S30 Aerodynamics not really that bad?

[rudypoochris]

I have been doing a little research about the aerodynamics of the Z. It deffinately falls down alot on the amount of lift generated, heat build up, etc. I am focusing on the Cd though, which was reported to be around .43 to .46. That is not exactly good considering .28's-.32's are becoming the standard for the average car/sedan. Is our .46 really that bad though? Nah.

 

From what I can gather CdA is more important than Cd when talking about overall drag.

 

Cd - Simply refers to the amount of drag per sq unit

CdA - Takes into account the square units (frontal area) which is a much better representation of what is going on.

 

The CdA of an S30 is 6.169 using ft^2, as stated in the QUARTER2.xls program that can be downloaded from here. Meaning, for those that care, the frontal area is 13.41-14.35 sqft.

 

Since the Z is much smaller in frontal area than most modern sedans, the drag really isn't that bad. To put it into perspective, here is a list (from wikipedia.org) detailing some modern cars CdA's using ft^2 aswell:

• 5.10 - 1999 Honda Insight
  
• 5.71 - 1990 Honda CR-X Si
  
• 5.76 - 1968 Toyota 2000GT
  
• 5.80 - 1986 Toyota MR2
  
• 5.81 - 1989 Mitsubishi Eclipse GSX
  
• 5.88 - 1990 Nissan 240SX
  
• 5.92 - 1994 Porsche 911 Speedster
  
• 5.95 - 1990 Mazda RX7
  
• 6.00 - 1970 Lamborghini Miura
  
• 6.13 - 1993 Acura NSX
  
• 6.17 - 1995 Lamborghini Diablo
  
• 6.27 - 1986 Porsche 911 Carrera
  
• 6.27 - 1992 Chevrolet Corvette
  
• 6.54 - 1991 Saturn Sports Coupe
  
• 6.40 - 1990 Lotus Esprit
  
• 6.57 - 1985 Chevrolet Corvette
  
• 6.77 - 1995 BMW M3
  
• 6.79 - 1993 Toyota Corolla DX
  
• 6.81 - 1991 Subaru Legacy
  
• 6.90 - 1993 Saturn Wagon
  
• 6.93 - 1982 Delorean
  
• 6.96 - 1988 Porsche 944 S
  
• 6.96 - 1995 Chevy Lumina LS
  
• 7.02 - 1992 BMW 325I
  
• 7.04 - 1991 Honda Civic EX
  
• 7.10 - 1995 Saab 900
  
• 7.14 - 1995 Subaru Legacy L
  
• 7.34 - 2001 Honda Civic
  
• 7.39 - 1994 Honda Accord EX
  
• 7.48 - 1993 Camaro Z28
  
• 7.57 - 1992 Toyota Camry
  
• 7.69 - 1994 Chrysler LHS
  
• 7.72 - 1993 Subaru Impreza
  
• 8.70 - 1990 Volvo 740 Turbo
  
• 8.70 - 1992 Ford Crown Victoria
  
• 8.71 - 1991 Buick LeSabre Limited
  
• 9.54 - 1992 Chevy Caprice Wagon
  
• 10.7 - 1992 Chevy Blazer
  
• 11.7 - 1993 Jeep Grand Cherokee
  
• 16.8 - 2006 Hummer H3
  
• 17.4 - 1995 Land Rover Discovery
  
• 26.3 - 2003 Hummer H2

Yes, we did best the H2. We are mroe aerodynamic than any of those cars in bold. Feel free to PM me CdA's to add to the list.

[thehelix112]

I'm intrigued by your comment on heat buildup. Care to expand?

 

Dave

[Guest Mike]

Yeah... heat expansion please

[rudypoochris]

I have read that Z's do not evacuate heat very well from the engine bays. Could be wrong though, as it may seem.

[Lunar240z]

I think the confusion is coming from the mention of heat, in a discussion that regards areodynamics. Which seem to have nothing in common.

[jt1]

Rudy, that's interesting info, thanks for posting it. I knew the cd of the S30 was pretty high, but I didn't know it was per sf. So with a small frontal area it's not as bad as it seems.

 

Lunar, heat and aero are very closely linked. Air flowing over a surface heats the surface, sometimes not enough to matter, sometimes enough to make a huge difference, like F-16 wings.

 

John

[Xander]

where does the s130 end up in that list? great information. I have always wondered what the frontal area of the s30 was. I can input this data into the roaddyno software wich I use to make more accurate power curves.

[Mikelly]

I commented a while back that although not ideal, the S30 aerodynamics numbers are taken with the factory body, no aero add ons such as an airdam or sideskirts...

 

For comparison, check the numbers on the new Viper Coupe. The CD is .39.

 

http://www.caranddriver.com/previews/9018/2006-dodge-viper-srt-10-coupe.html

 

Amazing as it seems, this particular car does well with not-so-great aerodynamics...

 

Mike

[Guest Mike]

Add hood and/or side vents;-)

 

I have read that Z's do not evacuate heat very well from the engine bays. Could be wrong though, as it may seem.

