Info & video - Opel Calibra - Cd 0.26
 

Every once in a while, GM has shown it's capable of hitting one out of the park, aerodynamically speaking. Unfortunately, this one never saw North American roads.

Here's a pic of the Opel / Vauxhall / Holden Calibra, for those who don't know it:


Launched in 1989, it was designed by an American who headed GM design in Europe. At the time, it was apparently the most aerodynamic production car available anywhere. (Source: Wikipedia)

Obviously they started with a decent shape.

But the designers paid a lot of attention to optimizing the details as well, such as the grille openings: only the 8-valve version of the car had the Cd 0.26 figure - the 16v engine needed more cooling, which meant a bigger hole in the front, and a worse Cd of 0.29 (other contributors to the change were probably things like wider tires, chunky rims, etc).

Here's another line about the small and sometimes counter-intuitive details that the Calibra engineers dealt with:

This YouTube vid contains a bit more of a look into what makes the car slippery:

I find it very amazing that the little vent increases drag by over 10%.

Pretty sweet Cd on that thing, and it doesn't look half bad either.

It might help once you realize how big the flat plate area (i.e., the CdA) of the car is. It's not that big - 0.30 equates to 30% of the car's frontal area. Which is probably on the order of 6-8 square feet. Compare other vehicles here.

If you understand that drag coefficients for really, really, ugly things (aerodynamically) such as the Eiffel Tower are 1.8-2.0, and that routing air through the engine bay, under your car etc is probably just as ugly, what is 10% of the CdA and hence what is the actual area of grille that would be blocked to achieve the difference?

10% of 7 square feet is 0.7 square feet. If we take the drag coefficient of that particular area as 2 (not a bad assumption), you only need 0.35 actual square feet of grille to get the difference. That's the difference a "little vent" can make.

Very nice video. 0.26 is amazing for a car from nearly 20 years ago.

That's a really interesting way to emphasize that point.

Yes, it's a good way to think about it. Considering that an airfoil has a Cd of around 0.05, and good cars have a Cd of around 0.30, the airfoil has 1/6 the drag of the car and 1/20 the drag of a flat plate. It's as if the airfoil is almost invisible to the air.

Of course, being near the ground has issues, you can get to 0.11 or so but to get better you have to start approaching an airplane by having cowled wheels and hoisting a fuselage as high as you can without toppling in a corner. But still, a Cd of 0.11 is still 10 times better than a flat plate, which is what a motorcycle or upright cyclist is basically.

Another trick I learned from airplanes which the designers of the aptera seem to have learned is that you put the trailing edges of airfoils next to the leading edges of other aifoils. See this plane. If you google the AR-5 you can find more about it - total CdA of 0.88 square feet. I think what happens is that the leading edge of an airfoil creates high pressure, the trailing edge creates low pressure, and you minimize pressure difference by doing this, and hence minimize drag.

Daox - here's another bit of info that emphasizes the aero impact of the cooling system. On a study of an un-named car with Cd 0.29, it makes up the biggest fraction of total drag:

http://us1.webpublications.com.au/st...4/2455_7mg.jpg

This is from the Autospeed article, linked above. Worth a read: Modifying Under-Car Airflow, Part 1

I was practising archaeology throughout this aerodynamics forvm when much to my delight (I'm the lucky owner of not one but two Calibras :D ) I found this old post. I can't help but resucitate it :) .

Wayne Cherry, in collaboration with Opel Designstudios' then chief design engineer Erhard Schnell.

http://img37.picoodle.com/data/img37...cm_683dfdb.jpg

Correct, world record most aerodynamic production car in 1989, and for a decade afterwards until the Insight arrived.

Info on that towards the end of this pdf:

http://img34.picoodle.com/data/img34...cm_87f06ba.jpg

http://img37.picoodle.com/data/img37...cm_d19fd36.jpg

That's an incorrect statement I've seen everywhere in the net, probably the result of blind quoting by uninformed journalists without first hand experience on the car. All Calibra versions have identical front bumpers with equal size openings for cooling.

Tyres are the sole culprits (195/60-14 in the 2.0i versvs 205/55-15 or 205/50-16 in the 16V/V6/Turbo).

:D I guess the gentleman who wrote that was more used to SUVs and minivans than sports cars.

Yeah, he was definitely not in top form and suffered abnormally high perspiration :) . No such issues in the Calibra, the ventilation works as in any other car.

On the contrary, the Calibra is one of the cars with better all-around visibility I've driven (excluding top-down cabrios).

