A belly pan question

[CamaroAJ]

on alot of high end newer cars they are running full length belly pans that also enclose the exhaust system. how are they controlling the heat in the tunnel? i know i won't be able to make a full pan like they are but i was wondering how they were doing it.

if i were to try a full pan my exhaust will be stainless and i though about having it coated and then wrapping it to help but it will still get hot.

[JEFFTATE]

Well , they just downright get hot ..
My 2007 Z06 has the exhaust through the driveshaft tunnel and above a belly pan .
The console gets hot enough to be uncomfortable on my arm..
I could keep my lunch hot in there !
I don't know how to cool the area , or keep the heat from transmitting except , like you said , ceramic coating and heat wrap.
I think the wrap can cause corrosion issues with the exhaust ??
Because of moisture condensation from the hot gasses /cold metal at startup.. ( I could be wrong about this )
I would think that after the exhaust heats up , it would vaporize any moisture.
Stainless steel piping would be a must.

[Nessumsar]

You could incorporate a vent for the driveshaft tunnel, much like a brake-duct; but I can't think of a way to make this work with a rear-exiting exhaust, they would have to exit infront of the rear tires. I have been pondering this as I have a huge sheet of alluminum that I could use for this.

I would think that as long as you seal (paint) the exhaust, you wouldn't have to worry about corrosion.

[CamaroAJ]

Well , they just downright get hot ..
My 2007 Z06 has the exhaust through the driveshaft tunnel and above a belly pan .


the vette doesn't really have a belly pan. the trans tunnel plate doesn't count since its part of the cars structure and doesn't do anything aero wise for the car.

and i know what you mean about the heat, i left chapstick in the cup holder once at a track day in my old C5Z and it melted lol.


You could incorporate a vent for the driveshaft tunnel, much like a brake-duct; but I can't think of a way to make this work with a rear-exiting exhaust, they would have to exit infront of the rear tires. I have been pondering this as I have a huge sheet of alluminum that I could use for this.

I would think that as long as you seal (paint) the exhaust, you wouldn't have to worry about corrosion.

i was thinking maybe loovers facing the rear of the car to draw heat out.

i'm not sure adding air into the trans tunnel would be a good idea because that air would go under the car. i did read though that adding air at the back into the defuser helps speed up the air comming under the car though. i'm not sure that would be far enough back though.

[Nessumsar]

i'm not sure adding air into the trans tunnel would be a good idea because that air would go under the car. i did read though that adding air at the back into the defuser helps speed up the air comming under the car though. i'm not sure that would be far enough back though.


I was thinking along the lines of building a box (for lack of a better term) around the exhaust itself and curve it so the exhaust and cooling air exit out the side in front of the rear tires.

This wouldn't introduce any air under the car as it would be contained and exited from underneath.

[CamaroAJ]

it could be built like corvettes. they have flat floors and down the center the exhaust and torque tube. its not ideal but it works.

[69496]

If the exhaust is stainless wrapping it should be ok, it would atleast last long enough. If its not stainless it will rust out pretty quick if you wrap it, like someone mentioned earlier.

[LeighP]

Guy I know on another board did this with a 2nd gen T/A. He made up a three piece belly pan from the front to just in front of the rear axle. All mounted to a triangulated brace he fabbed up. Stiffened the car nicely, but it was so hot he ended up taking it off as the car was too hot to drive in for any long distance runs (which was why he built it in he first place).....Last I heard he was trying to figure some way of using the airflow to extract exhaust heat.

[overZealous1]

simple, louver it.

[CamaroAJ]

that was my thinking too. however sitting in traffic no air is going to move so it will get hot with the quickness.

[wmhjr]

How do they design them from the factory?

With massive computer modeling to determine all the thermal and fluid dynamics. You have to worry about all kinds of variables, and it would take some serious track time and constant modifications to try and trial and error it. I would be very concerned about creating hot spots where heat would not extract, as well as unintended lift at speeds. You would also very possibly increase engine temp if done incorrectly, as you might create a high pressure area in the engine compartment, decreasing air flow through the cooling system

[CamaroAJ]

air flow through the radiator will not be a problem with my car. i wish i could get my hands on a car full belly pans to see what they did with the exhaust.

