Midas Cars Forum - www.midascarsforum.com

HI all
I'd like to get some air in and out of my car (mk4 / cortez).
Currently air enter's through the dashboard vents but there is no exit for it.
Driving to work this cold and frosty morning the windows soon misted up inside with the heating on yet cleared as soon as I opened the passenger window a crack which has the opposite effect of warming me up.
This implies that there is no airflow.
I'm reluctant to add anything to the rear like the gold has because I've only just stopped the smell of the exhaust entering the car by making a sealed membrane to separate the boot from the passenger compartment.

The only thing that springs to mind that I fancy is maybe a roof vent which suits my taste but I may also want to sell this one day so don't want to bastardise it too much.
Any thoughts?

Jeff used a brake light housing to add a vent to his car, it seemed to help.

It was similar to this one


https://www.autoelecwarehouse.com.au/HE ... p/5242.htm

Aaaahhh
That's a fantastic idea.
Maybe able to kill 2 birds with one stone too and have both.

Mike Perkins has a grille in his parcel shelf with a fan to blow the air out through trunking.

Thanks Neil.
That's an option I had considered.
My reservation however is that when I had my back seats out last to retrim them and I was driving the car, the membrane I used between the boot and interior (the rear bulkhead) balooned up inside the car with quite a force, so much so that I'm unsure that a fan would overcome the force.
Not only that but the smell from the exhaust is overwhelming inside the car meaning that either the car is such a shape that dynamically on the move the air moving over and under the car causes positive pressure at the boot outside causing the air to rush in bringing exhaust gas with it or the air moving over the outside of the car is causing a negative pressure difference inside the car (far more likely) causing the car interior to suck up all exterior air including the exhaust.
Regarding the exhaust smell I have bee around and sealed up as much of the boot area to outside as I can (back lights and holes in boot floor etc) but the design of the bootlid is such that's it's nigh on impossible to seal tight plus there will always be gaps in locks etc.
So the membrane was a simpler solution.
However as a result, although I'm not dying from asphyxiation I am unable to see where I'm going on cold mornings but swings and roundabouts and all that.

The stumbling block I have is not knowing if while on the move the air is pushing in from the boot or sucking in from the interior

Hi Justin,
Is your sealing membrane only fixed around the edges? If so, then only a small pressure difference would be enough to cause it to balloon. ( small p.s.i. but a lot of square inches) When you had the rear seat out, did the membrane balloon when the heater fan was running and the windows closed, or was the internal pressure then greater than the external pressure?
When I worked in Nuclear Medicine at City Hospital, we used a 3 way tee piece connected to trunking when doing lung scans. The patient breathed in air plus a radioactive gas through a one way valve in the tee piece, and exhaled air went out through the other side via another one way valve. The valves operated on very small pressure differentials, patients breathing normally, and were essentially flaps of silicone rubber about 1mm thick. Such a valve in the outlet trunking would prevent gases coming back into the car, but not obstruct the outgoing air.
Mike found the system worked for him and demisted the rear screen.

Hi Justin,

There is usually some positive pressure under the car, unless you seal the underside of the engine bay with an undertray and duct all the hot air out of the top and sides of the bonnet.

The other area of high pressure is in the wheel arches. Firstly, wheels create positive pressure by having to push aside the air that is in front of them. Then the wheels act like rotary pumps and create positive pressure by dragging air up into the arches. This effect will be more noticeable at the rear where the tyres run closer to the inside of the wheel arch. I suspect that you haven't completely re-sealed the boot after installing your rear springs mounts, so the positive pressure in the boot will be more likely than in a standard Cortez.

