Diffuser

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Major System Category: Body (Undercarriage)
Task: Diffuser
Parts:
Vent Louvers
Strakes
Diffuser Grille
Diffuser
Prerequisite Tasks:
Additional Costs: $250 (estimate)
Time Requirement: 8+ hours
Date Started: February 20, 2015

Date Completed: February 28, 2015

The diffuser is the unwieldy carbon fiber piece that fits under the transaxle and extends slightly past the lip of the rear end. Factory Five supllies these little posts (ugly as sin) to connect the top of the Diffuser to the bottom of the rear frame (just below the license plate and the exhaust pipe openings). Each is entitled to their opinions, but these little post that hold the diffuser in place look cheap.

The Diffuser grille comes flat. The tabs need to be bent to provide attachment points for the grill to the body shell. These are no 90 degree angles. In order for the grille to fit properly, the angle is more acute (i.e. less than 90). With all the pieces already in the area (e.g. transaxle bracket, chassis mount for wing, backup camera, lights) there isn't a lot of room to get a rivet tool in there. I went with  short bolts and locking nuts.

Fortunately, Vraptor Speedworks makes this marvelous looking diffuser grille. I think this is a necessary purchase (from an aesthetics perspective) that really makes the back end look finished. The shape of the Gen I and Gen II cars is different, so this grille only works for the Gen II cars. I have decided to bolt the diffuser grille and diffuser to the car. The manual says to rivet everything in place. I prefer to have a way to access everything on the car without having to drill out 40 rivets.

I painted the diffuser grille gloss black, bent the tabs using a small vice as my metal bending tool. This is the grille clamped in place for initial fitment. I discovered that I had to "bow" it a little to get to go flush on the edges. The oither issue I have with rivets has to do with maintenance access. If you go the rivet route, then the only way you will ever get it off the car is to drill everything out.

Before I could pre fit the diffuser, I had to get the  half shafts and the transaxle installed. This way, I could remove the jack stands from  under the rear chassis and have free access under the car for the diffuser.The manual indicated that you should measure 1.5 inches from the rear edge of the diffuser. This is how far the lip of the diffuser extends beyond the rear fiberglass shell. This is probably going vary by .25 inches from car to car based on how well everything matches up for the fit between the diffuser and the edges of the body shell.

I used painter's tape to lay down a strip then I marked off 1.5 inches along the rear edge with a Sharpie. The blue tape is where I masked off the opening for the transaxle housing..

There are three places I ended up having to cut the diffuser for the initial fit. There is a section of the transaxle housing that extend below the line of the diffuser. This seems pretty common for most GTM builds that I have examined. Certainly, the 991 transaxle continues to provide its share of challenges. The other place I ran into issues was the bell housing. The lip interfers with the forward diffuser edge.

This is the diffuser and the grill clamped into place to see how close I was getting on having the transaxle opening correct. This seems like the measurement should be straight forward, but it isn't (at least for me). The problem is the diffuser piece is somewhat unwieldy (I came close to dropping it more than once) and the transaxle housing is a combination of curved shapes and straight lines. In addition, you need to worry about the side ot side fitment between the diffuser and the bottom of the body shell. You can see the cardboard roller I used to assist me in getting diffuser on and off the lift.

I measured about a gazillion times and made a 6 inch square cut for the transaxle housing. I used a Rockwell VersaCut for the initial opening. I used a die grinder to make the cuts on the leading edge to clear the bell housing. To handle smoothing out the edges, I used a router table and a plunge router to smooth out the edges for the cuts for the opening. I set up the fence on the router table to handle the straight cuts for the bell housing. The other cuts I made was to widen the areas around the control arms for the suspension.

Finally satisfied with the cuts for the transaxle, I did a preliminary lay out for the louver vents. This is before any drilling, painting or bending. There is a very definite right and left side to these pieces.

All of this took several iterations between cuts and remounting the diffuser on the car. The lift proved its worth once again. I had the car about 5.5 feet high. I had a roller behind the rear wheels. This allowed me to slide the diffuser into place and to check the fitment.

These are the cuts for the vents. You need to be very precise here, because you have to be  aware of the edge beyond the body shell and running our of room as the diffuser bends around the frame. There is maybe a .25 inch fudge factor. I used the VersaCut, for the gross opening, a drill for the rounded corners and the router table to smooth and straighten everything out.

Once satisfied with the cuts for these openings, I started on the louver vents and strakes. The louvers are shipped flat. I used these as a template to determine the minimum cut opening for the louvers. I used the same method for the cuts and drilled out the corners to better provide for rounding. Next, I used the drill press to make the holes for black rivets. I purchased these a long time ago, because I think it looks really cheap to have this beautiful diffuser peppered with shiny aluminum rivets.

These are the strake and vents. Vraptor did a good design job here, because the strakes rivet to the outside and the louvers rivet to the inside. I was able to use a single rivet to fasten the louver vents, diffuser and strakes together. These are the black head rivets.

The strakes are designed to be attached with the same rivet. I used the louvers as a template for those rivet holes. The diffuser piece is easily drilled with a hand held tool. Before final assembly, I painted all parts a gloss black and opened the louver vents, by bending them open. This is really pretty easy to do, you just need to take your time.

The end result is the diffuser with plenty of venting. This is clamped in place. I will not worry about final attachment until the car is painted. I plan to go with  1/4 - 20 Helicoils for the frame and bolts and lock nuts for the body shell to diffuer connection. Some of those fasteners will be awkward, but I have a way into the underside of the car should the need arise.