[wmhjr]

You think air flow through the radiator will not be a problem until you close off the belly of the car, creating a pressure area which restricts air flow. IOW, you still have plenty of surface area in the front, but unless you're ducting air from the engine side out somewhere the air does not flow. Same reason top race cooling guys want you to retain your cowl seal, etc. Force the air flow along the path it was designed to go. Not saying there WILL be a problem. Just saying you can't say there WON'T be a problem. Not unless you've modeled it.

What is the problem you're trying to solve? If it's just to be like "some high end cars" I think they've spent millions of dollars figuring it out for their specific design. The variables at play here are very significant. You could certainly do it via trial and error, but I think heat, high speed handling and noise would all be factors to deal with. You just might be creating a very heavy airfoil, generating lift at speed. Of course, the other thing would be that it may not pass tech at some tracks.

Interesting thought.

[CamaroAJ]

the air flow will be venting through the hood from the radiator so its not going to be a problem. and there will be hood and fender vents to get the pressure out from under the hood with as much of the grill blocked as possible to keep alot of air from building up.

belly pans are mainly smooth with a few ribs to direct air flow and need to be angled up in the rear to create an expansion are to help suck the car down and end in a defuser at the rear.

thats the easy part of the big picture. heat is the hardest part. the exhaust can be coated and warpped to help but it won't take care of it 100%. another option taken from corvette racing is to use vented mirrors and pipe air into the space between the belly pan and then control the exit of that air to the rear of the belly pan at the defuser (corvette vents the spot where the plate goes with a screen.)

there would have to be alot of trial and error to see if any gains would to be had. but if it wasn't worth it then do you think all the race teams would do it?

[wmhjr]

I do think it can be worth it, but the race teams spend hundreds of thousands of dollars in computer research before just trial and error. Like I said, interesting, but probably pretty difficult to deal with unless there are more metrics available.

[vette427-sbc]

What front engine cars actually have full belly pans though? I wouldnt think any production cars would have a fully enclosed exhaust due to the heat generated when idling in traffic or other similar situations. You would think that GM would have put a belly pan on the 190+ MPH Z06 or ZR1 if they could. Same with the Viper. Also... most full belly pan cars are mid/rear engine layout and dont really have to deal with this problem.
I know this doesnt really help in your quest to put a belly pan on your car, but maybe the manufacturers leave them out for good reason.

[CamaroAJ]

thats a good point chris, i never really thought about that. i'll prolly just do a front pan and enclose behind the axle.

[exwestracer]

thats a good point chris, i never really thought about that. i'll prolly just do a front pan and enclose behind the axle.

Donohue and Penske recessed the exhaust into the floor of the 1969 Trans-Am Camaro, but they didn't have to worry much about sitting in traffic...

If you really want to sell your soul for a flat undertray, how about side pipes with skirts inboard of the exhaust? No different than rocker panel extensions, really...

There is a hell of a lot to be gained with a well-designed flat floor concept, because the air pressures are relatively constant as compared to a wing.

[Bjkadron]

Belly pans can be very complicated to design. Especially if your needs require that it be able to sit in traffic. I would look into ceramic coatings to control the heat. Some companies that do exhaust coatings claim that you can touch the pipes with your bare hands. Sounds nuts.. But I would say it is possible.

That said I am putting a full bellypan on my car. But other areas of the car will be radically changed to compensate for cooling and such. Also I am working to use the cooling air to help add downforce where I want it.

Whether or not you should do it could be answered with a simple question. How much is the benefit worth to you?

[vette427-sbc]

Some of the ceramic coatings do keep the heat way down, but Ive found that only to be true with open headers (on my car at least) I could hold my hand on the sidepipe for a 5 or so seconds without cooking my fingers. As soon as I put mufflers in, they would get hot enough where you couldnt hold your hand on there for more than a second without getting a nice burn. To end my story, the ceramic started rusting, I sanded it off, painted with regular high temp paint, mufflers out, and they are still too hot to touch.

