Functional Rear Diffuser
 

I have been researching around for a functional rear diffuser, Nothing for our cars. I have decided to start designing one myself.

Question 1: Since i do not have a wind tunnel in my back yard i was wondering if getting a scale model and making a mini tunnel and doing tests with that to see if it will work well is a worth while idea?

2: I know for it to work the way it is designed a spoiler is also needed. the APR starts to add downforce around 40 MPH, so i was thinking about getting that and using a diffuser to futher create a suction force under neath. has anyone used this particular spoiler first hand and noticed better handling?

3: Does anyone have a home made FUNCTIONAL rear diffuser and would help me make one of my own?

4: Should i even bother?

The reason i want one is because i plan on racing next year. I want the most out of my N/A car and use aerodynamics to my advantage. Also since i work with FRP/ CF and have lots of spare time i can possibly make a few more for those that would want one.

Thanks

Unless you have significant track experience I vote why bother?

Cause it looks bad azz :)

Seriously, for the mortal human, a rear diffuser will not make a difference in performance, just looks.

:facepalm:

If you're serious about racing and adding real downforce, I would start with getting a real wing and mounting it to the frame (not the trunk lid) of the car.

Of course once you do that, to balance the car you are going to need a functioning front splitter; again mounted to the frame so that it doesn't pull off your bodywork once it starts creating real downforce.

Don't focus on how much downforce you are generating (at least at first anyway), worry about overall aero balance and how it affects the handling of the car. In other words, don't put a huge wing and rear diffuser on the car and then neglect the front of the car. The car will handle like *** if you do.

ecomodder.com has a lot of excellent aerodynamic readings. Their main goal is to reduce drag. For the most part, you do not want a sharp angle of attack, otherwise you are going to get highly turbulent flow. The airflow is most likely detached and turbulent at speed by the time it gets to the rear of the underside of the Z anyway since it has no flat paneling on the bottom. You might have luck getting partially reattached flow with a mild angle of attack, starting parallel and increasing to ~10 degrees. Essentially anything that decreases turbulence underneath the car is going to generate a downward force that will counteract any lifting.

I was thinking about starting With doing a flat bottom. Either aluminum or FRP. From splitter and then the diffuser FRP with a hinge to adjust angle for testing. I need to find out our coefficient of drag because I found a equation to use for the angle of the diffuser. But also switching to the evo-r mirrors will help too. Less drag on those. I see this being a long process.

Flat bottom is indeed the next step after big wing and front splitter.

CfD for sport package is 0.29

Since i have truely stared researching this. I have come down to this.
Raise my car up this week and make templates for a falt bottom and possibly for a diffuser. I dont know weather to get the stillen front splitter or a generic one out of CF with adjustabe links. But home depot sells aluminum sheeting for roofing that are almost the perfect size for this so maybe i will start from there and then onto FRP or CF depending how deep i want to dig in my wallet.

Thanks TAKJAk for the Cfd. that helps.

:ugh2:

Might want to look into Dibond. A lot of my fast local AutoX guys are using dibond for splitters with a lot of success.

Damn good find. i just looked at that stuff and it is just what i was looking for this project. Thanks:tiphat:

I Have Fabricated One
 

Here is the diffuser I fabricated out of aluminum and fiberglass. It is around 9 lbs.

http://www.the370z.com/members/resip...diffuser-1.jpg

http://www.the370z.com/members/resip...-explained.png

what angle did you use? i was thinking 6-8 degrees based on our CFD. Also did you make a flat bottom and front splitter?

ResIpsa do you have more pics or specs?

I have started making a custom one myself. The S2K guys have a lot of neat stuff where I have been getting my ideas. Another week my final drawing will be complete that will work for my car and Amuse rear.

Nice. once it stops raining i am going to jack up the car and make a template. Look into mounting points and where/ if needed some heat shielding near the muffler and midpipe.

The angle of my diffuser follows the natural slope of the rear bumper. According to my cheap a$$ Harbor Freight angle gauge it is somewhere between 8 and 10 degree slope. My research suggests that 9 degrees is a theoretical limit for a diffuser. However, the addition of strakes should prevent my diffuser from stalling.

Yes. I run a fairly aggressive front splitter. I have not fabricated a flat bottom yet.

I have been working on this diffuser for nearly 2 years. Here is what the initial design looked like. My initial testing (as described below) showed that it was very effective but I was always concerned about the huge vortexes I was creating at the diffuser turn downs behind the wheels.


http://www.the370z.com/members/resip...8-diffuser.jpg

After some more research, an article in GrassRoots Motorsports, and an article in Race Car Engineering I modified the diffuser to its current form. I realized that a diffuser should be as wide as possible between the inside of the tires. As you can see, my original design had the turbulence from the rear tires impacting my airflow. That is when I created the second set of strakes to act as the outside walls of the diffuser.

My preliminary testing shows that my design is fairly effective. I also was reaching speeds on the front stretch of Summit Point (determined by how fast you take turn 9) that I had never reached before. However, this could be purely coincidental or related to other factors.

One trick to test the functionality of a rear diffuser is to wait for a day with heavy rain and have a friend drive your car and you follow in another vehicle. As you follow, take a look at other cars on the road and notice how rain is being dispersed behind them (better yet, have another 370Z without a diffuser drive along side of the 370Z with a diffuser). You will see most cars on the road have sprays emitting from the front and rear wheel wells (not very efficient). Also take a look at how rain flows from the rear of the cars. If you get lucky and spy an Evo or Lotus with a functional diffuser the rooster tail of spray will be noticeable.

When I tested my first design of my diffuser (with front splitter) I was quite pleased. I drove behind my 370Z and noticed that there was absolutely no water spraying from the front or rear wheel wells. Also, it was kicking up such a rooster tail that driving behind was almost blinding. My only concern was the large water vortexes emanating from the points where my diffuser turned down. My research suggested that vortexes may not be beneficial.

Research and experience has shown me that the majority of the force acting on the diffuser is at the diffuser throat.

The bolts that hold the little piece of bodywork (splash guard?) behind the tires are perfectly located to anchor the diffuser. In fact, the first thing I did was take a long piece of aluminum carpet molding, drill two holes in it, and mount it under the car. This formed the back bone.

The front of my diffuser is anchored with aprox 8 trim screws directly into the edge of the bumper.

First don't be talking about Harbor Freight. Love that place. :icon17:

i was wondering when you would chime in, SPOHN! :tup:

I apologize. You are correct.

Where else can you get a 4 x 8 moving blanket and a 6 piece screwdriver set for $3.98?
:bowdown:
http://www.the370z.com/members/resip...re68654-hf.jpg

Damn that's dollar store status right there. What ever works right. Too bad there isn't one near me.

They have some cheap things I wouldn't touch like drill bits. But most of the things are damn good.