Why? Because ive done virtually every mod I wanted to up to doing expensive engine work, and £500 spent on aerodynamics will be far more influential than £500 spent on engine power. In april I got it up on axle stands and started designing how my car would look, sculpting the new air flow paths.
The MR2 as standard has some key flaws when it comes to downforce.
firstly the turbulent air that passes through the radiator is sent under the car, which generates lift underneath. Secondly the rear window step causes the flow to detach, creates a low pressure zone above the car and generates more lift there. It also doesnt create cooling for the engine so its a bit pointless. The spoiler does nothing for downforce, and the engine being exposed ruins the rear under car airflow. Basically its awful for downforce.So what have I done to change it? I purchased
£500 of 1.5mm sheet aluminium to use for the mods. First off i cut a hole in the frunk behind the rad fans.
The i cut a hole in the bonnet and crafted a tunnel to connect the two holes.
This was quite challenging because the bonnet swings past 90 degrees with the frunk wall, which created closing issues. I also put a plate beneath the frunk wall to block the previous flow path. My solution isnt pretty, but it works. 
Then i focused on the floor.
A good aero car has a flat floor to reduce drag and give the low pressure a uniform surface to act upon. So...flat floor required. Front downforce is generated by a front lip, giving the high pressure stagnant air on the front bumper a surface to act and also by accerating the air under the car lowering the pressure. It only makes sense to have the front lip as an integral part of the floor to make for a smooth transition underneath the car.
The floor panels are bolted into the existing undertray holes, but there are not nearly enough of these holes.
Some holes were drilled through the floor pan and tapped, with a nut placed on the cabin floor. Where this was not possible the panel edges are bolted together using overlapping tabs. 
You can just make out the under engine bay panel in this pic
(and the one afterwards). A cut out was made at the front of the panel with the front edge rounded upwards, much like the standard undertray. This allowes air into the engine bay. Other cut outs were made below the gearbox because it protrudes through the floor plane, one under the sump for oil drainage and three under the rear subframe, 2 for axle stands and 1 for a jack face. Whilst this is not ideal its necessary. 
So nice low pressure uniform airflow travels beneath the car and reaches the rear, where a diffuser is required.
Diffuser angles should not exceed 7 degrees or should feature a smooth gradual curve to allow the airflow to stay attached. Having separating fences helps restrict turbulent motions and improves diffuser efficiency.
Focusing above the car now, The above engine bay area needs sorting.
This is primarily because i intended to have a rear wing and the area in front is it is vital. Instead of having the recirculating air that detaches off the roof, a better way is to have a saloon style back. I made an attempt at copying the GT style engine cover. Its hinged on the roof edge for engine bay access and bolts to the actual engine bay cover. The engine bay cover was replaced with one fitted with fans. These were wired in with a switch to the cabin so that i can use them when i want and have them running even when the engine isnt, for post track session cool down. It also provides more cooling than standard. Quite awesomely (and by design) the engine lid directs the cooling airflow down along its exterior surface, which will energise the over roof airflow and keep it attached. It also provides uniform flow to the wing, which is vital. 
And lastly the rear wing.
Most carbon fibre wings cost £500-£1000 and frankly have no aerodynamic testing. It wasnt feasable for me to do that, and i couldnt be bothered to CFD model my own one. I did have the fortune of being next to a radical SR3, which is an aero car and has clearly been tested in a tunnel. So i basically copied the chord dimensions and wing design from that. an SR3 rear wing costs £1500, mine cost £80. All made of sheet aluminium, purchased steel brackets and some steel cable to reinforce the drag component force. The sheet aluminium was bent using my knees, wood, a black and decker work mate and a ratched strap and was a pain in the ar$e. I think the finished product looks awesome though. Its mounted above roof level for best airflow.
I was going to add side skirts but i cant be bothered and they are highly impractical.
But because not doing this canards are vital . Canards generate vortices along the side of the car which restrict freestream air being sucked into the low pressure region under the car.
So there you have it, my finished race car.
Now heres hoping it passes an MOT. 
All comments and questions welcome
