[xk] keeping the heat out - DON'T LOUVER YOUR HOOD!

DON’T LOUVER YOUR HOOD!
Don’t panic!!

Put in a bilge blower on the left side of the engine compartment
venting through the wheel well, under the car with a piece of 4’’
dryer duct hose. I have a 12 volt blower that moves 600 cfm.
CAN YOU PLEASE SEND ME A PHOTO AND A PART NUMBER?

HERE IS MY NEWPLAN:
I have no front bumpers so I have two air intakes that are normally
blocked by the massive front bumpers.
These openings appear to be about 4" x 12".
I already have a sheet metal piece so the air can go into a 4 or 5
inch spiral duct.
This air blows over the exhaust headers.

I already have a fender side exhaust fan but we are going to replace
it with a newer quitter fan. I don’t know what the CFM’s of the
current fan.
More outside air in.
More hot air forced out.
If I can get the heat out from under the hood it won’t heat up the
fire wall as much.

In addition we are going to re wrap the headers (already ceramic
coated) with what we hope is a better heat shield product.
In addition we are going to put another layer of insulation under the
carpet up to the firewall.

We are leaving in a week to ship the car to Jackson Hole for a 4 day
driving event and I will do the new work early June.
Reports to follow.
Tnx for all of the tips.
Ron

In reply to a message from F Ronald Rader sent Thu 8 May 2014:

I am not advocating louvering your XK’s bonnet, but I would
just like to remind us all that increasing air flow by
passive means is probably more effective than active blower
systems unless it is strictly a low speed problem.
By passive , I mean capitalizing on pressures that are
generated by the forward motion of the body through the
air; recovery of dynamic pressure. The reason for this is
that the forces rise at a rate of V **2 , accompanying what
is likely a demand of increased power output necessary to
achieve that speed. The flow from a few 12 volt blowers are
piddle compared to that from recovering the stagnation
pressure at 50 MPH. Piddle !

Karl–
karl
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Karl
You are right on!!!
Have you been perusing Sclichting’s “Bible” again.:slight_smile:
It’s still the best publication on drag that I know.
Best
KlausOn 5/10/14 11:44 AM, “karl” klkirkman@aol.com wrote:

In reply to a message from F Ronald Rader sent Thu 8 May 2014:

I am not advocating louvering your XK’s bonnet, but I would
just like to remind us all that increasing air flow by
passive means is probably more effective than active blower
systems unless it is strictly a low speed problem.
By passive , I mean capitalizing on pressures that are
generated by the forward motion of the body through the
air; recovery of dynamic pressure. The reason for this is
that the forces rise at a rate of V **2 , accompanying what
is likely a demand of increased power output necessary to
achieve that speed. The flow from a few 12 volt blowers are
piddle compared to that from recovering the stagnation
pressure at 50 MPH. Piddle !

Karl

karl
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In reply to a message from Klaus Nielsen sent Sat 10 May 2014:

Klaus,

Somehow, I knew you would approve. Saves having a flight
engineer and a checklist to operate all the blower
sequences correctly too.

Actually, I was partial to Hoerner’s two books: Fluid
Dynamic Drag and Fluid Dynamic Lift; privately published by
his widow for many years after he passed.

Karl–
karl
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My brother and I just bought an XK120 bonnet from the west coast with a big
ole air scoop cut out and raised up in the middle of it for the 53 XK120
FHC rat rod we are building. Think we will get some airflow though that?

John Brady
678462

John and Tom
IMHO Š not much at all, unless you are backing up.:wink:
Best
The VikingOn 5/11/14 1:34 PM, "JBrady5282@aol.com" JBrady5282@aol.com wrote:

My brother and I just bought an XK120 bonnet from the west coast with a
big
ole air scoop cut out and raised up in the middle of it for the 53
XK120
FHC rat rod we are building. Think we will get some airflow though that?

John Brady
678462

Well gentle folk, IF one doesn’t louvre his bonnet AND one doesn’t add any blowers to do piddling work, what does one do? The aim, of course, is to actively or passively extract warm/hot air from the engine compartment without allowing it to roil up and find sympathetic entrance into the interior.

I assume this means louvering or putting holes/slots into the engine area some how. OR, have I got it all wrong? If OTS and FHC/DHC ideal configurations differ, please expound on both. Perhaps under chassis extraction is the answer. I know we can all insulate the cabin to a fair-thee-well, but is there another way?

