learn me.... REAR MOUNTED RADIATORS

So this thought has been bouncing around in my head for months. Done a fair amount of internet-ing and not convinced myself that it will 100% work. The basic question is with nearly zero natural forced air flow (eg. vehicle at highway speed air forced through the radiator) will electrical fans alone pull enough air to cool a LSX V8 on the track or highway???  

The application for this is my lil LS1-KART. A shift to the back would remove ~50lbs off the nose and put it over the rear axle. I think the water pump, which i have upgraded would be fine - lots of 4x4 guys have done this. I can plumb it through the frame-rails, probably using marine exhaust hose/tube.  Looks like adequately sized radiators could be placed leaning forward, like the yellow stripe or leaning aft, like the red stripe (my preference). 

the blue lines illustrate what i perceive to be the available air flow.... with much more probably coming over the top than below...

placing the radiator aligned with the red or yellow lines will result in minimal forced air flow and i am not sure which direction (upward or downward).... i definitely do not want to suck/blow air towards the passenger compartment, the air flow must go aft .   So... the ultimate question is for sustained highway use (and eventually track) would this cool a 500hp aluminum block LS.... 

Minimal forced air flow until you put some ducts to it- any reason you couldn't have ducts in front of the rear wheels?  Looks like there's a nice triangular passthrough there , and pulling air from the sides would mean you're not stealing any from your wing.

In reply to ¯\_(ツ)_/¯ :

My first thought was to just use the radiator as a spoiler, especially given how vertical the spoiler appears. I’m sure it would be way more heavy than that spoiler though, compounded by how high it is. 

In reply to Pete Gossett :

I wouldn't put it high up, but if you wanted to get into aero trickery it could go all the way at the back and be used as part of a "powered" diffuser.  We did something similar on an FSAE car and calculated it made an entire 1/4lb of downforce when the fans were on 

¯\_(ツ)_/¯ said:

Minimal forced air flow until you put some ducts to it- any reason you couldn't have ducts in front of the rear wheels?  Looks like there's a nice triangular passthrough there , and pulling air from the sides would mean you're not stealing any from your wing.

i had that thought, its hard to see with just pictures, but they would have to stick super far out bc the A-pillar is the widest part of hte car. further complicating things how the bars cross each other. i dont think its impossible, it would just be obtrusive and i am not sure it would have a huge impact.  

¯\_(ツ)_/¯ said:

In reply to Pete Gossett :

I wouldn't put it high up, but if you wanted to get into aero trickery it could go all the way at the back and be used as part of a "powered" diffuser.  We did something similar on an FSAE car and calculated it made an entire 1/4lb of downforce when the fans were on 

so putting it aligned with the wing upright is also an option. aesthetically is probably the least pleasing and would place it higher than the other options, but it would probably get the most air flow that way. 

I think there is a good chance that i could delete the wing if i get the radiator in the back. my current thought is to move the radiator aft and modify the existing wing to make its legs bolt on to the chassis for possible reinstallation... otherwise, probably going to go with a smaller, aftermarket aluminum wing. 

Well, I cut various holes in the hood of my E36 SBF trying to increase cooling and reduce intake heat.  And I got nowhere.

I manged to get good flow through the ducts, but the air flow wasn't useful at the kind of motor heat levels I'm dealing with at the track. Surface exchange against the block, heads, headers, and intake weren't very efficient.  The bigger undertray, on the other hand, was a real prize in the radiator flow department.

So I might as well start trying to ruin my perfectly good trunk lid to see what kind of flow I could get for a radiator.

Maybe a spoiler on the trunk lid, instead of on the rear edge, to create some high pressure on the forward edge and feed air into holes there.  And outlets between the tail lights, where I already have low pressure.  With some ducting and BA electric fans, maybe it could work?

I can't believe one of us hasn't already tried switching to a high-volume oil pump and putting a loooong rectangular oil cooler out back as a lip spoiler.  A stock cooler from an E30 is about 20"x3" and costs about $50.  But I guess the plumbing would cost at least double that.

These were pretty common on '80s era turbo road race cars, like some of the Zakspeed Mustangs. Putting the radiators in the trunk left more room for an intercooler.

Years ago, a buddy of mine was planning to build a 2002tii along the lines of the old E21 320 Turbo "flying brick" Gr.5 racers. He had done a ton of research on them and we talked about it quite a lot (he wanted us to go in on the project together).

From what he showed me, BMW cut out the trunk floor and laid the radiators flat. There were two fans drawing air from side scoops in front of the rear fenders and ducting them down through the radiator and out the bottom. So, you can definitely flow enough air to cool an engine that way.

Not sure about a 500hp LS, but it works OK on the 4.6 Modular Ford in the Justang. We've never tried ours on long high-load runs like a track session. It has only been autocrossed but it maintains pretty well idling in grid. You'll definitely need good strong fans and try to direct as much air to it as you can. Don't forget to account for the fluid volume (weight) that adding all of that plumbing adds as well as the increased load on your water pump. It also takes a lot longer to get all of that water up to temp. Consider keeping the filler above the rest of the system whichever way you mount it to simplify filling and bleeding.

Seems like you could tape a bunch of yarn segments around the rear areas of your car and with the help of an onboard camera get some cheap but useful aero flow visualization.

IIRC some of the original UOP Shadow Can Am cars used the rear wing AS the radiator by using a slot in the leading edge to feed the radiator housed inside the wing upper/lower surface panels and then exited the air at the trailing edge. 

I don't think it worked very well and was abandoned shortly thereafter. 

