SVX Engine cooling "Again & Again"

[Dessertrunner]

I have removed

[bazza]

Originally Posted by BRZCory

I fully understand this, and that was the "Common knowledge" that we ran into when bob first proposed hood spacers.

So much so that we tested this. http://www.youtube.com/watch?v=yo1hboEl6Qg

We found no evidence of airflow reversion, and I suspect that it has more to do with windshield angle than anything (old cars weren't the most aerodynamic anyway, and there's a reason that they don't use cowl induction anymore).

That being said, there is an easy way to increase airflow, and subaru has endorsed this method:

Although I would be curious how many people have the stock undertray still installed vs who has cooling problems. Another way would be to make sure that there's ducting around the edges of the radiator, forcing air through it rather than around it.

That video shows what I'd expect - high pressure air pushing the strings downwards and some times seeming like it want so push air into the back of the engine bay. Is that what you mean? Although to confirm you'd want to put a high speed in engine bay or something... hmmm I have a high speed camera :P

With the EG33 in the WRX, I had full undertray and splitter and rear diffuser and also ran the vents like what you've posted. However the trick with mine was a bonnet diffuser in the centre section of the bonnet - this sucked the air out very nicely from the engine bay.

With the SVX - why not professionally fix the front bar with far more area for the air to get in if airflow is in fact a major issue? I'll most likely being that and adding a bonnet diffuser with mine if air flow to the radiator proves to be an issue.

[Dessertrunner]

Have removed

[BackWoodsBob]

rarely did the strings get 'pushed' or 'sucked' under the hood.

[bazza]

Originally Posted by Dessertrunner

Bazza,
Exactly do you mean with the bonnet scoop in terms of location and direction?

Basic diffuser in center of the bonnet works very well. If you put an air temp sensor in the engine bay you notice a huge drop in engine bay temperature which indicates this item working.


Something also like this, although this is very much over the top.

Originally Posted by BackWoodsBob

rarely did the strings get 'pushed' or 'sucked' under the hood.

You can see they tend to dogleg down or slightly to one side but it's quite hard to see exactly. To test properly you need to use dye like seen in F1 testing or a high speed camera in the engine bay with strings. High pressure
tends to want to find a low pressure and go towards it. Remember the in front of the radiator is high pressure. The engine bay pressure is generally lower and at the base of the wind shield you then have another high pressure. So given high pressure goes to low pressure, it's a bit hard for the lower engine bay pressured air to get out there. Anyway give it go, chuck some engine bay air temp sensors in and see how you go.


This is my undertray btw, looking from diffuser towards the front:

[BackWoodsBob]

Wow that's a sexy aero tray. How do you have it fastened to the body?

[bazza]

Originally Posted by BackWoodsBob

Wow that's a sexy aero tray. How do you have it fastened to the body?

Steel structure supporting the tray - then 8 x mount points. It can hold the cars weight so it's rather strong.

[BackWoodsBob]

Holy hanna! Aluminum I assume?

[bazza]

Originally Posted by BackWoodsBob

Holy hanna! Aluminum I assume?

Yes, mostly. Steel supports where strength was required

[SilverSpear]

Originally Posted by Dessertrunner

Anybody got some ideas on how to decrease the air pressure under the bonnet to increase airflow through the radiator?

Originally Posted by Dessertrunner

Bazza,
Exactly do you mean with the bonnet scoop in terms of location and direction?

Hello Tony, whenever I finish with the fuel tank issue that I am facing now, I will tackle the hood second, since the one that I have right now became trash with time (and old mistakes). I am planning to use whatever old stock that I have of fibermat and resin to redo it.

My main idea was to do a "classy" hump on the engine to make space for the future modifications that I am planning. With the large opening in my front bumper, making a large opening at the back of the hood hump (the side of the firewall) and an undertray which redirects up at the firewall level, I think my flow will never go wrong. What do you think?

