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Discussion Starter #1
So I know that many out there will get an aftermarket diffuser and move the exhaust up, I just never could see exactly why. I am not new to the aerodynamics of a car and am currently trying to make an old muscle car aerodynamically disappear. During my digging for info, I had found that exhaust gas flow actually helps diffusers work better!

Another point is that Lotus flat out knows what they are doing. My Exige has to have the fewest compromises of any car that I know of in terms of performance vs amenities/ascetics. I am positive that if the car would be better suited with the exhaust thru the back panel, it would be there. Here is a quote directly from Wikipedia regarding this topic

Injecting the exhaust into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This fast-moving air helps evacuate the diffuser more quickly, which helps drop the pressure at the underbody. However, this makes the diffuser rather sensitive to engine speed. When the driver lifts off the throttle, the exhaust flow is greatly reduced, which makes the diffuser less effective, robbing the vehicle of downforce. Thus, handling is negatively affected.

Another point I have: A few years ago I started reading about a car called the Super Cuda, a 1970 Plymouth 'Cuda with a 1000hp twin turbo Viper V10. It was built by a shop that does work on high end exotics that wanted to see if an old muscle car can be made to have the performance of one of these exotics. They re-did every single body panel in search of aerodynamics, downforce and handling. The car obviously has a diffuser, guess where the exhaust dumps...

I happened to see this car a couple years ago at a show and got lots of pictures of it. One of them in particular of the diffuser to see what they did with the exhaust



Again, the exhaust would not be here if this were not the best place for it.

I have read comments about 5 element diffusers where some would consider the center element to be useless if the exhaust were to still come thru there. What I am getting at here is that I will probably end up with a 5 element diffuser on my '06 some day, it will for sure have the exhaust in the stock location
 

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The flat body of the Elige/Evora is an upside down wing - it sucks the car to the pavement...

If you break up the smooth line with exhaust pipes you lose the flat wing and lose down force.
 

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The flat body of the Elige/Evora is an upside down wing - it sucks the car to the pavement...

If you break up the smooth line with exhaust pipes you lose the flat wing and lose down force.
Well said. That said the Exige sacrifices aerodynamics for downforce. I can't say I've ever read superlatives about Lotus aerodynamics.

San
 

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My $.02 worth.

Having the exhaust energize the air in the diffuser can be made to work. It works so well, if done correctly, that they banned blown diffusers in F1.

Some of the problems with our diffuser are: The angle is a bit aggressive. "They" say that 15 deg is about the max you can make work. That is about the angle ours is. The problem is that all the stuff feeding our diffuser is not as optimized as it needs to be to make this angle work.

The cutouts in the belly pan for our rear suspension is ridiculously over sized. Then you throw in the three naca ducts. That is now 5 huge air leaks all in the general area of the rear suspension.

The air that we are still trying to accelerate, that is what air is left after all the leaks just covered, can be energized by the exhaust! Except... The hole in the diffuser that the exhaust exits through is.... yep. ridiculously over sized.

My next analogy is backwards, but still true. With six air leaks in the tire, or in our case the diffuser, it just cant take, or produce, the pressure.

Now the plus side is. Even with these six air leaks, it does still produce some down force, or negative lift!


And things can be improved.

One of the venders here sell plates that help the suspension cut outs. This is the easiest quick fix.

I have heard, but not seen, that there is a plate that replaces our engine cover that does not have the naca ducts. I have flipped my naca ducts over so they are now acting as vortex generators, and with the addition of the side scoops, are exiting air (backed up by a test i did with an HD Wing Cam and some yarn) into the air flow helping to energize it, not sucking air in deenergizing it. I have not had any heat problems, that I know of.

The standard fix for the exhaust is to plug the hole and move the exhause outlet over the diffuser. Plugs one more air leak and does produce measurable benefits.


Now back to your question. Can our exhaust be made to energize the airflow underneath the car. I sure think so. I would love to have 30 min of Adrian Newey time and I bet we could double or triple the negative lift our cars produce. I still think if we can fabricate a duct to go fits over the exhaust exit point, and maybe make the exhaust outlets thinner, height wise, and much wider, it may work. Adrian, if your reading this, feel free to comment!

I also think that maybe we could build a double diffuser to help reduce the angle of attack on the lower plain, and in the thinner, less volume, upper area use the exhaust to energize that air. Adrian?

So if there is anybody out there that has access to a wind tunnel, some instrumentation, has good sheet aluminum fabrication skills, and is willing to to take my direction:D
 

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The flat body of the Elige/Evora is an upside down wing - it sucks the car to the pavement...

If you break up the smooth line with exhaust pipes you lose the flat wing and lose down force.
Unless you can accelerate the velocity of that airflow underneath the car.

Again, I am thinking the Blown Diffuser principals that F1 was using.
 

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Discussion Starter #6
Every car in the world has a relatively flat under belly and rounded up top, they all are shaped like an airplane wing and all create lift. Clever things have to be done on cars where the engineer's actually cared to get rid of that lift. It starts with smoothing out the under belly, lowering the car as much as reasonably possible then adding wings and splitters.