Like exwestracer said, sidepipes may be your ticket to a full belly pan. I havent looked under a first gen f-body in a while, but could you run the exhaust on the outside of the subframe connectors and have it come out of a hole in the rocker? Might look pretty bad ass too if you could pull it off. :smoke:

[Wicked]

The belly pan does not have to be continuous or complete. Although for the best aerodynamic affect it should be as smooth as possible. The goal is to keep the air under the car moving as fast as possible over the underbody.

That said, the exhaust(at least mufflers and cats) don't have to be covered.

Here's what we did for the One Lap Odyssey I built at work(Honda).

Using a single roof/floor manometer reading and assuming the same pressure difference over the main roof area, we measured an improvement of 230lbs(191lbs of lift --> 40lbs of downforce) at 100mph.

FYI the van is 532whp twin turbo 3.5 V6 Honda Odyssey. 6-speed manual. H&R coilovers, Enkei 19x10, 275/35/19's. Turbo system built and tuned by yours truly with Autronic ECU. Built for the '06-'08 One Lap of America.

[dr.gonzo]

maybe if you can do a few louvers at the back of the car, and than hook up a couple of small fans up front on a toggle switch. i think might work..

[exwestracer]

maybe if you can do a few louvers at the back of the car, and than hook up a couple of small fans up front on a toggle switch. i think might work..
https://static1.pt-content.com/images/pt/2010/12/Chaparral2JChevrolet_3-1.jpg

?????????

[Nessumsar]

Chaparral 2j! Pure Awesome.

[MrQuick]

I remember that, it sucked all the chrome off itself. Multiple fans provide the proper amount of suction.

[twosaturns]

I wouldn't enclose the exhaust; make some real pretty bends in SS, then bring the pan right up to it.
OR, you could cover the floor of the car w/ space shuttle ceramic tiles.

[68Formula]

Here's what we did for the One Lap Odyssey I built at work(Honda).

Using a single roof/floor manometer reading and assuming the same pressure difference over the main roof area, we measured an improvement of 230lbs(191lbs of lift --> 40lbs of downforce) at 100mph.

FYI the van is 532whp twin turbo 3.5 V6 Honda Odyssey. 6-speed manual. H&R coilovers, Enkei 19x10, 275/35/19's. Turbo system built and tuned by yours truly with Autronic ECU. Built for the '06-'08 One Lap of America.

HHhhmmmm, wife could use a minivan, is this thing for sale? LOL!

[Bjkadron]

?????????

or with a corvette?

http://grassrootsmotorsports.com/articles/plan-attack/

[Zachalanche]

Perhaps a compromise is in order. belly pan as much as you can within reason but leave the trans tunnel open. combine that a good front splitter and rear diffuser and you should have a noticeable increase in down force provided you have effectivly expanded the air passing under the car. you could always have a removable piece for the trans tunnel just for track use. obviously this won't be as good as a race car, but i hear race cars aren't the most comfortable.

[James OLC]

Th agent 47 mustang incorporates a naca duct in the rear window with an air tube down to an opening over the exhaust system to aid in cooling... I'm thinking of something similar for the '69...

[MonzaRacer]

OK if you use a high grade stainless like 321, and have it thermal barrier coated inside and out,using the same stuff they coat jet engine "tail feathers" with and as for a "belly pan" from what I read from ol' Smokey Unicks (sp) ideas and testing a pan between the bumper and the crossmember and behind the rearend to the rear bumper.
I wouldnt get too radical near floor and driveshaft area. dont do as complete enclosure, keep it real and clean. like from pinch welds to tunnel. leave exhaust exposed so you dont enclose the heat, and I dont care what metal you use wrapping is a no no. The thermal stress it will have cracks and fractures in no time.
This came from 3 different stainless header manufacturers, no wrapping UNLESS your protecting a hose or critical part. The shorter the better.
Cleaner air under car is good as long as your work helps it build a suction at speed nad also let the engine compartment bleed air properly.
Good Luck.

