Front Undertray (vented?)

First off, I am no aero expert (far from it).

After seeing RAAMaudio's build, and his custom undertray (integrated into the front splitter), I started to read about how to maximize the pressure differential between the front and back of the FMIC/Radiator.



Using the above design, you get a lot of negative pressure at the back of the engine bay, and that should help pull air through the FMIC etc.
It SEEMS that a vent (like a NACA duct) is a BAD IDEA, as it would actually ADD pressure in the engine bay.
(If it was positioned facing forward, narrow end towards the nose, wide end towards the back of the car)

My question is, is there any advantage in putting some kind of duct in the undertray, such that it create a vaccume as air passes under the undertray, and helps to pull higher pressure air out from the engine compartment? Seems like the above design would be just fine, but I'm curious.

Aside: I like the little wickerbill (Gurney flap) at the back of the undertray, but wouldn't that create LIFT?

Finally, thoughts on how best to add Front Wheel Deflectors? (or even needed?)

--- Max

Answering some of my own drivel....

From: http://www.formula1-dictionary.net/gurney_flap.html



"The device basically operates by increasing pressure on the pressure side of the wing, decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil. At the same time, a long wake downstream of the flap containing a pair of counter-rotating vortices can delay or eliminate the flow separation near the trailing edge on the upper surface (aircraft wing) or lower surface (racing car wing). Correspondingly, the total suction on the airfoil is increased."

Hmmmm......

It depends really. Ideal you completely seal the undersize, duct the radiator and intercooler through a hood vent (heat extractor). The high pressure in the engine bay would naturally want to diffuse into the outside, low pressure moving air. It would do the same with an outlet on the underside, however with a splitter traditionally you want it uninterrupted for maximum pressure delta and flow.

BUT...there is the theory behind exhaust driven diffusers. Essentially you introduce the hot exhaust gasses into the low-pressure area to extend the wake of the diffuser, allowing it to integrate better. Theoretically you could vent the underside into the cooler -low pressure under the car, speeding the air up more and thus dropping the pressure and increasing the delta.

In GT racing its ideal to run a completely flat bottom for the greatest/longest pressure deltas. This was the case, but modern FIA rules require a hump/break in the splitter just aft of the wheels to limit the downforce. This is done because the caars were so sensitive to pressure changes/cross winds that a few cars even flipped (notable le mans in 98 and 99).

What you have above is essentially a Gurney Flap, which provides additional pressure deltas on the trail edge of the wing, without inducing much drag. More pressure delta means more down force. If you look at a lot ilof real heat extractor vents on race cars, they employ something similar on the leading edge of the vent (albeit more rounded). This allows for a nice pressure change to help create a bubble to suck air from the entrance of the vent, improving cooling efficiency.


If it were my car, I'd vent the hood and seal the lower and radiator.


Sorry, I'm on my phone and typing us difficult.

Additionally if you observe the Mitsubishi evo x, there is a heat extractor hood with a rear, forward facing NACA duct. This is there to cool the turbo (seriously).

Usually in base of the windshield has a stagnant zone sue to the air flowing over the hood in laminar flow, then having to move upward abs slam into the windshield. This also creates a bubble, and was the basis for cowl induction. This pressure bubble was pulled in the from the cowl vent via aerodynamic pressure delta and in aid with the vacuum pull from the engine intake being right there.

Big thumbs up for the info from the duke! That jives with everything I have ever read or dealt with concerning underbody aero. Nice seeing people "bring the tech" to their posts.

The best situation is to seal the bottom and vent the top, which sadly requires hacking up the hood. Take a look at the Fiesta WRC RS and R5 cars, you will see the large vents off to the side of the hood near the rear of the headlamps. There are ducts under the hood that seal to a section behind the radiator. All the air entering the radiator/IC (on the WRC RS) is ducted upwards to the hood vents. If you can find pictures of the 2003ish WRC Focus RS, they had twin fans placed between the hood and radiator to help with heat extraction. The large vents on the center of the hood is where all the heat was exited. The other part of this equation is under hood heat from the engine. This needs to be exited someplace as well. Typically this goes out behind the front wheels on GT cars or out additional hood vents that aren't sealed to the cooling system. Having things too sealed up causes a large amount of heat in the engine compartment and that can lead to a fire. Very common problem with stage rally cars that have a skid plate and limited venting on the hood.

