Reducing Drag on a Track Miata

I have a 95 Miata that gets used for track days and time trials. I have an R-package lip, a factory hardtop, and a fairly low ride height. At high speed tracks like Watkins Glen, any reduction in drag will make a noticeable improvement in lap time.The organizations that I drive with require that the windows be down while on track. It seems to me that having the windows down is horrible aerodynamically. Is there anything I can do to mitigate the effects of the windows being open? What else can I do to the car to reduce drag?

The underside of most older cars causes lots off drag with exposed components for the air to get caught on. Doing anything to prevent air from going under the car, or smoothing/directing the airflow that does go under the car could generate noticeable improvement.

An airdam/sideskirt combo would work to keep air from getting underneath the car in the first place, therefore reducing drag. A bellypan /diffuser combo would reduce drag too, but it does it by smoothing the path of the air, and directing it properly. I believe a well designed bellypan/diffuser can reduce drag and increase downforce.

Watch the videos this guys has, starting here.  Much of it is MX5 specific:

https://youtu.be/iPWoyNUmu7I

basically, remove the rear bumper and get a smooth undertray are a great way to start. 

Moti at Blackbird Fabworks has done aero testing with various Miatas and mods. Might contact him to get some info. Here's a longtail he made to help increase top speeds. https://trackhq.com/forum/the-equipment/aerodynamics/6904-fun-and-games-at-blackbird-fabworx-moby-dick-miata

Start with one of these undertrays

http://lrbspeed.com/product-category/mazda/

that hardtop must be quite an air trap with the windows down. Is there anyway you can add vents to help it escape?

Remove the side mirrors, vent the bottom of the hardtop at the back edge/center. Otherwise you are going to be looking to smooth out any steps or edges on the underbody.

It’s not that big an air trap.  In testing Spec Miatas at Sebring, cars with the hardtop were around 7mph faster on the top end than cars without a top on.

The hard top being better than no top is not suprising, but it can still be better than the hard top with a little more work.

Jaynen said:

Start with one of these undertrays

http://lrbspeed.com/product-category/mazda/

Not the OP, but thanks for the link! I've been planning to build something like that for a while and haven't gotten to it, perhaps I have time for credit card engineering?