The Exige and Evora still have a windshield and cockpit area. The windshield forces are to move up. That would create a high pressure zone at the base of the windshield (hence cowl induction is so popular with other cars). The Lotus cars help this by having the 'mouth' at the front which sends air thru the radiator then up and out in a linear fashion along the windshield. Some down/backward force is still there but not as much. The air would still be moving in a relatively upward motion at the roof creating a negative pressure area creating lift. The roof line of these cars is very fluid and gradual which is nice looking while also relatively eliminating that effect.

The rear of any car, even a fast back will have an area of negative pressure over it (lift). The Exige can some what help with this by allowing air to come up out of the engine cover and fill that void. That would allow the air flowing over the roof to travel horizontally backwards instead of being sucked back down towards the car. The air coming out of the engine cover can flow over a wing if there is one, creating some down force.

The diffuser is great because it can promote a vacuum under the whole car, it too works on negative pressure but because it is on the bottom of the car, it creates downforce. The Exige only makes 100 pounds of downforce at 100mph in stock form, the Elise is at 7 pounds.

The tail panel of any car is another area of negative pressure. This time being at the back side of the car, it pulls on the back of it creating more drag. Exhaust gas exiting at the center of the car relatively high up will fill that void (think Zonda), relieving some of that drag. Granted, exhaust in the diffuser is relatively low to fill that void, it is centered at the back side of the car and equal on both sides. The two sides of the diffuser are allowed to fill that area of negative pressure while the exhaust does the middle. A 5 element would be very nice but I will still allow the middle to be filled by the exhaust. That will produce way more volume of gas vs the vacuum of air being done my the diffuser and tail panel of the car
 

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Discussion Starter #7
I forgot to mention that thanks, I really like the aero plates that are available to cover up some of the rear suspension. I plan for those as well

As for that NACA ducts, I have not taken the belly pan off yet, are those just open to the bottom side of the engine bay? I am not a fan of that, I would rather have as much as as possible be allowed to escape thru the top the fill the void over the engine cover and flow backwards over the wing
 

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Discussion Starter #8 (Edited)
What i would rather see is something that you noted, a cover that goes around the front, sides and bottom of the exhaust. That would only allow exhaust gases to come out of the back side of that area, eliminating more leaks VS exhaust being relocated and a cover being placed over the old exhaust exit. I think that may be my first part that I make for myself, time to get some sheets of aluminum, a sand bag and my hammers! or make a wood buck and form it around that


I also like your idea of inverting the panel with the ducts. What are your thoughts on covering them entirely? Is there any space between the floor of the cockpit and the belly pan of the car? I would like to seal those ducts, route air in between those layers and the let the air escape thru the engine cover
 

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If Adrian Newey built an Exige...

Reminded me of a car I saw at PRI last year...which was for sale. It was Adrian Newey's Exige (listed on wirewheel.com). Shows his choice of above or in diffuser exhaust location.



'course he was probably limited by Britcar regs on aero mods, so I'm sure this isn't what is ultimately possible given free reign. :popcorn:

Bunch more pics and info at their site:

2008 Red Bull Lotus Exige GTR/ GT3 for sale
 

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Discussion Starter #10
Very interesting. I like the mesh part of the tail panel to allow airflow out of that area. I am curious if there are rules stating there must be a separation of exhaust and diffuser like you said
 

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My $.02 worth.

Having the exhaust energize the air in the diffuser can be made to work. It works so well, if done correctly, that they banned blown diffusers in F1.
QFT. I see no real benefit or gain in real world application in moving the exhaust out of the equation.
 

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Very enlightening!

By his notes, a 5 element design would be beneficial with a diffuser exiting exhaust as the sides, section off, the central part of the exhaust.

Also interestingly, he mentions that the suspension cutout/fill in section pieces made very little to no difference at all. At least until the bigger tip diffuser went on.

I consider Simon a genius. He put a tonne of work into designing a complete aero package for a mate's Time Attack race car, and certainly knows his stuff.
 

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That was a fantastic article, thank you for sharing that! Of course, now I have to reconsider my planned mod roadmap... :D

-Matt
 

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The racecar engineering article is a great article.

A video that I have bookmarked because I review it frequently is the elise in the wind tunnel

 

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Discussion Starter #16 (Edited)
Awesome article! I had not read that before. The one thing I did not read was if the engine was running. It looks like the wheels were not spinning so I assume not and also that the motor would not be under load so it would be producing much gas to the back anyways

I also like that video, neat to see it with the roof off too

For a road car, I would like to see more downforce but the increased drag is not something that I would want. Also noted that the downforce is much greater in the rear. I would like to focus more on increasing front end downforce vs the rear
 

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splitter
 

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Discussion Starter #18
For sure! While I like the looks of the front canards, I do not think they will actually do enough to make them worth while. They only have a top surface area of what, maybe 12 square inches? Plus I am not going to modify my clam for that. I think that increasing vacuum under the front end will suck the front end down way better

Some day with enough tweaks, we can turn out Lotus' into that car in your avatar pic
 

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Discussion Starter #20
Watching that video again, I love how the air coming at the front lower corner wraps around the fender and flows rite into the side scoops! Even the mirrors lend a hand in that aspect. Another reason to keep your stockers

One thing I saw, expected and did not like was the air leaking under the car just in front of the rear wheels. That would ruin the vacuum under the rear. Seems like a pair of
side sills are for sure on order

Also with the splitter on the Exige, the front end will be slightly different. More air coming over the top instead of the bottom. Still need to protect that vacuum under there tho and the sides of the car seem to let me down there
 
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