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If I'm understanding your plans, you want to bring the top of the diffuser up towards the bottom of the bumper?......is so, I honestly don't think it is necessary. I think that the diffuser will work fine even if there is a 4" to 8" gap between the bottom of the rear bumper cover and the top of the diffuser. This would allow you to save a lot on mufflers and exhaust mods plus allow the exhaust heat to vent out the back as it does stock.
If this diffuser is going to be used on the track, it is also easier to make it removable if the unit is a "flat" design that does not curve up with multiple attachment points to seal against the bumper cover. Diffusers work great, but to be functional the fins (strakes) have to be very close to the ground. You can make the fins a two piece design with a sacrificial wear material that will rub on the ground. Once on the track, if they never rub they are too high off the ground for maximum effectiveness IMO. Just trying to drive a car on the street with a functional diffuser is going to be scraping the fins like crazy.
If you are going to "flat bottom" your car as shown in the pics, the biggest challenge will then be transmission and diff temps and keeping them cool on the track (again, my opinion as my car is not flat bottomed).
I look forward to see what you come up with and following your build.
That is an option to space the diffuser down away from the bottom of the bumper for muffler clearance purposes. Right now I'd need roughly 1.25" gap to clear my mufflers PLUS having about 1/4" air gap to the bottom of the mufflers. My approach to making this would be somewhat "universal" where the owner could place longer or shorter "standoffs" to adjust muffler clearance as needed. Making the center portion curve upward increases fabrication challenges...making all panels flat would be much easier.
Originally Posted by C5Z06CE
I will agree and disagree with some of what is said here. I have been designing my flat bottom for 9 months now and just now finishing the design and fab. I have looked at this problem from every possible angle.
First, if you leave a gap to allow exhaust placement there are ONLY 2 ways that can happen. #1 the diffuser is pushed closer to the ground and LOWER than the flat bottom of the car in which case air will blow above the diffuser and largely making the diffuser nonfunctional. That was shown in someone's design in another thread. #2 the diffuser angle will have to be very shallow to allow exhaust to remain and again largely negating the diffuser function. Trust me I have measured it. The Max angle of the diffuser is about 5 degrees with the exhaust in place. This is largely because the muffler exhaust near the cradle limits the elevation of the diffuser. Even if curved diffuser it will not be possible to go beyond about 5 degrees.
Second, the diffuser does NOT need to be close to rubbing on the ground to be effective. Just look at the Corvette GTLM car over the last several years where the rules did not allow diffuser strakes. They just used a flat angled diffuser with no strakes. This idea I believe is a fallacy and likely taken from the design of the Viper ACR because it has wear strips on the diffuser and is closer to the ground. The diffuser design simply needs to have a shallow angle and the strakes are there to limit disturbance of airflow causing turbulence largely from the tires. Turbulence lessens the effectiveness of the diffuser. The purpose of the diffuser is to turn maximize a low pressure zone under the car (sucking it to the ground) and slowing the air back down (raising pressure back to ambient pressure) as the surface area increases and air speed slows. This also usually has the effect of decreasing drag at the same time. It has been shown repeatedly that a flat bottom will benefit a diffuser far more than a non flat bottom. The diffuser is also largely assisted with pulling air out from under the car by the low pressure zone under a rear wing and why the wing and diffuser combo work better than the sum of their parts. With all the added rear down force the strakes may contact the ground and may scrape so that is why many race teams use Jabroc (?spelling) wood wear strips but it is not a requirement to work. The airflow that provides the down force is laminar flow at the underbody/diffuser junction not at the level of the ground so again no absolutely necessary to seal it off.
Last, I agree that I have created heat issues and why the car will not see the track for a few more weeks because I plan to upgrade my transmission and diff coolers. I also plan to install trans and diff temp gauges to monitor on track. Worst case scenario I take off pieces of the flat underbody which are all attached with Dzus fasteners.
I have given this mod countless hours of thought and planning. It is not for the faint at heart because every appropriate adaptation of aero principles will impact other aspects of the car and will need to be addressed. I believe I have done that and plan to prove that in several weeks on track.
My 0.02
C5Z06CE
You make some good points. I have not got under the car and started measuring angles etc yet. My approach is a "functional" (as in it doesn't allow air to pack up under the rear bumper) but also "streetable" which will not cause scraping problems on a daily basis. This should be simple as I will just shorten the strakes as needed.
Originally Posted by c5racr1
Any updates?
If asking me, no. I have been wrapped up getting wheels figured out for my wife's Lexus, selling a car and getting the C5 ready for the season. I have sheets of cardboard ready to start making a template and a game plan. Just need to dive in at home or get to my buddies shop with a 4 post lift.
Regarding this quote: Making the center portion curve upward increases fabrication challenges...making all panels flat would be much easier.
Assuming there are no class rules you have to abide to and you can make the diffuser any way you want, is there a measurable performance advantage to having the diffuser curl upwards to meet the bodywork? Or is a flat diffuser that extends past the body more beneficial? I ask because I'm not sure which is better for ultimate performance and efficiency if you don't have to follow any class limitations.
I think a lot of the diffusers we see on real race cars are shaped the way they are because of class rules, not to exceed length limits, etc. But if given free rein, what would be the best design on a vehicle shaped like a late model Vette??
