Belly pan ???
 

I'm working on my belly pan this weekend. What is more effiecent. As the pan passes the rear suspension should it arc up to the rear bumper, Run the same parallel distance to the ground, or somewhere in the middle.

I am able to make side skirts to extend down to the pan behind the rear wheels if it needs to be level all the way. I hope to get the belly pan and grill block done this weekend. Also would there be any benefit to extending beyond the bumper.

Depends on whether you consider lift or downforce to be important.

Well I want the least amount of aero drag. I was thinking parallel to the ground but am unsure. The amount of lift or downforce generated is unimportant as long as it is not dangerous and adds FE. I realize this arm chair engineering so I'm looking for a best guess. Here is a pic of an SL

https://www.gassavers.org/attachment....2&d=1154019557

I'll be deleting the front airdam and running parallel with the side rockers. I'm just not sure about the rear.

The rear should be a smooth curve up the the bottom of the bumper, I'm sure mira or darin will give a better description than I have.

I have been researching this. I'm worried that if I arc up to the bumper that it might cause the venturu effect (discussed here https://www.nas.nasa.gov/About/Educat...undeffect.html)

My reason for worry is the angle up will be sharp. How about a shallower arc with side skirts. This will allow me to cover the muffler, which is a large bump down.

Indeed, from what I understand you do not want it to slope more than 11 degrees, I think it would be better to have it end lower than the bumper and not exceed 11 degrees than it would be to meet the bumper and be 12+ degrees of slope, but I'm no expert!

I'm no expert either, but I think what Ben just said holds true. I've read that 10-15 degrees is the range of slopes where flow remains attached. Closer to 10 is "safer".

And like the lip of the trunk discussed elsewhere, you also want to terminate the tray abruptly - ie. avoid trailing features that end with radii to other planes. If the bottom of the bumper cover is curved (as most are), you might want to consider having the pan extend a little past the curve:

(bottom of bumper as viewed from the side)

*********
*********
********
*******
****
---------- (end of pan)

I think XFi used this approach on the rear tray of his car.

This is what I had in mind. You guys have confirmed my theory.

A slope up to the bumper should allow the underflow to help fill the low pressure area at the back?

Blending it in with the bumper should allow the flow to stay attached further into the low pressure area?

Adding a row of vgs right before the transition to the bumper might help it stay attached?

If you can test it with tuft tests, that should be the way to go. I.e. keep increasing the incline until the flow separates, then back away a bit.

I don't think that you can get away with as extreme a slope on the underside than you can on the topside. On the topside you have some 60 thousand feet of air to fall into the gap, whereas on the bottom you only have a few inches of air to feed into the decreased area.

I'm not sure how compressible air is at highway speeds, but I'd say that a lot of air has to come from the sides rather than the few inches below the car when the underside slopes up at the back, and there will be a limit that is approached faster than the topside when air will not be able to flow fast enough and you start getting flow separation.

I'm not sure what that limit is, it will be more extreme the closer your car is to the ground, which is why I think tuft testing is a practical way to do it.

Do you have a quick read on the tuft testing? Also how would I view the test underneath?

That NSX image shows a diffuser, which is to a belly pan as a rear wing is to a deck lid extension.

By that I mean, the NSX underbody is designed nainly to create downforce (through the radius, shape and angle of the rear pan where it meets the back of the car), not to decrease drag.

But the NSX does illustrate an important point:

This talk of optimum rear undertray angle (for reduced Cd) is academic unless the flow reaching it is smooth to begin with. The NSX even covered its rear suspension & drivetrain components.

Run a search. We did a thread on it recently.

You'd have to mount a camera somehow.

Interesting thread for me, as I have a '98 SL2.

If possible, the rear diffuser is indeed the way to go. Look under the rear bumper of a Toyota Prius or Honda Insight / '06 Civic. All have diffuser-type panels from the rear axle to the rear bumper, and none of these are downforce-intensive applications :-). As previously stated, keeping the airflow smooth up to the diffuser is important. If the diffuser remains <12 degrees, the expanding air should actually "pump" the air ahead of it, resisting the build-up of a boundary layer under the car. You should see significant gain in efficiency this way. To further improve the performance of the diffuser, try to make some vertical fences on each side that protect the airflow from the very turbulent air behind the rear wheels. I would not take the edge of the diffuser beyond the lip of the bumper. If the escaping pressure stays close to the bumper as it rises into the low pressure zone, all the better. It could lessen the effect the low pressure zone has on the back of the car.

As noted in a previous post, I eliminated the front airdam on my SL and saw +2 highway mpg just from this (reduced frontal area). I am now introducing more airflow to the non-smooth underbody and have already seen a drag decrease. Adding a smooth floor from nose to tail should do wonders.

