Hi, I'm new here and my cD is .37, can you help me get a .30?
 

Ok, so here is my crossfire. Now I've already checked out the big mod list, but different cars get different amounts of benefit from each mod. So I was hoping you would all take a look at my car and identify the "biggest mods" and estimate how many points they would drop me. Thanks a bunch.

Grille block, air dam and boat tail and you'll be good.

Really? I can get down to a .3 with just that? I already pulled my wipers. Whats that good for? .005 cD?

How crazy are you willing to go? Do you care about looks at all?

I don't think an air dam is going to help you much, since you're already so low to the ground. A grill block on the top would definitely help and blocking as much of the bottom as possible while still having cooling would be good.

The rear hatch looks like it has a sharper angle than the template. Have you tried using the template yet? If you're going for looks a kammback to bring you back to template would be nice. If you don't care about looks, then a boat tail would help.

What does the undercarriage look like? With how low to the ground you are, a full belly pan might be the best way to go assuming you have a lot of parachutes hanging around underneath the car.

Try finding some wheels that are more "covered" or use some coroplast or other material to cover the ones you have. Wheel spats would also be useful.

You might consider running a sheet of coroplast from the hood bent up to the windshield to give the air a better approach angle. You could also hide your wipers underneath it. This would be a finishing touch though, the other stuff is lower hanging fruit.

Good luck!

Heres my underside. Also has a front pan. Exhaust has since been removed and routed out the front fenders

Removing the wipers is good for almost nothing; especially on this car with the long nose.

To get the Cd down 0.07, you will need smooth wheels, Kamm back, grill block, sealed front seams, mirror delete - at least - and probably a smooth belly pan, and maybe even a boattail, too. The shape of this car is better backward than forward.

As far as your belly goes, you have a really good start, but you could definitely improve upon it, especially if you moved the exhaust pipes. If there is nothing hot running down the underside anymore, you can do a near complete belly pan to fill all the "holes".

Here is a rough idea of the template on your car. You'll need to fill in the voids as best as possible to lower your cD.

http://i1289.photobucket.com/albums/...psed59e469.png

Is there an option for a spoiler from the factory or an aftermarket one that is specific for this car? If you don't want to do a kammback, a spoiler might help fill the void and still look OEM.

A rear diffuser would also probably help considering the sharp, vertical cutoff of the rear bumper. A clean belly transferring to a properly design diffuser would help a lot.

Do you have any pics of the new exhaust setup?

ok, well right off the bat, I will completely seal off the mostly fake lower grills and fog lights. I will use bondo and polish the whole bottom half of the bumper. I will also make a completely smooth pan from front to back and make some aero lenses for the headlights. About how many points is each of these good for?

And thanks for taking the time to make that photo!!!!!!!

Sounds like a good starting plan. I would probably start with the grill block and then move on to the belly pan. The other way around will yield the same results in the end, but I imagine the grill block will have a better effect by itself. The belly pan will have a better impact when you have the grill blocked off, forcing the air to go somewhere else (e.g. underneath the car). I'm not sure how many points your plan will get you, maybe NeilBlanchard can help with that.

No problem! I forgot to post the link so you can mess around with it: Aero Template

I also found a pic with the spoiler I was talking about. It's not the best photo since it's not a true profile, but you can get the basic idea. Notice how the spoiler helps fill the void and "touch" the line of the template (it probably would if the photo was correct)? Depending on how the spoiler actually works, this might help with the template, but hurt in terms of increasing down force at the expense of drag. I'm sure NeilBlanchard or AeroHead would be able to help in that department.

EDIT: Forgot to add in the pic!
http://i1289.photobucket.com/albums/...psb1213c9b.png

http://ecomodder.com/forum/attachmen...p;d=1421684497

That's a very nice car. The drivetrain is from Mercedes, is it not? Please don't cover it with coroplast.

What's the use case? Race? Commuter? Show car?

