turbojeff

iTrader: (30)

You guys are confusing...

Faster moving air decreases pressure. An airplane wing works by lowering the pressure ON TOP of the wing. The air must travel farther on top of the wing than on the bottom of it. The increased velocity decreases pressure and LIFTS the plane.

Now if the plane were a car and the pressure on the BOTTOM of the wing were lower it would pull the car down the faster the air moves.

damian

iTrader: (2)

so we all can agree that you want to reduce the amount of air going unde the car and any air that does you want to go smooth and fast under there...right?

damian

iTrader: (2)

so my question still remian, does the side skirts in teh pic at teh top of this thread (not mazdaspeed, or anyother, im talkign specifically about the ones in the pic at the top) help in aero?

im looking for an 'in general' answer, not a physics formula proof :-)

clayne

Your answer Damian:

I met the owner of that car. He was stopping at a city hall out here in the bayarea and a cop gave him a ticket for expired registration even after he was parked. But the cop did say the car looked like it was glued to the road.

DamonB

You guys lack in the aero dept...

If the car is traveling through the air and therefore there is air under the car, over the car and around the car how do you conclude that any air under the car is going faster than any air over the car when it's all the same "air"? In fact the air under the car will be going slower than the air over and around the car. Why? Air reacts to friction. The proximity to the ground when traveling under the car induces friction with both the ground and the non-smooth underbody of the car. This slows down the air under the car compared to everywhere else and therefore creates lift on the vehicle. You do NOT want air under the car unless you have purposely designed the underbody to make use of that air. The entire point of an airdam (front chin spoiler) and sideskirts that extend nearly to the road surface is to keep air out from under the car. At the same time you don't want to "seal" the rear of the car because you want any air that does find its way under the car to be able to find its way out easily. If you trap air under the car you get even more high pressure and therefore more lift.

In short you do NOT want ANY air under the car unless the car was purposely designed to make use of that air. That means the car at minimum has a smooth underbody which can generate downforce via chassis rake or even better the underside incorporates a diffusor arrangement (make note that slapping a diffusor on an otherwise stock FD does not count; the rest of the underbody has removed all the energy from the air, created turbulence and slowed it down. By the time the air finds it's way behind the rear suspension and into that tiny throat of the diffusor it has little to contribute). If the car is not designed with underbody aero you do NOT want air under the car. You therefore put the nose as close to the road as you can and you do the same with the sides. The rear you leave as open as possible to allow the easy exit of any air that makes its way under the car.

clayne

Here's where I grab the stupidity card for my statement on sideskirts being "beneficial." On their own: nope - as Damon pointed out.

I was more trying to say: get the air that does go under the car moving FASTER than the air above the car.

DamonB

If you have a low nose and spend a lot of time at 150+ mph they might. Problem is that even with the low nose there is still a lot of air making its way under the car: All the air that goes through the nose to the radiator gets spilled out under the car and has to go somewhere...Air will always move from areas of high pressure to areas of low pressure and try to equalize. So if you do have a superb underbody setup than can generate low pressures the rest of the air around the car is constantly trying to find its way in there and mess it all up. Aero doesn't work the was you want just because you have an "aero kit". It's a much tougher nut to crack than that.

damian

iTrader: (2)

so my question still remian, does the side skirts in teh pic at teh top of this thread (not mazdaspeed, or anyother, im talkign specifically about the ones in the pic at the top) help in aero?

im looking for an 'in general' answer, not a physics formula proof :-)

clayne

In *that guy's* aero, you mean?

spyfish007

Here is one for you to chew on Damon ... I always heard a vented hood helped push the nose of the car downward. What do you think?

tookwik

I remember getting behind a Prelude with a high wing and diffuser on the Interstate once in some light rain and it was amazing watching the mist flow around the rear of that car. It honestly looked like one of those things you would see in a wind tunnel... I couldn't stop watching it... A HONDA!

