CatDaddy
CatDaddy New Reader
12/2/21 4:45 p.m.

 

I am thinking about rear mount radiators.  (Not in a drift setting.) 

the top of the trunk is relatively higher pressure, right. The bottom of the car is lower pressure. The more we optimize aero, the lower the pressure gets under the car and the higher it gets above the car.  
 

So, if we duct for a rear mount radiator should we try to isolate the pressure to a horizontal plane? 
that's what Autobahn did with their JZz30. They duct in from one side, but I don't know how they make it exit...

 

thoughts? 

matthewmcl
matthewmcl HalfDork
12/2/21 5:15 p.m.

The area of the trunk lid is behind the rear window. That area is flow spreading apart and is therefore low pressure, similar to the back of the car. Side ducting is easier to make work (easier to scoop or naca inlet) and therefore more common. Vent out the back face of the trunk area (typical license plate area).

CatDaddy
CatDaddy New Reader
12/2/21 5:39 p.m.

In reply to matthewmcl :

That's an excellent point and does make perfect sense. Of course those 2 areas would be lower. In through the side and straight out the back! 
Autobahn added a trunk spoiler when they went to a rear radiator I believe.  Perhaps accentuating the rear exit of the radiator?  
running a 52.4xx at Tsukuba with an iron 2Jz up front, they must be doing something right. 

iansane
iansane HalfDork
12/2/21 5:44 p.m.

Nothing functional to add other than BMW experimented with it when the shoved a v16 in a 7 series back in the day. Obviously just an exercise in excess. I love excess.

 

Same idea though; in through the side out through the back.

RossD
RossD MegaDork
12/2/21 7:01 p.m.

Renault 8 is a rear engined and rear radiator mounted car.

The black partition seals off to the trunk lid and the space between the bumper, and that portion of the lid has vents. I cant recall if the fan is pushing or pulling...

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
12/2/21 7:59 p.m.

How fast do you plan on going with this vehicle?

'Cause, I mean, in an autocross/rallycross like environment, ram air is insufficient anyway, you need lots of fan no matter where the engine is.  Or enough coolant "sump" to handle a short run without completely boiling over and enough fan to cool off before the next run.

My 280hp Mod-class RX-7 has severely mongoid fans.  I've done long dyno pulls with very little measurable coolant temperature rise, and it generally can finish a run at 210 after starting at 190.  My 100hp Prep-class RX-7 is not allowed to have a better radiator or mongoid fans, it finishes a run at about 230F after starting at 180.  This is with front mounted radiators, of course, but the point is, low speed means you are relying on the fans for your airflow, not aerodynamic-guided flow.

 

I wonder what the Euro rallycross guys do, since they usually have rear mounted radiators so they don't get holed by gravel spray from the traffic ahead of them.  The really nice thing about rear mounted radiators is that you have a lot more room to make them BIG, and you can have decent ducting before and after the radiator instead of having a bumper in the way in front and an engine blocking the airflow exit.

Honsch
Honsch Reader
12/3/21 3:31 a.m.

Here's what we did:
Scoops on each side.  It's hard to see but the scoops extend to the same width as the rear fenders:

The exits are ducted from the rad box and exit the car like this:

And yes, the rad box needs fans:

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
12/15/21 8:00 p.m.

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

Honsch
Honsch Reader
12/16/21 3:41 a.m.

We used 1 3/4 tube for the run back to the rad specifically to lower flow resistance.
Would something smaller work?  Probably.  We also like having the extra capacity.

hybridmomentspass
hybridmomentspass HalfDork
12/16/21 9:21 a.m.
Pete. (l33t FS) said:

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

SW20 dont have anything fancy to move the coolant, just pressure. 

The idea of mounting it in the rear (of a front engined car) sounds good because you DO have an additional 18 or so feet of hose for it to bleed off some temps, assuming your radiator will/is still getting airflow. 

cyow5
cyow5 Reader
12/16/21 9:55 a.m.
hybridmomentspass said:
Pete. (l33t FS) said:

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

SW20 dont have anything fancy to move the coolant, just pressure. 

