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Installing a Difflow diffuser on a car w 2002+ Seibon rear valence

29 May 2001
San Francisco Bay Area
Hello fellow members,
I haven't really seen any threads about how to install the Difflow diffuser onto a car with the Seibon 2002+ rear valence.
The problem with this is that it doesn't fit like the typical OEM 2002+ rear valence. For one thing, the Seibon is too stubby in the bottom part where the diffuser would have mounted, so it will not reach. The second issue is that the Seibon bottom part is too flimsy. It flexes so much, and is held by two bolts to the rest of the rear valence that I feel the diffuser would be ineffective in providing the whole car the intentioned rear downforce. I think if it was to be mounted to that bottom piece, it would continuously flop up and down as the car is driving at high speed, and eventually rip off the Seibon bottom part.
I have no issue with the Seibon being light and flimsy, as it reduces weight (more so than the OEM rear valence), and it was not designed to support the weight of anything else.
I knew I had to figure out an alternative solution to mounting it.

I came up with a bar that the Difflow would mount onto. The front part of the diffuser still mounts in the same place, but I designed a bar that would attach to the chassis so that when the diffuser was going at fast enough speeds to create downforce, it would do so straight to the chassis of the car.

I went to Lowes, and found the section where they sell various bars. I looked specifically in the aluminum section for light weight, and found a 72" L-shaped bar. Cost= under $15.
I measured it to the diffuser, and cut it to the same width of it (approximately 39") using a dremel and cutting discs.




I then looked for a place to mount it on the body of the car. Looking around the area of the exhaust, I found two bolts on the chassis that I could attach the arms. I measured it down to approximately where the diffuser would go. It was around 8". I then cut two 8" pieces of the L-shaped bar. I drilled two holes where I could thread screws with bolts to the long L-shaped bar (I think around 36" apart), as well as holes on each end of the two 8" bars.



I unthreaded the two nuts from the chassis, and attached the 8" bars vertically through each of the top holes.

Then I attached the long bar to the two short bars using bolts and nuts.

I loosened the allen screws on the Difflow for the rear mounts to the very end of the threads without letting the screws fall out. I attached the front of the diffuser to the appropriate place (according to the Difflow instructions) and attached the locking bar immediately afterwards (counter to the Difflow instructions which state to do this last). This is because my bar was still angled a bit too far forward towards the front of the car. The aluminum bars have just enough "give" that they can be bent backwards once the front part of the diffuser is attached.

I then kept the diffuser propped up with my shoes while I used a long standard screwdriver to open/lift up the rear "flaps" of the Difflow, while at the same time using my other arm to pull back the L-shaped bar so that I can slip it under the flap. Once I was able to do this, I went to the other side and did the same. I tightened up the allen screws from the middle of the car and worked myself outwards, so that the diffuser would not bow.

I am very satisfied with the install. My diffuser sits just a few millimeters below my Seibon bottom part without ever touching it. No matter how much I try to move around the Difflow, it is pretty much immoveable. I know this will be more effective than mounting it to a rear valence.



I reversed the bolts, and then cut them short.

I hope this helps others who have the Seibon rear valence, or OEM owners who don't want the Difflow to make contact with their diffusers, or those who want a much better mount to the structure of the NSX.






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I wonder if this route could work for us guys with a pre '02 bumper. Cool mod though.
Cool write up. I wonder if this will also prevent the difflow from bowing out in the middle? That, along with scraping is why I went nsx-r style diffuser. But it doesn't have that aggressive difflow look.
Thanks AnywhereInTX.

mwagner10702, I think he's asking if using the L-shaped bars can work on the pre-2002 Difflow diffuser, so that it doesn't have to be mounted to the pre-2002 rear valence.
I think it's possible, but can't guarantee it, as I can't remember what it looks like back there. It would be a tight fit. You would have to ensure that the bars just about come down to where the bottom part of the rear valence is located. It should sit like a millimeter lower, so that when you attach the diffuser to the L-shaped bar, it will have that millimeter of spacing to ensure it doesn't touch your valence.

Hi Jinks,
I think it's a good possibility, as the rear valence is made of a flexable material and won't hold as straight a structure as an aluminum bar would. Since it is L-shaped, it is much less prone to sagging like a straight metal bar without the L support would provide.
Exactly. The pre-2002 model is a near impossible fit unless you have a certain exhaust setup. Stock cats will rub on the diffuser when holding a straight edge up to the front mounting point to the rear mounting point. This mod looks like it will allow you to mount the rear of the diffuser lower to clear the exhaust.
I mounted mine directly to the valance and OEM valance supports, I strengthened the cross member of the valance by using some L shaped aluminum bar across the hollow opening. In addition to that, I filled it with fire retardant expanding foam to keep crud from building up. The end result is extremely rigid and more than adequate to mount the center portion of the diffuser. For some outside support I retained the "fang" supports that were used with the original valance (early and late model) and modified everything to mount to those as well.

So I have evenly spaced hardware across the rear portion of the diffuser: fastener/washer/nut on each support "fang", and two fasteners attached to the center portion of the valance cross member. Hard to describe without pictures... Oh btw I knew I would have to rig up something custom so I requested Difflow not to drill any holes in mine, which is how I was able to mount it as previously stated.
Nice write up! Glad you made it work with your valance! :)

As for the NSX owners with 91-01 style rear valances, we do have a diffuser, but it does require a tiny exhaust since the valance sits higher up and doesn't provide a lot of clearance. I recommend anyone that wants to have a Difflow diffsuer just upgrades to the 02+ style valance and throw the diffuser on :)
Update: Need to secure Difflow clips

Hey all,
I wanted to update this, by letting you know that my Difflow using the Seibon had the issue of flexing towards the middle of it (perhaps by the aerodynamic suction or just weight over time) such that the clips at the rear end that attach to my bar slipped off. Without me knowing it, I was driving on the freeway in a trail of sparks and flames until someone caught up to me and pointed it out that I needed to pull over. By the time I did that, half of my fins were shaved away. Luckily, where I pulled over, an ATT technician was there, and he helped me secure it by giving me some very long zip ties that I could attach to the clips and my bar. Had I known the Difflow was going to flex like that, I would have done this from the beginning and would have an intact diffuser. At least now I don't have a problem clearing driveways and speedbumps like I used to before.

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