Rear diffuser mod

[HEDHUNTR]

Ive got an MK1 and one thing that is bugging me about it is the fact the gas tank sits really low and looks bad IMO from sitting behind the car. Does anyone know if anyone has fabricated a splitter/diffuser for the back to go over the tank? Or is it just easier to put a fuel cell in the truck and delete the tank?

[mikeinatlanta]

I did a full diffuser on my MKII, which is almost the same back there as the MKI. If you want the diffuser to flush with the body you will have to raise the tank a few inches and lower the body a few inches with the dropped butt mod.
The fuel cell is not really going to help unless it is smaller that the stock tank. Most of the better looks of the newer cars is the lower body in the rear. I would suggest a drop butt mod as a first course of action as it will also improve the overall stance of the car.

Making a good looking diffuser with an integrated look for the MKII was a challenging sheet metal task. I wouldn't go there unless you are the king of guy with a slip roll, shear, and an English wheel in your shop.

[skullandbones]

Ive got an MK1 and one thing that is bugging me about it is the fact the gas tank sits really low and looks bad IMO from sitting behind the car. Does anyone know if anyone has fabricated a splitter/diffuser for the back to go over the tank? Or is it just easier to put a fuel cell in the truck and delete the tank?

HEDHUNTR,

I thought I was the only one who thought of that. I should have known better! I have gone so far as to purchase some aluminum for this project so I have been thinking about it for some time. I figured if I didn't like it after it was in place it would be simple enough to delete. I have one advantage though. I have fabricated a Watt's link for my live axle so there is a cross member that runs in front of the gas tank and is about 3/4 of an inch lower than the bottom of the tank. So I have a beginning point for the leading edge of the diffusor. I agree with the poster that it can be challenging but I think I can fab it without a lot of tools. Unfortunately, it is not at the top of my to do list. I'm installing a new computer system so that comes first. I will drop you a PM when I have done a mockup. I really think it will work. I plan on using a splitter similar to the Breeze one, I think that's who has it. I wanted my splitter to reflect the down force with the reverse wing shape. Don't know how practical or effective it will be with the air flow over the radiator. Don't know if anyone has done any work in that area. I don't see why the diffusor wouldn't work though. That too must be configured correctly or you can cause more bad air flow than directing it the way you want to. I will have to go back to get those measurements after doing a loose mock up just to see how the pieces will fit together. Great minds!

BTW: you could probably install a "cross member" the same dimensions as mine but much lighter to serve as your leading edge support for the diffusor. I used the quad shock brackets to fab a set of flanges to support the cross member. Sure was nice for FFR to provide that for me.

Good luck,

WEK.

[CraigS]

Do the dropped butt mod and that pretty much hides the tank.

[HEDHUNTR]

Where can I get the instructions for the drop mode? Thanks

[mikeinatlanta]

Here is my diffuser. The hole in the middle is a jack point.

51168

The diffuser mounting structure is mounted to the IRS structure.

51169

The biggest challenge was mating it up to and transitioning onto the body surface.

51170

The big picture.

51171

[mikeinatlanta]

I plan on using a splitter similar to the Breeze one, I think that's who has it. I wanted my splitter to reflect the down force with the reverse wing shape. Don't know how practical or effective it will be with the air flow over the radiator. Don't know if anyone has done any work in that area. .

Here you go. It's an extruded aluminum airfoil.
51172

[skullandbones]

The dropped butt mod is a lot of work. I thought about doing it but after looking at the loss of space in my trunk which I've gotten used to, I changed my mind. Also, it isn't necessary to perform that mod to add a diffusor. All diffusors are not created equally but from the research I've done you can get nominal results from models that may not fit the ideal designs you see on full out race cars. In other words, it's not an all or none situation.

Mike: you are way ahead of me. Speaking of a lot of work; you've been busy! Also, I like the splitter. Wish it was a little thicker and wider. It's definitely the right shape and orientation. Need source info.

My understanding is that you really need to pair the splitter and diffusor. The 818 wind tunnel results have supported that concept as well.

Thanks of the pics,

WEK.

[mikeinatlanta]

The dropped butt mod is a lot of work. I thought about doing it but after looking at the loss of space in my trunk which I've gotten used to, I changed my mind. Also, it isn't necessary to perform that mod to add a diffusor. All diffusors are not created equally but from the research I've done you can get nominal results from models that may not fit the ideal designs you see on full out race cars. In other words, it's not an all or none situation.

Mike: you are way ahead of me. Speaking of a lot of work; you've been busy! Also, I like the splitter. Wish it was a little thicker and wider. It's definitely the right shape and orientation. Need source info.

My understanding is that you really need to pair the splitter and diffusor. The 818 wind tunnel results have supported that concept as well.

Thanks of the pics,

WEK.

Thanks,
It's 4" chord. I have 6" on hand, but in order to keep the trailing edge away from the rad it would have had to be pushed too far fwd. There is less room in there than it looks like at first glance. I agree with what you are saying about balance. Adding rear downforce without taking care of the front could be an issue. Problem is that if you make a non functional diffuser, many knowledgeable car guys will assume it's just poor design.
51173


The dropped butt mod requires redoing the sheet metal on the sides of the trunk, possibly roll bar holes, fender well sheet metal, and maybe some massaging of the frame a bit. Basically you free up the body to sag under its own weight in the rear, and secure it using two of the four rear bumper mounts by securing the top two body holes to the lower two frame holes. The other two rear bumper mounts then become fake and secure to the body only.

[HEDHUNTR]

I really like it, very nice

[CraigS]

Search the other forum for dropped butt info. There is a thread in the last two weeks w/ most of the info.