Opening up rear diffuser

[SonicDeathmonkey]

I don't really understand enough about airflow to understand how their diffuser can both reduce drag and increase downforce. Seems like those two aerodynamics work opposite of eachother (like how a rear wing increases downforce at the expense of more drag)

In the typical application of a diffuser, as in on a racecar with a smooth bottom, it works by expanding the cross-sectional area of the airflow in a controlled manner as to keep the flow somewhat laminar to prevent a turbulent low pressure wake behind the car (which is exactly what we have). For the purpose of reducing a turbulent low pressure area behind the car I think that the aftermarket diffusers certainly wouldn't hurt but I'm not sure how much it would help. Diffusers are most effective when the air coming into them isn't already turbulent, which it is since the bottom of the car upstream of the diffuser is not flat/smooth. At a minimum it might help to at least clean up and direct what little airflow there is. Like I said, couldn't hurt. Another approach would be to try to keep the flow off the roof attached via vortex generators or some other method, but with such a sharp drop off there I doubt much can be done.

Purpose-built racecars with a highly optimized undertray basically function as upside down wings. The low pressure zone underneath the car does a LOT of work and if it's controlled well the drag penalty is minimal, especially when compared with wings. By moving the point of lowest pressure (center of pressure) forward or aft you can influence the high speed balance of the car. Anyway, I digress.. lol

 

[MRX330]

Only difference I see with the Hotchkis vs the Eibach is the Hotchkis uses an extra bracket for the lower shock bolt but other than that they pretty much look the same and shouldn't have any issues with the diffuser. When I installed mine I believe I had to install the Diffuser first then snake in the sway bar, There wasn't a lot of room and having the car up on a lift made things a lot easier. Next time I have the car up I will try and take some pictures for you.

This is why you are a Captain!!!

 

[D1JL]

I have decided that I too will revisit this project as I did on my '04 Mini.
I have found a new airfoil that I wish to use and I have come up with an interesting way to mount it.
I can almost guarantee downforce, it is the ability to create a vortex that will keep the hatch clean that I am not sure of.
After confirming some details with the manufacturer, I will place my order and it should take a week to receive.
I will be sure to document build and install information.

 

[koozy]

I know you have the Hotchkis. Any idea why the Versus would fit with an Eibach rear sway bar?

Nope, I don't have first hand experience with the diffuser so I'm not qualified to comment. You can read what he stated here.

https://www.fiestastforum.com/threads/hotchkis-swaybars.19633/page-2#post-341041

 

[TyphoonFiST]

Well I got as far as putting on my side skirts from Vega motorsports today....My Vizla pupperoo destroyed his pen gate being anxious so it took me away from putting the rear splitter on....but tomorrow is D-day unless I'm requested for OT at my job. But I will have write up for both!

Sent from my SM-N950U1 using Tapatalk

 

[MRX330]

Well I got as far as putting on my side skirts from Vega motorsports today....My Vizla pupperoo destroyed his pen gate being anxious so it took me away from putting the rear splitter on....but tomorrow is D-day unless I'm requested for OT at my job. But I will have write up for both!

Sent from my SM-N950U1 using Tapatalk

Do you have a rear sway bar Dude?

 

[TyphoonFiST]

Do you have a rear sway bar Dude?

No I have the rear inside trunk bar and Torsion strut bar. The kit does come with spacers to allow more or less distance between components. Another member stated he does have a rear bar and it works just fine as I remember....it was yesterday he chimed in.

Sent from my SM-N950U1 using Tapatalk

 

[MRX330]

No I have the rear inside trunk bar and Torsion strut bar. The kit does come with spacers to allow more or less distance between components. Another member stated he does have a rear bar and it works just fine as I remember....it was yesterday he chimed in.

Sent from my SM-N950U1 using Tapatalk

Eibach fits. So far no confirmation that my Hotchkis will fit man.

