Radiator In Rear?
 

So, I'm in the middle of an interesting project, and am trying to figure out if everything I'm planning will work. The car is a Dodge Neon R/T. Its not quite a mileage project, yet a large part of the build should help mileage.

Part of the plan, is relocating the HVAC to the trunk of the car. This would preferably be done with an electric compressor, and would have the heater lines ran to the back as well. But I would also like to have a custom condensor built, and relocate it to the rear as well. My thoughts are to use the vacuum behind the car to suck the air through it. The car will have a full undertray, and would use fully sealed ducting to get the air from under the car, to an exit in the rear bumper.

The question is, would it also be plausible to do this with the radiator as well (of course using a custom small multipass radiator), and therefore be able to completely seal up the front grills? I'm willing to do the work, if there is a chance that it coule work. I'll get some pics of the areas I'm working with as soon as its not raining.

For an understanding of the level of dedication to this project, I am already marking out where I will be cutting into the wheel wheels and pushing into the front wheel wells from the cab of the car with new meterial, before stitch welding the whole thing.

When rally cars do it, it can usually work out pretty well.

http://farm3.staticflickr.com/2598/4...2c498b355f.jpg

Hmm, seems I need to wait until this summer, and then make sure the car is at final weight, and climb the grapevine with the ac on and see what happens temp wise. Then figure out how much radiator I really need. My only concern is that this is going to be a heavy car (33-3400lbs with driver).

rear
 

You might want to study contemporary urban and intercity buses as they all have rear heat exchangers.
Also,Ford's Probe-IV concept car of 1984 had the radiator and condenser ducted into the rear quarter panels,with electric fans which could push the car to 4-mph if I remember correctly.
A thought.If you were to pull a perfect draft behind a semi and the mpg was so remarkable as to compel you to proceed with the project,then great.However,there may be purely stock cars whose mpg already exceeds the theoretical maximum which the Neon demonstrates in the draft.

Currently I am able to get 44-46mpg average on the freeway, at 70mph in the neon, with small aeromods. However, offsetting that, its a little heavier than stock, and has a set of hotter cams, which are soon being removed for stock. Its also a DOHC, which gets worse mileage than the neon SOHC. Although I do have the lower geared tranny from the SOHC in the car, with a 8 pound lighter aluminum flywheel, and no power steering. I dont generally draft (and wont), but on my latest trip to Anaheim, a van did get closer than I'd like when they cut over, and the scangauge jumped to 56mpg, and stayed there until I could get around the van.

There are definately better starting points from a mileage perspective, but there arent many cars that are better for my other half of the project. The car is being built as a national level sound quality competition car. This car was the best compromise between mileage and ability to carry weight. There isnt a budget for this, so my goal to reduce mpg's is purely a personal goal, to get as good of mileage as possible, while simultaneously trying to have the best sounding car in the country.

The engine will need to be rebuilt, and when it does, it will have a multitude of friction reducing and ceramic coatings from Swain Tech applied throughout the engine. It may not be noticeable, but it should help. The body of the car is being extensively modified, including stich welding the whole thing, as well as building all aeromods out of fiberglass, as smooth as possible. I'm not worried about getting my money back from gas savings.

The biggest reason I asked is because the HVAC is being moved to the trunk of the car no matter what, for audio reasons. Need the room to rebuild the dash. While I'm at it, if its possible to reduce drag, even better. The nice thing is the aeromods also affect the stereo. I noticed just from the little bit of tape from the top, the whistle from the externally vented and currently speakerless kick panel enclosures went away. Took the tape off, and it came back, put it back on and it went away. So I'm hoping for the best mileage possible, and an extremely quiet car.

I will be trying this weekend to start working with fiberglass on either the front bumper, or the rear wheel skirts.

The car stock.
http://i254.photobucket.com/albums/h...9/DSC01430.jpg

Temporary duct tape mods.
http://i254.photobucket.com/albums/h...9/DSC01431.jpg
http://i254.photobucket.com/albums/h...9/DSC01432.jpg
http://i254.photobucket.com/albums/h...9/DSC01433.jpg
http://i254.photobucket.com/albums/h...9/DSC01434.jpg

rear
 

For the sake of argument,let's say that your cooling system constitutes 12% of the overall aero drag.If that were totally eliminated by moving it aft,at 55 mph,the 12% lower drag would net you an extra 6% in mileage.A little more at higher speed.

I was wondering how much benefit it would actually be, considering the fact that the front bumper still needs a flat portion for the license plate to go back on. The other thing I was wondering was if the radiator in the rear would have enough cooling capacity at low speeds or idle.

Heres another question. If the radiator stayed in the front with a much smaller and ducted grill, does it make any sense to move the condensor to the rear, knowing that the HVAC box is going to be in the trunk no matter what? Or should it just stay in the front, and keep the rear of the undertray closed?

Hi TOOSTUBBORN2FAIL,
Obviously rear engine cars; VW, Fiat or Corvair..., had enough cooling. But when VW made a water cooled Vanagon they kept the rear engine but put the radiator up front.
Anyway, consider how much fan you'll need. If your 3300 lb car needs about 6 hp for rr and 12 hp for aero on the flat at 65 mph. Then on a 7.5% grade it will need about 63 hp total. Generally you can expect to dump out the radiator about the same hp as the engine is putting into the tires. And the temperature rise through the radiator should be less than 15°F.

