KG engine compartment air flow

[crazy tarzan]

After reading the wool tuft thread I have questions about how the air flows in/out of the engine compartment of a Ghia, and what changes affect it.

How does the air enter? How does adding a spoiler work? What about raising the leading edge of the rear lid (a la standoff style)? What about changing the vents in the rear lid?

I ask because I've always wanted to put 2-3 porsche 356 grilles on the rear lid of a ghia, and because I'm looking at running a boosted motor with an inter-cooler someday (when I purchase another ghia. . . . . . ).

I've read about people pulling air from under the car, or via the wing windows. How about with a roof scoop? Scoops on the sides just in front of the rear wheels (using body lines for an opening)?

[silkvw]

I started by running my Ghia without the front engine tin and the openings in the deck lid covered. I bought a manometer (measures air pressure) on Amazon for $60.00 and use it to monitor the pressure in the engine compartment. Before the engine starts, the pressure is set at zero, engine starts, pressure goes slightly negative, as I drive and increase speed the pressure has always increased. With positive pressure, I think the engine fan will provide enough flow to keep the engine cool. Turbo air is drawn from under the car, not from the engine bay. This car goes only a mile or so at a time, so may not be the best set up for road racing or street driving.

[FJCamper]

Hi Tarzan,

The VW factory decided to draw air into the fan housing from the engine compartment, and that made sealing the compartment vital. The intake vents
under the Bug's rear window and on the engine lid deck of the Ghia are in "high pressure" areas when air is forced in, not just drawn in by the fan.

There are answers to what you're asking.

For racing, where you can do things you might not want to do to your street car, we've resurrected the old trick of directly providing air to the fan housing,
using hardware store ducting. Depending on the duct's intake shape, you can pull air directly from the cabin or just above the transmission.

We draw air from the cabin, behind a baffled firewall for safety.

You have to make sure you have exactly the same number of square inches (or more) of intake area for the duct as you do on the fan housing intake hole.

Once you duct air in this way, you have no further worry about poor engine compartment sealing and sucking back in hot air.

Our experience with this over several long endurance races has been excellent. No whine, elevated engine temps, nothing.

FJC

[crazy tarzan]

FJ--so on a stock ghia, the motor air for both cooling (and heat since I live in WY) and combustion is forced in via the slots/grilles on the decklid due to this being a high pressure area (I'm repeating to make sure I understand correctly). Thus the reason for making sure you have all engine tin, and a good engine/decklid seal on a stock car.

How would increasing the area of the vents affect an otherwise stock vehicle? The porsche 356 vents are longer, but skinnier than the stock air venting--two porsche grilles won't quite cover the width, but are longer, 3 is just over the width with the extra length.

I understand the ducting and having at least the same intake area for the fan intake--you don't want to make the engine work harder to cool itself, that is a recipe for overheating quickly I would think.

Now I need to go review the pic of how a duck tail affects the high pressure area over the vents.

If you pull your motor air from above the trans, how does this affect the airflow from the vents on the decklid?

[FJCamper]

Hi Tarzan,

With the engine taking all its cooling air front a direct duct, and not from inside the engine compartment, the vents that ordinarly fed the fan are just there for general ventilation to disperse some engine heat.

Remember, Porsche went from one air inlet on the pushrod 356A models to two on the 4-cam Carreras, and the two inlets on all models beginning with the B's.

I'd think you'd probably want two inlets just as homage to the Carreras.

We're very happy with our ducting.

FJC

[crazy tarzan]

So if I pull air from somewhere other than the engine compartment, I could then in theory also pull more air from a vent/duct and have it flow out if the hood vents (as long as I am trying to exit more pressure than exists over the stock vents) possibly through an intercooler? Or will the native high pressure area render this not very efficient?

[silkvw]

My opinion is that the engine compartment seals are over rated. There are many Baja bugs and dune buggies that have no seals or sheet metal around the engine, and over heating is not a problem. Just my opinion.

[crazy tarzan]

They also have a more open engine compartment--no lack of fresh air for them!

[silkvw]

You're right. Too bad we can't get some kind of government grant and get paid to study this. Ha Ha!