I’ve read several opinions on this matter, but nothing definitive AFAIK. Since us art majors never took much of any science courses, this is something I’m not likely to be proficient or even stumbling in. Please enlighten with the definitive info. I guess race cars did this to some, if not great, effect. Trouble is I haven’t seen any treatises on this subject.

Best, Brian
eventually, getting into this area most earnestly

I am not advocating louvering your XK’s bonnet, but I would
just like to remind us all that increasing air flow by
passive means is probably more effective than active blower
systems unless it is strictly a low speed problem.
By passive , I mean capitalizing on pressures that are
generated by the forward motion of the body through the
air; recovery of dynamic pressure. The reason for this is
that the forces rise at a rate of V **2 , accompanying what
is likely a demand of increased power output necessary to
achieve that speed. The flow from a few 12 volt blowers are
piddle compared to that from recovering the stagnation
pressure at 50 MPH. Piddle !

In reply to a message from Brian Ternamian sent Sun 11 May 2014:

If the car is moving over 30 mph, blowers or louvers won’t
make any difference. There should be enough air coming through
the front and exiting out the bottom. I would only use my
blower in parades or stop and go traffic. Did it help keep the
engine compartment cooler, MAYBE, did the engine run cooler,
NO. When I stop the car, I open the hood. That does a pretty
good job of getting the heat out:-)
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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In reply to a message from Brian Ternamian sent Sun 11 May 2014:

Brian,

At the risk of oversimplifying, and earning a storm of
protest, let me try this.

The problem we face is two fold, and sometimes get mixed.

  1. The first issue is providing sufficient cooling to the
    engine itself to prevent internal hot spots that fail or
    increase wear. When it is all said and done, I believe the
    consensus is that the first step to take is to restore the
    originally fitted cooling system to as new condition before
    trying anything else and this cures many many problems.
    This restoration done thoroughly is not a minor matter;
    there are lots of posts that tell how to do it.

  2. The second issue is heat transmitted to the cockpit.
    This is not likely to be greatly affected by small
    variations in engine temperature, as what we are dealing
    with is many pounds of iron and aluminum that have been
    brought up to optimum operating temperature, and then
    continuing to ‘‘stoke this fire’’ by burning additional fuel.

It seems to me there are two approaches:

a. Insulate the cockpit as well as possible, or

b. Increase a flow field outside the normal one for
radiator cooling to transport the heated air surrounding
the engine in the engine compartment more rapidly outside
the car.

It is my belief that providing passive means of recovering
some of the stagnation pressure on the car that arises
because it affects the flow field around it is superior to
blowers except at very low speeds as a means to achieve
this.

I simply do not understand the comment that louver do no
good at over 30MPH, but stand ready to learn. I would seem
to me that if properly located in the flow field, they
would continue to recover the stagnation pressure that
increases with speed. If the only function of the louvers
is to permit hot air to escape upwards, that is not really
an effective pressure generator to induce flow.

BTW, my car happens to have louvers to the two valance
panels that drop down on each side of the engine as a way
to permit more flow to escape the engine compartment.

Karl–
karl
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Karl,

What I’ve heard, without the benefit of any independent testing, is that the
windscreen creates a pressure ridge in front of it. If you look at wind
tunnel testing of cars with windscreens you’ll see that the smoke begins to
rise long before it reaches the base of the windscreen. The area beneath the
smoke is the pressure ridge created by the windscreen. The more vertical the
windscreen and the faster the speed, the farther out the pressure ridge
extends. By modern standards, the windscreens of XKs are pretty vertical. I
don’t know if 30 mph is the magic number but if the louvers are in the
pressure ridge area, it will add to the pressure in front of the windscreen
and impede the flow of air out of the engine compartment. If the theory is
correct thus far, it’s theoretically possible to create enough over-pressure
to force air into the louvers slightly increasing air pressure in the engine
compartment and making it more difficult to air to pass through the
radiator. It might actually make the engine and engine bay hotter.

As you probably know from your studies of fluid dynamics, there are all
sorts of odd things that happen at speed. For example, pickup trucks are
more aerodynamic with the tailgate up. The reason is that you create a
pressure ridge over the bed that allows the air coming over the cab to
smoothly flow over the tailgate which acts almost like a kamm-tail. Put the
tailgate down and you create an area of vacuum that sucks the airflow over
the cab into the bed and induces turbulence which increases drag.

