Flat bottom aero

It's getting close to summer vacation again and I'm trying to come up with cheap projects to do with the students. One of the ideas I came up with was to make a flat bottom for the engine compartment.

This will pretty much seal the underhood area and make all the air exit through the vents on my hood.

I know this is pure guessing, but for those of you who have experimented with aero, how much does a flat bottom affect your handling balance?

Benjamonkey, the flat bottom on the engine compartment will have some major and some minor effects. Major: reduced drag, generally more downforce on the engine bay end, hotter engine bay. Minor: difficulty working on the engine, possible component overheat (starters are most likely), vibration from the flat panel. You didn't mention what type of car you are planning the project for so the balance change you are asking about is dependant on the car layout. Front engine cars will add some front downforce to an already front heavy (aero-wise) platform. Rear engine cars will gain rear downforce where it is needed most.

The drag aspect is going to be the most sensitive change for a street car with relatively high clearances. If the panel is fitted without regard for the incoming air direction, you could wind up adding drag. The drag change will be seen most effectively in mileage or terminal top speed (not suggested on public rodas!!).

The downforce and balance aspect will only become noticable if the car is quite close to the ground (less than 2"). You may be able to determine the downforce change by checking the change in shock travels before and after (knowing the spring rates at the wheels and using a smooth test area).

Cooling may be your biggest issue. If the engine bay is completely sealed, then no air will get in or out. If there is an inlet and no outlet, the engine bay get pressurized (very slightly) and then no more air gets in. You mentioned exits in the hood, which will work at speed, but may need to be enlarged. Also keep in mind that the airflow directed to the hood exits will probably allow other items near the bottom to get hotter than normal. I would suggest adding some small exit vent in the flat panel at the bottom of the engine bay to draw heat away from some of the components.

Roughly the same amount of energy that goes to the wheels is cast off from the engine as heat, so there is quite a bit of energy to deal with.

For your project you may want to define it a bit more as far as drag reduction or downforce increase/balance change.

Steve Stafford.

You could space your hood at the hinge bolts in back to aid in cooling

If you haven't got them already, I would get both Tune to Win by Carroll Smith and Race Car Vehicle Dynamics by Milliken. Both of these books should help a lot with the Daewoo project.

Make louvers at the rear of the flat plate to draw air out from the bottom .

For those of you who didn't notice, I believe Stafford1500 is the person who wrote the article on aero for GRM so I'm guessing his will be the best advice you get.

Taiden wrote: You could space your hood at the hinge bolts in back to aid in cooling

That lets air IN the back of teh hood. There is a high pressure zone at the base of the windshield. Air will NOT come out of that area.

However, more air IN could lower temps.

wvumtnbkr wrote:

Taiden wrote: You could space your hood at the hinge bolts in back to aid in cooling

That lets air IN the back of teh hood. There is a high pressure zone at the base of the windshield. Air will NOT come out of that area. However, more air IN could lower temps.

I know this to be true due to the existence of cowl induction hoods, BUT I know that my parents 67' Corvette runs cooler with the hood propped open a bit - might just be speed related though...

maybe you don't want to seal it compleatly.. but I think leaving a few inches "open" at the trailing edge to let heat out would be a good thing

mad_machine wrote: maybe you don't want to seal it compleatly.. but I think leaving a few inches "open" at the trailing edge to let heat out would be a good thing

Correct

The under tray can aid cooling by directing the flow of air in the engine bay. Rx-7's are notorious for overheating when the under tray is discarded for being just another plastic engine cover. The FC under tray only covers the area underneath the coolers and the front half of the engine. It ducts all the air entering through the middle bumper opening through the coolers, which helps keep the temp needle where it should be.

make the hood looks something like the hood on a Ford GT- all the air that goes thru the radiator is directed up and out from under the hood- and add some vents towards the rear of the fender like on late 70's Trans Ams had..

The car is a 1998 Daewoo Nubira. We built a hood for it last year that looks like this.

I'm running pretty close to stock ride height. However, I was wondering if forcing that extra air through the hood would act sort of like a splitter?

I like the idea of louvering the back of the tray, that could keep the low hanging accessories from cooking.

@ Stafford. What are some good methods for controlling vibration? I was thinking lots of mounting points and rubber washers.