DamonB

Depends. Any air coming through the nose (from radiator for instance) has to get out somewhere and that place is usually under the car. Ideally we don't want air under the car so a vented hood could let that air out and over the upper surface of the car instead of sending it underneath. BUT (there's always a but) the hood vents would have to be positioned on the hood and designed in such a way that they are positioned in a low pressure area to draw air out of the engine compartment and through the vents. If they were in a high pressure area the air would flow from the outside, through the vents and then under the car again; not what we want. Therefore the "real" vented hoods for the FD for instance have a small lip on their leading edge that creates an area of low pressure and this low pressure pulls air out from the engine compartment. This would be beneficial. Given the fact that the vents are merely in the hood and there is not a dedicated duct sealing the hood to the radiator the gains are there but not completely optimized from an aero standpoint. IMO a "plain" vented hood with no duct sealing it to the nose intake in some fashion will aid cooling somewhat, but it won't help the reduction of underbody airflow much. The air will still take the path of least resistance and though some of it will find its way through the hood vent the majority of it will still flow back through the engine compartment and find its way out the underside of the car.


If you really study pure racing cars you'll find they always exhaust their cooling air out the top or sides and they always duct both into and away from the cooler(s). Frequently you also see vents or louvers in the tops of the front wheel wells to vent any high pressure air that gets "stuck" there. You don't want any air under the car unless you have created an underbody designed to make use of it.

David Beale

The easy way to think of it is the front chin spoiler (what I like to call it) lowers the amount of air going under the car (pushes it over and around the front). If you effectively lower the sides of the car, then air passing the sides will have less tendancy to flow under as well. So you can end up with a large area under the car of slightly lower air pressure.

End result is it sort of adds downforce, or at least reduces lift if done properly (effective seal with the ground).

Now, what most body kit installations do is "change" the looks of the car, and that's their prime purpose.

If you look at pics. of my car you will notice a very low front chin spoiler (3.5" or so) and side skirts with reduced height (reduced about 1"). Combine that with the H&R Sport springs and you get a nervous driver, as it will hit most speed bumps etc., but only at the front - the sides have never hit the ground or undulations of the ground. I think the parts reduce lift on my car, as I don't get the light front end feeling others have reported at high speed. But that's a "butt windtunnel" test, so accuracy is debatable.

damian

iTrader: (2)

thanks for all the great posts guys... ill munge it over in my head

fd3s777

iTrader: (3)

Best side skirt is Mazdaspeed GTC.

clayne

In regards to the hood question:

http://e30m3performance.com/installs...s-3/hood-vent/

From that URL:

"So, why the vent? Well, the primary reason was to reduce front end lift. "German" John points to wind tunnel tests done in Germany which showed that the Evo III components could attain negative lift, i.e. true downforce. The hood vent would be pushing lift further into the negative direction at the front end. The secondary reason to add the hood vent was to improve cooling to the radiator. This idea of negative lift first, cooling second, is counter to what many folks would think. The anti-lift effect of a hood vent is only recently becoming widely known. The theory of this effect will be detailed in another article. Suffice it to say that the air which passes through the radiator is at high pressure (due to the stagnation point at the front of the vehicle). The air looses some pressure as it squeezes through the air passages of the radiator, but not that much. After that this high pressure air is "trapped" inside the engine compartment, with no where to go except down and out around the engine. This is bad from an aerodynamic point of view. Meanwhile the air which does not pass through the radiator mostly flows up and over the front edge of the hood. It must speed up to make this journey (and still conserve overall mass) thus the local pressure goes down. Add to this the fact that the air possibly separates into a local eddy as it turns the corner over the front edge of the hood and you have that there exists a local low pressure zone above the front portion of the hood.


The net result of these flow patterns is a large plan area (the hood), with high pressure underneath it (pushing upwards), and low pressure above it (sucking upwards). Multiply the difference in pressure by the area and you get an upwards force. With such a large area it does not take much pressure differential to create a rather large lift. Solution: vent the two pressure regions together. Let the high pressure air vent out to mix with the low pressure air thus reducing or even eliminating the pressure delta, and therefore also reducing the lift force. If one takes time to examine photos of any contemporary front engine race car it is quickly apparent that radiator (and oil cooler) outlet air is always vented out to a low pressure region in order to reduce under-hood lift. Sometimes it is out the top of the hood, sometimes out the sides of the front spoiler, or even out the lower portion of the car behind the front wheels (i.e. modern DTM cars). All of these exit regions are generally low pressure."

