Getting started with Aerodynamics

[John S]

Hi Guys, I'm interested in cleaning up the aerodynamics on my Monte Carlo. I've read through several threads including the links Ash posted and learned some, but I still have some questions. After my reading I think there are 4 areas to consider working with.
My first step will be to perform the yarn puff test. This will most likely have to wait a while while i work a few other things out, but in the meantime I wanted to get as much information as possible. As far as the 4 area's go;

1. Front air dam. SO far my reading has not fully defined how the front air dam affect my particular car. I was thinking that a splitter may help but I am not sure. Instead I was thinking that a horizontal lip attached the the bottom lip of the air dam would be the way to go. Would this be as effective? Also, I have a fiberglass nose on the car. there is a large open space behind the air dam between it and the radiator. There is also a small spoiler mounted in this area. Does anyone really know it's purpose and how effective it is? I was thinking of installing a panel across this opening to smooth this area out.
2. The hood, I have a Harwood Pro Touring hood on the car. The front of the scoop is open and I plan on ducting this to the intake to create a Ram Air effect. The rear of the scoop by the cowl is closed in. It is approx 4" tall and I was thinking of opening this area up in order to help vent the engine bay air. Right now any air entering the engine bay through the grill can only vent down and under the car which I think is really not desirable.
3. My car is a Notchback and although I have not tested it yet I think that the area behind the rear window and the trunk lid may be a massive turbulent zone. I am thinking that vortex generators may help in this area, thoughts?
4. Lastly the rear of the car. The rear of the car must be a huge low pressure zone. I say this because it is always filthy after driving. There are several rear spoilers available for the Monte. I currently have the standard upright spoiler, but there are extended spoilers and laydown spoilers available. How does spoiler design affect airflow in the rear of the car? Is a diffuser something I should consider?

I would like to learn more about aero so if anyone knows of a good book on the subject I would appreciate the information.

[exwestracer]

Are you trying to increase downforce, or reduce drag for top speed (you CAN'T do both)?

The air dam will always help to some extent, but if you are looking for real downforce; it needs to be pretty close to the ground. Like touching the ground... The idea is to stop the "high pressure" air going under the car from meeting back up with the "low pressure" air going OVER the car. The air dam will also help with penetration (uhhh...lower drag), but reshaping the entire front end would be a lot more effective.

The hood scoop isn't going to help either way, although it may kill some lift by interrupting airflow over the hood and roof...with a huge drag penalty. If you need it, run it; just don't expect to get any great aero gains from it.

A spoiler on the decklid will do a lot to kill lift, but the drag penalty is directly proportional to how well the spoiler works. The Monte aerocoupes were built for low drag in NASCAR. The notch isn't a terrible thing from a downforce standpoint, so again it depends on your goals.

Personally, I wouldn't worry about a diffuser in the rear unless you are really going to clean up the underside (flat bottom, or close to it). THe diffuser is probably the only place you will be able to generate "drag-free" downforce, but it won't do much of anything if the airflow under the car is cluttered up by hanging components...

So, what was your aero goal again?

[Randy67]

For the rear of the car, look at the Monte Carlo Aerocoupes for what to do. It could be duplicated in Lexan to save over the costs of the original ones. Since the car was used in Nascar, there should be some good info out there on aero.

[Bryce]

John,

I think your goal should be reducing drag, unless you are doing a lot of driving over 100mph, you will not see a lot of down force with simple body mods.

1. The air dam needs to be very close to the ground to be effective. I would extend the air dam in the vertical direction only. A front slitter will work by creating a pressure differential between the top and bottom, this can be tested using pitot tubes or monometers. If you build a splitter you should be able to stand on the thing without any deflection. If it deflects it will be les effective.

The nose on the car should be blocked off as much as possible only have as much air as needed going through the radiator.

The stock air dam is there to prevent air from coming into the engine compartment from underneath the car.

2. Do you have a picture of the hood, most hoods I see have intakes just for looks, most of the time the intake is in a low pressure area the hood. The base of the windshield is the second highest pressure area on the car. You cannot vent air into a high pressure area.

