Difuser design and function

[Bill Musarra]

Terry,
It is my understanding that where the panel is located is where the down force is, because that is where the venturi is located. The back of the car is after the venturi and separate in its function. That being drag or more down force. So its angle has to be considered and it is added or subtracted to the venturi. I think that is why the backs of the 40 are shaped like they are, an upside down wing. Just my opinion.
If you want to see some interesting images of the principle(s), go here:
https://www.google.com/search?q=ber...wfk-ZTUDg&sqi=2&ved=0CE8QsAQ&biw=1280&bih=668

Bill

 

[Howard Jones]

I have been thinking about my SLC and downfore so here is what I have come up with so far.

I believe that to have any hope of improving down force by using the bottom of the car it will need to maintain laminar air flow from front to back for at least a large majority of the width of the under body.

To do so it would be necessary to prevent turbulence from being created by obstructions along the path of airflow. This more than likely will necessitate a complete flat bottom at a minimum. That means a fully covered bottom of the engine room. My SLC has a dropped floor for the drivers seat so I have screwed that up already to some degree.

Now we can begin to try and create a low pressure area under the car by lowering the nose in relation to the tail. My guess would be at least a 1/2 inch less ride height at the front and maybe as much as 1 full inch. Other factors related to maximising mechanical grip with setup geometry may well limit what ever rake angle might be achievable.

I would think that as a baseline we would not want to sacrifice mechanical grip at all to attempt to enhance any limmited possible ground effect down force that might be possible. That's just a guess at this point but a full setup R & D to attain max grip would be a baseline goal in my opinion before moving on with areo development.

Then, any help preventing high pressure air being pumped from the wheel wells into the low pressure area we are trying to create under then car will be very helpful. Now that we have a good laminar flow along the bottom of the car from front to rear we can try and improve the efficient of our "ground effect tunnel" with a diffuser at the tail. But before the diffuser will do any good at all, the airflow to the entrance of the diffuser must be laminar or we are just wasting our efforts.

The rear wing will also act to extract air out from under the car at the rear and thus improving the low pressure area under the car.

So, one thing that I think will help us is front wheel well slats on the top of the fenders to expel pressure buildup and prevent spillover out of the bottom and to the area under the car.

Another is trying to manage airflow through the engine room so as to not allow air drawn into the engine room for cooling to exit out of the bottom of the car. I think it would be helpful to attempt to manage the exiting engine room cooling air so that it does not interfere with the low pressure area under the rear wing and at the same time try to keep it out of the exit of the diffuser. This will be difficult, but I think I would lean towards wing efficiency over diffuser. This is one reason I raised my wing over all height about 8 inches.

At the front it might be possible to exit all the radiator air out the top of the body and across the windshield instead of filling the wheel wells. We will need to see how the car cools first I would expect.

That's my two cents worth.

 

[Fran Hall RCR]

I can tell you that there is no simple answer to aero on the SL-C..or ANY other car...we have done seat of the pants testing and also masses of CFD work ....especially during this off season....

Some of the things that you may not feel are going to make much difference , prove in fact to have the greatest effect overall....one is a really nice fitting body with nicely mated surfaces is really really important overall air flow.....making both the louvres, rear wing, rear diffuser all more effective even at a base setting.
We ran witbout a flat floor under the engine for most of the season.....engine bay temps rise very quickly when you seal the floor of the engine bay.
The 01car has a flat floor under the cockpit but we also ran many races without the rear diffuser due to not running the engine underfloor....we had them designed to work as a system
We have some very very cool new parts this season ...this all has to be kept in check mind you , as we are racing at the front of the field and it needs to be remembered what we managed to do last year with the brand new SL-C with a pump gas street engine.
...run away from everyone that had far more power
...set lap records with far less power
...and win races
...and win a championship
....and finish a 13 hr race after the cars longest previous race had been 50 mins.

I cont think there is any better barometer of how succesful the overall package is than race results
...if you install all the same aero parts that we ran last year , then any SL-C is capable of having the same aero balance that we had last year..which appears to have been pretty darn effective.....and all of those parts are available for sale...

We had no cooling issues with the stock radiator and 595hp either...

