[jazig.k]

a rear diffuser is a big task to get woking efficiently. and it wont be easy without a wind tunnel or some very very good aero infomation of the car your making it for [same height, body and exhaust placement].
im presuming you understand how downforce is generated on an aeroplane wing. a car can be looked at like an upside down plane wing. you need a lower air pressure under the car than over the top, pushing the car down to the road.

understanding a diffuser is the first step. the idea of it is to help the air flow more smoothly[minimal turbulance] to join the rear air flow [coming from the sides and top of the car]. smoother air flowing from the rear means the air flows under the car faster[shorter in the path of travel instead of bouncing off the underside parts like the diff, wheels and brakes].

but before the air reaches the diffuser it should be minimised [air getting under the car] at the front of the car and the side skirts [skirts arnt just rice, they do have a purpose you know].
now you cant just block all of it. you wouldnt be able to cool the engine or the transaxel, but the down force would be high. so you want what ever air that does enter, to enter smoothly, and neatly, but only enough to suffeciently cool the car. not living in a perfect world there will be excess so make it work with you.
ideally air only enters from the front and exits at the rear in a straight line, and side skirts stop spillage [air 'spilling' from the side of the car into our low pressure zone under the car].

so far what iv been TRYING to explain is that the whole car works together, and not any diffuser will work with any car. so changing the front bar [EVO, EVO with your little add-ons and stock] will change the flow under the car, effecting the way the diffuser works.
i can help you with more details if you want do it.
i highly recomend a book called 'competion car downforce'. accually relates to the 155 touring car, and how flaps in the vents control how much air enters and cools the car [close them on cold/rainy conditions for higher downforce].
and another 1960's 12pot alfa powered racer, very tricky team when it came to bending the rules for extra downforce [used a underside fan to suck up air to cool the engine AND reduce the air pressure under the car] because the class didnt allow mechanical devices that generate donforce [this one served another purpose so was passed, but only for one race].

anyway, hope you work out something.