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.
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!
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.
02 Saturn SL
for pics click the link below
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,
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. 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, 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.