What will a Gas Coupe that's .30 Cd stock be in race trim?

[FoundSoul]

Just trying to make some estimates here, I know I'll have to visit A2 to get precise on this.  I'm building a Gas Coupe from a 1st Gen (89-94) Nissan 240sx.  The OE Cd is .30 making for a pretty slick shape, but in order to more accurately estimate what speeds may be possible I need to get some idea of what the Cd this car in race trim will be.

Standard Gas Coupe allowed body mods only--
** Air Dam will be added, possibly with a VERY small splitter as the rules don't allow for much here.  It looks like if I come straight down from the leading edge of the bumper with the air dam, that I could still use a 1/4" splitter at the bottom of the air dam.  not sure if that's beneficial or not?
** No windshield wipers, no side mirrors (smooth surfaces/sheet metal replacing both). 
** Minor trim may be removed as noted in the rulebook, this is likely to involve the trim around the front and rear windshield and that's about it. 
** 'Moon Eyes' Wheel Covers all around  (do these really make a difference, or just look the part?)
** Lowered ride height with some rake in the stance front to back (should aid in keeping air out from under the car and providing some downforce right?)

I think that's about all I can do in Gas Coupe to slicken things up a little bit.  But what I'm completely unable to hypothesize is how much of a difference to that .30 Cd this might make?

Anyone have any experience with actual measurements of this and able to shed any light here?  Am I talking about a .01 or .02 reduction in Cd?  Or something a bit more dramatic than that?  I know the only way to know for sure is to make the changes and measure in a wind tunnel... but I also know some of you guys have done exactly that and might be able to give me a rough 'rule of thumb' to fill in these numbers for me until I can make that happen?

[PatMc]

Just ramblin:

.30 Cd doesn't mean a lot.  You multiply that by the frontal area to know what the drag is going to be.

I'd be stunned if you could drop that more than a couple hundredth in GC.  You can't block the radiator IIRC, you can't run a belly pan, and you cant close off the body panel gaps.  The nose on a 240 is pretty good.  An airdam will mostly stop front end lift.  I'd use .30 for your calcs, then do whatever you can to assist that.

Anyone feel free to correct me.

[PatMc]

If I can talk Wifey into it, I can get a free car that is .25 cd in factory trim with 5 sq ft of frontal area.  A streamliner with a license plate.

[FoundSoul]

I understand Cd is only part of the equation, but it's the part of the equation I can do a little something about as I can't really shrink frontal area at all...

Anyone ever built a DIY miniature wind tunnel and take measurements using a model?

[FoundSoul]

What lead me to start thinking on this was last week out on the salt I met a gentlemen running the same chassis in Gas Coupe who stated that the Cd was 'significantly lower than stock in GC race trim', and it got me wondering just how significant. 

It's only a few mph difference per 100th of Cd, but that's a few mph...  just trying to get my estimates as accurate as possible, and to gain a better understanding of all of this...

Been crewing for customers out there for years, just helped Lee Sicilio's team record with an exit speed of 283mph last week and every year come back more excited than the last.  Can't wait to get my own car out there and drive instead of just crew/help others.

[4-barrel Mike]

This quote from Mike LeFevers may help:

...the stock c/d for a '82-'92 Firebird is .32, lowering the car and removing the side mirrors and detailing the trim reduces that to about .28... 

 

Mike

[SPARKY]

its nose helped some also In reading Turks article in HR  he got helped is efforts from how he did how he did the rear spoiler!!

[panic]

Not on point, but I have a gut feeling that spacing the wheels to place the sidewall as close to flush with the fender cut-out is a tiny plus.

[FoundSoul]

http://www.hotrod.com/techarticles/body/113_0703_car_aerodynamics/viewall.html

Excellent article, thanks!   Assuming those numbers are accurate that's pretty exciting....

If I can talk Wifey into it, I can get a free car that is .25 cd in factory trim with 5 sq ft of frontal area.  A streamliner with a license plate.

If you can't talk your wife into I'm pretty sure I could get mine onboard, just let me know where to pick it up .

[PatMc]

http://www.hotrod.com/techarticles/body/113_0703_car_aerodynamics/viewall.html

Excellent article, thanks!   Assuming those numbers are accurate that's pretty exciting....

If I can talk Wifey into it, I can get a free car that is .25 cd in factory trim with 5 sq ft of frontal area.  A streamliner with a license plate.

If you can't talk your wife into I'm pretty sure I could get mine onboard, just let me know where to pick it up .

