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Old 10-25-2012, 01:46 PM   #1
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Default Managing heat with a flat underbody

Not sure how many guys have gone ahead and actually made a full flat body undertray for their car, but even if you haven't, if you've read info about this subject, seen other's setups, etc. by all means participate!

I'm looking for the proper ways to deal with heat generated by the engine, exhaust, transmission, and differential once there is a full undertray on the car.

Right now I'm 75% done with building it, and I would rather design it to deal with the temps properly from the start rather than break things and figure it out with trial and error..

Obviously, yes, I have an extractor hood (and am currently cutting it to vent even more), but even with that there's a good 50% of the air coming out of the radiator that probably goes around the motor and exits out the underside of the car. With the undertray, that air has to go all the way back past the exhaust, trans, diff, and exit over the top of the diffuser.

I have a substantial cut-out in the rear bumper, and actually may even run no bumper in the future, so there is plenty of room out back on top of the diffuser to vent air, but the trans. tunnel itself seems cramped to me - there's not a ton of room around the 3" exhaust, PPF, trans. etc. so I wonder if the air from the engine bay will have trouble making its way out the back?
Option 1 would be to leave the undertray sealed and hope all the hot air can get to the back of the car.
Option 2 might be to do something like downward-facing louvres just behind the front subframe attachment points, to give engine bay air an escape route. Might be better for heat, but would be worse for aero.

Second issue is the diff; with the flat bottom, the only air the diff will see will be already-hot air that is traveling down through the trans. tunnel. With good diff fluid should it be OK?
An option might be to do a NACA duct in the undertray that takes air traveling under the car and moves it up to on top of the undertray and directs it at the differential - that air then evacuates out the back over the top of the diffuser. This would be less detrimental to aero than the louvre suggestion for venting engine bay heat since a NACA duct hardly disturbs the air around it, but I am just not sure whether it is necessary for the diff.

Any and all input appreciated

Thanks!
-Ryan
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Old 10-25-2012, 02:20 PM   #2
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Plug (some of) the extractor holes in the hood to push more air through the tunnel (no other escape for the air)?

Oil pumps and coolers for the trans and the diff is the proper solution, but you need the air flow anyway.

Place this in the dumb and untested by poster category
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Old 10-25-2012, 03:00 PM   #3
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The diff and tranny run hot as is. I'm doing a flatbottom on Theseus and I expect them to run significantly hotter. I'll be doing dedicated coolers with 12v pumps on both.
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Old 10-25-2012, 03:03 PM   #4
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What will you do for pumps? The Tilton diff pumps, I assume?


Edited for not sounding like I typed that with mittens on.

Last edited by vehicular; 10-26-2012 at 12:53 AM.
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Old 10-25-2012, 03:36 PM   #5
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Speak slowly and enunciate.

Yes, I'll be using Tilton pumps.
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Old 10-25-2012, 08:16 PM   #6
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I would suspect that if you sealed off the trans tunnel all the way back to the diff and out the diffuser and created a scoop to direct air into the tunnel it would be a big start.
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Old 10-25-2012, 08:58 PM   #7
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subscribed*
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Old 10-25-2012, 11:21 PM   #8
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I have been thinking about this a lot recently too.

My current idea is to run ducting from the back of my radiator to a large heat extractor opening in the hood. Just behind the radiator. Like so.
DSM Ducted Radiator

I have been playing with the idea of where/how to mount a transmission cooler and a diff cooler also.
I was thinking about mounting the transmission cooler at the front of one of the front wheel wells and duct to it from a split brake duct / transmission duct in the front bumper. (The plan would be to mirror this with the same duct to an oil cooler on the other side.)

Then probably put the diff cooler rear vent to the rear bumper/license plate area? to take advantage of some low pressure, but I have not come up with a good high pressure air source for the diff cooler.
Do you guys have any bright ideas?

-Mike
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Old 10-26-2012, 12:25 AM   #9
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Here are some crappy cell phone pics I took of the stock undertrays on a Nissan GT-R.

View from underneath the front looking back. Front tray is basically flat with some pop-its securing things, and some access doors for service.


Moving towards the back, you can see the front and rear undertrays are two separate pieces with a sizeable gap in the middle. It's still fairly flat on the sides from the passenger tub, and a tray in the very middle section might get melted by the exhaust.


The rear undertray has 4 or 5 little ducts to allow air to enter. I'm not showing all of them but you get the idea. There's one of these little ones on each side, not exactly sure if they point at anything specifically.


Here's a duct pointing directly at the diff.


