Full Flat-Bottom Undertray EVO
Jun 16, 2008, 08:11 AM
#35
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As for all the thermal comments, I'll be testing and gathering data. I'm a mechanical engineer, this is what I do. This reminds me of my very first cold-air box, it was built out of polypropylene (plastic). Everyone said "nice, too bad it is going to melt in the engine compartment". I ran the car at the track in 100+ heat and never got close to "melting" it - there's a lot of unfounded comments about heat.
For example someone said "heat rises", that is true. Well, heat rises with or without the underpanel. Also, someone said that under hood temps were high with a splitter and no undertray. Well the car comes from the factory with a larger front undertray...
I have a track day this week so it will be the first bit of info I get on a potential thermal problem.
Here's what I know, and I'll get some data behind this with tempertaure sensors later.
- At a stop there is no airflow, and the thermals under the car are governed by natural convection and radiation. This is where the "heat rises" comment comes into play. Natural convection of the hot air will carry the hot air up and into the body, this happens with or without the underpanel. The difference is the size of the thermal reservoir below the exhaust, and the temperature differential of the surroundings. Without the underpanel the thermal reservoir is good sized and has the potential for favorable temperature differentials. Although hot asphalt in the sun gets pretty hot. With the underpanel the reservoir size is now smaller, which effects the thermal time constant, and the panel itself will be the reference temperature for both natural convection and radiated heat transfer from the exhaust.
- Cruising conditions everything is convection cooling. Although conducted and radiated heat transfer are still factors they are small contributors in comparison to the convection cooling. So this will be all about airflow.
Don't worry, heat is on the radar for items to look into. So don't tell me my plastic airbox is going to melt
.
Last edited by jid2; Jun 16, 2008 at 08:17 AM.
Jun 16, 2008, 08:20 AM
#36
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My biggest worry is the temperature of the rear differential; I think a NACA duct under the car set up to direct air at the diff might be a big help. However... the whole goal of this is low pressure under the car. I hope there'd still be enough pressure for the duct to get adequate airflow. With that said, I'm extremely impressed and I've been dying for someone to rig up a full undertray.
Last edited by Tom-05-MR; Jun 16, 2008 at 08:24 AM.
Jun 16, 2008, 09:57 AM
#39
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It has about about an inch of rake in there already, it's hard to see in the photos. The height of the rear edge of the splitter is lower than the rear chassis brace, so the aluminum sheet that span that gap is angled to help create low pressure under the axle.
As for all the thermal comments, I'll be testing and gathering data. I'm a mechanical engineer, this is what I do. This reminds me of my very first cold-air box, it was built out of polypropylene (plastic). Everyone said "nice, too bad it is going to melt in the engine compartment". I ran the car at the track in 100+ heat and never got close to "melting" it - there's a lot of unfounded comments about heat.
For example someone said "heat rises", that is true. Well, heat rises with or without the underpanel. Also, someone said that under hood temps were high with a splitter and no undertray. Well the car comes from the factory with a larger front undertray...
I have a track day this week so it will be the first bit of info I get on a potential thermal problem.
Here's what I know, and I'll get some data behind this with tempertaure sensors later.
- At a stop there is no airflow, and the thermals under the car are governed by natural convection and radiation. This is where the "heat rises" comment comes into play. Natural convection of the hot air will carry the hot air up and into the body, this happens with or without the underpanel. The difference is the size of the thermal reservoir below the exhaust, and the temperature differential of the surroundings. Without the underpanel the thermal reservoir is good sized and has the potential for favorable temperature differentials. Although hot asphalt in the sun gets pretty hot. With the underpanel the reservoir size is now smaller, which effects the thermal time constant, and the panel itself will be the reference temperature for both natural convection and radiated heat transfer from the exhaust.
- Cruising conditions everything is convection cooling. Although conducted and radiated heat transfer are still factors they are small contributors in comparison to the convection cooling. So this will be all about airflow.
Don't worry, heat is on the radar for items to look into. So don't tell me my plastic airbox is going to melt .
As for all the thermal comments, I'll be testing and gathering data. I'm a mechanical engineer, this is what I do. This reminds me of my very first cold-air box, it was built out of polypropylene (plastic). Everyone said "nice, too bad it is going to melt in the engine compartment". I ran the car at the track in 100+ heat and never got close to "melting" it - there's a lot of unfounded comments about heat.