I will let you know.   

Guess I was wrong about being able to make big changes.  Sorry.  The most aero mass produced (over 500/yr) car was the Chevrolet EV1.  Pity they crushed all but three of them. 

[FoundSoul]

Based on that article, I'm figuring on being able to get my Gas Coupe (OE .30 Cd) down to about .25 Cd, which is a big difference.  About 11mph to be exact, from 187mph potential to 198mph.  It appears it may be possible to further improve on that....

I may build a DIY mini windtunnel and some model cars to make body mods to and measure the differences.  I think I could learn a lot in small scale that will save me some time/money when I scale it up....

[FoundSoul]

Thanks-- The tufting article is interesting, though I'm guessing I'd need to do more reading to better learn how to interpret the results as well as what to do to address turbulence issues.  There's the added issue of the fact that my race car is far from streetable and also not quite together yet...  I have a nephew that has a similar chassis though in stock form that I might be able to use, gotta help him replace or repair the engine first though-- it's his first car and he just popped a coolant leak, didn't notice, and cooked the head gasket

CFD might be interesting -- I'm guessing you need a complete 3D model of the vehicle in question-- how is that generated?  I'd be interested in know what it would cost to do something like this though I'm not sure if it will be the right route for me, still, tell me more....

I intend to visit A2 as they're only one state away from me, but that's after I do some small scale or other testing to try and pick the low-hanging fruit initially, as well as to help formulate a plan for what to test at the tunnel.

[hotrod]

If you have a location near your home that is known for being windy and winds are generally from the same direction, like an on shore wind on a scenic over look or something similar you can do basic tuft testing by parking the vehicle facing into the wind and taking video or lots of still picture of the car as the wind blows. It also allows you to sit the car at a slight angle to the wind to see what the airflow does with a slight cross wind.

Burt Rutan and even major manufactures have done simple aero tests by placing items on fixtures attached to the front of a car or truck. This for example is a way you could see how much drag a side view mirror had, by measuring the pressure exerted by a side view mirror attached to such a fixture at say 70 mph.

Tuft testing is pretty basic you are looking for attached flow and separation points. The tufts pinned flat to the surface all pointing in the same direction indicate attached flow with little turbulence. Wildly gyrating tufts sticking up in all different directions indicates turbulent detached flow (bad). The point where the tuft behavior changes is the area where the flow detaches from the surface and you need to see if there is a logical reason why the flow separates at that location.

It is also useful to determine the actual direction of the local flow across the body, it is not always in line with the center line of the body. On the sides it is often at an upward slant especially near the rear C posts as the flow pours over the rear edge of the C posts and tries to fill the area behind the rear window.

In some areas you may find tufts pointing in the opposite direction that the general air flow is moving. This indicates that the air has set up a reverse flow (much like an undertow on a beach) where the air turns and forms a rotating cylinder (rotor) just behind a separation point. This behavior can cause air scoops intended to pickup cooling or engine air to not do their job if they are poorly located in an area of reverse flow.


If you watch the tufts behavior and thing of how a streams water swirls around rocks, and apply that same concept to the tuft motion you can usually quickly figure out what the air is doing near the car body surface, as it flows just like water at the speeds we are concerned about.

Larry

[Jon]

I may build a DIY mini windtunnel and some model cars to make body mods to and measure the differences.  I think I could learn a lot in small scale that will save me some time/money when I scale it up....

It will be pretty hard to replicate and measure small changes on a scale model unless the model and the measuring equipment is extremely accurate IMHO.

Then there is the Reynolds number issue that effects scaled models.

Tuft testing or oil streaking is very intuitive and visual, not measurable though.

Cheers
jon

[manta22]

I may build a DIY mini windtunnel and some model cars to make body mods to and measure the differences.  I think I could learn a lot in small scale that will save me some time/money when I scale it up....

It will be pretty hard to replicate and measure small changes on a scale model unless the model and the measuring equipment is extremely accurate IMHO.

Then there is the Reynolds number issue that effects scaled models.

Tuft testing or oil streaking is very intuitive and visual, not measurable though.

Cheers
jon

Oil streaking works quite well. As an example of how careful observation can show aero flow, John Horsman solved the handling problems that had plagued the Porsche 917K by noting a pattern of smashed gnats on the bodywork during a practice session at Zeltweg. The rear was generating lift and he cured that by adding a sheet of aluminum and a few pop rivets.

Regards, Neil  Tucson, AZ    Lake Tahoe, NV