Rear diffuser angle

[Floordford]

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.

[Vekke]

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...

[Piwoslaw]

Check out these threads:
Possible rear bumper mod
rear diffuser

[Floordford]

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.

[Piwoslaw]

Originally Posted by Floordford

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?

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.

[wyatt]

Originally Posted by Vekke

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...

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.