[Bjkadron]

Th agent 47 mustang incorporates a naca duct in the rear window with an air tube down to an opening over the exhaust system to aid in cooling... I'm thinking of something similar for the '69...

I looked that up... and they have it in backwards... Which makes it do absolutely nothing.

[Bryce]

I looked that up... and they have it in backwards... Which makes it do absolutely nothing.

Actually they are correct.

Once you consider boundary layers it makes sence. The air closest to the car " boundary layer" move at the same speed as the car. The NACA duct creates vortices that deflect the boundary layer away from the intake, but draws in the faster-moving air above it.

[Bryce]

Reversing the NACA duct will actually draw air out of the car, rather than drawing it into the car.

[Bjkadron]

Actually they are correct.

Once you consider boundary layers it makes sence. The air closest to the car " boundary layer" move at the same speed as the car. The NACA duct creates vortices that deflect the boundary layer away from the intake, but draws in the faster-moving air above it.

Reversing the NACA duct will actually draw air out of the car, rather than drawing it into the car.

I know how they work in the one direction. But I'm pretty sure they were only designed to work one way. I don't know a whole lot about fluid dynamics but I'm pretty sure it wouldn't draw much, especially where they have it mounted. They have nothing to reduce flow separation along the back window and the spoiler is creating a high pressure zone.

https://static1.pt-content.com/images/pt/2010/12/harbinger_18lrg-1.jpg

Do you by any chance have a CFD picture or something of a NACA Duct that I could look at?

[Bryce]

Benjamin, I though we were talking about the quarter window ducts. Hmm. I have to give those some thought. I dont think those would work very much. They would have to do some windtunnel testing to verify flow over the back window. Usually the flow has not come back down to the window at that point. But the interior of the car is usually high pressure and the back window is lower pressure. So you would have a outward flow at that point but not due to the NACA duct specifically. If you had a laminar flow over that duct it would work.

Let me look through my areo books and see if I can find the calculations for the NACA ducts.

[The Stickman]

The reversed NACA duct would be easy enough to test. Say make a temp side window out of something with the duct mounted to it. Heck even cardboard would work. Then go for a drive and see if it pulls air, draws it in, or does nothing.

[Bjkadron]

Benjamin, I though we were talking about the quarter window ducts. Hmm. I have to give those some thought. I dont think those would work very much. They would have to do some windtunnel testing to verify flow over the back window. Usually the flow has not come back down to the window at that point. But the interior of the car is usually high pressure and the back window is lower pressure. So you would have a outward flow at that point but not due to the NACA duct specifically. If you had a laminar flow over that duct it would work.

Let me look through my areo books and see if I can find the calculations for the NACA ducts.

Nope, the side ones are correct. I was talking about the one in the picture. Every fastback I have seen has flow separation or at least increased boundary layer thickness or turbulent flow. With the spoiler there creating a high pressure area, and with the tubing going somewhere the inside/outside pressure difference would be negated I wold think.

[Bryce]

Nope, the side ones are correct. I was talking about the one in the picture. Every fastback I have seen has flow separation or at least increased boundary layer thickness or turbulent flow. With the spoiler there creating a high pressure area, and with the tubing going somewhere the inside/outside pressure difference would be negated I wold think.

I dont think that spoiler does much unless the car has vortex generators on the trailing edge of the roof.

It would need testing to verify. I would just put some pitot tubes (monometer) in that area to test for pressure. If you have a lower pressure on the outside versus the inside of the back window you could make those vents work.

I saw a studebaker streamliner that had a huge duct from under the car to right behind the rear window. I will try to find the picture.

[Bryce]

Check this out for the story on the studebaker. http://www.rodandcustommagazine.com/featuredvehicles/1953_studebaker_coupe/index.html

[Zachalanche]

sorry to deviate from belly pans, but here is an interesting article on various aero stuff.

http://www.hotrod.com/techarticles/body/113_0703_car_aerodynamics/index.html

gives some good ideas, but not if sure I fully believe all the numbers.

[MrQuick]

its hot rod...why not? lol

[Zachalanche]

its hot rod...why not? lol
they probably made a sequel to that article on how to build a 200 mph car for $500.