Air scooped up by the ducts exits between the exhaust tips.
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Old 10-26-2012, 12:36 AM   #10
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You could wrap your entire exhaust with the stuff use on headers i suppose. Seems like keeping the heat from your exhaust... in your exhaust... would be a good start.
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Old 10-26-2012, 01:36 AM   #11
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Quote:
Originally Posted by mfoote View Post
I have been thinking about this a lot recently too.
I have been playing with the idea of where/how to mount a transmission cooler and a diff cooler also.
I was thinking about mounting the transmission cooler at the front of one of the front wheel wells and duct to it from a split brake duct / transmission duct in the front bumper. (The plan would be to mirror this with the same duct to an oil cooler on the other side.)
If the line was that long, it and the cooler would add a substantial amount of volume to the ~2qt system. Long lines also = increased failure potential.

I'm subscribed! My initial plan was to do a full length on each side with a large gap under the tunnel for exhaust ventilation but if the smart guys figure out how to make the full bottom work without melting gears then win!
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Old 10-26-2012, 02:08 AM   #12
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Here's a pic from a lotus undertray:


You could probably place a few NACA ducts into the flat bottom to introduce air where needed... I'm not sure if that'd be enough though.


edit: Here's another example of an S2k full undertray.

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Old 10-26-2012, 02:10 AM   #13
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Big heat management difference on a mid engine.
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Old 10-26-2012, 03:07 AM   #14
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Ok, I'll preface this with. I don't know nothin about nothing, but every now and then I'll have an ok idea.

In addition to the things mentioned above, could a NACA duct placed in a high heat area like above the trans and tubed out the rear in a low pressure area like the area where the license plate.

Kinda like this pic, but from the trans tunnel out the back.





Dumb?
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Old 10-26-2012, 05:18 AM   #15
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That S2K undertray definitely has a big NACA duct right in front of the diff.

After the work I put in today I'm 90% done with mine, just need to decide on ventilation/heat management. It's made out of ABS, but there is an aluminum section riveted in where the exhaust comes very close to touching it - I was worried about the ABS melting (I can form it with a heat gun after all, so temp. resistance is not its strong point). The whole thing weighs 10.5 lbs.

Andrew - I am hoping to find a solution that works without spending hundred$ and hundred$ on coolers and pumps, your budget far exceeds mine... plus I don't want to add the weight if I can avoid it Theseus will obviously put double the stress on driveline components as mine will, and I would agree that that car needs dedicated coolers for each component. Crossing my fingers that I don't...
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Old 10-26-2012, 05:23 AM   #16
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Quote:
Originally Posted by mr_hyde View Post
My initial plan was to do a full length on each side with a large gap under the tunnel for exhaust ventilation but if the smart guys figure out how to make the full bottom work without melting gears then win!
I've been playing with ideas for a flat underbody for years, and my original idea was similar to this - but the reality in my experience is that the tunnel is fairly narrow already - if you just make flat sections for either side of the exhaust, but leaving a big gap down the center, you're basically doing nothing, it's just added weight - it's that transmission tunnel that so badly needs to be closed off if you want to make a really efficient under-side to the car. My biggest motivation was that I wanted my diffuser to actually work instead of just look cool, and no diffuser is going to do much of anything without a flat underbody, so I finally just jumped right in and made myself find a way to do it.

-Ryan
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Old 10-26-2012, 06:59 AM   #17
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If you find it suddenly does start chewing and spitting out ring and pinions, then I know that at least in Europe S13/S14's came with diff-oil cooler setups and are a cheap junkyard source of reliable hardware.
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Old 10-26-2012, 11:25 AM   #18
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Quote:
Originally Posted by dstn2bdoa View Post
Kinda like this pic, but from the trans tunnel out the back.

I've thought about something similar. The area around the license plate is very low pressure. Would running a duct or two from an extremely high pressure area and exhausting it through a hole the size of the license plate area be effective at all? The pic above has a fan which would only be legal in the most crazy unlimited classes but would the passive pressure differentials move enough air?
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Old 10-26-2012, 11:37 AM   #19
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I plan to use brake duct fan blowers attached to naca ducts/hose shooting straight on the tranny and diff. Most likely some 12v nascar consumables, or even non electric pushers? I planned for two naca ducts connected via Y joint and fan attched to that, tranny and diff will each have that set up. Venting above the diffuser.
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Old 10-26-2012, 12:54 PM   #20
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Quote:
Originally Posted by dstn2bdoa View Post
Ok, I'll preface this with. I don't know nothin about nothing, but every now and then I'll have an ok idea.

In addition to the things mentioned above, could a NACA duct placed in a high heat area like above the trans and tubed out the rear in a low pressure area like the area where the license plate.

Kinda like this pic, but from the trans tunnel out the back.



Dumb?

What IS that? It looks kind of like a 1G DSM, but it isn't...
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