The idea of mounting it in the rear (of a front engined car) sounds good because you DO have an additional 18 or so feet of hose for it to bleed off some temps, assuming your radiator will/is still getting airflow. 

The Elise and Exige suffer from this "benefit". The aluminum chassis acts a little like a heatsink around the coolant hoses but then that just adds heat to the cabin right next to the driver. It isn't a lot and surely not enough to make a difference to the engine, but it sure does make a difference to the driver!

If you really wanted to augment the radiator, you could add a finned line section instead of rubber, but you'll want to make sure that newly heated air has somewhere to go. 

DjGreggieP
DjGreggieP HalfDork
12/16/21 10:05 a.m.

I've seen lots of mud runner guys mount the rad on an opposite angle to the 'roll-bar' in the back of pick up trucks. Most have the OEM shrouded electric puller fan mounted as well 

This is also on very sketchy mix and match lifted trucks used exclusively in deep mud holes with copious amounts of alcohol involved (both during their creation and use) so no idea if they put much thought into if it does anything other than it stops it from getting it covered in mud and blocked in seconds in the mud hole. 

APEowner
APEowner SuperDork
12/16/21 10:16 a.m.
Pete. (l33t FS) said:

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

Feieros didn't have anything special for water pumps. 

Many Formula Fords have front mount radiators and they  use a water pump designed in 1959, and have no cooling fans.

Here's the water pump in my Crossle 32F.

The coolant runs through these aluminum tubes.

To and from the front mount radiator

Keith Tanner
Keith Tanner MegaDork
12/16/21 10:37 a.m.
Pete. (l33t FS) said:

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

It's backwards, but having a Subaru engine in the back of a Vanagon with a front-mounted radiator doesn't seem to be problematic for the water pump. The rod bearings last just as long as they do with the radiator closer to the engine :)

APEowner
APEowner SuperDork
12/16/21 10:49 a.m.
Keith Tanner said:
Pete. (l33t FS) said:

Question for those who have done it.  How does the front radiator designed cooling system hold up to having a 10 foot long lower radiator hose?

I've always been picturing needing to have an electric water pump mounted in the rear to help generate head pressure.  But then I don't think Fieros or MR2s had anything special cooling system wise compared to front engined cars with the same engine.

It's backwards, but having a Subaru engine in the back of a Vanagon with a front-mounted radiator doesn't seem to be problematic for the water pump. The rod bearings last just as long as they do with the radiator closer to the engine :)

Is that long enough to even get up to operating temperature?

Keith Tanner
Keith Tanner MegaDork
12/16/21 3:33 p.m.

In reply to APEowner :

It's long enough to get you far enough away from home that getting back is a real problem.

DaveEstey
DaveEstey PowerDork
12/17/21 3:52 p.m.

I ran rear mount rads in my hillclimb RX7 for years and switched back to front mount. It was better up front and shed a not insignificant amount of weight that way too. It's very fan dependent in general. I also ran a Davies Craig water pump with a PWM control to handle moving water around. 

 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
12/17/21 9:41 p.m.

In reply to DaveEstey :

I have been considering a rear mount radiator for things for which a front radiator simply is not an option.

My Quantum, for example, has its engine shoved as far over to the right as possible, and is angled to the right, and has the intake and exhaust ports on the right, to make room for a too-small radiator on the left next to the engine.  (It about lines up with cylinder 2, the alternator sits in front of it!)  This makes for a bad cooling scenario, even before all of the little cardboard ducts disappear, and there is very, very little room for a turbo and downpipe and airbox all over on the right.  VWAG's first DOHC head was for this chassis and the compromises made for all that were carried forward, which is why the timing belt driven 4 and 5 valve engines had such goofy valve angles and cruddy port shapes.  And that application (the Sport Quattro) had the radiator cockeyed at an angle towards the fender in order to make room for the intake manifold!

The Evolution 2 rally car had a radiator about 2-3 times as large in the boot, which allowed room for a really, really nice intake manifold.

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