[neverLift]

As Bazza stated, an opening at the base of the windshield/firewall/back of the hood, will not let air flow out of the engine bay. Depending on specific car design, there will not necessarily be full reversion/flow INTO the engine bay from this high pressure region, likely just a slow bleed into the engine bay. Even with an undertray directing the flow, it will not be able to flow out of the opening you create due to the high pressure region and will become stagnant.

But, unless the vehicle has little to no change in angle from the hood to the windshield, the airflow will not remain attached thus creating flow separation and turbulence which forms the high pressure region Bazza mentioned. Without active means, pressure can only flow with the gradient, i.e. high to low.

A vent/opening in the hood needs to be further foward, in a region of smooth/laminar flow.

Like the Corvette C6.R and new Stingray (note, colours in the flow-line picture correspond to temperature NOT pressure)



This CFD plot shows the pressures across the vent and windshield (red/yellow = high pressure, blues = low pressure).


cheers
-Evan

[SilverSpear]

Originally Posted by bazza

With the SVX - why not professionally fix the front bar with far more area for the air to get in if airflow is in fact a major issue?

Hello Bazza, what do you mean by front bar? Is it the front bumper?

Originally Posted by bazza

Basic diffuser in center of the bonnet works very well. If you put an air temp sensor in the engine bay you notice a huge drop in engine bay temperature which indicates this item working.

I am trying to dodge putting a scoop at the center of the hood since I want something practical in terms of car wash and water flooding the engine. The duct that I was thinking about is like 5 inches from the edge of the hood on the windshield side forward towards the center of the hood and not right at the windshield/firewall level.

Originally Posted by neverLift

But, unless the vehicle has little to no change in angle from the hood to the windshield, the airflow will not remain attached thus creating flow separation and turbulence which forms the high pressure region Bazza mentioned. Without active means, pressure can only flow with the gradient, i.e. high to low.

A vent/opening in the hood needs to be further foward, in a region of smooth/laminar flow.

-Evan

I was wondering, does this apply to all cars with different front end design? Or do we need to carefully explore the SVX case in details?

[neverLift]

Originally Posted by SilverSpear

I was wondering, does this apply to all cars with different front end design? Or do we need to carefully explore the SVX case in details?

It applies to *most* cars with some angle between the windshield and hood (i.e. it is not one smooth, flat, line).

Only a car with a design like this DeLorean concept would not have that high pressure region (though there would still be a disruption from the wipers and hood edge, even with the kick up on the rear hood edge)


Obviously exploring the SVX case in detail would yield a better idea of airflow around the view, but you can't escape from the flow separation that occurs. Airflow doesn't like to change direction quickly.

We can see in the flowlines below, there isn't a massive gap below where the airflow changes direction, but it still exists (its just not as bad as a 90* Jeep/Model T windshield). The shrouding of the wipers in the hood and heavy windshield rake is part of what gives the SVX its low cD.


The gap between the hood and windshield where the cowl is and wipers reside is also intentional, to increase and make use of the high pressure region. Most vehicles have their fresh air intake located there, using the high pressure region to force it into the cabin when the vent flap is open.

cheers
-Evan

[SilverSpear]

Originally Posted by neverLift

cheers
-Evan

Gotcha Evan. What if I fab a hood hump that corrects the airflow via the windshield. It should not be necessary a Deloran prototype design, but at least the flow just skews a little bit up. If you all can see, the SVX hood is designed with recessed lines as opposed to lets say... a BMW 5 series.

[neverLift]

A hump to soften the transition between the hood and windshield? I could see it definitely decreasing the size/effect of the high pressure region, perhaps to the point that you could draw airflow out of the engine bay. But this would be required trial and error/testing to get right. And it still would not be as effective as a design similar to the Corvette, partially due to the hood/windshield interaction (though as you have said this could be minimized), but moreso that the engine itself disrupts airflow through the engine bay. Ducting the air from the radiator directly out is the most efficient set up, though of course this may not be the best compromise for a street car dealing with rain/snow etc.

cheers
-Evan