I have been considering a rear diffuser for a while, my hesitation has been that it might make things worse rather than better due to the shape of the rear end and having open rear wheel arches that allow air in to the rear valance; the top black line is approximately the boot floor so everything below that point that curves forward must be adding drag (happy to be proven wrong).

Options:
1. Allow the air to exit from this area by cutting out most of the lower black line.

2. Add diffuser that goes under diff to the seat pan, but I suspect air will still end up in this area from the wheel arches.

3. Create wheel arch liners to reduce the amount of air funnelled to the rear valance.

4. Combination of 1, 2 or 3.

5. The difference is so negligible I'd be better off doing sod all and keep the rear lines clean.


Stylus rear

I have not finished my car yet however it has a diffuser that looks like it would fit your car


Description

Sorry, can't help with your question but here's another pic for inspiration if you do go ahead and make one


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Originally posted by cerbera
Sorry, can't help with your question but here's another pic for inspiration if you do go ahead and make one

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My rear arches are open and I can't say I noticed a difference to be honest.

Option 5.

Just cut a hole in the rear valance to let out air that can be trapped. At normal road speeds diffusers are only visible features not functional

Tend to agree with the above. Most diffusers are too far off the floor to have any real effect. I think extending the flat floor to reduce lift is the only real way of doing it.