You could possibly push it well below 0.30, but what is working against you is called fineness ration, IOW it's short and stubby. jeff88's overlay doesn't show any extension at all. A complete boattail would be approx. as long as the wheelbase of the car. So it comes down to how much 'pain' you can tolerate.

I'd leave the car completely stock and add a single wheel trailer like BamZipPow's, but finished to a level to match the car.

http://i677.photobucket.com/albums/v...psf8b58829.jpg

Else you could consider a hitch mount like HHOTDI:

http://i42.tinypic.com/21epa9g.jpg

He saw a positive benefit dragging around a brick. Imagine what an aeropod would do.

Your car has a speed sensitive wing does it not?

I was behind one when they first came out going down an off-ramp and once we got below 45 mph the wing dropped down level. If you do anything in the back, be sure to not interfere with that thing.

As you said, its a very nice car and thank you. So I really don't want a boat-tail. Plus I need that downforce from my spoiler. The .37 is measured with the wing up.As for "coroplast" I have been working on cars since I was 8, I have the paint code and it will look like it never had lower grilles. My goal is to achieve a higher top speed at Michigan mile and gas mileage to boot. Only the inside half of the lower grilles are real. I wonder if opening them all the way and sealing the upper grille would be better.

This. I'm excited to see your final product! I love the look like it came from the factory. :thumbup:

I edited a previous post as I forgot to add the pic that I was referring to. If you have a speed sensitive spoiler, I don't think what I'm talking about in that pic will work as you already have a spoiler. Maybe you can change the parameters so that the spoiler opens at a different speed, or you could have an on/off switch to control it manually?

I think the lower grill will be better to block if you do a full belly pan and the bottom is almost perfectly smooth. Otherwise, I say do the top and help direct the flow over the top. You can probably do both and just leave a very small section in the bottom grill for cooling (maybe the portion that is "real").

You have got the template drawing too far forward - the 0 point should be lined up with the high point of the roof. When you do that, the rear roofline will be fairly far away from the ideal line, which is why I think you need a Kamm back to get the Cd a lot lower.

Yup, and by real I mean only the inner halves of the lowers allow air through. I intend to lower the suspension as much as possible. I'm guessing it would be better to drop the front more. Now Should I try to avoid letting air under the car?

Oops, you're right. I fixed it for clarity's sake. It's still a bit off, since the pic is slightly angled. I agree a Kamm will be helpful, but how to do it while not interfering with the spoiler. And if it's important to the OP, how to do it while still looking "nice".
http://i1289.photobucket.com/albums/...psc827b22b.png

AutoDeutchland, I would block the whole upper grill and bottom up to the inner halves. If you're still getting enough cooling, then you can start to block the inner halves as well. Lowering will certainly help, but how much lower can you go without causing rubbing?

At this point, I'm not sure what's worse... air over the car with nothing following the template, or letting the air under the car if you do a full belly pan. Hmmm...

Without the exhaust, I can do a diffuser as well. But I'm not sure what the proper angle would be. I guess it depends on how much pressure is underneath. I do have a basic understanding of this, nothing like most of the folks on here tho.

http://www.the-blueprints.com/module.../07596-mid.jpg
I found this blueprint of the car.

Very nice looking car.
As others have said, it would be a shame to mess up the look of the car.

I would do the same as others have mentioned and block the grille down to the very bottom slat that you can leave open.
If you take your time and put some effort into it, you can do a grille block that won't even be noticed to the casual observer. Personally, I would have the grille block behind the grille mesh and painted flat black so it will be stealth.

A few here have done some really nice blocks that look totally factory : http://ecomodder.com/forum/showthrea...eza-30402.html

Lower the car and this will not only improve the drag, but the handling and looks of the car.

The rear of the car is very rounded and this is a problem as the wind separates and causes a lot of turbulence. There is not much you can do that wouldn't mess with the look of the car and the function of the spoiler, but you can create some professional looking trip strips that wouldn't mess up the look too much.

