Ok here is the rear diffuser I am working on. It is going on my 1984 Oldsmobile Custom Cruiser wagon. I am building the wagon up in a streetFighter/Pro-touring way. It has already seen the track at New Jersey MotorSports Park. And should see more track action in the future. One of the main reasons for the diffuser is that there is a large area between the tank and rear bumper for air to get caught causing drag under the car. And the rear area from the bottom of the bumper to the roof is large so this should lessen that helping things as well. The way I started was to cut sections out of the rear bumper where I wanted the diffuser to go. Then I made cardboard templates. I made the verticle fences larger than needed because I wasn't sure how tall or how much I wanted them to stick out behind the bumper if at all. I figured I would just trim them after. I then covered the individual cardboard templates with fiberglass. I figured this was It would be like a honeycomb type material. The resin soaks into the cardboard. It works nice as long as you coat both sides and seal it. Once I had all the individual pieces done I started to mate them together. Then once I had both tunnels complete I joined them with the center section. After that I laid Fiberglass on the top side for strength at the angles but also again to seal it up. It is held to the car using Dzus fasteners so it can be taken on and off whenever I need to. It is still rough and yea kinda crude but it works and is very strong. I forgot to take it off when loading up the hauler at the track and drug it into the ground and it was fine. But I again forgot to take it off unlaoding it when I got home and the pavement wasn't so forgiving and I ground some of the fences down on the bottom. But that shows how strong it is to take the weight of the wagon and not break. I have to wait till I solve my exhaust issues to finish the outside portions. The exhaust pipes should be directly under the frame rail. But the drivers side isn's as then the pipe would be to close to the fuel take. So I either make it side exit of cut it and put a bend in it to get it to the right spot. Here are the pics. Let me know what you think or if you have any questions. The first pic shows where I made the cuts on one side to show how much area is getting vented.

Here is all of the openings cut.

Here are the side fences

And the one side fence on the top portion

And the tunnel done

One the car.

Two on the car

Figured I would split the post because of the number of pics posted. Here are some pics the tunnels trimmed down.

And both tunnels mounted to the center section.



And a good pic of it on the hauler showing it in a bit better light and vantage point

Looks like a good start Definetly watching this thread as I want to do something similar for my own car later