 

[dennism]

Howard I think that you've got it
Remember that all wings have induced drag and on your vehicle having the additional drag from the wind attachment supports to the vehicle body
Exhausting engine heated air below the wing will help engine bay cooling and lessen that negative pressure drag below the wing that is already probably all turbulent and should be kept away from interfering with diffuser effects
I don't know what your dropped floor looks like but shoud have at least a30deg (some would say 15deg), not a 90deg edge to minimize underbody air flow turbulence production
Fran's success is remarkable and I hope that he continues his improvements as I'm sure his competition must also since he has substantially raised the bar
Dennis

 

[Cliff Beer]

I love it Fran - you're winning races (and keeping temps down....) without a rear diffuser at all. And here we are...a bunch of street drivers...getting all bent out of shape about optimal diffuser design. Thanks for the perspective!

 

[Fran Hall RCR]

Cliff...that was kind of my point and perspective also...

Nothing wrong with forward planning for when you may do the big MPH numbers but really its just great bench racing/engineering until that point...and if you get it wrong it could be pretty disasterous at 150mph or more

 

[Howard Jones]

The bench racing/ learning/ building and development part of this hobby is why I didn't just buy a new Corvette or Ford GT.

The aero development aspect of the SLC design is the reason I bought the car in the first place. I have every single picture ever posted of #1 and a bunch of #18 in a file named "good ideas" and have tried to incorporate as many as I can into my build. If you study my build thread you will see a lot of the changes I have made come from observing both of those cars. There are many more to come as I begin to finish off the assembly of the basic car and start to develop it. I will be careful not to try and run the car at 250MPH.

After all I have a perfectly good GT40 to take to the track already. It would have been pretty easy to add a 100Hp and slicks and been done with it.

I don't have the buget Fran has or the parts. Everything I do, I try to do myself. Some times it works and when it doesn't I don't have 1000s of dollars in labor invested, just some pretty cheap material costs and a lot of learned experience/ hobby time.

In the end thinking is fun. I wouldn't do it if it hurt.

 

[Fran Hall RCR]

Howard.
I agree bench racing is part of the hobby and also the fun aspect , as I said its good to plan for the future , besides you dont need to have a business budget to install proven parts or copy what we have done...many of the things we did were near free....

I can tell you without any changes to your car it will be wicked fast ....and I would drive it first and make your changes after you get a feel for the way the car works with your driving style....

 

[Jack]

Howard, I have extended my floorpan rearward, but only to the engine crossmember.
This helps initial undercar airflow and still allows for helping to reduce heatsink (I hope). Added fender vents and I'm playing with canard shapes and endplates for the standard wing. All of this is not really necessary, but "just because". My SL-C is not intended for the track only, and will be merely a mean street car. This stuff just looks so cool though! I also made a rear window of FRP should I decide to use a solid vented window (still undecided as yet, but I have the part made). Small scoop ready to go to make additional room on the race tail for air intake (thanks Fran). I need to get moving and quit making mod's!

 

[Chris B]

... I have extended my floorpan .... Added fender vents and I'm playing with canard shapes and endplates .... My SL-C ... will be merely a mean street car. This stuff just looks so cool though! I also made a rear window of FRP should I decide to use a solid vented window .... Small scoop ready to go to make additional room on the race tail for air intake .... I need to get moving and quit making mod's!

Molleur:

I have been stopping by the forum over the years as I patiently plan a hopeful SL-C build after my kids to finish up college. It is great to see how each builder personalizes thier car (thanks guys!) but I don't recall seeing much on your build. From your description, it sounds like you have done some nice work. Why no photos or build thread of your SL-C? Do you have it on another site?

Chris

 

[Jack]

Chris,
I am compiling a photo series but have not posted any for a long time. I have been sharing some stuff with other builders though. Send me a PM with your e-mail address and what you would like to see and I'll send some. A couple of SL-C builders and possible future builders have stopped by my place to have a look. I did loose a great deal of time on the build because of some physical issues, but getting back on it recently. All good!

 

[Howard Jones]

I completely agree Fran. I will run the car for at least 3 or 4 weekends and work on all the little stuff like leaks, driver seat, peddles, mirrors, seatbelts, carb tuning, etc. Then after running the basline setup on camber, caster, spring, shocks etc. Begin to tune the suspension. The antiroll bars will be interesting and may take a while to get right.

Once the mechanical grip is close I intend to begin with a baseline aero setup and work from there.

All in good time, and I am sure I will be on the phone with you a few times for help. The experiance you guys have with #1 will be very useful. You have always done what you could for me and I look forward to getting (no name yet) going.

Maybe Francine...............a little sister for Betty........

 

[Fran Hall RCR]

Francine.....I laughed out loud...