The number of mounting points is the first order of business. Use as many locations around the edges as possible. This will keep the edges from 'peeling up' and ripping the panel off the car. For large open areas, you can try to add some beading or flanges. Rolling beads in the part might not be possible with equipment at hand, but using small aluminum angle rivited to the inside of the panel across big flat section will help. Basically, the aero load you apply to a large flat panel can add up quickly and without some form of stiffener you are asking for it to start deforming. That could lead to buffeting (like an old convertible top at speed) or worse fatiguing the panel at the fasteners and having it fly off the car at speed.

POst any other questions and I will try to answer them.

Steve Staford

Oh yeah, the air going thru th top of the hodd after going thru the radiator sort of acts like a splitter, but keep in mind the air pressure acts in all directions, so if the air is going thru a duct it is affecting all 4 sides of the duct (up/down/left/right). What the hood openings do mostly is change where the air flows around the front of the car. It effectivley moves more air over the car than under or around the sides, due to the pressure difference between the nose and the holes at the hood. Pressure difference is the only way to generate flow (fans make pressure, not flow).

Take a look at the 3cyl. SAAB 96. Full under tray/flat body. They let the air out into the wheel wells, just like NASCAR vehicles do. You could duct the radiator air out through the hood as many high preformance cars do.
Not at the rear .

Stafford, thanks for all the information. Much appreciated.

I'm running a super tight budget and my "mechanics" are middle school students who mostly don't know what a screw driver is, so I was actually considering using corrugated aluminum home siding so we can f up two or three times without costing a fortune. Would that be the equivalent of putting in a bead?

I was planning on using studs welded to the subframe and front bumper support as mounting points and then sandwiching the aluminum between two nuts on the stud with rubber gaskets and steel washers. Then I would cut out a hole for the lowest part of the oil pan.

With the hood, I actually did some testing with a garden hose and high way driving to determine the lowest pressure zones. My hood vents are in the low pressure zones. My thinking is that this will create a net upward flow and a little down force without having to rip the splitter off every time I come across one of Korea's insanely angry speed bumps.

iceracer wrote: Take a look at the 3cyl. SAAB 96. Full under tray/flat body. They let the air out into the wheel wells, just like NASCAR vehicles do. You could duct the radiator air out through the hood as many high preformance cars do. Not at the rear .

I'd love to do this, but it would involve making a hole in the unibody. Isn't that typically a bad idea?

Not trying to be a smart ass, I honestly don't know.

Benjamonkey,

The corgated material will definitely be stiffer than flat. It may even give the advantage of giving you free drains and air exits, if you run the curgations front to back. I would still stiffen the area around the oil pan cutout. The studs welded to the subframes will be perfect. Just make sure you have enough to prevent too much flex between them. If an edge gets pulled down into the airstream below the car it could be destructive. Try to get the leading edge above the front most attachment point or roll the front edge up a bit in order to clean up the edge facing the incoming air.

Good luck with the project and post up before and after pics. Also, let the kids have a crack at what the pan should look like. Most of the time kids are not corrupted by what things 'should' look like.

Steve Stafford.

If it pels off it will crush under the car and might remove things like wiring, brake lines, exhaust and air from the rear tires. Expect anything.

MrBenjamonkey wrote:

iceracer wrote: Take a look at the 3cyl. SAAB 96. Full under tray/flat body. They let the air out into the wheel wells, just like NASCAR vehicles do. You could duct the radiator air out through the hood as many high preformance cars do. Not at the rear .

I'd love to do this, but it would involve making a hole in the unibody. Isn't that typically a bad idea? Not trying to be a smart ass, I honestly don't know.

The holes would be in an unstressed area. SAABs were unit body way back then.

"Flat bottomed cars make our rockin' world go round!"

In reply to stafford1500: Several GRM articles have shown plywood used for making a splitter. Wouldn't using plywood as the "core" to a fiberglass panel (1/8" or 1/4") plywood give a flatbottom more stiffness, with the appropriate heat foil placement, coat it with fiberglass for durability.
Thanks this is interesting. Dean

A cheap way to "see" what's going on underneath the car once you do build the tray is blobs of oil or very slow drying paint. Dab some blobs in a grid pattern, drive at speed, and have a look under the car. you can see the patterns the oil/paint will make and you'll see how the air is flowing .

I knew the oil and coolant leaks resulting from my blown headgasket would come in handy.

The equivalent of a bead on corrugated aluminum is to bend it along the axis of the corrugations. It will hold the bend by itself unlike cardboard or coroplast.

Having the corrugations running longitudinally with the car is the better way to do it anyway. Just like coroplast, If you bond two layers together with the corrugations at a right angle to each other, it will be vastly stronger.

This is what I ended up doing, btw. http://bengarrido.com/gallery-of-ghetto-modifications/