Wind Tunnel test notes

[Jim Schenck]

Recently we brought both the 818S and 818R to the A2 wind tunnel in Mooresville North Carolina to get both a baseline of the standard body shape and to test the effectiveness of the additional aero parts and see if any changes were needed before they are made available. Also the data from the parts and combinations helps in tuning the race cars not just for total downforce but for front to rear balance to a help tune the car and optimize it for different race tracks and the street car to maintain a safe balance.

The first car we tested was the 818S and all of the more street specific parts designed for that car. Our baseline test was run just as the car comes with no add-ons and at street ride height, in this test we made a very small amount of lift front and rear, 28lbs in front and 18lbs in the rear at 100mph, which compares very favorably to almost all but the most exotic sports cars. The news from here only got better as almost every addition we tested showed a large gain in downforce, with the only exception being the side skirt panels which added a small amount of lift but did give a slight decrease in drag. Our best combination in terms of both downforce and balance was with the car at zero rake (street ride height of 4.5 inches) with the street front splitter, the rear diffuser, a modified rear spoiler, no side skirts, and the gap between the nose and splitter blocked off. The only part we will need to modify is the rear lip spoiler which needs to be slightly larger to balance out the effects of the front splitter. We did this with a section of an old splitter for testing purposes. Also the results show that using the splitter only with no rear add-ons would result in a very tail happy car at speed and is something we do not recommend. Overall the car showed good balance with close to neutral lift in its base configuration and even better balance and significant downforce when optimized with the optional aero parts.

Next up was the 818R with its lower ride height and much more aggressive splitter and rear wing. The baseline on the R showed that the ride height and lack of windshield combined to make the car almost dead neutral lift wise using just the standard body panels with no Aero additions. (1 lb front lift and 11 lbs rear lift at 100mph) After that we tested the set-up that was run and dialed in during the previous on track sessions to get numbers on what a comfortable aero balance was at the track. This combination included the old splitter, the side skirts, and the rear wing at zero degrees, and produced just under 200lbs of total downforce at 100mph giving us our baseline to improve upon. Switching out to the new splitter added 30 plus lbs of downforce to the front end and another 20lbs by taping off the opening between the body and splitter. With all this added front downforce we then adjusted the wing angle and added the diffuser to balance out the car. In the end our best numbers came with the extra radiator ducts taped off, the new splitter with the air gap taped off, 5 degrees of wing angle, the rear diffuser in place, no side skirts, and a level ride height of 3-3/8 inch. The max downforce produced was 120 lbs front and 164 lbs rear at 100mph, this increases to 271lbs front and 369 lbs rear downforce at 150mph. The only concern with this set-up was cooling with the extra air inlets taped off, however we were able to test this configuration two days later in 97 degree weather at the track and the engine temperature was not affected.

Our conclusions from the testing were that the rear spoiler needed to be extended for the street car, the race splitter is to aggressive for street use without a rear wing, and the car has a nice useable balance without any aero add-ons for someone who would prefer the clean look or not have the budget for the extra pieces.

[Mechie3]

How much lift did the side pieces add? I ordered them, and like them for the look they provide. Same rear diffuser from the S and R?
How much downforce did the S end up generating (unless I missed the number).

[Darkpiggy's dad]

Thanks for this information. It sounds like the street splitter, diffuser and modified spoiler would work well for me. I hope you publish more details when time allows.

[Jim Schenck]

The side skirts took away 4 lbs of downforce, the optimized street car made about 75 lbs of downforce at 100mph and 170lbs at 150mph.

[JAubin]

Awesome info, thanks for keeping us up to speed! So nice to see real world #'s on each part so we can make informed decisions on what parts to buy. So in essence the street car optimization is a 120 lb delta? Nice work!

[fateo66]

Can you post up pics of the two front splitters and how you tapped off the front splitter to the body?

[RelfF2]

So, if i understand this right, there are two front splitters available, race and street. The takeaway is that the street splitter works best with a, yet to be finalized, more aggressive lip spoiler. The race splitter does not work with the lip spoiler and requires a wing. Is that right?

PS: very impressed with the work you guys are putting in, and the transparency with which you communicate it all to the community. 1st class stuff.

[Jim Schenck]

The race splitter and the street splitter are both finalized and now that they are tested we are starting to build them. They are the same basic shape in that they follow the body about halfway up into the lowest opening on the nose and have a large flat floor with small rear diffusers. The street version is shorter that the race version by about 3 inches and the diffusers are smaller.

