[DeTomaso] Rear Window
MikeLDrew at aol.com
Fri Mar 18 12:35:53 EDT 2016
I don't think you will see any measurable water temp difference by restricting the airflow past the engine. That is, the temp inside the engine will likely be unchanged as the radiator is more than capable of coping.
Your ambient under hood temps will likely increase, maybe even dramatically. That means your carb is breathing superheated air so performance will drop. The workaround to that would be a sealed air box and ducted ambient air feeding it, but that's a whole 'nuther project.
I don't know how much pressure you will see, but between the high pressure inside the engine bay and the vacuum created by airflow over the top, there will likely be quite a differential, which would be significant at triple digit speeds but less so at more routine velocities.
In any event, I would advise mounting the glass (and I would start off with a cheap, easily fabricated piece of plexiglass rather than actual glass) and secure it well to the inside of the opening. I should think a thick bead of windshield sealer silicone would be all that you need? That means no drilling of holes etc for brackets.
Looking forward to your results!
Sent from my iPhone
On Mar 18, 2016, at 8:35, "Stephen" <steve at snclocks.com> wrote:
> I've been pondering for quite a while now the idea of putting a piece
> of glass in the "window" in the deck lid behind the engine in 5332.
> This would keep the engine from getting wet every time it rains and
> would also keep from sucking grit and water off the road when the car
> is running down the road.
> Both seem like decent goals. Oh, and it would also make it more
> difficult for someone to just reach in and remove 5332's dog-bone air
> cleaner. OK - no one has taken it so far. But, hey, it would be all
> too easy to remove.
> Looking at the opening - it sure looks like the original intent was to
> put in a window.
> I have sent an e-mail to Tom Tjaarda (attached) asking his thoughts.
> Hopefully he will respond.
> In the interim, I put together a set of sensors so I could see what I
> could learn about conditions in the engine compartment just inches in
> front of the opening, to the right side of the dog-bone. The attached
> photo shows the instrument cluster shot from inside the car.
> I'm measuring air velocity through the rear window (vane anemometer)
> Vacuum just in front of the window
> Rotation of the a/c condenser fan (volt meter connected across the
> motor, which acts as a generator when the fan turns)
> Two of the above are fairly straight forward. The vacuum - not so
> When one talks about a vacuum, one is usually talking about a vacuum,
> relative to ambient atmospheric pressure. So, if a vacuum gauge is
> hooked up to your intake manifold, it reads the difference in pressure
> between the atmosphere where the gauge is sitting, and the inside of
> your manifold, where the hose from the gauge is connected.
> So, what does one compare to in a moving car? The interior of the
> car? That is what one would get if one puts a vacuum gauge in the cab
> and routes the hose to the area in question. But, what does the
> pressure (or vacuum) inside the car do with the car moving down the
> road. Open the windows, your ears pop, and the pressure changes. Most
> of us don't have an external air inlet (at least an intentional one) to
> the cab except for the windows.
> My solution is to use a fairly sensitive pressure gauge (a
> sphygmomanometer - gauge used to read blood pressure. It reads in
> inches of mercury. FYI, there are roughly 50 inches of mercury per
> psi). I have connected the gauge with small-diameter plastic tubing to
> a 200 ml stainless sample bomb. I wrapped the bomb in towels and
> placed in a cooler to minimize temperature changes. See, the pressure
> in the bomb will change by roughly .03 psi (1.5 mm Hg) for each 1
> degree F change in temperature.
> I then pressured up the bomb and let it sit overnight to stabilize. It
> ended up at 280 mm. This is my "zero" at the then current barometric
> One has to view the pressure gauge as a delta-pressure device. It
> shows the difference in pressure between the bomb (high pressure) and
> the atmosphere around the gauge. So, if the reading on the gauge goes
> up, the differential is increasing, which means the atmospheric
> pressure is going down. As in a wee bit of a vacuum around the gauge.
> Flip side, if the reading on the gauge goes down, well, there is less
> differential between the 280 mm Hg in the bomb and the atmosphere
> around the gauge, so the pressure is going up.
> OK - that is complex.
> While a gauge that can be read down to around 1 mm hg (0.02 psi) is
> pretty sensitive, I suspect we are talking pretty minimal vacuum
> levels. Preliminary testing suggested a reading of 272 mm Hg at 80
> mph. The pressure around the gauge went up!
> Huh. A lot to learn here.
> I also noted that the volt meter did not show any rotation of the fan
> on the a/c condenser. Need to make sure the wiring is ok. But, note,
> the meter reads in mV, and, in my earlier testing, it showed voltage if
> the fan turned at all.
> The rubber for the new window shows up today, the window next
> My plan is to get a decent video and stills of the test instruments at
> several speeds. Then, when the new window is installed, rerun the
> test. I suspect the anemometer will still be valuable, since I suspect
> there will be a lot of turbulence in the air in the engine compartment
> even with the window in place.
> Needless to say, having the window in place will complicate putting the
> deck-lid back in place.
> Stephen Nelson
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