Efficiency of Radiator Ducts with Air Vents

For those of you with vented hoods (deep dish or otherwise), have you noticed that the vent air seems to be warmer when compared to no vented hood? Dial has been in the same auto position, and I tried it on manual setting as well. If so, what fix have you come up with if any?

I am experimenting with different radiator ducts to see which will resolve the issue but it seems the more you can enclose/place the vented mouth of the hood in the radiator duct the more you solve the problem.

I have the ProCar hood and did several measurement using the DaliRacing bucket radiator duct (had to trim the bottom for my application) and a home made one with a cardboard - the latter resting on the spare tire bracket and going all the way up to the the fuse box level with two bendable side arms protruding forward. I even tried a simpler version by just using an extension lip (V2 picture) with the DaliRacing bucket; it sits right inside the bucket. It seems the cardboard options were more efficient in reducing the warm air from the vent than just the DaliRacing bucket :tongue:

When comparing the two with the NSX-R version (LHD or RHD - see picture), notice that the NSX-R's side arms angle upward and "may" enclose the vented hood. The vent edge sits on top of the hood latch and the NSX-R arms are situated below almost at the latch level on their highest point. The rear of the duct is only 1" higher than the DaliRacing bucket and that may be enough to solve the issue but I haven't tried the NSX-R version with my current radiator fans.

I find the NSX-R version to be less efficient than the DaliRacing bucket for it leaves wider openings on the sides, and in my particular application where I have an enclosed dual spal fans, the right arm of the duct sits in-front of the radiator hose (Dali's sits behind it - see picture) hence blocking some of the air from the fan.

Before I fabricate one that is not cardboard :biggrin:, any thoughts? Aero engineers are welcome :wink:

It is a little hard to understand your post clearly. You use the term vent to describe the duct of a ducted hood and then ask people if they have a warmer air coming from their vent with a ducted hood than without.

Is this a fair way to restate what you are asking?


Is it correct to assume your question has nothing to do with aerodynamic efficiency (lowering the car's CD or increasing downforce) or thermal efficiency (pulling more heat out of the radiator) and is all about the climate control temp?

Ok, thinking your question is what I've paraphrased...

When the climate control is not in recirculate mode it pulls air from under the hood on the passenger side of weather stripping (see top of your third picture) - not from what would normally be described as "under the hood" - where all the mechanical bits and pieces (i.e. radiator) are. A ducted hood allows (or forces, but more on that later) air from the radiator up over the hood. It stands to reason that this would increase the temperature of air entering the climate control (to be further cooled/heated). Warmer air in may mean warmer air out. I imagine the impact cruising on the highway would be minimal (lots of ambient air) but might be a factor when sitting still or going slow.

Another possible way to increase the temperature of the air going into / out of the climate control system (recirculate or not) would be heat physically soaking into the climate control system through its materials. I would be surprised if this was increased by a ducted hood, as the air from the radiator mixes around up there against the climate control unit with a normal hood. If anything, a ducted hood could decrease the heat soak by keeping the warm air away from the climate control unit that protrudes into the front mechanical compartment.

That leads me to the "more on that later" point of air from the hood being forced out of the hood duct to the top side of the hood. The bottom of your hood's duct has weather stripping on it. I believe the purpose of the Type-R box is to grab all the air from the radiator and force it through the duct...and that such a box would meet up with and seal against this weather stripping on the hood's duct. This could actually increase the "warmer air from climate control" issue as more warm air will pass over the car instead of under (though I would still think there is plenty of ambient air to mix with. This would be the proper way to design the box per the ducted hood's design/purpose (create downforce by directing all that air over the top of the car instead of underneath...and allows that section to be sealed off on the bottom to make the underside of the car more slick).

(I know that last paragraph is not to your point of climate control temp...but I included because it is to the point of box design).

Craig, you are on target - even if my question was not clear :biggrin:

I follow your analysis. And I may have one other scenario that might be at play too, but I am curious if I am the only one noticing this warmer air from the inside vents even at highway speed in non-circulating mode.

I agree with all the downforce, etc., and I still working to make the mating between the radiator duct and the hood duct as good as possible but as you noted that was not the question in this particular post.