 

[MRX330]

I loved the way Lebowski said “man”. It was whiny and elitist at the same time lmao

 

[averyislost]

In the typical application of a diffuser, as in on a racecar with a smooth bottom, it works by expanding the cross-sectional area of the airflow in a controlled manner as to keep the flow somewhat laminar to prevent a turbulent low pressure wake behind the car (which is exactly what we have). For the purpose of reducing a turbulent low pressure area behind the car I think that the aftermarket diffusers certainly wouldn't hurt but I'm not sure how much it would help. Diffusers are most effective when the air coming into them isn't already turbulent, which it is since the bottom of the car upstream of the diffuser is not flat/smooth. At a minimum it might help to at least clean up and direct what little airflow there is. Like I said, couldn't hurt. Another approach would be to try to keep the flow off the roof attached via vortex generators or some other method, but with such a sharp drop off there I doubt much can be done.

Purpose-built racecars with a highly optimized undertray basically function as upside down wings. The low pressure zone underneath the car does a LOT of work and if it's controlled well the drag penalty is minimal, especially when compared with wings. By moving the point of lowest pressure (center of pressure) forward or aft you can influence the high speed balance of the car. Anyway, I digress.. lol

Ahh see that makes sense! I was wondering the same thing, without an underbody tray I wonder how much work the rear diffuser can do to "clean up" the turbulent air.
On a side note, I always wondered how vortex generators would look on our cars. Always thought they looked killer on the Wrx / Evo's even if they were mainly aesthetic pieces.

 

[D1JL]

I just ordered all of the items to build my custom diffuser.
I decided to use the Verus Diffuser as a base/template to save much time.
Major modifications however will need to be done so as to fit my car in addition that I don't like the look of the strakes.
I will document all of this in my build thread once I get started.

 

[MRX330]

You sound confident. Can’t wait to what you cooked up 👀

 

[D1JL]

You sound confident. Can’t wait to what you cooked up 👀

Yes, but as always, as I continue to think about how I am going to do things, I change my mind.
So then I need to order different parts.
The only problem then is that the final project will cost others half as it cost me.

 

[MRX330]

Yes, but as always, as I continue to think about how I am going to do things, I change my mind.
So then I need to order different parts.
The only problem then is that the final project will cost others half as it cost me.

The real payoff is in the fact that you came up with it yourself. Now could you speed it up a little?🤭

 

[D1JL]

The real payoff is in the fact that you came up with it yourself. Now could you speed it up a little?🤭

I once was able to do things very fast.
Now I am only able to do things half-fast. (say that quickly)

 

[MRX330]

I once was able to do things very fast.
Now I am only able to do things half-fast. (say that quickly)

Hahvest...that’s what they in New England in the fall

 

[D1JL]

As I get older I notice that even though I am half as quick, I am twice as good.

 

[D-resonant]

I bought one deeply discounted. Did he try to finagle it in there or did just realize they can’t exist together and throw in the towel?

Front vertical section of the Verus occupy same space as the leading horizontal section of the Hotchkis bar. A no-go combo. (If the Eibach fits with the Verus, the Eibach horizontal section must fit higher, and further away from the rear twist beam.)

My perceived effect of the diffuser suggests an reduction of drag. I first noticed the effect when air-drying the FiST on the same stretch of road (in Mexico). When lifting near the top of fifth gear, the FiST would normally immediately start slowing... No so with the Verus. I suppose one can do repeated coast-down tests, but I'm not quite as studious as @Siestarider on his front air-dam experiments.

Also noticed a bit more dust on the back. Not what you wanted @MRX330.

 

[D1JL]

Front vertical section of the Verus occupy same space as the leading horizontal section of the Hotchkis bar. A no-go combo. (If the Eibach fits with the Verus, the Eibach horizontal section must fit higher, and further away from the rear twist beam.)

From what I see, both bars are tight up against the twistbeam.
The Eibach is straight and runs along the bottom edge of the beam.
However, the Hotchkis has a bend in it and is much higher on the beam in the center.
As the disffuser goes toward the front of the car, it angles upward and is very close to the top of the beam.
There appears to be no way to trim the diffuser at this location without loosing the front mounting point.

I do believe there is a solution.
It would require tweeking the bends on the front edge of the diffuser just a little.
Then adding spacers between the front mounting bracket and the car.
This would allow the front edge of the diffuser to be moved about 1/2 inch to the rear and clear the Hotchkis bar.
This would also lower the front edge of the diffuser but by an insignifagnt amount.

This is only a theory based on photos but it is easy to do and worth a try.

 

[MRX330]

As I get older I notice that even though I am half as quick, I am twice as good.

When I said “half-fast” out loud fast it ended up “hahvest”