The rear spoiler we made is about 1-1/2 inches tall but we extended that by about an inch to get the balance we wanted so that part will be modified before being produced. I would say something is required on the street car to balance out the splitter, the spoiler works well and we know the data but you could also use a rear wing if you prefered. On the race car you could mount the splitter further in to cut the downforce and balance the car out with the spoiler or a smaller wing, but with it maxed out you really need a full size wing to get a good balance.

I don't have any good pictures of the nose taped because we were really scrambling to get as many tests in as possible, but we pretty much just closed off every opening except the biggest one.

[Flamshackle]

Stunning info FFR. Will you be going back to the wind tunnel for the soft top or rumored hard top?

dying to see how this weapon would look/perform as a coupe

[RelfF2]

The race splitter and the street splitter are both finalized and now that they are tested we are starting to build them. They are the same basic shape in that they follow the body about halfway up into the lowest opening on the nose and have a large flat floor with small rear diffusers. The street version is shorter that the race version by about 3 inches and the diffusers are smaller.

The rear spoiler we made is about 1-1/2 inches tall but we extended that by about an inch to get the balance we wanted so that part will be modified before being produced. I would say something is required on the street car to balance out the splitter, the spoiler works well and we know the data but you could also use a rear wing if you prefered. On the race car you could mount the splitter further in to cut the downforce and balance the car out with the spoiler or a smaller wing, but with it maxed out you really need a full size wing to get a good balance.

I don't have any good pictures of the nose taped because we were really scrambling to get as many tests in as possible, but we pretty much just closed off every opening except the biggest one.

Thanks, makes perfect sense, I don't think i was using "lip" correctly, but I think you understood me. I don't want to run a wing, but I do like the front splitter, so a street splitter and a non-wing (what i meant by "lip") spoiler for me. Thanks again for all the info.

[RM1SepEx]

When can we expect a price listing and availability for the street lip spoiler? Are the street and race splitters priced the same? Smaller is usually cheaper...

[Bob_n_Cincy]

QUOTE=Jim Schenck;109768]The race splitter and the street splitter are both finalized and now that they are tested we are starting to build them. They are the same basic shape in that they follow the body about halfway up into the lowest opening on the nose and have a large flat floor with small rear diffusers. The street version is shorter that the race version by about 3 inches and the diffusers are smaller.

The rear spoiler we made is about 1-1/2 inches tall but we extended that by about an inch to get the balance we wanted so that part will be modified before being produced. I would say something is required on the street car to balance out the splitter, the spoiler works well and we know the data but you could also use a rear wing if you prefered. On the race car you could mount the splitter further in to cut the downforce and balance the car out with the spoiler or a smaller wing, but with it maxed out you really need a full size wing to get a good balance.

I don't have any good pictures of the nose taped because we were really scrambling to get as many tests in as possible, but we pretty much just closed off every opening except the biggest one.[/QUOTE]

Here is a picture of the front end taped up
19978[
Bob

[CHOTIS BILL]

Jim,

Thanks for the report. Those numbers are very interesting and to me the most interesting is that on the 818R the center of pressure remained stable at about 58 percent from 100 to 150 MPH. When you were track testing did you have a data system in the car so that you can compare track data to wind tunnel data? In the future will you be testing smaller ride heights and maybe some rake angle along with the side skirts at the lower ride heights?

Thanks again,

Bill Lomenick

[FFR-ADV]

Congratulations Jim!

Great report and impressive results! From your experience with the other FFR cars do you have expectations of how a removable hard-top or soft top will impact the spoiler or wing performance?

Thanks again!

Steve

[riptide motorsport]

Excellent numbers.

[skullandbones]

X2^^^^^ Those are off the hook numbers. I can't imagine being in that little monster at 150 but I'm sure there will be a lot of takers. Every report just gets crazier! Thanks, WEK.

[shinn497]

Thanks for the report JIM!

Sigh the car looks good with a front splitter. Any chance a more aggressive rear diffuser can provide enough downforce on the tail?

[Matty_STi]

Thanks for the report JIM!

Sigh the car looks good with a front splitter. Any chance a more aggressive rear diffuser can provide enough downforce on the tail?

