Trunk mounted radiator for mid-engine car?

Hi all,

I have a Honda v6-powered Porsche 914 track car. Im exploring putting the radiator in the back of the car.  Up front there are a lot of issues with safety and space that Im trying to resolve. I would love to do some real CFD on this but I wonder if anyone has some aerodynamic intuition about this idea.

Id like to mount the radiator in the engine compartment and then duct up through the trunkl-id up to the spoiler.  Ive seen a lot about pressure zones in the front of cars, but very little about the rear and trunk area of cars.  My 914, of course, has no lower floor in the engine compartment and that big immediate dropoff off the roofline at the rear window.

How bad of an idea is this? Anyone have intuition or immediate thoughts?  Its not lost on me that the most efficient place is at the front of the car but the tradeoffs are really high, Id like to find a more innovative solution.

The following pics are just mock-up and photoshopping:

Your general layout will work, but will require some sort of duct/scoop sort of panels under the car to help drive air thru the radiator. Even though there is low pressure behind the cabin, there is also going to be low pressure under the car.

An alternative would be something like a duct on either side of the B-posts or over the roof to capture air for the radiator. It could be vented thru the trunk/rear fascia.

Your initial sketch is less visually destructive to the  cars shape.

I think louvers in the hood would help a lot since they can act like a bunch of little Gurney Flaps and generate low pressure on their leeward sides. For getting air in, side scoops would work, too, and I love the look. 

I'll throw out a less visually appealing idea: instead of leaning the rad toward the cabin, I could lean it the other direction toward the rear bumper and then duct the lid down into it, venting out the rear center panel of the car.   The goal would be to direct the airflow into the radiator as it passes over the roofline instead of trying to suck it out from the engine compartment up over the spoiler.  Would it flow better?

MichiganMat said:

I'll throw out a less visually appealing idea: instead of leaning the rad toward the cabin, I could lean it the other direction toward the rear bumper and then duct the lid down into it, venting out the rear center panel of the car.   The goal would be to direct the airflow into the radiator as it passes over the roofline instead of trying to suck it out from the engine compartment up over the spoiler.  Would it flow better?

That was the second line of my comment above. You will need to direct some flow down from the roofline to get the flow you need. That means some sort of duct that could be molded into the panel over the trunk.

For reference, pics are an air cooled VW bus with an EJ25.  Like you I didn't want to put the radiator in the front because the entire package just didn't want to have a radiator there, nor did I want to put the radiator horizontal under the floors because of potential damage.  I could not arrange the radiator like an old rear engine bus (side mounted fore-aft) because there was not enough vertical room.  So I thought I'd put it behind the engine and be able to get air to it.

The problem is that the entire engine compartment is designed for air to come in to the scoops at the very rear/top of the van and flow downwards and out of the bottom of the engine compartment (the baffles forcing the air through the fins of the engine).  To get air to flow through the rear mounted radiator and out through the two slits in the decklid required:

- rad outlet ductwork very well sealed to the decklid inside surfaces

- air diverters at the base of the air scoops on either side to force inlet air to the front of the radiator

- flap on bottom of radiator to 'encourage' the air to go out of the rad exit instead of spilling straight down the front face of the radiator to the ground

It works, but it took a lot more effort than I would have thought.  For yours I would just bite the bullet and put a radiator in the middle front, or two small radiators in the front corners, or two small radiators in the rear quarters in front of the flares

You can make air go wherever you want with well sealed ductwork as long as the inlet is at higher pressure than the outlet, but actually doing it can be a huge pain. 

I suspect everything will get less filthy ducting from above the car than under it...

Obviously, neither of these data points are a modified V8 powered porsche

Renault 8 and 10 cars use a trunk mounted radiator behind their "rear" mounted engines

Air was pulled in from under the car in the engine bay and exhausted through the engine cover via the visible vents on top

Rear engined Fiats had the radiator located in an odd place. Up against the firewall 

The air flow path was air came in through the vents in the deck lid, was pushed through the radiator by a fan and hit a duct that routed the air down and then back out the rear of the car

Both of these solutions worked just fine. 

My experience is, Without natural air flow over the radiator, you will need much more fan than you think. In my car (turbocharged Fiat 850) there was no Spal fan that could possibly fit that would move enough air over the Fiat radiator.  I put in a front mounted rad on the rear engined car and everything became very easy

I'd probably start with one of the behemoth OEM fans (like the Taurus unit) that pull almost 50 amps on startup

I vote for grbp's idea of small rads side-mounted in front of rear fender flares. Getting air in would be super easy, though it forces you to change the shape of your flares. I'm having visions of the box flares found on BMW 3.0 CSL and others:

or some variant thereof.

I say just go front mounted,weight up front isn't going to hurt the weight balance(improve it more likely) and fighting with cooling issues at the track sucks.

 Also robbing air from above the engine cover to feed ducts for rear mounted reduces the available air to feed the underside of the wing.

Don't underestimate how much cooling just the airflow over the engine itself is doing.  If you're pulling air that's flowing off the engine through your radiator that's going to make cooling it that much harder.