Ha, I have zero experience with creating aero....this project started as a "that looks cool and I want one." Then my Mech Eng background got the best of me and started to investigate proper aero. I daily drive my car and only locally autocross in CAM-S. I have yet to be on a road course so my experience level is low....sorry. If people like Danny Popp do not have diffuser (but do have front splitters and rear spoilers) something tells me there is not much to be gained by having one. UNLESS he wouldn't bother adding one unless it was 100% functional and that class rules limit the design to the point he didn't see the effort to make one.
AJ Hartman would be the person I would ask in terms of specific design features. Curved vs flat upward angle, how many strakes, strake placement and length etc. I do remember reading that there is a "optimal diffuser angle" as mentioned in here which keeps the air attached to the diffuser. If I research and find that angle, would I incorporate it into mine? Sure, but it needs to stay DD capable. Another aspect I have found is the length of diffuser. I see many that stick out past the bumper a good bit. For me, this looks "too race car".
Think of OPTMIA events and having D&E (Design and Engineering) portion of the event. I plan to make one that fits the overall appearance of the car, keep air from under the rear cover and stay on the car 90% of the time being able to clear all daily driver situations.
As of NOW, there is no CAM-S rule mentioning anything about diffusers. So anything goes. Realistically my diffuser will follow the outside perimeter of the rear bumper. It MAY protrude ~1 or so rearward Strakes will obviously be no lower than rear cradle but most likely be 1-2" higher. This makes it a tasteful "appearance mod" that should still somewhat function better than no diffuser (I THINK).
This is NOT my approach from a street driven aesthetics standpoint. However this large style seems to be quite functional when there are limited/no restrictions.
We have done significant testing, with a diffuser. Our ON TRACK testing has shown that the increase in downforce of this design, on our car, was not worth the effort. We know there is a gain to be had, we just have to continue to search, to find it. Our diffuser must not have been far enough forward, or enough angle, or deep enough strakes.
Side shot of diffuser in testing, before nasa nationals at COTA 2108 Better shot after testing, lowered side strakes, and enclose diffuser from turbulent wheel airflow. bumper cut, and closed out to allow clearance for diffuser diffuser on the ground, literally. Flow attachment. This streak of dirt water after a rain session never detached, meaning we could increase angle or lower diffuser.
As for me, I have installed heat shielding around my exhaust near my diff and tranny to lessen the amount of heat they see. I have also added heat shielding to the appropriate spots on my underbody and diffuser. Soon enough I plan to upgrade my current tranny and diff coolers with much larger ones and then do my own track testing.
I feel I have done the appropriate back ground work to design it well. Only a time will tell when I finally get on track again.
Stage 1 has been completed. LT's and custom exhaust built to free up room under the trunk:
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It goes in for a tune adjustment tonight. Tailpipes are removable to service anything and can remake them to better accommodate a diffuser build later on. Tips are tucked as high as I feel comfortable to not touch/melt bumper cover. Maybe 1/4" gap. I COULD have brought tips closer together and move them up 1/2" or so, but wasn't worth the added fab time when building this.
Last edited by smitty2919; May 16, 2019 at 11:41 AM.
Stage 1 has been completed. LT's and custom exhaust built to free up room under the trunk:
It goes in for a tune adjustment tonight. Tailpipes are removable to service anything and can remake them to better accommodate a diffuser build later on. Tips are tucked as high as I feel comfortable to not touch/melt bumper cover. Maybe 1/4" gap. I COULD have brought tips closer together and move them up 1/2" or so, but wasn't worth the added fab time when building this.
Looks good. Without the mufflers and high as you have set your tail pipes you should have enough room to put a diffuser with around a 10 degree or so angle.
Looks good. Without the mufflers and high as you have set your tail pipes you should have enough room to put a diffuser with around a 10 degree or so angle.
In your experience, 10* is a good/functional angle to use?
In your experience, 10* is a good/functional angle to use?
Most people would say a range of 7-10 or as high as 12 degrees if you have everything (flat floor and well designed rear wing working with diffuser). So 10 degrees is probably the most you should push it.
There are so many variables, that determine if the diffuser will be effective or not. We ran ours at 14*, and still had flow attachment, but the car did not perform better on track, nor did top speed increase due to wing angle taken out due to the added downforce from the diffuser.
One step forward, two steps back. My previous midmounted magnaflow exhaust setup droned too much for as much as I drive the car. Sounded great but was too much to enjoy the car. I went back to the original catback that came with the car which has 6" dia mufflers under the trunk and hang below the bumper cover...
So now I'm in search of a stock Ti catback which seems to be able to tuck up nice and high while providing a drone free sound.
This video shows the fitment of the stock catback very well and should work well for what I want to do with the diffuser:
I think that's a Pressure Wave Termination Box or a big mid mount muffler.
In F1 over the years a number of versions of "blown diffusers" were experimented with and that's what I first think of when I see this. Although successful designs are infinitely more complex it does exhibit visual cues of the basic premise of exhaust gas accelerating airflow in diffuser to aid in generating additional downforce. In the link below some interesting reading regarding Team Red Bull's Sebastian Vettel and Adrian Newey winning several championships utilizing this concept far better than the other teams, Great thread.
I finally got a stock Ti system on my car to keep my sanity since previous setups were too loud inside the car. Another step backwards since the mufflers do not tuck up high and above the bumper cover like the red C5 in the video above...
Next step is get a cam in the car and dyno tuned. Exhaust should not change and then revisit either cutting/adjusting the Ti system or change the approach to the diffuser fitment.
