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Cooling package ductwork

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Hi All,

I just watched the recent Racecraft Video on YouTube on ducting, I was interested particularly in the ratios for size of the opening aperture and the length of the duct for the entry.

When it came to the exit from the coolers this wasn't mentioned in the same detail, do the same rules apply, in that ideally the exit would be the same length as the coolers are high and that the exit aperture would be 1/4-1/3 the area of the cooler?

Cheers :)


Hi Mike, I hope you found the ducting video helpful! Here is the video link for anyone that wants to watch the video.

As a baseline starting point, the rules of thumb for sizing the inlets and minimum duct length also apply to the outlets. One thing to consider on the outlet is the change in energy within the fluid after it has passed through the cooler(s) of which there are two competing effects.

1. The fluid has had some thermal energy added to it because it has had its temperature increased (because of the heat exchanger interaction)

2. The fluid has lost some kinetic energy because of viscous and turbulence losses from passing through the coolers and ducting

Both of these competing effects have opposing effects on the ideal duct outlet sizing.

If you want to get really carried away, to properly size the outlet, you are trying to get the outlet flow velocity to approximately equal the free stream velocity in the vicinity of the outlet (which obviously isn't necessarily the same as the average free stream velocity of the vehicle). This requires a lot of CFD or wind tunnel time, which restricts this analysis to teams that have access to these resources. In this case, they will spend time tuning the inlet and outlet sizes for different drag and vehicle configurations.

So, in summary. For the rest of us in the real world, if you make the inlet and outlet areas similar, make the duct lengths as long as possible (ideally at least equal to the high of the heat exchangers), make all radiuses as large as practical and all transitions smooth, you will get good results!

Hope that helps! 💪

Hi Tim,

Thanks for the detailed and quick response! Cheers :)


Hi Tim,

Have you also included a fan in this setup? How would this affect the design strategy?

Background is (small) rally cars: Extended high load-low speed sections, followed by a few mid-speed stretches plus occasional traffic and idling times make the use of a fan a must.

The benefits of ducting are clear, but in the 30~100km/h zone, is there any benefit to be had from extensive duct work?

From my experience, a larger rad, a bigger fan and an increased cross section thermostat will do most of the job and basic ducting to guide the air through the rad and potentially shrouding the fan, are the next steps.

Usually, bonnet mods for exhausting are not allowed, so evacuating the hot air is also tricky.

Regarding fans, i lean towards front mounted ones, for extra space in the bay and that extra bit of impact protection in front of the rad. You have a hope to nurse the car back to service, without a fan, but not with a pierced rad.

No problem Mike!

Armaki - Yes we have a pair of fans mounted within the outlet duct. It doesn't have much affect on the design strategy apart from placing another small restriction to disturb the flow.

The speed range for where the ducting becomes beneficial - this would be different for every application. But I would think that at any forward speed where the cooling system isn't relying on the fan to help the flow the ducting would be helping the situation.

Packaging and rules always have a big impact on the cooling system ducting. Even if you don't have the space to build a fully ducted system, then at least sealing the radiator to the inlet and fan to maximise the flow is a good starting point. Mounting the fan to the front for a bit more impact resistance makes sense in your application too! 😅

Thanks for the input Tim,

Assuming no other type of ducting, would a fan shroud on a front mounted fan have any benefit? I believe not.

I would integrate the fan to the outlet ducting, as a kind of shroud, but at the front.



Without seeing a picture or diagram of the layout you propose it's hard to say if that would help. However, my first thought is that I agree with you about a shroud for a front-mounted fan. It would likely improve the cooling in the situation where the fan is providing all the flow (car stationery or slow) but would hurt the cooling in other situations.

Any recommendations for material to use for mocking up parts and for straight pieces to “block” flow? Was thinking of something like black corrugated plastic, but not sure it would be stiff enough. Would eventually transition to fiberglass or carbon fiber

Hi Aaron,

Depending on the span and stiffness required, there are lots of affordable options. Perspex or Polycarbonate sheet is cheap and easy to work with. Another favorite of mine is aluminium/plastic sandwich board material, I've put an image below for reference. This is what you'll often see signs and trailer bodies made out of. Being a bonded composite it's very stiff and as a bonus quite easy to work with! A bit of Googling around your local area is bound to find a few different suppliers.