Originally Posted by ZPirate

Alright I have finally had a chance to do a write up on the diffuser install. I purcahsed and installed the FRP diffuser rather than the carbon fiber diffuser, but both diffusers should fit the same. First let me say that my caveat to the install is that I didn't follow the provided install instructions. The reason being that I did not want to drill holes through the rear fascia that would be visible in the event I decided to remove the diffuser at a later date so what I post here may not be so helpful to those of you who do follow the instructions. BTW I am sure that EVO-R would prefer that you follow the install instructions provided with the diffuser. That being said here is what I did.

As I said in a prior post the fit and finish of my FRP diffuser are very good right down to the finish on the back side. The finish on the back side of the diffuser is just like the front so there are no rough surfaces to worry about and no prep work to be done to the diffuser before installing it.

First I used some spring clamps to hold the diffuser in place while I explored options for mounting it without drilling holes through the outside of the bumper cover. What I quickly learned was that tape alone wasn't going to be enough to hold it in place. Mainly this is due to needing to pull the outside edges of the diffuser back and up to meet the bumper where it curves in and the size and weight of the diffuser.

In the end I determined that I could use some one and a half inch black trim screws at the bottom of the diffuser near the outside edges to not only pull the diffuser in place, but to help hold it on the car. Those screws go through the bottom of the diffuser and the bottom edge of the bumper cover, but since they are under the diffuser you can't see them unless you look under the diffuser. When you put pressure on the diffuser from the bottom near the outside of the diffuser the top is held in place by the groove in the bumper cover.

I also used the supplied 3M tape along the top of the diffuser just below the edge of the top groove from one side to the other. I supplemented the screws and the tape with a generous amout of clear silicone caulk on the back of the diffuser and along the bottom edge. I held it all in place with six spring clamps for 48 hours until the silicone dried.

The combination of the screws, tape and silicone hold the diffuser on solidly. You could also ad a couple more screws to the bottom toward the middle of the diffuser, but I did not and don't plan to at this time. I did end up using three trim screws near the bottom outside edge of the diffuser on both sides. I have them spaced out in a mini triangle.

The only issue I had was along the very outside edges where I ended up with a small gap on both sides where the diffuser is not quite up to the groove in the bumper. You really need an extra pair of hands to hold the ends of the diffuser in place while you drill the pilot holes for the screws. Clamps alone aren't enough to hold it in place while you drill.

In the end I didn't mind three screws on each side under the bottom of the bumper to hold the diffuser in place. Also the tape and the silicone can be removed without damaging the bumper if you ever decide to remove the diffuser. I'm certain that following the directions and drilling holes through the bumper cover as provided in the instructions would make the diffuser even more secure, but I don't think mine is going anywhere with the combination of the screws, tape and silicone.

I hope this helps everyone who is interested in a diffuser and doesn't make installation more confusing. I'll try to get some pictures and post them later this weekend.