FCdemon

they will affect the aerodynamics very minimally unless you are at HIGH speeds.. basically there is no way you will ever feel the difference. It is a "for looks" mod mainly.

damian

iTrader: (2)

dooood...these would be for my track car , I AM AT HIGH SPEEDS!!!!

that is why I started the thread, im talking about a range from like 80 to 180 mph :-)

...and at 150+ mph I'm sure I would feel a difference (heheheh)

http://64.158.28.140/MPEG/TwinVision...s_with_dad.mpg

InsaneGideon

Speaking of hood vents, I belive the TVR Sagaris has an electric "Porsche-style" speed dependant front spoiler/wing/vent.

It additionally has "pressure-relief" vents on the fenders. Interesting car.

http://www.pistonheads.com/doc.asp?c=102&i=7756

blueskaterboy

well since your car is shaped like a wing and experiences lift i would think that the less air and pressure is under your car, the better. i always thought that was the real reason for body kits.

SiKoPaThX

iTrader: (2)

So I would assume stuff like this would help much more...



Also....I swear I've seen somewhere a full flat bottom for the FD.

CrispyRX7

iTrader: (139)

Not to detract from the original thread topic (would be my choice of sideskirts also BTW...I love they way they look) but an empirical datapoint on vented hoods. I have a Knightsports 4 vent hood. 2 large vents over the radiator and 2 more on the rear of the hood on each corner - one near/over the turbo's/ABS pump and the other over/near the MC area. Of note however is that the drivers side rear vent is pretty much directly over a vented oil catch can. Now as some of you might be aware I have been struggling with this oil catch can over filling when at the track. Me being one of those who spends A LOT of time over 120mph also. Well, in the early days of the over filling catch can, when nearing the full mark, the vent/filter on top of the oil catch can would spray a fine oil mist when it was venting oil pan pressure. Where would this mist go? It would be **SUCKED/PUSHED** out of the hood vent. In this region of the hood there is clearly a pressure differential between underhood high pressure and outside air lower pressure. Directly in front of me as I drove I could plainly see the oil mist come out of the hood vent and swirl over and around the A pillar. So I know for a fact it was venting air and "relieving" under hood pressure. Food for thought.
Regards,
Crispy

damian

iTrader: (2)

good report crispy!!!

FDNewbie

iTrader: (10)

Did you mean this?



Oh and Crispy, you read my mind. I was wondering if there's a simple way we can determine whether the vented hoods we're running are causing lift or downforce... but since I don't have a oil catch can etc...here's a simple (and possibly even totally bogus/stupid) idea...but I'd like to hear if you guys think it could work.

Basically, I'd tie a very thin and medium length peice of nylon string to a part of the vent in the hood...and go for a nice high speed run. I would *think* that if the hood is generating downforce, the nylon string would be fully extended up and outward, facing the windshield (air from engine bay exiting the vent, blowing against the string). If I was getting lift, the nylon string would be sucked into the vent and into the engine bay, so essentially you wouldn't see it (vaccum caused by vent would suck air into the engine bay, and the string w/ it). What do you guys think? Would it work?

Lastly...in regard to the Mazdaspeed GTC skirts...I'm real curious about something. Knowing that Mazdaspeed actually uses wind tunnel testing on their aero parts (which yes, must be used in conjunction w/ one another to actually be effective), I'm curious what the bulge toward the rear of the GTC skirts does, as it's a new addition (wasn't found in the previous Mazdaspeed skirts). I don't see that doing anything for air flowing under the car...rather, for air flowing around the car.

damian

iTrader: (2)

ohh i like that unertray,.... good start at least