For note, the corvette factory team cars show this wonderfully.


Notice the radiator is sealed with its own ducting, and additional heat extractor are used to cool the rest of the engine.

The thing is, this all works well at high speed, but low speed is different. As BGR mentioned, vans are places behind the radiators in this ducting to help move air at lower speeds. If you look at front ducts of a Ferrari Enzo, there is a radiator and fan under each hood vent.

Great posts, thanks!

I will be venting my hood and after talking with the Ford race shop manager at MMP when he saw my car on a lift, and liked what I have done with the whole car, loved it when he drove it....said I needed to duct some air into the bay to help pull heat out.

I looked at all the places I could do it and decided to put it towards the rear, it is fairly large and directs air to the rear in the DP location, now when I pop the hood it is dramatically cooler than before adding the duct.

I have pics but on my camera back home, I will post them when I return there soon.

Since I owned a C5 Z06 race car I had a C6 WC fiberglass body built for it I was installing I tilted the radiator way forward and started on the ducting to vent all of it out of the hood like the Vettes ran in ALMS. I would do it to this car if possible but not so with the engine location and needing to keep AC.

Would it be possible to dumb down the nerd talk for us simple folk.

BUMP! [MENTION=636]RAAMaudio[/MENTION] did you ever post pics of the ducting? My engine bay gets sooo hot with the GTX kit.

I was thinking of buying this Mustang undertray/air deflector and trying to fab it to fit my car. http://www.cjponyparts.com/lower-air-deflector-gt-2005-2009/p/AD4/ I have the Pierce 6-point brace which I would latch it onto with some clamps that way it is easy to remove for oil changes.

My car also still has a big hole in the radiator shroud where my big mouth snorkel used to be. The hole leads directly into the airbox inlet to a non-existent airbox because I have the 2J cowl intake. Is this a good thing to have a steady stream of air going directly into the engine bay?

I played with a custom undertray/air dam last year on track. Found I had to open up the sides of tray next to front wheels to get enough cooling air exhaust. I believe those experiments and others are in the aero mods and cooling thread. I also cut my hood to vent where the low pressure areas (like WRC), but have not installed the lip around each that I assume will decrease pressure at vent locations. Need to get to this this season. I posted vent pics somewhere.

I did not get enough additional performance from the undertray to be excited about. Will try a splitter/dam combo sooner or later, hope that will decrease pressure beneath front end.

Main point is to get lowest possible pressure behind heat exchangers, and seal the nose so high pressure at speed cannot leak through stock gaps (have to pull nose off to seal these). It is easier to understand maximizing pressure gradients across heat exchangers than to understand air flow. A manometer is easy to use and will show you what works and what will not.

Can you link me to that thread please?

Honestly it's mostly to protect the engine from getting hit with debris and from when I hit puddles.

I had a lowered mustang in high school/college. One day I parked over a parking bumper that has the rebar sticking out and ripped the entire tray when I backed out of the spot. Fast forward to a rain storm where there was a big deep puddle. You know when you hit a puddle, that stream of water that shoots up on the side of your car/tire? Well that stream of water occurs on both sides of the tire, not just on the outside. I hit the puddle going like 30mph because I didn't see it, and instantly lost power steering. Managed to pull into an apartment complex parking lot and when I popped the hood, there sitting on top of the engine was my power steering pulley. The jet of water completely sheared the pulley from the engine, hence no power steering. I'm sure I would have drained my battery too since the belt was not connected anymore had I kept trying to drive the car.

Long story short I am traumatized by this and avoid puddles like the plague. With the open bottom the FiST has I'm worried about something similar happening.

I've purchased a second hand OEM Fiesta undertray from a deisel Fiesta but I haven't fitted it yet. I'm hoping it will give me the protection I'm after without increasing the engine bay temperature too much. WRC vents are the next mod if the temp goes up too much.