The Vishnu part was proven to be very effective at eliminating the rear bumper's parachute effect. I think most rear undertrays will accomplish at least that much.

Here's a couple of pics of my modded Vishnu rear diffuser. As I stated in Hamflex' thread, I think this merely provides a foundation for a good design. IMO the fin placement could be much better, although I understand why it was built the way it was, as it had to accomodate a large rear muffler. In my case I have a bullet-style resonator instead of a rear muffler so that could accomodate a full flat undertray.





Also, before anyone points it out ... the fins are facing the wrong way right now. Easier that way for street use. To maximize downforce the taller portion should be facing the rear bumper. One of the things that I really like is that the fins are depth adjustable. Invaluable, IMO in fine tuning the car's balance.

l8r)

 

What do the leading edges of the Vishnu diffuser look like? Are they just normal, or do they bend up to try and block some air?

 

Are you planning on having the fins like the APR one? If so how many and why did you choose that number or spacing?

 

I'm going to be building vertical fins yes. I think the more you can have the better, up to a point. But that is too much work. I plan on somewhere around 5, evenly spaced.

These guys use lots of them....

 

Have you looked at what ride height and what height range range will be needed to produce the desired force?

I've always been skeptical of folks trying underbody parts without pulling out the calculator first. For a track car, the evo has a lot of suspension travel. This makes it great for bumpy surfaces or rallying, but not so good for ground effects. Even stiffly sprung evos roll a lot more than that C6.R. I imagine low pressure havoc ensues when one side of the diffuser is 20 mm from ground and the other is 200mm.

d

 

In my discussion a few years ago with an aero guy about the C6R's diffuser, it came up that all the strakes are there because they don't induce much drag or anti-yaw... so even if you can't spend the time to find the optimal placement in the wind tunnel, you can put quite a few strakes in the diffuser and still have no ill effects. This is especially true for a car that has large pitch changes.

If you don't tie in the diffuser to the undertray as Kevin said, I would make a short pocket in the bumper for air to escape over the diffuser. It will be pretty hard to seal the wheel wells.

 

damn mechanical engineers

good job man, keep up the good work

 

yup +1
jid, youve got some mad skills bro. loved the engine build thread w/the army paint scheme engine frickin sweet!

1 question, you always seem to have all these neat tools, over on this side of the pond they cost a fortune to get. Do they sell those cutters, torches, welding material in wall mart or tool shops?

 

Harbor freight brother! Haha, really cheap tools!

The diffuser looks good! I finished taking weight so I think I may start construction of my rear diffuser, you have motivated me! Haha

 

Here's a sketch of what you should be shooting for, HamFlex. There are several critical design elements to a proper diffuser:

1. Large outer fins to keep air from the wheel wells and side of the car out of the diffuser
2. smooth transition from underbody to the diffuser
3. The angle of the diffuser is actually dependent on the overall aerodynamics of the car. Your best bet is to start with between 7 and 10 degrees, but if you can make it adjustable, it will allow you to tweak it to match the type of rear wing you're running
4. Additional fins - more are better, but not strictly required. Again the effectiveness depends on the overall car setup



Some sketch notes. Optional fins in green. You may want to consider building a 3 element design as shown to accommodate the rear diff (it's the lowest point). Seriously, man ... stop by sometime.

l8r)

 

I was with a shop once that was trying to build a diffuser like the one above. If I remember correctly there was no easy way to block the air flow from the wheels with a place because the Evo's axles and brakes and suspension hangs so low and links so far in toward the differential. The actual useful area was a fraction of the rear inner tire width. If you look at OP's diffuser it doesn't start till pretty far after the rear wheels. I don't see an easy way to cover up the underbody much further forward on a car like the Evo without making body work that drops down below all mechanical components. The Vette is a Pratt and Miller tube frame, so they don't have as much of a problem and there's nothing back there but the big transaxle, the A/C system, and the fuel cells which actually sit relatively high in the chassis.

 

I have a design in the works, I have no sketches yet but got some Ideas when I was at the nationals this year for nasa. The diffuser on julian mann' lister gave me some ideas along with all that I have been reading.

For my desing to work it will have to sit low and I will incorperate a full flat bottom and splitter. All will be easily removeable because there will be no way to load it on a trailer with it on the car.

 

That's a really good point. I've spent a lot of time staring at the rear suspension links to see how wide of a diffuser/undertray I can squeeze into there. Getting out to about the middle of the lower rear control arm wouldn't be too hard. To go any wider ... I've got some ideas, but haven't tried to mock up anything yet. The other big problem is the exhaust. There is no easy way to route that 3" pipe anywhere else...

l8r)

 

sure there is.... right out the side skirt...

 

^^^^ bingo.

If I had more time and energy I would redo my exhaust to exit out the side. The Sierra Sierra guys did that and built a proper flat bottom and diffuser. Mine is not that well implemented, but I'm not trying to do anything other than have a little fun building parts, and have fun running the car at the track. If I lived in California I would hit up the local time attacks, but there is nothing going on in the Northwest, and I don't have enough free-time right now to travel.