Do be careful that you give the radiator sufficient exhaust under the car, or you will see major overheating issues. My plan is put in a plate between the edge of the lower front fascia and the front swaybar. Perhaps I'll continue the cladding at about the front axle line. However, from the research that I have done, it appears that the largest drag gain is had by adding an engine cover. Such a design on the SL will allow the radiator to exhaust into the airstream, in the same direction as the airstream. I would discourage a radiator intake block unless you live in Canada (no intended offense, MetroMPG) or like blowing head gaskets. As noted in a previous post, I moved my front license plate so as to not block the two air slits in the nose. From appearances, the Saturn actually has a very efficient cooling set-up, if one improves the exhaust side. I bought some foam and will be sealing the gaps between the plastic cool air diffuser (shroud in the nose) and the sides of the radiator, to ensure that all of the ingested air passes through the radiator.

I'll be interested in your results!! Please post pictures!
Bman

Well Bman its nice to have another 2nd gen SL'er around. BTW thanks for those pictures that I used. Have you thought about exhausting air at the rear of the hood? Using .5-1" spacers. The radiator flow is something I have given alot of thought to. I guess I could try it both ways.

I didn't think the grill block would work to well in a high temp & high humidity enviroment. Especially considering I use the A/C(wife and kid you know). I think a partial grill block would be okay, the fog light holes are def not needed.

There have been a few honda owners to do grill blocks and belly pans, but I'm not sure I've seen it on any other makes. They haven't had any issues, but honda is notorious for it's oversized radiators, :p

I've got a grill block on both my cars no problem with overheating the temps run around 200 when on the highway and the highest I've see in stop and go traffic is 213. Pretty hot temps here.
I think as long as your block in not right up against the radiator you'll be ok. Make your block watch the temps and modify as needed to keep the temps where you want them.

My point on the radiator block-off is to proceed carefully. I think most of us are running fairly high stressed 4-bangers, and it only takes one good overheating to blow a head gasket or, worse, warp a head. Don't ask me how I know. :p One good head gasket replacement pretty well eats up any $$$ saved in improved mileage. I do realize that cars are engineered to avoid self destruction in Death Valley, while towing up a hill with the air on. My point mainly is to proceed with caution, be aware of the possible consequences, and pay at least as much attention to how the air exits the engine compartment.

To "lovemysan's" point, I would not prop up the rear of the hood, unless you seal off the gap across most of the back. You actually have a high pressure air pocket at the base of the windscreen, and you will more likely be pushing air into the engine compartment from there. The outer edges of the hood are in accelerating air, so they would make good venting zones. On my '83 Mustang HDPE car, I've done both. I've raised the rear of the hood and ducted the center to my carburetor (yep, it's that old!). I've left the outer edges open to the airstream to vent what my hood cut-outs don't. I'll eventually cover these with wire mesh and actually duct the radiator exhaust to them. I also plan a complete cover of the engine compartment from underneath and will be planning air management very carefully.

To have yet to hack holes in the Saturn's hood because I don't see an immediately attractive way to do this. The openings from the fender liners to behind the front wheels are already pretty good. Perhaps trying to push the front fender behind the front wheel out a bit might help. I haven't really explored this because I was intending to retain the bottom-venting arrangement (be it re-directed).

Bman

Bman, I have some questions for you. It seems that after the rear spoiler removal the car has lost some high speed stability. After 50mph it seems to be more darty, and requires frequent steering correction. I drive the car very little on the highway, this is something that I have noticed during the last few drives(going to the chiro). What is your take on this?

Does your car have the formed deflector just in front of the wipers. On my car it serves as the fresh air intake for the cabin but it is also shaped to deflect air over the wiper arms. I'm curious how effective this thing is? One of the aero mods I had been mulling over was a fiberglass formed deflector for the wipers to park under. After noticing this I'm curious if it would be worthwhile to invest time into.

Also do you frequent www.saturnfans.com ?

That is supposed to be the result from adding airtabs...less darting around and disruption from trucks. I would guess that your OEM spoiler helps mpg.

The rear wing / spoiler could have been helping you, but probably not much. At best, the air behind our rear windows is rather turbulent. I noticed after 120k that my struts were getting soft, and the rears were pretty well shot. At highway speed, the car lacked stability, and the rear bobbed horribly under braking. I got a set of KYBs from SPS and, with an alignment, it is rock-solid again. I'd check your struts before you give too much credit to the rear spoiler.

I don't think spending energy on the wiper area will bring you much tangible result. Per reading, posts here, etc., I'd go after a flat bottom, flat wheelcovers, front wheel deflectors and the like first.

I'm registered on Saturnfans (Bman91SL2), but I'm not there much. I owe them a post because I tried a Marvel Mystery Oil flush and cured my oil consumption problem.

Bman

I only have 54k on the car. The struts are still nice and tight. The tires are getting to the end of there life though.

Like I said, I really think the dartyness was there before. There are long periods of time between highway trips for me. So it is hard for me to notice such things. Also it has been windy lately so that could be a contributing factor.