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Cooling recommendations - it depends

Posted: 04 Apr 2018, 10:24
by Midlana1
The attached recommendations were found on an offroad site ( I like to read articles like this to learn more, but in this case, I feel it's pushing unbalanced advice (never mind pushing only one brand of radiator, but whatever). I'll explain and then you can decide for yourself.

A fan shroud is always recommended as a general rule. It ensures air is drawn (by the fan) through the entire face of the radiator and not just the portion under the fan itself. True, and necessary for an engine that does not receive enough airflow on its own. You always see shrouds on earth-moving equipment, stationary engines such as generators, and on cars used in very low-speed conditions such as off-road vehicles, delivery vans, and taxies. For some reason however, the downside of a fan/shroud is never discussed with a passenger car in the environment where it spends most of its time, at 40-70 mph. At this speed, a fan isn't necessary at all, and in fact, the fan/shroud hurts radiator effectiveness by at least 20%*.

If the car can afford this drop in cooling efficiency at-speed in order to enhance cooling ability at idle, that's great; I just wish they'd spell that out. Fortunately, most cars have radiators much larger than necessary while at-speed in order to handle stop-and-go conditions. In the case of a track car though, a shroud needlessly inhibits the cooling system. Whether or not to have a fan on a track car "depends", since it blocks part of the radiator, but comes in handy during pit stops if the engine's kept running. Because Midlana is driving on both the street and track, I compromised and use a fan, but no shroud.

Oddly, their list of rules don't address something at least as important as a shroud: preventing air from bypassing the radiator.

Lastly, though my book contains a pictorial of the cooling system, the same off-road website has a nice clear figure showing virtually the same cooling system, as it applies to an LS installation.

* This is the case of a square radiator and a cooling fan that fills the face as much as possible. For example, a 12" x 12" radiator has a face of 144 sq. in., and a 12"-diameter fan on this same radiator consumes 113 sq inches, leaving 31 sq. in. blocked off by the shroud, and not working at-speed. A shroud placed on the radiator recommended for Midlana reduces its effectiveness by a third!

Re: Cooling recommendations - it depends

Posted: 04 Apr 2018, 13:07
by rennkafer
It's not really surprising to see a group oriented towards low-speed offroading making those recommendations. They generally aren't moving fast enough to generate their own air flow, so they depend on a fan/shroud for it. Not blocking out air going around the radiator is fairly low priority as well, for the same reason, they're not going fast enough to be losing much.

I'd be interested to see where you got your "a shroud blocks radiator efficiency by "X" percent" figures from. A well built shroud might slow the flow somewhat, but it certainly wouldn't block the flow enough to lower efficiency by 30%. Mind you, a lot of the aftermarket shrouds you see are just crap, basically a flat plate across the back of the radiator with holes for fans. That's just poor design. You can also mitigate some of the blockage by using flaps like VW/Audi do on many of their shrouds. At speed they just lift out of the way to allow freer flow. For a dual purpose car, particularly in a hot climate, I'd be running both a fan and shroud. For a track only car, I'd have a fan with no shroud over maybe 1/3 of the radiator just for those trips to the hot pits where I didn't want to shut down the car.

Re: Cooling recommendations - it depends

Posted: 04 Apr 2018, 13:47
by Midlana1
Fair point - I'm going by total radiator area versus that after a fan and shroud are added. Granted, I may be wrong in thinking that blocking off X% of the core equates to that same X% not being utilized. Like you say, it also depends how well the air gets in and out.

I had totally forgotten about VW's doors in the shroud, a very clever and simple solution, and will keep it in mind.

Re: Cooling recommendations - it depends

Posted: 10 Apr 2018, 08:52
by ShadowCat38
My installation will be using rear cooling rather than piping coolant all the way to the front of the car. The entire nose cone of my build will likely look a lot different than Midlana as a result, and will probably not "fit the mold" of most Super 7 clones, either. Since my radiator(s) will be in the back, I won't benefit much from ram air effects, even at highway speeds. Think more of the Lamborghini Aventador radiator setup below the taillights. I'll need to draw in some air forcibly to meet my cooling requirements, not to mention pull in cooler air than what will be readily available in the engine bay, and shrouds go a long way towards improving fan efficiency. While I know my case is certainly not the norm, my special case has to have the shrouds to work correctly at all.

While pirate4x4's article is sort of presented as a one-size-fits-all, I don't think that is necessarily the case, either. Every application is unique, and the intended use of the vehicle should certainly be a driving factor for how your particular cooling system is designed.

On a third note, and possibly more detrimental than air bypassing the radiator is air recirculation from the outlet side of the radiator back in front of the radiator to pass through again. The air is already heat-saturated, and cannot absorb much more heat from the radiator. Even with excellent air flow through the radiator itself, if the design aft of the radiator stifles the passing of heated air, it will make it back in front of the radiator without a shroud. If the design is bad enough, this is even a problem at highway speeds. I don't think Midlana, or cars like it, will have that problem at highway speeds, but the lack of a shroud, or even something of a tunnel for air to travel through that separates it from the front and rear of the radiator, can cause difficult-to-troubleshoot cooling problems at lower speeds (stop-and-go, track idle, etc). As long as the nose cone is sealed around the radiator it covers with formed guides, gaskets, or just a good fit, this really shouldn't be an issue. If you leave gaps around the sides of your radiator inside that nose cone and don't use a shroud, you'll likely have recirculation problems.

Posted: 03 Jul 2018, 08:33
by TRX
Aerodynamically, there's nothing wrong with a rear mounted radiator. The underside of the car is usually a high pressure area, and the rear is always a low pressure area. You get a lot more natural airflow through a rear mounted radiator than a front mounted one, where it's easy to get air in, but much more difficult to get it out...

Unfortunately it's not all sweetness with the rear mount. The air you're picking up from underneath the car (assuming you're not using side ducts) is heated by the road surface. And the engine and exhaust system put out a *lot* of heat. So the efficiency of the radiator would be greatly reduced.

Some rally cars have moved to rear radiators, which seem to work fine... but they're mostly front-engined cars, and they're moving the radiators to make room for humongous intercoolers, and possibly to shift some of the weight rearward.

Leaving the radiator in the back gets rid of the troublesome radiator piping, pump losses, cockpit heating, eventual rust or pinholes, etc. Plus it leaves more room up front for storage. But as far as I know no OEM mid-engine car has ever gone with a rear mounted radiator.

I hope to be able to do some back-to-back comparisons, if I can ever get back to my build...