[chillsworld]

Hope y'all don't mind reading, my fingers got happy with the keyboard

Originally Posted by freebeard

My mention of pic#1 was distracting; I meant what range of vehicles are you considering. I assume front-engined and water cooled.

Here are some mid- and rear-engined examples that might be instructive.

The Porsche Boxter uses two radiators in front of the wheels. They also cheat the duct length-equals-radiator-height rule of thumb with deep pockets for wind tunnel hours.

(Adding the cowl hood scoop to your race car ducting would add skin friction and exit in a high pressure area.)

Primarily envisioned for small displacement front engine water cooled automobiles (FWD/AWD). Of course every engine bay is different, so the idea is applicable to ANY car/truck/suv which has the space for such ducting (and is water cooled I suppose). Body style, in my opinion, would determine bonnet vs wheel well vs under car exit of the ducting.

In my opinion though, the technology is more prevalent in mid engine (and rear engine) vehicles... They have more room to adjust and play with ducting and departure angle/direction. Think Honda NSX for example... They do it to maintain a uniform flow under the car, a goal that seems to be high on the ecomod list. In the image below you can see a pristine and perfect flow under the body of the NSX-R. And the graph below that you can see they state that the vented hood smooths airflow over the car adding to down force and reducing drag. They actually moved away from venting under the car or wheel wells in favor of venting from the hood, because in their design it *helps* smooth airflow over the body of the car. You had mentioned earlier that others had come to the opposite conclusion with their research/ideas, suggesting that under or wheel wells was the optimal exit. I do believe that each of these three options can be the "correct" choice depending on the car in question... All cars are not created equal.



(From Honda Worldwide | Automobiles | NSX-R)

But the above seems to go with my original posts questions... The smaller inlet, more efficient cooling, creation of down force via the combined components, additional lowering of drag via assisting with airflow over the body of the vehicle/windshield. Granted those were just the results with this one particular car

The placement of the duct, is also at the "break over" point I mentioned earlier... The place between the high and low pressure zones of the hood/windshield. In the racing version of the NSX, this vent is moved forward so that the rear lines of the duct match this magical line on the hood... I'm assuming this is to take advantage of this spot for sustained high speed aerodynamics.

Originally Posted by serialk11r

I'm not entirely convinced that many of those race car examples even have a well-designed vent. I'm not an expert but here goes.

The holes in the hood are way smaller than in some of those pictures if you compare to the intake area. I have a suspicion that a lot of people who make gargantuan hood ducts unknowingly are reducing the pressure over the hood by doing that and losing downforce. That Evo for example, has an exhaust that's like 3x the area of the intake.

I'm guessing the difference here is that air going over the hood of the car is moving a bit slower since often times that's an area of positive pressure, but some of these guys are slowing the air down too much as it leaves.

Read below for the simplest explanation I could find, and far better than I could ever explain

"Vents can also be used to reduce lift and create downforce by relieving unwanted pressure differentials. Pressure differentials can occur anywhere that a fluid flow is allowed to enter an area that is difficult to exit. If this pressure differential contributes to lift, or reduces downforce, it is obviously unwanted.

One area where a large pressure differential can develop is the engine bay. The pressure differential is created in two ways. Firstly, as the air flows into the engine bay it strikes the engine, accessories, and firewall, stagnating and raising the pressure in the engine bay. The air flowing over the hood is moving quickly and is at a lower pressure, creating a pressure differential. Because the pressure below the hood is higher than above it, lift is created.

The effect is amplified by the fact that the air entering the engine bay is heated by the radiator and the engine itself. Buoyant forces (the same forces that cause a hot air baloon to rise) are created because the hot air has a lower density than the cool air above the hood. This effectively turns the hood into a hot air baloon, compounding the problem. The problem is again made worse because the engine bay is sealed on the sides and top. This allows the air only one escape path - down. The air is forced to flow down and beneath the car, increasing drag and reducing downforce. Obviously these conditions are detrimental to a car's performance.
However, simply placing a vent in the hood can help reduce the pressure differential in the engine bay, reducing the effect of these phenomena. This solution can be seen on many race cars including JGTC and DTM cars and also on very high performance road cars like the Ferrari F50, Jaguar XJ220, and Lotus Elise.