Possible though with rear diffusers they can accelerate the air from the front all the way back creating a near even (in some cases) amount of down force across the entire under body. This when combined with that splitter would create a tail happy scenario whereby the rear diffuser is pulling the front end down with that low pressure fast moving air being pulled through the underbelly. Thus you would be left with having to add a rear wing or spoiler of some kind to make up for the load difference. Rear diffusers are really complicated pieces and can totally change the dynamics of a cars aero package.. when you look at F1 most of the down force is coming from the diffusers the wings are just used to direct the air such that it can be used by the diffusers or won't spoil the air flow coming out of the diffusers... Also with an aggressive but poorly integrated diffuser it can generate massive rear loads which pull the front end up which creates a fast moving very low pressure (remember Bernoulli here) wall of air that then sucks the front end down but causes the rear diffuser to gain more space and create an even more aggressive profile which lifts the front end. Essentially you are now riding a bucking (oscillating) Bronco at 150mph, not fun..

TL;DR yes/maybe/sure/be careful what you wish for.

lol

-Matt

[longislandwrx]

Nice Job. Looking forward to seeing pictures of all the new pieces.

was the diffuser you added on the R the new diffuser or the aluiminum one that's been on the car for a while now?

[Jim Schenck]

Bill,

We are data logging corner speeds, side forces and the engine data but nothing aero specific. For balance we are relying on John's feedback and how it correlates to lap times and he is very good at feeling what the car is doing and communicating it, the car is most often faster after we adjust for what he is asking for. We ran a few different ride heights and rake settings in the tunnel but stopped from going lower than what the car can currently run.

Steve,
I am sure a top will have a big effect on both the downforce and how clean the air is to the wing, both in a positive way. We may take the car back to the tunnel to test a hardtop but we will for sure simulate it using CFD during the design process.

The rear diffuser can be extended for a pure track car but the angle is pretty maxed out as far as generating downforce goes. (11-12 degrees depending on body rake)

We tested both the old flat and the new splitter and came to the conclusion to only offer the new one.

[CHOTIS BILL]

Matt,

The “bucking (oscillating) Bronco” you are refering to is more commonly refered to as “porpoising” and it only happens at very low ride hights and at very high downforce levels. Ride heights of 4.5” or even the 3.375” and a few hundred pounds of down force doesn’t even come close to creating the conditions necessary to creat “porpoising”. From what Jim reported what can be a real problem is if someone were to run a front splitter and nothing to balance it out on the rear. The center of pressure migrates to far to the front and you get into a tank slapper. I have been there once at about 135 MPH while doing aero testing. It wasn’t fun and I removed that setup choise from my setup book.

Bill Lomenick

[BipDBo]

"Our baseline test was run just as the car comes with no add-ons and at street ride height, in this test we made a very small amount of lift front and rear, 28lbs in front and 18lbs in the rear at 100mph,"


The side skirts took away 4 lbs of downforce, the optimized street car made about 75 lbs of downforce at 100mph and 170lbs at 150mph.

That's pretty impressive that with just a few minor looking additions, you went from 46# lift to 75# downforce. 121# is not insignificant for a car that (with the driver) weighs about 2000#. What was the F/R split on that 75#?

I'm curious what might happen if you did two things:
* Remove the windshield. The 818R is the dedicated track car, but 818S owners may want to hit the track for time trials. Removing the windshield would be a tempting weight and drag saving measure for track day.
* What if you enlarged the hood vent? I've mentioned, this before, so I apologize if I seem like a broken record. The hood relief vent free area is much smaller than the front intake, which, to me, is backwards. For comparison, the type 65 coupe has a smaller frontal intake than the 818, but a much larger hood relief vent. I'd bet that right now, at most 1/3 of the air that goes though the 818 radiator leaves up through the hood. The rest probably snakes around down or out through the wheel wells. If all of the radiator air was ducted to go up through the hood, I'd bet that we would see an improvement to the front downforce.

[CHOTIS BILL]

BipDBo,

I haven’t looked that close at how the air flow get into and out of the radiator on the 818 but in general the inlet opening should be smaller than the radiator and the ductwork should increase in cross section so that the air flow is slowed down which increased it pressure. After the radiator the ductwork should decrease in cross section so that the air flow is speeded up to rejoin the outside air flow in an orderly manner. This can give you meaningful down force. You never know what the air flow is really going to do unless you make changes and test. But I wouldn’t be surprised if preventing the air from exiting under the car would result in an increase in total down force. From my past experience the air entering from above the floor pan causes disturbance in the air flow for a substantial distance along the bottom of the floor pan thus reducing down force. If I were testing I would like to try closing off the bottom of the car and retrying the side skirts. One of the coolest things about having cars like this is being able to come up with changes to try and test to see if they work. In order to do proper testing you need a data system and sensors. There are some pretty cheap data system on the market and you can hook up sensors using rotary pots, some string, and a spring.

