT reduce as fast as I believe they should have once up to a decent speed, considering the cooling arrangements I had (powerful fans, three core rad, all in top shape).

Above about 40 MPH the ram air through the radiator should cool far more effectively than any fan can, and coolant temps should drop pretty well straight away. In Ambient 100 F this was not happening. So I had the idea that this MUST be an airflow problem, NOT a radiator problem.

The next step was to unbolt the front bumper and go for a blast. WHAT a difference. Basically coolant temps dropped straightaway after getting up to speed (say 30-40 MPH) after a hot stop. Not only that, but my main electric fan started generating power as it was being spun by the extra airflow! the warning light glowed at speed, which it had NEVER done with the bumper in place, however fast I was going.


So, whether purists want to believe it or not, the fact is that many cooling problems the XJS experiences are because of poor and aerodynamically dirty airflow through the radiator stack. So I did the following:

• Cut a slot in the bumper (see poor photo attached in real life this is hardly noticeable, rather extraordinary but true).
• Moved the horns to the side out of the airflow
• Moved the auxiliary coil (Lucas car) out of the airflow.
• These modifications taken together give at least an extra 60 square inches of aerodynamically clean airflow though to the radiator stack.

Result, far better airflow through the radiators, and far better airflow through into the engine bay. That is the reason I suspect my under bonnet temps are lower than most standard spec cars.

I do have fitted the AJ6 Engineering plus torque larger throttles and extra air inlet to the airbox, and high flow ITG air filters, all of which I highly recommend and made a big difference to the modern-world drivability of my car. The extra intakes for this system are fed via ducting from just behind the undertray. This system does not affect under bonnet air temps
Greg

 

I have louvers punched into the hood of my car and have assumed that they expel a large amount of heat. I should measure it.

Also, I have an interesting set up on my car with the electric fan replacing the factory mechanical one.

My fan is off of a Volvo S40 (If I remember correctly) and is operated from the stock temperature switch in the water pump housing. I installed the fan with a constant 12v source (can't remember why though) but this has the effect of keeping the fan on for a few seconds to a few minutes after the car is shut off. I can feel air blowing through the louvers and under the car after shut down and it is reassuring to think about all that heat escaping.

I don't know the longest I've heard the fan run but I would estimate 3-5 minutes.

 

Undoubtedly they do, but the major benefit is a low speeds and at rest. At higher speeds the under bonnet temps are not too bad at ambient plus25 or 30 C

Me too.

The stock system on XJSs of our cars' age anyway, have a system as OEM that keeps the OEM aux fan going post shutdown if the temp switch in the water pump inlet is still "on". It will keep going until the temp reduces enough to switch the fan off. Good idea to have that circuit running the main electric you have fitted. does it work the OEM electric auxiliary as well, or have you done away with that one?
Greg

 

It's more than just a temp benefit. The ones I had on my Camaro were advertised to reduce front end lift at higher speeds as well. I was not able to verify that with actual measurements (other than a bunch of 130mph+ passes where I didn't go airborne), but I'm sure there is a benefit.

Padre

 

Rather an interesting point, Padre. The nose of the XJS is pretty good from the air penetration point of view; but the extra air coming out of the louvres might well reduce the "reverse wing" lift that the bonnet generates. Any aircraft guys out there who can explain it for us?
Greg

 

Bernoulli effect.

The air flows faster across the top of the hood than
under the hood. That is a relative vacuum. That equates
lift.

An old stock class rally trick was to put washers between
the hood and hinge on rear hinged hoods. That lifted the
trailing edge away from the firewall ... instant vent and
reduction in lift. The rules mandated stock panels, but
not panel alignment. All the better teams had bad body
shops

 

There's two effects at work.

One is the aerodynamics of the body shape that may/may not turn the car into a wing and inadvertently create lift. 'Spoilers' front and aft do exactly as their name suggests- they spoil the lift.

The other force potentially in play is a static pressure build up at the wrong place at the wrong time. Although there is gains to be made by tapping into the area at the base of the windscreen to supply air to the engine, there's other areas where trapped pressure can be a negative.

It is thought that certain car designs (and again I'll refer to older Corvettes) would trap enough air under the hood at extremely high speed that the front end of the car would become light to the point that steering control would be compromised. Sort of like wind getting under a tent and lifting it.

As to whether this is a concern on street driven cars, I have my doubts. If GM's claims about underhood lift are as accurate as the effectiveness of the cold air intake on my Corvette, both are f*rts in a hurricane.

 

I knew there was a better explanation than I could give. And I love the old school racing tricks. Smokey Yunick is my hero.

Indeed, the first three generations of Vettes (even the Mako) are pretty bad both for the Coefficient of Drag and their instability at high speeds.