It is important to note that if the vent is placed too far back on the hood, then it may cross into the stagnation zone at the base of the windshield. If the vent is open to this high pressure area, it may actually force air into the engine bay, compounding the problem that it was installed to relieve."
(Superhachi.com - Downforce Basics)

That last sentence is actually quite relevant since some NSX-R owners notice that their Air conditioning gets hotter when they fully seal the duct work. The vented hot air is being sucked back in at the base of the windshield. This is also a reason you see the full race version of the NSX-R shift it's duct exit forward to take better advantage of the pressure zones and air flow.

So t's not the vent that creates down force per say, it's the vent/duct that allows the other aspects of aerodynamics and pressure issues to create down force. When you read about the amateur road racers and time attack builds, they always do rear down force first, because the addition of under tray and splitter can lead to a situation where the rear of the car looses traction due to too much down force in the nose of the car. That is how massive the impact is to down force on these cars at speed

In addition, the shape and placement of the ducts or vents is impacted by the shape and size of the hood/windshield/cabin shape. Your example of the McLaren P1 is picture perfect for this when compared to a NSX-R. The vents are split, sending air into two directions (towards the side of the rounded cabin body/windshield), the vent's are angled and spread the air flow over a larger space side to side, and the hoods bodywork creates a wider exit as opposed to an exit that is the same size as the duct it is venting. The NSX on the other hand is very flat, barely gets wider than the duct, and points directly over the very flat and wide windshield.

The EVO is far from an efficient design if I were to wager a guess... However, it is a time attack vehicle that cares more about down force as a whole and less about aerodynamics in general... If that makes sense. If i were to wager a guess, the car literally will drop to the ground at speed, and then it's aerodynamics are improved more than if they were to spend the time to optimize the duct exit for it's own creation of down force and aero impact. I'm not advocating this approach with ecomodding, I was more just posting the manner in which one can create a duct in the engine bay It was visual assistance so people knew what ducting I was talking about.

I would be willing to bet that those two "smaller" vents are actually larger and remove more air than you realize. But I of course have zero proof of this, nor would I even know how to look that up

Actually, if done correctly it could/should work in a similar fashion as a rear diffuser just upside down Which is known for it's potential to produce massive amounts of down force and reduce drag. It's smaller opening leading to a larger exit creates pressure zones and leads to the air slowing down at the exit... But it doesn't decrease pressure, it actually increases pressure as the air tries to fill the void and sucks air from the under-body of the car (rear diffusers do a lot more than help with separation of flow). So perhaps an appropriately designed hood vent or radiator duct could indeed be a stand alone source for down force and drag reduction? I was more interested in it from the stand point of "the machine is greater than the sum of it's parts alone"... But I guess it could stand alone if one had the formulas and wind tunnel time

Back to the original post I made, in high performance vehicles the radiator hood duct and associated mods (under body tray, reduced inlet size, air dam) leads to the following things...

Ecomodding goals...

-Smooth underbody flow
-decreased area of intake/resistance
-equalization of high pressure zones (under hood, wheel wells, etc)
-reduce lift
-reduction of drag
-increased engine efficiency
-warm air intake

Non ecomodding goals...

-increased Down force
-Looks sweet

You guys already like under trays, splitters, air dams, radiator ducting, etc etc... I'm just curious if this too could assist in maximizing these other practices that make a large impact on your FE. Take a good thing and make it better? At the very least, could hood vents alone improve these things? if the idea of radiator ducting is too much... Perhaps start with a smaller discussion of hood vents in general? You all seem to be saying the same things as the people who advocate and use the ducts, but you are coming to the opposite conclusion that those people do... So little ole me is just trying to understand

~C