Bill Lomenick

[Jim Schenck]

The 75lb downforce run was the highest overall but also skewed a little more than optimum toward the front. The best overall run in terms of balance with the S we made 69lb total downforce with 27lbs front and 42lbs rear. That run had a little less tape on the nose and the rear extended spoiler along with the street splitter and rear diffuser.

The ducts on the nose of the car can be trimmed larger if you followed the recess back until about 1/2 inch from where it blends back into the hood. That would more than double the size of the opening and would only require some extra mesh and/or a custom trim plate. Changes the look a little but I am sure it would add some downforce as well.

[Xusia]

VERY nice data Jim. I wasn't really considering any aero parts for my street car, but this has changed my mind. I'll at least be doing the front splitter and rear lip spoiler. The jury is still out on the side skirts, but with only 4 lbs less down force the decision will be based on looks.

[Mechie3]

The ducts on the nose of the car can be trimmed larger if you followed the recess back until about 1/2 inch from where it blends back into the hood. That would more than double the size of the opening and would only require some extra mesh and/or a custom trim plate.

Another product idea. ;)

[flytosail]

Long time ago I had a Hyundai Elentra rental car in Germany on the Autobahn up to 125 mph. The car was doing a sort of weaving floating dance before I realized that this could turn out bad.

Downforce balance out good.
Upforce bad.

[flynntuna]

I'd like to know if the latest version of the splitter, both S and R versions have been or will be changed to cover the lower radiator opening between the splitter and the upper radiator opening? Or will this mod be left up to the builder? It seems from the report that the lower opening doesn't improve cooling and it doesn't improve down force. A long way to say form should follow function. :p

[bigAl]

This isn't a Formula I car where the rear underbody aero is significant in terms of major downforce. No one should expect big numbers from the so-called diffuser, which on conventional cars like this is relatively unimportant - other than being done in a proper balance with the front underbody aero stuff.
Of course, it should be worked out before zeroing in on front underbody work - and if changed, the front underbody work should be looked at again. Balance is the important issue and it looks like FFR took this into account properly.
Lots of people think the diffuser configuration is a big deal. Well, it is - for styling purposes.

[FFR-ADV]

Steve,
I am sure a top will have a big effect on both the downforce and how clean the air is to the wing, both in a positive way. We may take the car back to the tunnel to test a hardtop but we will for sure simulate it using CFD during the design process.

The rear diffuser can be extended for a pure track car but the angle is pretty maxed out as far as generating downforce goes. (11-12 degrees depending on body rake)

We tested both the old flat and the new splitter and came to the conclusion to only offer the new one.

Hi Jim,

The diffuser looks great and the lack of turbulance in its wake is impressive. I look forward to seeing what you will do with the hardtop. Is the Solidworks modeling of the top under way? Might a longer splitter be required to balance increased downforce of a rear wing?

The 818 is visually stunning with performance to match!!!

Thanks you for sharing with us!

Steve

[Matty_STi]

Matt,

The “bucking (oscillating) Bronco” you are refering to is more commonly refered to as “porpoising” and it only happens at very low ride hights and at very high downforce levels. Ride heights of 4.5” or even the 3.375” and a few hundred pounds of down force doesn’t even come close to creating the conditions necessary to creat “porpoising”. From what Jim reported what can be a real problem is if someone were to run a front splitter and nothing to balance it out on the rear. The center of pressure migrates to far to the front and you get into a tank slapper. I have been there once at about 135 MPH while doing aero testing. It wasn’t fun and I removed that setup choise from my setup book.

Bill Lomenick

I was speaking to the complexity of just making a diffuser more aggressive and the imbalances those can cause. The chance of oscillations (heard many terms, I like the bronco because most people have an easy time seeing that) are indeed limited to more extreme examples (been there myself). Point I was making is just because you can "make something aggressive" doesn't mean that you are achieving an increase in performance. Aero is about being pragmatic and making smart choices. At least in my professional opinion. ymmv


-Matt

[Flamshackle]

Thjs is my favourite thread on the forum right now.