For example someone said "heat rises", that is true. Well, heat rises with or without the underpanel. Also, someone said that under hood temps were high with a splitter and no undertray. Well the car comes from the factory with a larger front undertray...
I have a track day this week so it will be the first bit of info I get on a potential thermal problem.
Here's what I know, and I'll get some data behind this with tempertaure sensors later.
- At a stop there is no airflow, and the thermals under the car are governed by natural convection and radiation. This is where the "heat rises" comment comes into play. Natural convection of the hot air will carry the hot air up and into the body, this happens with or without the underpanel. The difference is the size of the thermal reservoir below the exhaust, and the temperature differential of the surroundings. Without the underpanel the thermal reservoir is good sized and has the potential for favorable temperature differentials. Although hot asphalt in the sun gets pretty hot. With the underpanel the reservoir size is now smaller, which effects the thermal time constant, and the panel itself will be the reference temperature for both natural convection and radiated heat transfer from the exhaust.
- Cruising conditions everything is convection cooling. Although conducted and radiated heat transfer are still factors they are small contributors in comparison to the convection cooling. So this will be all about airflow.
Don't worry, heat is on the radar for items to look into. So don't tell me my plastic airbox is going to melt
.I build race cars, that's what I do. I was also the one who mentioned the appreciable temperature difference in going from a valence to a front splitter on our race cars (not an Evo). I'm talking about @ a 100 degree difference between the two setups and that isn't even panned off.
I think all we were saying is make sure you are aware of the temps. You never know who is doing these projects.
IMO the biggest area you will see problems will be heat soaking after finishing a run. Even with a pan as long as it isn't fully sealed you will still get some airflow through the engine bay and out by the rear diff when moving at a decent speed.
What are you doing for a rear diffuser?
Good luck, it looks like a fun project.
Last edited by hotrod2448; Jun 16, 2008 at 10:06 AM.
Jun 16, 2008, 09:59 AM
#40
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My biggest worry is the temperature of the rear differential; I think a NACA duct under the car set up to direct air at the diff might be a big help. However... the whole goal of this is low pressure under the car. I hope there'd still be enough pressure for the duct to get adequate airflow. With that said, I'm extremely impressed and I've been dying for someone to rig up a full undertray.
Even with a few NACA's this would still be a big improvement over stock.
Jun 16, 2008, 10:12 AM
#42
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Jun 16, 2008, 10:27 AM
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You're an engineer that's cool.
I build race cars, that's what I do. I was also the one who mentioned the appreciable temperature difference in going from a valence to a front splitter on our race cars (not an Evo). I'm talking about @ a 100 degree difference between the two setups and that isn't even panned off.
I think all we were saying is make sure you are aware of the temps. You never know who is doing these projects.
IMO the biggest area you will see problems will be heat soaking after finishing a run. Even with a pan as long as it isn't fully sealed you will still get some airflow through the engine bay and out by the rear diff when moving at a decent speed.
What are you doing for a rear diffuser?
Good luck, it looks like a fun project.
I build race cars, that's what I do. I was also the one who mentioned the appreciable temperature difference in going from a valence to a front splitter on our race cars (not an Evo). I'm talking about @ a 100 degree difference between the two setups and that isn't even panned off.
I think all we were saying is make sure you are aware of the temps. You never know who is doing these projects.
IMO the biggest area you will see problems will be heat soaking after finishing a run. Even with a pan as long as it isn't fully sealed you will still get some airflow through the engine bay and out by the rear diff when moving at a decent speed.
What are you doing for a rear diffuser?
Good luck, it looks like a fun project.
I agree that heat soak after a run is the biggest issue, which is why it's important to do a proper cool down, etc. Cool downs are important in general for things like not catching brake pads on fire
. I've already got the temperature probes ready for the first bit of testing, so I'll get some data soon.
For the rear diffuser I plan to cut out the rear bumper to the level of that spare tire well so the diffuser can transition up the back of the car. To balance cost, and performance the rear diffuser will most likely be thermoformed plastic of some sort. Sheet metal is kinda ugly and not easy to make with nice curves without expensive tooling. Carbon is great but $$$$ and I'm not into fiberglass with a carbon top sheet. So thermoforming seems to be a good fit. That project is a little bigger undertaking.