“Mark 1” Mark Stephenson Phoenix, AZ
52 XK120 S673129; 59 Mark 1; 84, 85, 86 XJ6

I simply do not understand the comment that louver do no good at over 30MPH,
but stand ready to learn. I would seem to me that if properly located in the
flow field, they would continue to recover the stagnation pressure that
increases with speed. If the only function of the louvers is to permit hot
air to escape upwards, that is not really an effective pressure generator to
induce flow.

BTW, my car happens to have louvers to the two valance panels that drop down
on each side of the engine as a way to permit more flow to escape the engine
compartment.

Karl–
karl
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Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

Karl & Mark;
Look at NASCAR cars back when they used real cars), You
will notice that the Air Intake in right in front of the wind-
screen… Not only is the area in front of said windscreen a
“high pressure” area, it is an area of “reverse pressure”, that
is, the air movement can actually forward… as in towards the
FRONT of the vehicle… Believe it or not, NASCAR got a
“ram air” effect to their carbs (before EToH and FI) because
of this bit of weird aerodynamics…
A simple “wind tunnel” test can determine exactly where
the high and low pressures are on ANY vehicle, and you DO
NOT need a wind tunnel… just a skeen of yarn and a roll of
masking tape… Simply cut a LOT of three inch lengths of
pieces of yarn, tape in neat rows on the bonnet, wings, etc.
mount a few “GoPro” or other digital cameras, turn them on
and go for a ride… documenting your speeds from the when
the cameras were turned-on… I think it’s funny to watch the
bits of yarn in certain areas blowing forward at speed!!!
Charles #677556.
http://xktx.org----- Original Message -----
From: “Mark - Jag-lovers”

Karl,

What I’ve heard, without the benefit of any independent testing, is that
the
windscreen creates a pressure ridge in front of it. If you look at wind
tunnel testing of cars with windscreens you’ll see that the smoke begins
to
rise long before it reaches the base of the windscreen. The area beneath
the
smoke is the pressure ridge created by the windscreen. The more vertical
the
windscreen and the faster the speed, the farther out the pressure ridge
extends. By modern standards, the windscreens of XKs are pretty vertical.
I
don’t know if 30 mph is the magic number but if the louvers are in the
pressure ridge area, it will add to the pressure in front of the
windscreen
and impede the flow of air out of the engine compartment. If the theory is
correct thus far, it’s theoretically possible to create enough
over-pressure
to force air into the louvers slightly increasing air pressure in the
engine
compartment and making it more difficult to air to pass through the
radiator. It might actually make the engine and engine bay hotter.

As you probably know from your studies of fluid dynamics, there are all
sorts of odd things that happen at speed. For example, pickup trucks are
more aerodynamic with the tailgate up. The reason is that you create a
pressure ridge over the bed that allows the air coming over the cab to
smoothly flow over the tailgate which acts almost like a kamm-tail. Put
the
tailgate down and you create an area of vacuum that sucks the airflow over
the cab into the bed and induces turbulence which increases drag.

“Mark 1” Mark Stephenson Phoenix, AZ
52 XK120 S673129; 59 Mark 1; 84, 85, 86 XJ6

In reply to a message from karl sent Sat 10 May 2014:

This discussion came up about four years ago,raising the
same points. What I learned then, I applied to my XK150
restoration: ceramic coated exhaust manifold, heat wrapped
the exhaust pipes from the manifold to the mufflers,
insulated the fire wall and a passive louver to vent the
area around the exhaust manifold. I haven’t measured the
air flow at speed, but after a run, with the electric
radiator fan on, there is a noticeable flow of hot air from
the louver that you can feel by putting your hand in the
wheel well. See my photos, posted March 8, 2011.

Monte
XK150DHC

http://www.jag-lovers.org/snaps/snap_view.php3?id=1299552627&n4=--
The original message included these comments:

just like to remind us all that increasing air flow by
passive means is probably more effective than active blower
systems unless it is strictly a low speed problem.


p8099
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In reply to a message from p8099 sent Mon 12 May 2014:

The worst source of the heat is definitely the exhaust. The
‘‘good’’ thing is to restore the factory ceramic coating on the
manifold+warp the exhaust pipe until it goes into the box. Even
better to use a tubular manifold instead of the cast and warp
it all way long. An alloy radiator is also something to
consider. A good water pump is vital.

Gergo–
Ad Dei gloriam et ad gaudium nostrum
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