IIRC there have been some issues with just using the water pump on the engine.  An auxilliary pump has been necessary

Reviving this thread!!!  

For those who have wandered through my LS1-KART build thread, i did move the radiator to the rear. I would consider it a success. 

Rear grip, much improved. Reliability of the cooling system, great. Ease of install, not impossible. 

Ability to cool the engine at autocross events and around town, flawless. 

Ability to cool the engine for an entire track day session, fail. 

Long story short, pushing the kart hard on track is causing the coolant temps to climb with no apparent ceiling.  When i back off a bit, and run cool down laps or take it back to the pit the temps normalize quickly. I think my primary issue is a lack of air flow through the radiator. 

I think the air is doing what i thought it might in the sketch that started this thread. 

To test that. i made a string line arrangement and shot a short video on the highway. i converted the video to 4x speed, to help visualization of the string lines. 

youtube link

That test seemed to confirm that there is a great deal of air coming back into the cabin and very little flowing towards the radiator. 

my current plan.. is to make a removable aluminum and polycarbonate (clear) panel to fit inbetween my rearward roll cage uprights. Kinda where the red box is.. 

thought being, that air, would then be forced down this panel as opposed to coming back into the cabin. When it runs down the panel, it will be forced to the face of the radiator. 

i think this is the least obtrusive and least hideous solution. anyone agree/disagree?????

can you give me a measure of the area of the face of the radiator?  (or just the height/width, and I'll go from there)

Based on my discussions with the guys at Ron Davis and this http://www.rondavisradiators.com/RadiatorInletDiagram_RD.pdf 

It looks like you may need to direct air to, and away from, the rad as some may be going around the obstacle (the rad) at higher seeds.

Might consider some way to separate yourself from the radiator and coolant lines. If either blow, melted skin could become a reality. 

sleepyhead the buffalo said:

can you give me a measure of the area of the face of the radiator?  (or just the height/width, and I'll go from there)

absolutely, the face is 25x16" (not including the tanks/shell). thanks!

I think you are looking at the airflow wrong between the cabin (or where it would be) and the wing. I expect something more like this. The upper streamline gets forced up due to the wing and never fully falls. The middle streamline isn't high enough to avoid falling, tumbles in the cabin much more forward than you drew, and then gets drawn out and upward by the wing. The third streamline comes in from the side, passes around the occupants, and then out below the wing, again getting sucked upwards after it exits. I think a vertical radiator very much like they run on off-road buggies behind the occupants would work very well without disturbing the airflow as it already is. 

Appleseed said:

Might consider some way to separate yourself from the radiator and coolant lines. If either blow, melted skin could become a reality. 

thats another benefit of putting the 'rear panel' on.. it would serve that purpose too. the lines run through the frame rail, so they arent a huge conern. 

L5wolvesf said:

Based on my discussions with the guys at Ron Davis and this http://www.rondavisradiators.com/RadiatorInletDiagram_RD.pdf 

It looks like you may need to direct air to, and away from, the rad as some may be going around the obstacle (the rad) at higher seeds.

agreed, whenever i redo it, will make the front shroud angle inward a bit. thanks. 

In reply to cyow5 :

Yea, agree! In the pic i posted today, the rear radiator is in place, below the wing. 

Its that mid stream i want to redirct down the backside of a panel and into the radiator. like wind going over a normal car and approaching a trunk mounted spoiler.. 

so, first comment first:
I don't think putting a single panel on the backside of the downtubes is going to do you much good.  The angle between those tubes and the 'roof' is just too extreme for the flow to stay attached.  And that's before we look "downstream" and see the wing.  I'm guessing, but I'm fairly confident (and you could test it with tufts), that any flow that sticks around the backside will only do so to a maximum around 1" below the bottom of the wing and then it's going to try and jump from that panel to the underside of the wing.  maybe
(it's possible the flow down there is just too turbulent at 60+ mph for even that to work)

So.... what might be some better options?

Go_Gators (Forum Supporter) said:

sleepyhead the buffalo said:

can you give me a measure of the area of the face of the radiator?  (or just the height/width, and I'll go from there)

absolutely, the face is 25x16" (not including the tanks/shell). thanks!

yeah, "we" just want to know the core area.  So, 25x16 = 400 square inches (sqin) of area.

So, most radiator design puts an inlet size at a minimum of 30% the size of the core:  400x0.3 = 120sqin

that gives you a couple options... maybe.  If you put a duct inlet on top of the roof, then the flow will be more willing to take the corner and get to the radiator.  I dunno the width between "upper cage arms", but if I assume around 24", then the inlet would have to be 5" high.  The problem is, that means you're blocking 24" of rearward visibility (most likely).  The good thing about this, though, is you'll definitely have "high pressure" at the inlet.

Another option is to "split" the inlet to snake forward between the cage and the wheel.  So, 120/2 = 60sqin
now, if we remember our multiples table... then an 8"x8" opening would be 64".  Which is ballpark; but might not be enough since we can't guarantee "high pressure" at the inlet.  9"x9" would mean 162sqin total inlet area or ~40% the core.  Reasonable first guess.  Feel free to sneak a 9x9 baking pan out of the kitchen late at night for visualization purposes.

Alternatively.... pi()*r^2  or pi()*(d/2)^2.... 3.14159*(10/2)^2 = 78.5sqin.  So, a 10" pie plate if you want to go "circular inlet".

But, there's some other issues with the flow/arrangement that "we" might want to look at, and think about... but that might take me until tomorrow AM to work up.  Hopefully, though, that's enough food for initial thoughts.