I am guessing that the aftermarket hood (in this case ProCar) may not be sitting flush/tight on the cowl rubber seal and most likely hot engine air is escaping to where the vent draws its ventilation air. I am still trying to figure how I can test that - or what type of a higher rubber seal I could use to give it perhaps a tighter seal when the hood is closed. I have looked at some seals offered by car shops nothing impresses me enough yet.

Hrant said:

I am guessing that the aftermarket hood (in this case ProCar) may not be sitting flush/tight on the cowl rubber seal and most likely hot engine air is escaping to where the vent draws its ventilation air. I am still trying to figure how I can test that - or what type of a higher rubber seal I could use to give it perhaps a tighter seal when the hood is closed. I have looked at some seals offered by car shops nothing impresses me enough yet.

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Smear white diaper cream all along the top edge of the cowl rubber seal. Close the hood. Open hood. Look to see where the diaper cream does/doesn't transfer to the hood. Them clean up...hee hee.

That is the best test I can think of (certainly no indication that it is the best test). It also doesn't test this dynamically (maybe the hood lifts up a bit when driving from the pressure under it). But it might be a place to start.

If that is the issue you could think of ways to make the area of the hood where the rubber should meet it thicker (so the rubber actually does). Would be easier than making the cowel rubber thicker. I'm thinking of something like that adhesive-backed rubber that people put where their garage door meets the floor to have that seal better. Something like that at least.

Or probably better ideas that will hopefully be shared.

I had already thought of a similar test! Slightly water sprayed baby powder is a better option though not necessarily more effective :tongue:

Yes, there is some lift on HWY speed. The garage door lip is a good one too to consider.

Yes, I have noticed since i installed a NSX-R Bonnet/Hood with the Radiator Duct, the Vents seem to blow hot air.

I came to the conclusion that since the air supply to the vents is taken in at the base of the Windscreen some hot air from the radiator duct is sucked back in.

Price you pay for some extra downforce i guess.

The old toothpaste trick!

The process of elimination continues and I am now pretty much inclined to agree with Wayne (whrdnsx) that the downforce from the hood duct is being sucked back in.

Instead of diaper cream or wet baby powder ... I used toothpaste to test whether the cowl rubber weather seal/strip was tight with the hood. Well the before after pictures don't lie :tongue:

Taking Latzke's advice, I tried bought a 9' garage door rubber weather strip (from Lowe's md brand), and surprisingly this works pretty well. Proof: look at the edge of the right corner where there is at max a 0.5" indent from where the third hood duct deflector touches the strips. I can make a V cut there [and insert a piece for a better finish .....] and use the same application as in picture 5 in the first post to make it even more closed application. But there is little more I can do for the sides given the angle of the hood duct. Even the OEM NSX-R application of hood duct and radiator duct doesn't look like as tight fit when I compare pictures.

So bottom line, life is a compromise And the fun part is the journey figuring it out :wink:

whrdnsx said:

Yes, I have noticed since i installed a NSX-R Bonnet/Hood with the Radiator Duct, the Vents seem to blow hot air.

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Do you think this problem will be less obvious with my downforce DF-R duct? If this is installed properly it will completely guide the hot air from back of radiator to bonnet vent, so I'm thinking there will be no chance to get into the climate control inlets?

sparky said:

Do you think this problem will be less obvious with my downforce DF-R duct? If this is installed properly it will completely guide the hot air from back of radiator to bonnet vent, so I'm thinking there will be no chance to get into the climate control inlets?

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I'll let Wayne chime in for his observation.

But I think what we are saying is the air coming up (through) ducted hood is being sucked back in where the hood ends and meets the windshield - as would have been the case with an unvented hood. In the latter case it is the ambient air that is sucked; in the former the ambient air is being mixed with the hot air coming out of the duct and so far I don't think there is a solution unless you totally close/seal the duct so there is no vent air ....... at least that is my theory for now :wink:

best for this problem would be to find a means of making leave hot air laterally (difficult with the NSX) or under the frame (under the car) but I do not know if that would have an influence on aerodynamics and the handling has high-speed

Hrant said:

... I think what we are saying is the air coming up (through) ducted hood is being sucked back in where the hood ends and meets the windshield

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OK now I understand. hmmm, not so easy to avoid then ...

Thunder said:

best for this problem would be to find a means of making leave hot air laterally (difficult with the NSX) or under the frame (under the car) but I do not know if that would have an influence on aerodynamics and the handling has high-speed

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A ducted hood gets the air on top of the car to increase downforce or at least decrease lift. So, getting the air back under the car would be against the purpose of a ducted hood.