3. That may work, but without testing it would just be a hypothesis. In theory vortex generators work, nut its main use is to get air onto the spoiler. What you always want to consider is how do I make the air transition back into the flow stream easier. Trailing edges that are squared off rejoin easier with less turbulence (drag) than curved edges. Those trailing edges should be vertical and/or horizontal. The roof could benefit from a horizontal spoiler, think SUVs.

4. Spoilers add drag when they add down force. A diffuser slows down the air so that is can rejoin the flow stream with less turbulence. But a diffuser needs to be designed not just for looks. To be the most effective the car should have a full belly pan. I am designing a rear diffuser for the falcon right now.

A side note, my family car is a 2000 Dodge Durango, by using aerodynamics I was able to increase my MPGs from 17 to 24 on the freeway. I have a few more tricks that I will not share with the public yet.

[exwestracer]

For the rear of the car, look at the Monte Carlo Aerocoupes for what to do. It could be duplicated in Lexan to save over the costs of the original ones. Since the car was used in Nascar, there should be some good info out there on aero.

Might make it tough to get the longer trunk lid open, though...

[SS PUNISHER]

The "spoiler" on the bottom of the core support id actually what cools the radiator, not the grill in the front end.

[novaderrik]

The "spoiler" on the bottom of the core support id actually what cools the radiator, not the grill in the front end.

the Monte SS had an airbox that directed the air from the grill to the radiator and closed it off from air getting to it from anywhere else, just like the race car they were modeled after. the chin spoiler under the core support on the SS cars only serves to keep air from getting under the car.

to the OP- do a google image search for 80's NASCAR Monte Carlos. you can see a lot of little things they did to the production sheetmetal to make the cars happy at 200mph.

[John S]

Thanks for the responses guys. A lot of food for thought here! My intention is to ensure that the car is manageable at hi speed. I would like to open track the car at some point and I just want to be sure that the car stays planted at speeds in the 130-140 mph range (if I get it up that fast)

Novaderrik is correct, the radiator has a sealed airbox and the only air into the radiator is through the grill. With the sheet metal work in my engine bay and the hood I do not believe that there is anywhere to vent the incoming air other than down and under the car. Whether it is enough to cause any high pressure under there I do not know. Also he confirmed what I thought about the chin spoiler, but again I wonder how effective it is and can it be improved.
I'll try to take a few pictures of the hood in the next couple of days and post them up as well.

[UNGN]

I've have at least a few hundred miles over 150 mph on my '86 2+2 and found there are a few things that really seem to help high speed handling.

Minimizing front end lift is #1.

The Under bumper air dam on an SS is about 9" too high off the ground for a "splitter" to work, so, I'd put a deep core support airdam that is 3" or less off the pavement. We made ours out of conveyor belt rubber, stiffened with spring steel. This keeps the car cool, too when you...

Block off the top of the grill from behind. Make the radiator a bottom feeder and use a large electric fan (or dual fans) to suck air through it.

Venting the Hood mid hood (front to back) is good and also helps with motor cooling. Vents can be in the center at the front of the hood, but as you go rearward, move them out to the sides. Venting the engine bay near the back of the hood probably won't vent the engine bay.

A rear spoiler would have to be pretty big to provide measureable downforce. A slight front to back rake will provide adequate downforce. Keeping the nose down is more important that rear downforce.

For stability at high speed, probably nothing is better than Caster. Adjustable UCA's with 4-6 degrees of caster cranked into them, it what you need for going down the road above 160 mph.

[SS PUNISHER]

the Monte SS had an airbox that directed the air from the grill to the radiator and closed it off from air getting to it from anywhere else, just like the race car they were modeled after. the chin spoiler under the core support on the SS cars only serves to keep air from getting under the car.

to the OP- do a google image search for 80's NASCAR Monte Carlos. you can see a lot of little things they did to the production sheetmetal to make the cars happy at 200mph.

I never thought gm woulda actually made the grill that functional when the rest of the car was such a turd! Lol glad you corrected me there..

Lauren Engineering has some air dam extensions and a 5 inch stand up spoiler for the ss...

[novaderrik]

I never thought gm woulda actually made the grill that functional when the rest of the car was such a turd! Lol glad you corrected me there..

Lauren Engineering has some air dam extensions and a 5 inch stand up spoiler for the ss...