The thermal capacity of air is about 0.00042 hp•min/°F/cu ft
63 hp/0.00042=150000 °F•cu ft/min
for 15°F temp rise you need about 10000 cu ft/min through the radiator.
If you have a thermometer you can measure the temperature across the radiator and get a better approximation of how much airflow you'll need. An electric fan capable of blowing 10000 cfm through a radiator takes a bit of power. If your aero drag at cruise is 12 hp and the fan needs 1.5 hp, then your front end radiator must represent more than 12.5% of aerodynamic drag for there to be a benefit in moving the radiator.

But you are planning on moving the A/C condenser and using an electric compressor, so you'll need air flow for that anyway. You'll have plenty of electrical power (I assume) so you could remove all the original A/C stuff and build ducting for a portable or window type room A/C in the trunk and plug it into an inverter. So I'm thinking the same as you, why not move the radiator too? You won't win or lose much mpg, but I think it is wonderful and crazy.

-mort

Call me crazy, but I like the look of your duct tape mod.
I'm sure it looks worse in person, but from the picture ( on my smartphone - no wonder eh ? ) it is hardly noticeable since you used white tape.
The grille looks much better plugged up like that to an aero trained eye.

As an artist, I can also appreciate the symmetry of the modded grille. The side by side, centered grille openings mirror the stripes above the bumper. It's a well balanced look.

Looking forward to your progress !

Thanks mort, that was my thought, if ones going to the back, I didnt think it made sense to keep the drag from the radator at the front and then add another heat exchanger in the rear, even if it just breaks even. I cant help but think I'll get a little something though, using the cars wake to pull the air through.

Just curious, your saying that from the inlet to the outlet, there should be a 15 degree difference in temps? I could probably do a rough test of that pretty easily.

Thanks, ya its actually not too back. You can tell, but most people dont even notice the grill, just the skirts. The fiberglass will be more obvious since that will include full skirts, but I'm not worried what people think of the looks. The thing definately coasts better, and the kick panel noise is gone. Hopefully with the bottom sealed up, I can use less deadening as well, allowing me to strengthen other parts of the build more.

You post with lightning speed ! I just edited my previous post.
( I have a bad habit of editing post )

And everyone on the site says in one voice " You are going to tuft test the car right ? "
We all love tuft tests on this site.
It's the ultimate nerd porn.

Seriously, if you tuft test the car please share your results !
Tuft testing on your car will show the problem areas you have to work on .

Tuft tests will eventually be done once the weather is more up to it.

Depending on the weather this weekend, I may block more and more of the grill to see how far I can go. The upper grill can go for sure, but I'm curious about the air that goes in under the bumper.

Hi TOOSTUBBORN2FAIL,
The 15°F air temp differential is just an educated guess. If you can think of a way to measure the air temp. in the radiator outlet flow, maybe 5 different spots averaged, then you'd know.
There isn't really much vacuum behind the car. Imagine the back of the car is just the trunk lid height and width, say 7.5 sq ft, and the total aero drag force on the car is about 90 lbs (at 65 mph). If all the aero is suction in the wake on just that tail part, that's about 0.08 psi. So you build your radiator to fill the back of the trunk and the air is sucked through, that air comes from somewhere, duct work from under the car? Is that air at a high enough pressure to push 10000 cfm through the radiator? I think you need a fan.
However, this is a good idea. If you can insert air into the wake volume, using a few hp fan, and remove the radiator from the front of the car then there should be some reduction in aerodynamic drag.

You compared removing the radiator opening with the flat license plate. The license is smaller, but if it is centered it adds almost 0 to the cd. Somewhere on the front of any blunt object is a small flat area, and it can be quite a large percent of the frontal area before it makes much difference. Also the standard cd for a flat plate, considering it is on the front of a long object is 1.0. The opening for the radiator is a different problem. The radiator dumps heat by causing the air to be turbulent as it passes through. Basically radiators have the worst cd of anything on purpose. So the cd for a radiator opening maybe 2 or 3. Modern cars reduce the drag due to having to have a radiator by using slat vent like openings and setting the radiator at the back of a plenum, then suck air through with a fan when needed. It helps, but the biggest single contributor to drag at the front of the car is still the radiator.
-mort

enough/sense
 

The electric fans would cover the periods of idling and low speed operation so that part is covered.
If you do the Walter Korff full-width,1/6th height opening,airtight inlet duct you'll have the drag minimum for the inlet side of the cooling system.
Obviously,if you maintain the original condenser location,then you've saved some work,except for the tubing extensions to the rear.
If you went ahead and did the trunk-mounted heat-exchanger,you could try and mimic a trick Ferrarri has done with their 599XX racer.This car has a porous belly pan diffuser in which two trunk-mounted electric fans scavenge air from,then blast out to the wake area in the locations vacated by the tailights.They claim that by pulsing the jet,that the wake can be momentarily detached from the car.
They make no drag benefit or speed claims,but the idea is tasty.It has been known for decades that a suction slot at the back of a car can beneficially alter the flow for reduced drag.
You might want to look at that Ferrarri.

Unfortunately there isnt any room for anything in the trunk, it would need to be under the car, and vented through the rear bumper. Hence the need for a thicker but smaller rad and condensor. It would need to be custom.

I'm familiar with the ferarri, I have multiple pics of it on the computer. I'll look up Walter Korff. Anything I do, front or rear, would be fully ducted as smooth as possible with fiberglass. Gives me practice that I need, since the dash, center console, and rear seat area are being rebuilt out of fiberglass, as well as the front and rear bumpers, and wheel skirts.