Best fix might be to make sure the A/C is running as perfectly as possible. That the charge is correct, the condensers are not too bent up/blocked/dirty, the fans behind the condensers are working, the compressor is still good, the expansion valve is working correctly. From seeing other Honda service manuals there should be a table in ours that will tell you how cold the air coming out of the vents should be given the temp/humidity of the outside air (that you could use to gauge this). Anyway, if the AC isn't as cold as it should/could be it won't be able to counter this warmer air as well.

Thunder said:

best for this problem would be to find a means of making leave hot air laterally (difficult with the NSX) or under the frame (under the car) but I do not know if that would have an influence on aerodynamics and the handling has high-speed

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Well that's a very good question and it be a hard one to get that perfectly designed.
A Austrian member (forgot his name) did do some wind tunnel simulation testing maybe he can enlighten us some what.

I'd be interested in seeing how a GruppeM hood would preform as it would be the most "stealthy" ducted hood probably the least effective though.

It does have a radiator duct.

Well, it seems my home based R&D regarding the radiator duct proved to be consequential :wink:

At Infineon today with 90F ambient temp and mild breeze, my oil temp stayed at or below 225F and water temp was not even 200F. Today's results were on street tires. I used to see oil temps in the 250F range with water getting to 215-220F (with R compound) especially when ambient temp was 95+/-F.

I attribute this welcome change mostly to the dual Spal fans with their enclosed shroud as well as some tinkering with the radiator ducts fit and seal. The radiator is still my OEM from 1998!

Hrant said:

Well, it seems my home based R&D regarding the radiator duct proved to be consequential :wink:

At Infineon today with 90F ambient temp and mild breeze, my oil temp stayed at or below 225F and water temp was not even 200F. Today's results were on street tires. I used to see oil temps in the 250F range with water getting to 215-220F (with R compound) especially when ambient temp was 95+/-F.

I attribute this welcome change mostly to the dual Spal fans with their enclosed shroud as well as some tinkering with the radiator ducts fit and seal. The radiator is still my OEM from 1998!

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With proper ducting infront of the radiator, fans are useless above ~45mph.

Fans and shrouds are primarily used in street cars for stop and go traffic. Shrouding and fans ensure the fan sucks air from the entire face of the radiator at speeds below 45mph to improve efficiency but above that speed they are a restriction, especially the shrouding!

IMO - I would leave the dual spal setup but remove the shrouding for the best of street/track compromise.


Also making the ducting from the radiator to the hood air-tight would improve front downforce and cooling. The vented hood alone already made a big difference in both, but it could be better .


Billy

Billy, all I am saying and comparing are the difference in two scenarios and the biggest variable was the Spal/shroud.

Yes, the fan is particularly important in street driving; and yes I am aware many race cars don't even put a fan - but their radiator is not an OEM either. In theory, when both fans kick in at their highest speed given the threshold temps, they will suck and process more air/(time unit) than without. I think that is/was the difference.

I had pretty much the same set up before and after - with OEM fan with Dali's radiator duct and now the Spal with its shroud plus the additional tinkering with the radiator duct"s rubber piece. The differences in temps seem to be significant. Albeit the true test will be at THill which is notorious in causing overheating for "a few" NSXs.

stuntman said:

.... making the ducting from the radiator to the hood air-tight would improve front downforce and cooling.

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the only duct I've seen so far that directs all the air to the hood vent is the DF-R http://downforce.biz/cart/product.php?productid=16327&cat=290&page=1 but this looks too restrictive to air flow compared to the more open NSX-R design where some air still escapes into wheel wells etc . I'm not convinced a direct sealed duct will improve cooling, unless perhaps the combination creates a 'venturi' effect through the tube/hood.

And (thinking thinking) - why would a sealed duct would improve downforce, compared to (say) the NSX-R setup?

Hrant said:

Billy, all I am saying and comparing are the difference in two scenarios and the biggest variable was the Spal/shroud.

Yes, the fan is particularly important in street driving; and yes I am aware many race cars don't even put a fan - but their radiator is not an OEM either. In theory, when both fans kick in at their highest speed given the threshold temps, they will suck and process more air/(time unit) than without. I think that is/was the difference.