[Jim Schenck]

Still working on the soft top so nothing on a hard top yet. The race splitter is basically the street splitter shape but extended much further out the front. If you were looking to get more aggresive with street car rear downforce with a wing or larger spoiler and you needed to match the front, you could start with a race splitter and cut it down (from the back side) to find a happy medium of both balanced downforce and street ground clearance. You just have to be more careful on driveways and road construction if you have more splitter sticking out the front.

[tirod]

I am sure a top will have a big effect on both the downforce and how clean the air is to the wing, both in a positive way. We may take the car back to the tunnel to test a hardtop but we will for sure simulate it using CFD during the design process.

Now, put this in the context of why Pete Brock designed the Coupe. SA went from an open roadster in racing, which worked well on short American tracks with limited top speeds. Once overseas, the longer Euro circuits made it expressly clear the open roadster was slower than a hardtop. So, they were added - the LeMans top - which was pretty cool with the filler tube sticking up thru the roof. (Cool, but how safe when it's upside down?) That added some speed - just to see the Ferraris still moving ahead, albeit somewhat less.

Although the degree of difference may not be as great this time, when the 818 Coupe comes out, expect to see their taillights on long straightaways. A lot of us living outside the Sunbelt are gleefully waiting this out. Especially when somebody (please SOMEBODY) runs one at Bonneville.

AutoX, nah, extra dead weight. It's always a tradeoff.

[Canadian818]

Well there goes my plan to run a splitter and only put on the giant wing for track days. Either I buy both wings or run the shopping cart look all the time...hmmm

[BipDBo]

I'd like to know if the latest version of the splitter, both S and R versions have been or will be changed to cover the lower radiator opening between the splitter and the upper radiator opening? Or will this mod be left up to the builder? It seems from the report that the lower opening doesn't improve cooling and it doesn't improve down force. A long way to say form should follow function. :p

I hope that FFR does make changes to the body rather than leaving it up to the builders. Making these changes would be difficult if not impossible to do if you want a paintless build.

I wonder if the closing of the front intake may cause a problem of overheating while standing still. When not moving, air isn't pushed into the front grill. If the car is idling in a traffic jam on a hot day, will the radiator fans push enough air to keep the car cool? Larger openings are needed for this reason than what may be optimal at high speed. This is evidenced by the fact that many recent cars, such as the Chevy Cruz have active grill shutters that close at higher speeds.

The problem with the front intake is I've noted above. It's probably not too big. The problem is that the hood vents are too small and too constrictive. This is a point I've been trying to make for a long time, and it's proven by the fact that the downforce was improved by closing off part of the intake. The problem is that, at speed, it's easier for the air to ram into the car than it is for the air to leave. This pressurizes this volume, pushing upward on the hood. Also, much of the air is going down, rather than up through the hood. This all creates lift. Closing off this opening merely reduces some of that lift, not eliminate it, allowing some of the downforce from that sloped hood to be revealed in the tests.
We can simply, easily change that lift to down force.
1) The frontal intake area needs to be a fraction of the radiator area. I'm not sure what that fraction is exactly, but the current frontal area looks about right to me.
2) All of the radiator air needs to be ducted upward, not down, or sideways out the wheel wells. Curve the duct up at an angle. This, in effect creates a big internal downforce wing.
3) The air must exit up through the hood with little resistance. The existing hood vents are too small and it's mesh is too restrictive of airflow. I know that if they removed the mesh from the openings, they would immediately measure a downforce improvement in the wind tunnel, but much more improvement could be made. Either make it open like the type 65 coupe or use aerodynamic curved louvers. It also must have a free area significantly larger than the intake, probably 1.5x to 2x the area. The current hood vents are smaller than the intake, and covered in mesh. These openings simply don't cut it.
Although a more expensive option, I suspect that curved hood vent louvers may work better because they can direct the exiting air backward drawing the air out with Bernoulli's principle into the speeding air over the hood rather than colliding upward into that airstream. The hood vents on the GTM are likely large enough, but the mesh was a bad idea. A lot of builders have modified these vents or installed louvers.

[Xusia]

In the end our best numbers came with the extra radiator ducts taped off, the new splitter with the air gap taped off...

Hi Jim, can you clarify which ducts are the "extra" ducts, and what is the "air gap"?