The open spoke wheels could be replaced with smooth wheels when you race, and a full underbelly pan would be invisible unless you flipped the car :-)

Cd, that's a great find! the-blueprints.com seems like a good source, especially for doing the template. ;)

OP, notice the pic that Cd posted shows the low hanging stuff on the bottom of the car? You can see it in the profile shot and especially in the front on look. If you can clean that up, along with lowering the car even further, I think you'll be well on your way to that .3 cD.

Wow, I cannot believe how much effort people go to to help you on this forum. Ok, whats a trip strip? I would also be willing to cut a hole on each side of the license plate as shown and use fans to fill the vacuum with air from the interrior (not my car pictured, only the one with the bike to the side of it was mine)

The reason there is a spoiler (like the Audi TT Coupe) is that there is lift in the back.

I fixed the image link in my last post.

A trip strip is a small strip of something added to the car body in an area where there is too much curvature to maintain attached flow.

Here is a strip added on the roof of a car. I'm guessing it is a bit to low to 'snag' the air past the boundary layer. A tuft test would reveal how effective it would be.

http://i223.photobucket.com/albums/d...S/IMG_1063.jpg

To get a good idea of what the airflow around your car is doing, it would be very helpful to do a tuft test on your car.

AutoSpeed - Aero Testing - Part 1

Once you see where the problem areas are, you can begin to address them.

The point of the trip strip is what, to cause clean separation?

difusser—This suggests ~3° is optimum:

http://ecomodder.com/forum/member-fr...1370776504.jpg

The air those fans could suck out of the interior would be a drop in the bucket compared to the air flowing over the outside. And it would make your ears pop.

A trip strip is a strake or flap that is small in scale but affects air flow locally. https://en.wikipedia.org/wiki/Gurney_flap

Correct.

Here is an excellent example that was just posted : http://ecomodder.com/forum/showthrea...ars-30990.html

http://ecomodder.com/forum/attachmen...p;d=1421746047

So 3 degrees means it will look very subtle, almost flat underneath. My car is 159.8" long. About where do I want that 3 degree bend and does the optimum angle and position change with the ride height and amount of air I let or don't let under the car?

.37-.30
 

The Porsche 911 Carrera had Cd 0.39-0.32.
All the difference was in refinements to inlets and smoothing the belly.
This is the range of drag reduction you're interested in.So perhaps an optimized,air-tight cooling inlet/grille-block,and belly-panning/diffuser could do it,without really altering the 'look.'
http://i1271.photobucket.com/albums/...ntitled7-4.jpg
That rear spoiler would be very beneficial.

aerohead, are you saying that a grill block and belly pan were good for 7 "points" for the Porsche? Do you think that can equate across the board to 7 points for the Crossfire?

I will definitively do those things. My modding skills are very extensive and radiator relocation would not be out of the question. I can seal the front of that car tight. My question is: before I finish and paint the front bumper, do I want a splitter to keep air from under the car? Remember to account for an imminent 2" suspension drop. Also, is it better to port the exhaust out the sides right behind the front tires or directly underneath the headers into the underside. I'd like to make my aero and my engines exhaling mutually beneficial to each other. i.e. low pressure sucks exhaust gas out improving exhaust flow.

Relocating the radiator would be a major modification. How does it presently compare to this?
http://ecomodder.com/forum/member-fr...1215138521.jpg

Once again: What will you use it for? Could you live with this or would it be curbed repeatedly?
http://ecomodder.com/forum/member-fr...6-bedammed.jpg

The pressure differential in the airflow is much less (by orders of magnitude?) than in the exhaust. More likely you could use the exhaust for a 'blown flap' or Coanda nozzle
http://ecomodder.com/forum/member-fr...40-imagesb.jpg
But the exhaust is corrosive and will degrade anything it touches.

That front splitter is not the lowest drag configuration - it is probably the lowest lift, though.