To get some fresh air flowing through the car you need to take it from an area of positive pressure, such as right in front of the car, or at the base of the windscreen. In the original design Richard Oakes tried to use the bonnet stiffener to duct air from just in front of the radiator to the heater intake. In principle this is a good idea, but in practice I haven't come across a Cortez, or Excelsior that hasn't had a hole, or slot, cut out of the stiffener at some point to clear something under the bonnet. It is also possible to take cold air from the front wheels arches, but remember that the brake pad dust gets spread around this area so it isn't air that you would want to breathe in.

benofbrum wrote:Hi Justin,
Is your sealing membrane only fixed around the edges? If so, then only a small pressure difference would be enough to cause it to balloon. ( small p.s.i. but a lot of square inches) When you had the rear seat out, did the membrane balloon when the heater fan was running and the windows closed, or was the internal pressure then greater than the external pressure?
When I worked in Nuclear Medicine at City Hospital, we used a 3 way tee piece connected to trunking when doing lung scans. The patient breathed in air plus a radioactive gas through a one way valve in the tee piece, and exhaled air went out through the other side via another one way valve. The valves operated on very small pressure differentials, patients breathing normally, and were essentially flaps of silicone rubber about 1mm thick. Such a valve in the outlet trunking would prevent gases coming back into the car, but not obstruct the outgoing air.
Mike found the system worked for him and demisted the rear screen.

Thanks mate.
Good advice and good point on the pressure on the membrane.
Sadly I can't recall what circumstances it occurred but I have a feeling it was with windows down on the move.
The membrane at the time was a thin sheet of polythene that balooned up with a fair old force or as much as I could feel whilst driving and reaching round to push it with my other hand - no easy task lol.
I've since upgraded the membrane to a thick piece of rubber bonded over the hole so I doubt it will operate in the same way.
I can recall however that I always git exhaust smell in the car but more so on the move whereas now I don't at all.

I need to invest in a manometer

ACourtney wrote:Hi Justin,

There is usually some positive pressure under the car, unless you seal the underside of the engine bay with an undertray and duct all the hot air out of the top and sides of the bonnet.

The other area of high pressure is in the wheel arches. Firstly, wheels create positive pressure by having to push aside the air that is in front of them. Then the wheels act like rotary pumps and create positive pressure by dragging air up into the arches. This effect will be more noticeable at the rear where the tyres run closer to the inside of the wheel arch. I suspect that you haven't completely re-sealed the boot after installing your rear springs mounts, so the positive pressure in the boot will be more likely than in a standard Cortez.

To get some fresh air flowing through the car you need to take it from an area of positive pressure, such as right in front of the car, or at the base of the windscreen. In the original design Richard Oakes tried to use the bonnet stiffener to duct air from just in front of the radiator to the heater intake. In principle this is a good idea, but in practice I haven't come across a Cortez, or Excelsior that hasn't had a hole, or slot, cut out of the stiffener at some point to clear something under the bonnet. It is also possible to take cold air from the front wheels arches, but remember that the brake pad dust gets spread around this area so it isn't air that you would want to breathe in.

Thanks Alistair.
My bonnet is ducted as you describe but I have cut into one of the ducts to fit my bonnet vents - they were sited for style not function as such.
Anyway getting air in isn't a problem the fan blows and sucks well.
Windows down either stationary or moving the windows are clear and a good blast from the vents on the dash.
Windows up and the air doesn't escape thus causing no airflow and misty windows. .
The boot floor is now sealed but as I've isolated it from the interior compartment it's irrelevant.
I guess that windows down on the move causes a negative pressure as the air passes the outside of the car as aids the exit of ducted air through the dashboard.
I just need to replicate the effect with the windows up

Yes, if you have the windows open then that would cause a negative pressure inside of the car.

When you hear people talk about air flowing around a car that is a bit misleading, but that is probably because we are used to seeing cars in wind tunnels where you can add smoke trails to the airflow to highlight how it is behaving.

In reality, the air is more or less static (ignoring headwinds, crosswinds, etc) and the car has to push it out of the way. The formerly static air gets pushed over, under or around the car and in doing so the car gives the air some momentum. It is that momentum that creates the negative pressure when the air wants to move away from the surface of the car, which is known as separation. This effect is most pronounced at the top of the roof and the A pillars (and explains why convertible drivers can stay dry in the rain so long as they stay above a critical speed). Anyway, what that tells us is that the using a vent on top of the roof to extract air will work well, but using a forward facing intake from just behind the top of the screen will work less well. In fact to get it to work at all you may want the intake to protrude in front of the top of the screen - take a look at any WRC car to see what I mean.