Well, it undoubtedly affects the Cd when open, but "destroys it" seems an exaggeration to me after reaching 235kmh / 146mph http://www.foren4all.de/images/smilies/mauer.gif in the Autobahn with sunroof deployed.

http://img26.picoodle.com/img/img26/...sm_bd2ba78.jpg

That's quite true :). Also the large sloped windshield fogs more easily than other cars.

http://img26.picoodle.com/img/img26/...sm_8a2c687.jpg





Some comparisons to get perspective (Cd x frontal area in square meters = SCd, the real value):

http://tbn0.google.com/images?q=tbn:...da_insight.jpg 2000 Honda Insight: 0.25 x 1.905 = 0.47625

http://tbn0.google.com/images?q=tbn:...dHoldenM_m.jpg 1989 Opel Calibra 2.0i: 0.26 x 1.93 = 0.5018

http://tbn0.google.com/images?q=tbn:...AudiA2/Ex1.jpg 1999 Audi A2 1.2: 0.25 x 2.18 = 0.545

http://tbn0.google.com/images?q=tbn:...yota_prius.jpg 1997 Toyota Prius: 0.26 x 2.16 = 0.5616





Pictures of my babies. The 2.0i (17 years & 223000km / 140000 miles and still in top form):

http://img98.imageshack.us/img98/228...evla018cc3.jpg

http://img34.picoodle.com/img/img34/...Hm_1a3d456.jpg


The Turbo AWD:

http://img26.picoodle.com/img/img26/...Lm_9fe0717.jpg

http://img26.picoodle.com/img/img26/...sm_384ba7b.jpg


(I know I know, guilty as charged, it's crazy to have two similar cars in the same colour :rolleyes: )

http://img82.imageshack.us/img82/369...elvm0011kt.gif


At your service for any quest for more information on these streamlined cars :) .

What happened to the video?

YouTube - Vauxhall Calibra Road Test High Quality

http://www.youtube.com/watch?v=vDX5FMznyME&feature=related

[QUOTE=newtonsfirstlaw;3204]

Another trick I learned from airplanes which the designers of the aptera seem to have learned is that you put the trailing edges of airfoils next to the leading edges of other aifoils. See this plane.

Nice site, but I don't see anything about paired airfoils except a line about the ailerons being a few thou thicker than a pure contour would have them. I don't recall tandem shapes on the Aptera, either. Two optimum shapes would not have a beneficial interaction, but using them might reduce surface area and skin friction.

So---Is the effect of paired airfoils similar to the effect of a car pulling a trailer?
Ray Mac

Ah the calibra!

i'd love to have one but it's near impossible to find a decent low milage one, without some hidious "aerodynamic" upgrades (what makes a person think one of the most aerodynamic cars in the world would benefin from a widebody kit....)

but than i'm pretty close with the vectra wich was also designed by Wayne Cherry... the calibra was based on the same platform and shares the dashboard so i get a little bit of that calibra esperience, dispite of the slower pase
http://i77.photobucket.com/albums/j5...a/DSC01959.jpg
looks bulky next to the calibra, but it's actually still quite low compared to newer cars...
when you considder it replaced this:
http://i77.photobucket.com/albums/j5...a/DSC01719.jpg
you can see what a revolution it must have been at the time, and with a 0.29Cd was great in 1988 but it's still quite good today

@TELVM
do any of your cars have an undertray? i have yet to find any picture of the calibras undertray and i heared not all calibras had those. since the similaritys between the calibra and the vectra i wonder if a calibra undertray could (be made to) fit my car... i do have a custom undertray already but it would still be nice to see what opel originally came up with


for people with a strong stomach here's what some people do to a calibra
http://i77.photobucket.com/albums/j5...p_calibra1.jpg
eum, right...

Either blatant ignorance or that curious philosophy "I don't mind it's slower if it looks faster/cooler" (as in the 'huge wheels syndrome').

Quite true, Calibras and Vectras-A shared platform, engines, and most of the streamlining. Some mating experiments have proved DNA compatibility :D:

http://farm4.static.flickr.com/3266/...0a0cc799c4.jpg

In fact the Vectras were slightly favoured in the engine department, as the '88-91 Vectra-GT/Cavalier SRi got the 20SEH version of the SOHC 8V engine with 130HP, while all SOHC Calibras had to make with the 115HP C20NE. The camshaft, pistons and ECU now inside my 8V Calibra came from a Vectra-GT donor.

