Undertray air flow...

[CommodAnt]

Hello everyone,

I've got my car off the road to put in a new engine and am giving serious thoughts to installing an undertray as has been done on several other cars on this site.

My question pertains to air flow through the radiator and engine bay. If I install an undertray, where does this air go? I wish to retain a stock appearance, so don't want to cut holes in the bonnet or guards. How have others got around this problem?? I really don't want to stop too much cooling flow through the engine because I live in tropical Australia (very hot), and have a large engine (304cui... I know, not very fuel efficient ).

Is there an ideal place to vent excess air?? Would I be better off just installing a really big retractable air-dam/spoiler under the car?? Any ideas would be appreciated.

[basjoos]

I run most of my cooling air exhaust out through driveshaft openings (front wheel drive) into the front wheel wells and the remainder runs through a tunnel enclosing the exhaust pipe and catalytic converter to exhaust into the right rear wheel well.

[CommodAnt]

Hmmm. That sounds good. I would image that it would be better to vent the air to the wheel wells to equalise pressure there (normally low). Does this sound right to you or is this just false economy?

[basjoos]

With all of the fast air movement within the wheel wells, there is a lower air pressure (Bernoulli's principle) inside the wheel wells. This helps to pull the radiator exhaust air out of the engine compartment, especially in the front wells where I am venting the exhaust near the hub and the air is being spun outward as in a centrifugal (squirrelcage) fan. Since there is already so much air turbulance inside the wheel well, the added turbulence of the exhausted radiator air doesn't add to the turbulence of the car's aerodynamics.

Ideally the best place to exhaust the radiator air is into the low pressure eddy behind the car where it can help to fill that void. If your exhaust pipe runs the length of your car and you plan to install underbody panelling, you could build a tunnel into the panelling following the exhaust pipe to carry the radiator exhaust air from your engine compartment to the back of your car.

[CommodAnt]

Thanks basjoos.
You've steered me in the right direction. I'll post pics when I get going (soon)...

[red91sit]

So I noticed your name and location, put 2 and 2 together, and was wondering. Do you drive a Holden Commodore?

::EDIT:: DOH!! i looked at the garage entry. Very nice looking car!! But a few things.. crusing with the clutch in is bad for throw out bearings, better idea to just shift it into neutral.

also your engine's quite similare to mine!! 5.0 Liter ford right?

OH yesh, welcome to the site.

[usedgeo]

Well I got a belly pan on the front of my Saturn today. Here are some before and after pictures. I am not sure how I will change the oil now. It seems to help the mileage a bit but I have not driven too far and don't trust the scan gauge completely since my IAT experiment. The Aluminum heat shield is valley metal. I had to open the grill to about 16 square inches for now.

Ernie

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[Bman83GL]

Ernie,

I also have a Saturn ('98 SL2) and have a great deal of interest in your post. I have thought of doing an undertray as far back as the front swaybar, so that the radiator can exhaust sufficiently. What you have done is far more radical! Have you had a chance to ascertain any differences in stability or mileage yet? Has it affected engine temperatures (maybe not until summer, eh?)? What is the blue material you used? It looks like you are using an aluminum insert (valley metal?) below the exhaust routing.
Great stuff! Thanks for posting!
Bryan

[usedgeo]

Originally Posted by Bman83GL

Ernie,

I also have a Saturn ('98 SL2) and have a great deal of interest in your post.
Bryan

Before the tray I was running with the grill 100 % blocked and the air dam removed. It was running 195-200. Since installing this the warmest day has been near 70. I opened up the bottom grill slot the width of the license plate. That has been too much but warmer weather is coming. It still runs 195 -200. The low side of thermostat opening. There is lots of outlet area. I think cooling will not be an issue at all. I might be on the shoulder pulling grill block some day though. It did get to 206 one day but I think that was due to an EOC just before the thermostat opened. To put this in perspective it ran 198-200 with the new t-stat before any mods.

Mileage improvement will be confused with warmer weather and heated intake air. I am over half way through my best tank ever. The EOC is better. I might break 50 mpg this tank.

It is just coroplast and valley aluminum. It slips between the bumper cover and supprts. I had to use longer plastic rivets. I may try to find some screws and clip nuts when I chanage the oil. The rest is just tie straps. I hope to go back 4 more feet this weekend if the yardwork will let me.

Ernie

[Bman83GL]

Thanks for the details, Ernie.

Please keep the pictures and observations coming as this evolves.

Bryan

[The Toecutter]

This is why A-B-A testing is so important. If you can nullify the weather and engine temperature as variables, you have a better chance of figuring out which the fuel efficincy gains come from the aeromods.

[usedgeo]

Originally Posted by The Toecutter

This is why A-B-A testing is so important.

If and when I get the underpan removable I might do some testing then. I bought u-nuts and screws to replace the plastic rivets at the front. It will need to come off when the oil needs changed.

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Thanks for the details, Ernie.
Please keep the pictures and observations coming as this evolves.
Bryan
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I did extend back another 38 inches. This was the remnant from the front piece. I used 3 inch furring strips screwed to the sills of the frame so that the 48 inch wide piece was wide enough. It was sagging a bit in the middle so I used heavy wire to tie it up closer to the exhaust. I still have 1 inch air gap. I don't think the furringstrips would be a good idea in wetter climates.

I need to come up with a strip to join the two pieces also. I think aluminum strip and screws will do.

This did reduce air flow through the radiator. I was running about 216F in 60 degree air. My 15 mile commute is not long enough to show this problem. When you look at the picture of before you see that the swaybar brackets hold the pan down under the radiator. I think I need some pipe insulation to fill that gap and route the air through the radiator.

With less air flow through the engine compartment my intake temp ran up to 180-190. Even though this was real heated intake air as opposed to the fake heated air experiment the mileage was too good to report. I don't think it was true. I do have my best ever tank going and I am quite confident of breaking 50 mpg. I should revert to unheated intake air to get a baseline for the belly pan mod.

Sorry I did not take any pictures. I might get some later but I have a couple weeks of interference coming up. I probably should not refer to the 25th wedding anniversary as interference.

I have started on the rear wheel pants now. I should put a belly on the back but I have not given up on mounting a motor some where yet.

Ernie

[BluEyes]

Originally Posted by CommodAnt

Is there an ideal place to vent excess air??

My ideal place would be through the hood. Take a look at the Ford GT40's.
Not only is there usually a low pressure area just behind the front edge of the hood caused by the air curving around the shape of the car, but this also keeps air flowing over the car rather than under it. That is a 'cleaner' solution and also works to reduce front end lift.

Downside is that most cars have a pretty tight engine bay so doing any dedicated ducting is probably not practical and you've gotta have the guts to cut a hole in the first place!

[red91sit]

Originally Posted by BluEyes

My ideal place would be through the hood. Take a look at the Ford GT40's.
Not only is there usually a low pressure area just behind the front edge of the hood caused by the air curving around the shape of the car, but this also keeps air flowing over the car rather than under it. That is a 'cleaner' solution and also works to reduce front end lift.

Downside is that most cars have a pretty tight engine bay so doing any dedicated ducting is probably not practical and you've gotta have the guts to cut a hole in the first place!

Another idea (one I've been playign with) is to route the air to the wheel wells, I believe these are rather low pressure as long as you have good shaped wheel wells, I noticed on my fathers Elantra, there are some vents going into the wheel wells

[Bman83GL]

Thanks for the update, Ernie, and congratulations on your interference.

Both BluEyes and red91sit make good points. Through-the-hood is optimal for several reasons. But if you don't want to cut holes in your bonnet, ducting BEHIND the front wheels does indeed tap into a low pressure zone.

Bryan