Great thread. Lots of good info. It does raise more questions at least for me.
I've just bought another 86 for track use only. One thing I want to look into more is the rear diffuser. I find a lot of directly conflicting information on their design and am hoping to get some clarification on which information is accurate.
The diffuser design I was thinking of doing is similar to the photoshop @
sw20kosh did a few pages back. Because my subframe and diff will be raised, I can continue the floor paneling under the subframe, and the the diffuser chamber will begin.
1: Diffuser chamber design. Some say to make this as large as possible, curved up. ON this platform, once you're past the subframe, the chamber would start with a near vertical section, that then curved in an arc to become near horizontal at the exit point. Yet, other literature states that diffusers have an ideal angle or around 7-8 degrees, with their effect reducing as you go steeper. Quite wildly different information. Would you take a compromise of both approaches, and start the diffsuer from the end point (at the bumper) angle it down at 8 degrees towards the subframe, and where it approaches the subframe, have either a vertical section (maybe 6 or 8 inches or so) or a very tight arc.
2: Exhaust flow. Is it more benefical to incorporate the exhaust to exit the rear of the vehicle above the diffuser, again as per sw20kosh's photo. In other words, does the exhaust gas promote diffuser function? The alternative being to cut the exhaust through the tunnel so it enters the car at the passenger side (I'm RHD) foot well, and then exit the car from the side, from inside the cabin. (Yes this is a full blown race car) This would then allow the diffuser to be larger. However I am not sure if OEM supercar manufacturers exit the exhaust to the rear because it is benefical, or because they have to. My race class only requires that the exhaust enters behind the rear most cabin opening, ie past the b pillar.
from what i understsnd exhaust has to leave near the rear of the vehicle to be street legal, as i know most if not all supercars do that, several like the cobra and the viper have it exit near the front of the rear wheel. From the side as in passanger sideright behind the front wheel may disrupt airflow around that side of the car. I say point it down straight from the header, or angled down next to the passenger side.
3: Leading to the diffuser. I understand that the smoother this path, the better. However, cannot find a straight answer on whether you would rather aim to lower or raise the volume of air that passes under the vehicle. One side of me thinks that the more air that flows under, the greater the eventual effect of the diffuser. The other part of me says the less air that passes under the car, the more that the entire car acts as a massive spoiler.
I know that you want the whole car to use itself as downforce, years ago a racing company named chapparal was making race cars that were being banned, why? They had a giant fan under the car to suck the car to the ground, called the 2J. front aero fins, wings, diffusers, vented hoods, all contour the air around the sides or the top of the car, never have I heard push air underneath.
3a: Related to question three, is it then beneficial to undo the factory engine bay "exhaust" path (where hot air escapes) put new non vented panels in, and instead redirect air upwards and out a vented bonnet. This would have a two tier effect of reducing underbody airflow (if thats what we're trying to achieve) and expelling air from the bonnet which I understand further increases downforce.
Yes, sucking air from the front and expelling it from the top would produce downforce which is good.
3b: Again related to question 3. A front air dam is going to reduce underbody air flow, good or bad thing for the diffuser / overall ground effect.
good thing, also the air that would have traveled under the car is now being forced over it, ro into the radiator and out through the vented hood
4: Seeing that it appears universal that the lower the overall car is, the more compounded these effects are, why then are more people/racers not using a 15" wheel and a 580 slick, rather than the much more common 17 or 18" and a 610 or 640mm slick? Before you say brakes, the AP racing sprint kit for one, fits in a couple of 15" wheels I have.
Yes lowering the car does help reduce air under the car, but at some point it obviously hinders handling otherwise slammed JDM my car is kewl kids would be winning races right and left
5: If now had car very low, low airdam, diffuser, the car will then try to seek to suck in air from the sides. So now have a functional reason for side skirts? I think I'm starting to answer my own questions as to whether we're trying to encourage or discourage underbody airflow.
yes, yes you are 
Sorry for the barrage of questions, but all point really do seem related, which is mostly around my understanding of a few critical points of I'm ultimately trying to achieve with the underbody airflow.