My cars came without OEM undertray. I've only seen it under 16V C20XE Calibras, and only rarely, as people tend to discard that piece to get easier access to the engine from below, or because it brokes with age.

I'm not sure if it's a plug&play affair in the Vectra-A, but with some DIY fiddling it should fit. The reference is GM 90 487 258 or EPC 2 12 626, if I remember correctly it costs about ~350€ here in Spain.

http://img174.imageshack.us/img174/5...owerab9rh6.jpg

http://img380.imageshack.us/img380/1...ecarterzv2.jpg

This is the detachable central piece for access to the oil pan:

http://img15.exs.cx/img15/7389/011815219ct.jpg

http://img89.exs.cx/img89/609/011815220vs.jpg

TELVM thanx for the pictures the undertray. it's interesting to see the undertray doesn't cover the sides before the wheels, like my custom job, but it does extend below the engine... mine stops just before that, but i might extend it at some point.
350€ is a bit to much for something that's likely not to fit well...

mine cost under 20€ all brackets and screws included.
http://i77.photobucket.com/albums/j5...y-470-0608.jpg might look, or rather work good on a calibra too, as you can't really see it...


http://farm4.static.flickr.com/3266/...0a0cc799c4.jpg

that's awsome... i've been wondering if that was possible... i'd try my hand at something like that, but chances of getting that sort of conversion roadlegal over here are slim... to bad becouse its probably one of the easyest conversions possible... on a hatchback vectra it would certainly confuse people

You're welcome :) .

I agree, no need to feed Opel spare parts vultures, for a few euros the DIY enthusiast can make a very decent undertray. Yours is a good example.


Which engine is under your bonnet, Lunar?

my vectra is powered by a 1.6i X16SZ has about 72-75 HP:D... not really a spectacular engine on paper , and when i got the car i was a little worried it would be a slug, as some people call it underpowered, but i think they're used to more powerfull engines and are a little lazy with the gearchange.

it's a great engine for economy that will keep up with normal traffic, acceleration from 0 to 50 km/h is very fast, but after that is becomes a little slower because of the tall gears... on the highway however it's a very lively car, and you feel it wants to go fast, but without wanting to go too fast. some say the 1.8i was a better engine for the vectra, but if you don't expect a racecar the 1.6i is a very nice engine...

nope unfortunately not... i've been looking in retrofitting one wich i think isn't all that hard as i believe the wireing is there, but i need to find a computer for the engine or find out if it can be reprogrammed... do you have one, and is it usefull?

Correct. My 8V Cali also came without onboard comp in principle, but all the wiring was there up to the yellow plug. I just scavenged a comp and center console button from a german 16V Calibra, changed its chip for a 2.0 SOHC one, kicked off the nasty analog clock, plugged comp, installed & wired button, and job done.

http://img70.exs.cx/img70/9770/OrdeadorMisil002.jpg

http://img70.exs.cx/img70/4116/OrdeadorMisil003.jpg

I have several Opel comps, but unfortunately all their chips are for larger engines than yours. To work in your X16SZ Vectra a specific chip would be needed, which I lack as for now. Any comp, provided the correct chip, would do, so if I ever find a X16SZ chip I'll let you know ;).

The onboard computer displays instant L/100, average L/100 since last reset, estimated remaining range and average speed. Very useful.

http://img186.imageshack.us/img186/5...burgosavo5.jpg

thanx that would be very helpfull. i've seen one or two coputers for offer on the interent but they where for the wrong engine... but maybe i should get one anyway if i can find a good one.

My first tuft test. The Calibra is OEM in this area, save for the deleted roof antenna. The experiment was somewhat affected by cross winds.

YouTube - Opel Calibra Tuft Test

Lessons learned :) :

- White tufts match badly with clear skies & clouds.

- A lower angle and a bit more zoom in the camera wouldn't harm.

- Longer and more numerous tufts wouldn't harm.


I think the gentlemen who designed the Calibra earnt their money. Please feel free to comment/criticize.

OMG This takes me back! Top Gear was my favorite program on TV as a Kid. IIRC the presenters name was Chris Gofee(sp). The G registration on the car shows it was registered in 1990. I was 12... :)

i found some details on the calibra bumper with with numbers about the size of the cutout

of course it did not have such a large dam as your can but it's more detailed than what i've found about other cars

the original article is in german
but i've ran it trough yahoo bablefish wich makes it somewhat readable in english with some goodwill and imagination.

interestingly the research diagrams show that they did a great deal of optimising, increasing the size of the cutout untill they noticed an increase in drag again.