GM put a lot of time and effort into designing the aero package for the Monte SS race cars, and they probably had to justify that cost by making the street version match up with the race version.. also, the slight increase in aero efficiency had to raise the mpg numbers a bit and i'm sure they used the lessons learned from that program in the cars that came later- notice how grilles started getting smaller and smaller and even disappearing altogether on more and more of the GM product lineup as the 80's wound to a close and the 90's started up. even some other cars that had actual grilles had most of them blocked off- my 86 Caprice had about half of the visible grille opening blocked off from the factory with a black plastic filler piece behind the grille that you could only see if you looked at it closely, which is probably one of the reasons that 305 powered luxo cruiser would average 28mpg in normal daily driving.

as to what to actually do to the car to make it more aerodynamic- just look at the car and see where you want the air to go and where you don't want it to go. figure out what you can block off to manage the airflow thru the engine bay- one easy place to look is the radiator support. every G body after 81 or so has the same core support, and they have some HUGE holes behind the headlights that are easy to block with some aluminum. there might also bee some holes in the inner fenders that you can block off, too. all of that leads to less air getting rammed into the engine bay where it creates drag and lift. and don't limit yourself to looking at other Monte SS's or even G bodies- walk around a junkyard and look at the aero packages of every different vehicle you can. there's a lot to learn about air management from all those disposable late model econoboxes and even later model full size trucks.. hell, if you ever find yourself cruising down the interstate and come up alongside a newer Volvo semi you should give that a quick look. hell, look at the trailer it's pulling- it probably has side skirts on it.. those big rigs have got a LOT of aero management going on, and it can all be scaled down to your car.

[John S]

I'm attaching several pictures of the car. I know most of these have been posted elsewhere on the board but I think they would help with this discussion. Again, thanks for all the input. One thing I have learned is that I need to do more research on aerodynamics in general to get a better understanding of where airflow on the car is good and bad and what affects that airflow. This is so new to me...

This picture just shows the stance of the car from the front. It sits quite a bit lower than a stock SS but still could be lower. The only problem is driveability. Any lower and I would have trouble keeping the air dam in one piece. Maybe a flexible plastic extension attached to the lower portion?


A side view shows the stance. I added air bags in the rear so that I could use the back seat ( for passengers) which actually raised the rear about 1.5" since this picture was taken.


These next couple of pictures show the tin work in the engine bay. Not ideal pictures but they will give you the general idea.The large openings in the front of the fenders have been covered which prevents air from coming into the engine bay from under the car through the fenders. I have not covered the large openings in the rad support behind the headlight buckets. I am going to do that before spring. The wheel tubs restrict most of the airflow from coming in around the wheel wells so I do not think I need to do anything more there. In the second picture you can get an idea of how tight the tubs are to the frame and upper A Arms




Here are pictures of the Pro-Touring hood. The air inlet is functional even though the the hood is open underneath. I plan on building a plenum into the hood to channel and restrict airflow from the inlet to the intake. The rear section of the scoop is about 3" tall and solid on the rear. This is where I had though of cutting it open in hopes of allowing engine bay air to flow out there and reduce the amount going under the car. But if this is a high pressure area I may need to think of something else to vent the engine bay.
One of the things that I read that is if the engine bay air does not flow properly it can affect engine cooling. True?




My last question for now goes back to my original post concerning the large open area between the inside front of the air dam and the front of the Rad support. Do you feel it would be beneficial to close this up and install a larger chin spoiler in the process? Possible even an adjustable chin spoiler?

[UNGN]

Opening the back of that hood is most likely going to add air under the hood, not remove it.

Abandoning the factory grill opening and getting all radiator air from a deep core support airdam is the best way to go 150 mph in a street car. Trans Am's, corvettes, etc don't have above the bumper grills for a reason.

A NASCAR car, running on a smooth race track can have a front facia 1" off the ground, while a street car cannot. Unless the front facia extends to the ground, the relatively high grill opening will promote front end lift... unless its blocked off.

A Deep core support airdam is liveable on a street car, much more so than effective Chin spoiler or splitter that is deep enough to overcome the lift generated by the grill opening.

If front end lift is still an issue after you've blocked the grill off, then you can extend the chin spoiler until it is isn't anymore. The Chin spoiler you end up with would not be as deep as you would need with the grill open.