[FJCamper]

Above: Blitzwagen Sebring configuration


Above: Stock Ghia. High pressure in red.


Above: One of our Ghia rear window vent designs.


Hi Tarzan,

Believe me, the air pressure at the base of the rear window on the Bug, or the grille inlets on the Ghia, is poweful at speed. We ran Sebring (2012) in our 1973 Superbug (the Blitzwagen) with no rear window, and oil droplets from a breather up high in the louvers were picked up by the gale blowing in the back window and spotted the inside of the windshield!

On the Ghia, the high pressure area just behind the rear window is also strong. We've installed rear-facing shrouded vents on the deck area behind the rear windows to allow fan-assisted oil cooler air to escape, and made the discovery that the faster we go, even with the fan, the high pressure (trying to enter) blocks the escaping fan air and we get higher oil temps!

Once you duct air directly to the fan housing, any other air entering the engine compartment is just general ventilation.

It seems logical that if we were running Bonneville, and ducting air to the fan housing, totally blocking off the engine louvers might make the car faster, as it would smooth the airflow.

FJC

[crazy tarzan]

FJC--anyway you can get those airflow pics with colors other than red/green? I'm partially red-green colorblind, and in pics like that I can't tell what's what.

So if I understand correctly, there is plenty of air pressure/flow (especially the faster you go) over the rear of the ghia to cool/feed the motor. However, even if I pull air from another area, the high pressure spot stays the same.

Thus, if I want to run an intercooler I would probably be better off having it plumbed out of the motor compartment, or at least cooled by a separate air flow that does not exit via the decklid, or removing the decklid completely. This is because like your oil cooler venting, there will be enough pressure to negate any real gain from trying to vent out into the high pressure spot on the car.

[FJCamper]

Hi Tarzan,

Yes, no matter where you duct your air into the fan housing, the high-pressure air flow still exists because it is a byproduct of the body aerodnamics.

As a matter of fact, let's say you have a perfectly sealed engine compartment, and are drawing your air to the fan in the factory intended way. You are creating a slightly negative pressure area inside the engine compartment, which at idle would encourage sucking air in through the vents.

Once at speed, the high pressure outside the body forces air inside the engine compartment.

Sorry about the red/green. That's all the Flow Illustrator gives us.

FJC

[Jadewombat]

Look at the link again on the wool tuft thread of the radiator flow (I don't remember exactly where I linked it but I will look for it again). Cooling is not just about surface area exposure or flow, it is about the ability for that heat to be carried away--meaning radiators, intercoolers, oil coolers, etc. work best when you have that high to low pressure differential across the oil cooler. If you have a poor engine seal you no longer have a good low pressure area in the engine compartment to channel that air directly from the high pressure area (decklid surface) down across the oil cooler and over the engine cylinders where it is carried away by the air passing under the car.

To put it another way, if the pressure is equal on both sides of the oil cooler the fan would have to work much harder to push the air across the cooler. Bajas don't really recirculate that air from under the car back up into the fan compared to all the other air being sucked in from the whole backside of the car.

[r1cpowdercoating]

tarzan,
hey i ran a 69 ghia in the SCCA. i removed the rear side glass and rep-laced it with lexan and installed a nacta (?) duct and piped the fresh air thru the firewall into the engine compartment. seems to work pretty good as long as you are moving.

[FJCamper]

Hi Ron,

NACA ducts, after the National Advisory Committee for Aeronautics, which was the precursor to NASA.

Why a duct?

Anytime you get confused, just remember this scene from the Marx Brother's 1933 film "Coconuts."

Hammer: I'm alright, how are you? I say, here is a little peninsula, and here is a viaduct leading over to the mainland.

Chico: Alright, why a duck?

Hammer: (pause) I'm not playing "Ask Me Another," I say that's a viaduct.

Chico: Alright! Why a duck? Why that...why a duck? Why a no chicken?

Hammer: Well, I don't know why a no chicken; I'm a stranger here myself. All I know is that it's a viaduct. You try to cross over there a chicken and you'll find out why a duck.

Chico: When I go someplace I just...

Hammer: (interrupts) It's...It's deep water, that's why a duck. It's deep water.

Chico: That's why a duck...

FJC