[Jim Schenck]

On the front vents the intention was not to duct all of the radiator air through them, only to allow some of it to exit the the hood to help in reducing lift. There is no duct forcing the post radiator airflow through the vents and much of the flow exits out behind the front wheels. On the R one of the tests was to remove the mesh, because I thought as you do it might be restricting flow, but there was no measureable change. Both the Blue and white car have an updated hood from the earlier prototypes that have larger vents in them, and I think it would still be relatively easy to just cut the vents larger if you want by staying inside the recessed area.

The areas I am refering to as the extra ducts are the two roughly 4 inch by 3 inch rectangles on either side of the main grill opening. The air gap is the opening created between the splitter and the nose where the nose curves up in the middle and the splitter only follows it halfway up. (The old splitter stayed flat here and the opening was bigger.)

[D2W]

Jim without any post radiator ducting and the high pressure area in front of the windshield is any noticable amount of air flowing out through the hood opening? In the wind tunnel testing video when smoke was sent into the front opening I did not see any exiting through the hood vents.

[Jim Schenck]

Smoke was coming out of the nose vents when it was in the airflow coming in to the very top of the grill opening, you can see it briefly as the smoke wand switched from the rad opening to the top of the hood but we didn't get video of it other than the quick transition. The movement in the tunnel was to much for our hard drive video camera and would shut it off after a few quick seconds, so all we have are brief clips.

[johngeorge]

The race splitter and the street splitter are both finalized and now that they are tested we are starting to build them. They are the same basic shape in that they follow the body about halfway up into the lowest opening on the nose and have a large flat floor with small rear diffusers. The street version is shorter that the race version by about 3 inches and the diffusers are smaller.

The rear spoiler we made is about 1-1/2 inches tall but we extended that by about an inch to get the balance we wanted so that part will be modified before being produced. I would say something is required on the street car to balance out the splitter, the spoiler works well and we know the data but you could also use a rear wing if you prefered. On the race car you could mount the splitter further in to cut the downforce and balance the car out with the spoiler or a smaller wing, but with it maxed out you really need a full size wing to get a good balance.

I don't have any good pictures of the nose taped because we were really scrambling to get as many tests in as possible, but we pretty much just closed off every opening except the biggest one.

taped off nose pic:
http://www.johngeorgeracing.com/gallery/albums/utcc2013/IMG_6147.sized.jpg

[C.Plavan]

Is the Center "Pod" mirror needed? I see it in some pictures, then it is removed in the video. Why was it removed? Anyone know?

[FFR-ADV]

Still working on the soft top so nothing on a hard top yet. The race splitter is basically the street splitter shape but extended much further out the front. If you were looking to get more aggresive with street car rear downforce with a wing or larger spoiler and you needed to match the front, you could start with a race splitter and cut it down (from the back side) to find a happy medium of both balanced downforce and street ground clearance. You just have to be more careful on driveways and road construction if you have more splitter sticking out the front.

Thank you Jim!

[Bob_n_Cincy]

This wind tunnel thread was a little to quiet for me.
I am hoping to see some aerodynamic drag numbers come out so I can calculate the range of my electric 818s.
For the street 818 the downforce is interesting, but drag is much more important.

Let see who can come up with the best caption for this picture.
My first caption: We don't need no stinkin down force.
My second: Marty, push the hover button at 88 mph.
20124

[flynntuna]

Alright who spilled the pixie dust!

[BipDBo]

This wind tunnel thread was a little to quiet for me.
I am hoping to see some aerodynamic drag numbers come out so I can calculate the range of my electric 818s.
For the street 818 the downforce is interesting, but drag is much more important.

Let see who can come up with the best caption for this picture.
My first caption: We don't need no stinkin down force.
My second: Marty, push the hover button at 88 mph.
20124

I'd like to know the drag numbers too. The actual force numbers would be more useful than just a Cd, since the Cd is derive from the force numbers and a measurement of frontal area, which is always up for interpretation.

On an electric 818, you won't need nearly the airflow going through it for cooling. You'll be able to blank off a lot more, which will decrease drag. You'll likely do some other things that may decrease aero drag:
* flatter, non-spoke, aerodynamic rims
* skinnier tires?
* removable hardtop, or removal of windshield?
* smooth belly skin extended all the way to the back of the car, including engine bay

Without major reshaping the 818, any other ideas?

[apexanimal]

^lower ride height...

If possible...

[rjh2pd]

Rear diffuser would help as well