Rear diffuser angle
 

Is there an ideal angle to try to get? What angles do what? Like do steeper angles tend to cause more downforce and drag than shallower angles. Just trying to get an idea of what to do before I start trying to set up templates. Seems that all the diffuser info I keep finding is aimed at the racing folks that want more downforce. Downforce in the rear to me seems that it would cause drag or vacuum. Obviously what im trying to avoid.

There are optimal angles but everything affects to the results. First you should decide will you begin your diffusor before or after rear axle. If I remenber correct,

Before rear axle the optimum angle was 3-4 degrees
After 6-7 degrees

Max is about 10 degrees and after that you make lots of downforce

Please correct the values who have the huchos book on their hands...

Check out these threads:
Possible rear bumper mod
rear diffuser

This is an after thought related to the same topic. With the benefits of boat tails and reattaching wakes and such, is it safe to conclude that a good diffuser is more important than a front belly pan? Seeing how the wake seems to be more important than the air the vehicle hits.

Without a bellypan to keep the airflow smooth, the rear diffuser cannot be "good". It's purpose is to gently guide the airflow, which won't happen if it is sitting in a bunch of turbulence and there is no airflow to guide.

Hucho shows 3deg as best for Cd reduction for those with a "longer" diffuser (Cd changed by 0.025), and 4deg as best for Cd reduction for those with "shorter" diffusers (Cd changed by 0.020). You will continue to gain more and more down-force (at least up to 11deg) as you make the diffuser more and more steep. Even up to 11deg, you will have better Cd than flat.
As was stated above, you must have good flow coming in to get good flow going out, so belly pans are your friend. Also, Hucho does not discern between sedan or station wagon, it's just for an underbody.

Diffuser angle
 

Reviving this old thread for a question. I read the image below to mean that the most advantageous angle for my rear diffuser is about 4* or slightly less, since I am starting at about the rear axle. My question is how do I judge measure the angle I accomplish reasonably precisely? EDIT: maybe go to my favorite flat spot, park, and use a level to judge the ground and a "quick square," such as this one, to judge the angle? (I have never laid my hands on a "quick square" but a neighbor friend might have one I can borrow). I really have no idea, so maybe I'll just give it my best guess.

http://img72.imageshack.us/img72/4629/venturism8.jpg

precisely
 

*You do need to be on a flat level surface
*You should have 300-lbs of ballast in the front seats so the car will have its 'driving' inclination
*Locate the spot where your diffuser will begin
*Measure its elevation above the ground
*Then measure back to where the diffuser will terminate to get your horizontal distance,your 'run.'
*If you have a scientific calculator,you can use the trigonometric function to get your dimension multiplier
*If say,you do use 4-degrees,enter: 4 tan,to get 0.069926.
*If you multiply your 'run' dimension by this value,it will give you your 'rise' at the back of the diffuser for 4-degrees.
*This will be the exact distance above your original starting elevation for the rear of the diffuser,to get your 4-degree slope.:)

For that brief effective lesson you deserve a dispensation, sir. Thanks twice.

dispensation
 

Well,once your diffuser is going down the highway,we'll all receive a dispensation,by default.So double thanks to you for making it happen! I'm breathing easier already.:D