Higher drag because it adds frontal area. I picked it because it was handy and the most extreme example I could think of.

I took a moment to look around. It looks like a reasonable example would be the stock Crossfire SRT-6:
http://www.nextcar.com.au/i.chrysler...blue.05mar.jpg

7-points
 

When the 1989 Carrera 4 made it's debut,it came in @ Cd 0.32.CAR and DRIVER mentioned that,compared to the generic Cd 0.39 911 Carrera,all the drag reduction came with these subtle refinements.Porsche probably shared the info.
We don't have a drag breakdown on the Crossfire so it's hard to predict what any specific mod will do.
The AUDI TT has a 'fast' roof kinda like the Crossfire,and it's spoiler help it reach Cd 0.29.
http://i1271.photobucket.com/albums/...d2/06-2827.jpg
Some grille work,airtight ducting,underbody cleanup,and better spoiler might do the trick.
The cooling system out to at least have as good a duct as NASCAR
http://i1271.photobucket.com/albums/...titled33-1.jpg

For starters, this is a proper application of the template on your car.
http://i59.tinypic.com/o7tzjb.jpg

Be sure that whatever you do it fits with the look of the car. I question whether the few dollars saved on some weird looking aerodynamic enhancements would pay for themselves in the cost of both time and materials. I also wonder if you'd want to fiddle with the stock look of such a great car, I mean, if it comes down to spending a few hundred dollars more on fuel over the next 5 years or putting some crazy looking stuff like a Kamm on my cool car, I'll pay the money.

Next off, I'm sure that this car has been aero designed to within an inch of its life. I'm guessing that based on its rough shape, MB and Chrysler were scared to death of an Audi TT lift off at high speed, so they made sure there was plenty of downforce. Downforce=Drag.

I'm guessing you have a retractable spoiler that deploys at 45MPH or something, this is meant to break the lift. I see some crossfires have a permanent spoiler which to me looks a lot less prone to having mechanical issues, and has a more efficient shape to it. This spoiler could really help your Cd because it brings the rear end of the car closer to the template. In addition, it will help with what certainly must be a lift at higher speeds. It will probably make your car feel much more planted and stable at highway speeds too. If you can get one of these and put it on you’ll be happy.
http://i61.tinypic.com/2a0kg2x.jpg

There are some hidden things you can do to improve a bit. The grill block and belly pan would be big. Adjust your tire pressure up closer to the highest recommended pressure printed on the tire, I'm guessing that Chrysler wants it at 32psi or so, get it up to 38. That will save you about 1-2MPG alone. Hiding in plain sight would be the factory rear spoiler.

To get the biggest gain in fuel efficiency, you need to adjust "The nut behind the wheel". In other words, your driving habits, this will pay off more than any aero tweaks.

The largest areo drag is caused by the general shape of your car, the air affected by it, and thus the air you need to control extends out many feet around and in front of your car. Worrying about how the air is flowing along the skin of the car has a use in gaining insight on the mass air flow, but tweaking this with anything small is unlikely to have any measurable effect.

So, Block the grill, Add spoiler, Pump up tires, see about cool things you can add to act as air flow diverters around your wheel wells, look into a belly pan. For Petes Sake....Put your dern wipers back on, and never do anything to save a few dollars that can jeopardize your life. Safety First.

Thanks for that info. Again, my goal is top speed. Fuel economy is just a bonus. The car is usually only driven to tracks and never in anything less than optimal weather. So I'm just trying to figure out how I can get down to the .29 - .32 range without ruining the car. I don't mind bold mods as long as they don't degrade the aesthetics of the car.

Ah, so.... Just leave a little earlier? :)

Here's an article you may find useful.

That article doubled my understanding of this. It just seems too good to be true that I can dorm my cd from .37 to .33 just by dropping my suspension two inches.

drop
 

With the Volkswagen Jetta Land Speed Record car,it was reported that dropping the car had zero effect on Cd,it only reduced the frontal area.