[Guest Mike]

The question I have is... at what speed does this really begin to matter... 25MPH... 50MPH... 100... 150? And how much difference does this make at the speeds we're likely to care about? Obviously, a Humvee will suffer greatly at just about ANY speed because it's a frickin' giant cinder block. At some point the differences should be negligible for our purposes.

[ZROSSA]

Mike, check your fuel ecconomy. There is a speed at which every car starts to use a lot more fuel. Thats the speed at which aero comes into play. For my daily driver its about 105kph say 67 mph. Might be a little higher for a car with better aero, mines a truck.

 

Douglas

[rudypoochris]

I believe the heat build up and the aero have alot to do incommon. Simply because the air under the hood is not evacuated effectively which causes it to back up the flow infront of the radiator and create a high pressure zone under the hood. High pressure means the air isn't going to want to flow through the radiator as easily and to an extent lift will be created. Atleast thats what I gathered.

 

As for the S130, it has a much lower Cd, somewhere in the high 3's. I found a post on Zcar.com that says 79 models were 3.9 and 75-78 was 4.3. Not sure I trust it, but the 280zx had a better Cd by far. Not sure about the frontal area though.

[Guest Mike]

ZROSSA... This is my daily driver and it only sees speeds above 60 MPH about 60 miles per week. It just doesn't matter much in my case. I suppose it might make a difference if I could shave a few tenths of my quarter mile ET;)

[74_5.0L_Z]

The number that I used in my Quarter2.xls program should not be taken as gospel. It is a number that I arrived at by analyzing the coast down graph published in the 1970 review of the 240Z by Car and Driver.

 

What I did was pick two point on the coast down graph (80 mph and 20 mph) and estimated the slope of the curve at those two points. The slope of the coast down curve at any point corresponds to dV/dt. Using the instantaneous accelerations at those two points, I solved the following :

 

m*dV/dt=CdA/2*Rho*V^2 + Cr*V + C

 

In this equation dV/dt was estimated from the graph at two points.

V is the velocity at the two points chosen.

Rho is the density of air

C is a constant value that I used to account for the force needed to start the car moving from rest on a flat surface.

 

The variables CdA and Cr are the drag coefficient times frontal area and the static rolling resistance. The units of CdA are ft^2 and the units of Cr are lbf/mph.

 

One factor that I neglected in my original analysis was the inertia of the wheels and tires. To account for this the inertia of an original equipment tire, wheel and brake assembly would have to be measured and the equations would need need an additional term.

 

m*dV/dt + I* dw/dt = CdA/2*Rho*V^2 + Cr*V + C

 

where the term I x dw/dt is the mass moment of inertia of the rotating parts times the change in rotational velocity.

 

Although I was trying to be as accurate as possible, it should be remembered that I made the Quarter2.xls program for fun. Even neglecting the inertia of the rotating assemblies, I believe the number to be accurate within +/- 15%.

 

As stated on the downloads page, I welcome input to increase the program's usefulness and accuracy.

 

Dan

[Guest Mike]

Rudy / Dan,

 

This is an interesting thread. I believe you did the right thing in leaving wheels, tires, rotatonal mass out of your equation. It seems that these should be left to a separate formula due to the inumerable variations possible. Once the drag coefficient is found for the body... and the tire/wheel rotational mass, drag, and friction is calculated... these two factors can then be used in a final equation.

 

Very cool.

[rudypoochris]

Maybe someone can CAD in a picture of the Z's frontal area from the FSM (http://album.hybridz.org/showphoto.php?photo=2123&size=big&cat=) and somehow calculate the total area in sqft. I could put it into AutoCAD, but I don't know how to count the units up. 14-15 units looks like a good guess though, in any case it still is not as bad as one might initially think.

[JIM73240Z]

the frontal area is 17.78 sq. ft. per acad and the chassis pic. i traced the outline of the car and went down as far as the lower valence. add a air dam and i estimate a total of 20.7 sq. ft. depending on the dam used.

 

hope that helps

 

jimbo

[veritech-z]

Mike, seeing as you have a GTO kit, your Cd is probably totally different than mine, with a bumperless front end and the fiberglass BRE style spook...when you guys are calculating frontal area, are you accounting for the firewall and windshield, or just the leading edge of the hood to the ground? Does that stuff matter? I would think it does, but from what I've read of aerodynamics, it's generally counter to what I think makes sense...

[Dtsnlvrs]

Very good write up. With the heat issue though....It is not that the air builds up and spills back into the front, the air actually flows under the car. The heat becomes a factor when the temperature of the hood gets way over ambient. It affects the boundry layer on the top of the hood, and thereby affects the drag on the car. As far as getting rid of the heat....just remove the cowl to hood seal....instant 15-20 deg temp drop on a hot Alabama day.

[Guest Mike]

MAX... That's true. The aerodynamics will be slightly different. I read somewhere the Ferrari 250 GTO was one of the first cars tested in a wind tunnel and this affected body style. Of course the kit cars are slightly different than the original. Probably the biggest difference between an original GTO and a kit car, regarding wind drag, is the taller more vertically oriented windshield.