I had pretty much the same set up before and after - with OEM fan with Dali's radiator duct and now the Spal with its shroud plus the additional tinkering with the radiator duct"s rubber piece. The differences in temps seem to be significant. Albeit the true test will be at THill which is notorious in causing overheating for "a few" NSXs.

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Many racecars do use 1 spal fan thats the same size as yours (nascar included). While you improved the CFMs by now having 2 spal fans compared to the 1 stock fan, that is still only at speeds less than 45mph. Above that speed, any fan is a restriction. On track, the fans don't really do much atall and the shroud becomes an even bigger restriction. Since the stock fan/shrouding probably restricts more than having 2 spal fans and a shroud, I think that was probably the biggest difference on track. No doubt you improved the cooling below 45mph or when idling.

sparky said:

the only duct I've seen so far that directs all the air to the hood vent is the DF-R http://downforce.biz/cart/product.php?productid=16327&cat=290&page=1 but this looks too restrictive to air flow compared to the more open NSX-R design where some air still escapes into wheel wells etc . I'm not convinced a direct sealed duct will improve cooling, unless perhaps the combination creates a 'venturi' effect through the tube/hood.

And (thinking thinking) - why would a sealed duct would improve downforce, compared to (say) the NSX-R setup?

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If I had to guess, I don't think it would be too bad of a restriction, but it would need to be tested.

Sealed ducts (specially on radiator inlets) greatly improve cooling. Its all about pressure differentials, air seeks the path of least resistance and you want that through the radiator and out the top of the car.

Actually about a year or two ago we tested running without the OEM fan and shroud and that didn't improve cooling at THill. You might be right that the two Spal fans are overcompensating any loss from the shroud. As long as oil temp stays below 240F and water temp below 210F I am happy :smile:

stuntman said:

Sealed ducts (specially on radiator inlets) greatly improve cooling. Its all about pressure differentials, air seeks the path of least resistance and you want that through the radiator and out the top of the car.

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The aperture in the bumper for the radiator intake is much much bigger than the outlet in ANY vented hood that I have seen. So, I reckon it would be better to not "seal" the radiator ducting to the hood as this will cause a positive pressure and ultimately reduce flow through the radiator.

So for the best compromise between reducing aerodynamic lift caused by air flow under the car and most efficient cooling; I reckon that instead of sealing the duct to attempt to make all the air coming through the radiator is pushed out through the vented hood's much smaller aperture; wouldn't it be better to have the duct more as a guide to direct the air from the radiator out of the vented hood but as the speed and pressure builds, the air is allowed to spill around and under the duct to maintain the air flow through the radiator?

I reckon you're right on the money AU_NSX and that is probably why Honda did not fully enclose the airflow on the NSX-R

I may try the Type-R radiator duct too for back to back comparison - I just need to find one handy ...... :wink:

AU_NSX said:

The aperture in the bumper for the radiator intake is much much bigger than the outlet in ANY vented hood that I have seen. So, I reckon it would be better to not "seal" the radiator ducting to the hood as this will cause a positive pressure and ultimately reduce flow through the radiator.

So for the best compromise between reducing aerodynamic lift caused by air flow under the car and most efficient cooling; I reckon that instead of sealing the duct to attempt to make all the air coming through the radiator is pushed out through the vented hood's much smaller aperture; wouldn't it be better to have the duct more as a guide to direct the air from the radiator out of the vented hood but as the speed and pressure builds, the air is allowed to spill around and under the duct to maintain the air flow through the radiator?

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Exit speeds of radiators is no more than 15-20mph despite airspeed infront of the radiator being 100-200mph. This creates a huge pressure differential infront of the radiator. Since the airspeed behind the radiator is so low, The small duct should still suffice. I still recommend making it air tight.

stuntman said:

Exit speeds of radiators is no more than 15-20mph despite airspeed infront of the radiator being 100-200mph.

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wow that's surprising. I expected air to be slowed by radiator, but not so much. hmmmm ... thinking thinking

stuntman said:

Exit speeds of radiators is no more than 15-20mph despite airspeed infront of the radiator being 100-200mph...

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Really??? So what causes such a drop in air speed?

Is it the fan/shroud combination limiting the air flow through or the fact that the air has to find a way out of that compartment and is momentarily "blocked" until it finds its way of least resistance, or are radiators really THAT inefficient at air-flow (assuming the fins are not clogged or bent)?