One of my "back of the mind" thoughts for my LS-Swap (into my 91 XJS) is to one day run it in the Texas Mile. I was going to run my Camaro (figured 180+), but I melted the diff a week before the race.

What always attracted me to the XJS was what I remember from the 1980's: its smooth performance up to 150 (I fondly remember getting blown away on the highway by an XJS back in '83 when I had my 1970 GTO Judge screaming at 140).

Thing is, nowadays, 150 barely gets you in the door at the Texas Mile. So anyway, I intend to buy those Camaro vents (Hex Vent 5GC Edition - Functional, heat extracting 2010-2013 Camaro hood vents) and try them out on my donor 90 XJS hood. And then if all goes well with the LS-swap, I may put them on my 91.

Padre

 

Interesting stuff. Heat buildup when stationary is certainly the worst problem to have, & far harder to deal with than buildup when moving, where even small louvres in the bonnet will make a big difference if sited in an area of low dynamic pressure.

Has anyone found an effective routing for a cold air intake? I noticed on the later 6 litre XJS V12, the air boxes are connected to the top of the radiator slam panel via flexible tubing, but on the 5.3 I don't think there is space to DIY a similar setup. I wonder if it would be easier to take a feed from below somehow.

 

I remember reading quite a detailed article concerning the P51 mustang regarding the extra thrust imparted by the radiator set up on that aircraft. I believe the thrust imparted by the thermal expansion was in excess of the drag created by the structure hanging off the belly.

 

It was asked in another thread about cold air intakes on 5.3 cars. My plan is drilling 3" holes in the back of the headlight housing's would probably be the best place to pick up the cold air in my 91 US car with the 4 head lights. I think some other models must go through the inter fender, don't know. Either way some components have to be moved in order to connect duct tube's to the modified air cleaner's. My question is what others that used this way did with the Lucas FI box that's mounted right on the housing where the 3" hole is to be drilled.
Lawrence

 

The resistor pack (that is what it is) can be relocated anywhere convenient, horizontally or however you wish.
Greg

 

Thanks Greg, I though I started a new thread on "cold air intakes", like that subject hasn't been covered before............
Lawrence

 

Have most of you removed the underhood/bonnet seal towards the windshield? Does this really lower temps? Thanks

 

I tried it and tested it. In fact, my wool tuft tell-tales were sucked back inside the engine bay once under way. So it doe not work. If you have an electric fan, at rest air will come out, but not once you are on the move. Also you will get fuel odours into the cabin in traffic and at stops.
Greg

 

I am just in the course of looking at this matter. I have AJ6 Engineering extra intakes that AJ6 suggest should be placed towards the bottom of the rad, between the engine and the undertray trailing edge. I fondly believed this was a source of cold air.
In fact, my temp readings proved that in this position the intakes actually are being fed warmed up air from the engine side of the rad! Actually admitting hotter air at speed than I measured going into the OEM intake trumpets under the bonnet! Shocked or what!

So I moved the intakes out into the full airstream fully underneath the tray and pointing forward. Air entered at ambient; BUT the intakes are obviously very vulnerable so located. Right now I am looking for less exposed sites for the two intakes, both use 38mm (1.5 inch) diameter tubing.
Grant Francis has advised me that taking air from the oval gap into which the front indicator loom enters the body (on pre facelift cars) is a good place. This is the cavity in the front corners of the car beneath the bumper. From here you have to make a hole into the engine bay (through the inner wing or "fender" just before the forward wheelarch baffle, which lets you route the tubing to the airbox. You may have to move the odd relay but there is room for a large tube, Grant has done it with great success. My trouble is I hate removing the wheelarch baffles as I always mess them up and end up having to buy new ones! So I am still experimenting with my temp sensor whether there are other places.
The 6 litres cars use the humped bonnet just like the 3.6 and 4 litre inline six cylinder cars. If you swap your V12 bonnet for one of these, then there is room for a home-made system that mimics the 6 cylinder cars. My problem is that I prefer the low, flat, peerlessly elegant original bonnet line!
Greg

 

In other words, cowl induction. It will get air into the back top of the engine bay, but not really for cooling purposes. Not useful for our applications.

Padre

 

I guess I will leave the seal alone. The engine temp runs slightly below the bottom of the N on the gauge all the time anyways(on the freeways it is even a little lower). Just thought it may be a way to help the heat in the engine compartment. I always pop the hood when I get home and park it... Thanks Greg.

 

I remember reading Roger Bywater's comments that placing the air intake behind the radiator on the 5.3 was deliberate. It ingested hot air at low speed, which is good for fuel mixing and economy, and the air cooled off at speed giving more power.

When the HE was being developed everything was about increasing the fuel economy to allow the V12 to still be sellable in a post oil crisis and in recession world, not about making power.