[UNGN]

As far as high speed cooling goes, we initially had bad overheating problems at high speeds. We tried a Clutching fan and then a Flex fan with the factory air dam and grill openings and it would overheat (230+) cruising @ 140 mph, even with a big AL radiator and the mid hood venting we installed.

Going to a Chrylsler dual electric fan set up with the deep core support airdam made it run cool enough that we could block off all of the above the bumper grill openings and as long as the fans are running it won't overheat in 110 degree traffic and runs about 180 degrees cruising @ 165 mph.

[John S]

Chris, you've mentioned the deep core support several times. Can you provide more information on this?

[UNGN]

Sure.

Basically, we replaced the Factory radiator core support Air dam with a Longer, taller airdam made out Thick Conveyor belt Rubber, with spring steel sheet metal bent at about a 60 degree angle

Its Angled about 30 degrees fwd from vertical static, but you can see from this picture at speed, the relative wind under the car blows it back a little past vertical.




You can see from the windows blowing out and the 255/50's looking like 255/60's that the car is going over 165 in this picture, but the top of the tire is still above the bottom of the fender lip, which is the static ride height.

Compare that to various other cars going through the same speed trap, at the same speed or slower than ours:






The last one is Tom Whalen's car and it shocked the crap out of when he saw the picture. He made a bunch of changes to keep the front end down and its been over 200 mph.

[John S]

Ok, So I assume that the 2+2 does not have a sealed airbox as the SS does. I would need to modify the airbox the draw air in from the bottom in front of the chin spoiler if I block off the grill. Seeing these pictures make me think about realistically about about using a T-Top car, or at least keeping things within limits.
Whats up with all the bubbling on the front of the first TA? IS that from air pressure behind the fascia?

[UNGN]

I think the 2+2 had heavy rubberized plastic sheeting going from the bottom of the radiator to the bottom of the bumper. That all went away.

We used Aluminum sheet to block off the upper grill holes. It looks like a large amount of the lift on the cars shown is the hood trying to lift the car up. The vents we put in our hood seem to work.

The in cabin air pressure can blow stuff out. We had to put straps along the back edge of rear window, because it was raisng 2" off the seal at speed.

[novaderrik]

Ok, So I assume that the 2+2 does not have a sealed airbox as the SS does. I would need to modify the airbox the draw air in from the bottom in front of the chin spoiler if I block off the grill. Seeing these pictures make me think about realistically about about using a T-Top car, or at least keeping things within limits.
Whats up with all the bubbling on the front of the first TA? IS that from air pressure behind the fascia?

the airbox just unbolts, and you can then block off the grille and use a spoiler under the radiator support to ram air up thru the rad core.. it might also be a good idea to make a new air box that directs all of this air thru the radiator core instead of allowing it to go up behind the headlights and inside the fenders and what not.

for the hood, if you like the look you could block off the front opening and open up the rear and build an airbox around the air cleaner that seals to the bottom like the old cowl induction Camaros and Chevelles (and even the Citation X11 and Cavalier Z/24's until '94). if you want to vent underhood pressure, then get some GNX vents in the fenders and some Lumina Z34/Grand Prix GTP vents in the hood.

[John S]

Ok, so it seems that things are getting narrowed down a bit. It looks like I should concentrate on the front end to start, and more specifically getting the nose air cleaned up.
Since the Monte has a sealed air box would it be better to leave the airbox intact except for removing the bottom?
I am thinking that by removing the bottom of the airbox, seaingl up the bottom of the nose section and ducting incoming air into the radiator by blocking off the grill and installing a wider and deeper chin spoiler to direct the air under the nose into the radiator I will increase cooling efficiency and help break up the high pressure under the nose.
The next step would be to install a set of GNX fender vents with the associated ducting to vent engine bay air out of the engine bay. I did find the complete GNX vent kit at GBody parts.
My only question on this is whether the GNX fenders are close enough to the Monte fenders to make the installation fairly painless?
My next step is the possibility of the GNX style fender flares to move air away from the wheels and outwards.
As far as the hood goes, is there really any advantage to moving the intake air from the front (ram air) to the rear (cowl induction)? The design of the inner hood does allow for a sealed airbox to be configured fairly easily, but in the case of the cowl induction would it be better to leave the inlet open or fabricate a flipper door?