Lower Cd, but larger wake

[Cd]

How is it that cars such as the '92 - '95 Civics have a slightly higher Cd ( .32 ) in 2door sedan form over the .31 of the hatchback ?
Even though the airflow separates on the back of the sedan due to the steep angle of the window, I would think that the size of the cars wake would make up for this and lower the Cd.

[gone-ot]

...no pictures in your post.

[ChazInMT]

Because the hatchback more closely approximates the "Ideal Aero Template". The larger back surface area (So counterintuitive that 90% of the people in this forum who think they know better will argue) does not matter, it is how close we come to the template that matters, rear surface/wake area be damned.

[AeroModder]

It's the virtual boattail effect.

[Frank Lee]

Originally Posted by ChazInMT

Because the hatchback more closely approximates the "Ideal Aero Template". The larger back surface area (So counterintuitive that 90% of the people in this forum who think they know better will argue) does not matter, it is how close we come to the template that matters, rear surface/wake area be damned.

Er... hah?

The rear surface/wake area is THE DEAL MAKER.

BTW, how are the roof spoilers coming along?

[JasonG]

Originally Posted by AeroModder

It's the virtual boattail effect.

Look at your Avatar Cd.
It may be a larger wake area, but it reattaches smoothly.
The reverse eddies are what create the backwards suction we call drag.

[Cd]

Originally Posted by JasonG

Look at your Avatar Cd.
It may be a larger wake area, but it reattaches smoothly.
The reverse eddies are what create the backwards suction we call drag.

And more eddies equal more drag and a higher Cd right ?
The sedan ( actually coupe ) would have less of these eddies due to a smaller wake, as well as having the air re-attach just behind the trunk / boot.

Perhaps it is all just a misprint ?

I used to collect much of the same literature as Aerohead.
I had a sales brochure from Saturn that had the Cd figures for their cars and remember being puzzled at how that the Saturn station wagons had the same Cd as the SC-2 .
( it was either the SC-1 or the SC-2 . I can't remember after all these years. )

[AeroModder]

I think you need to start looking at CdA figures as well. The Cd can be the same for large and small cars, but when the frontal area is factored in, the drag numbers change.

[Cd]

It's the exact same car. The only difference is one is a coupe and the other a hatchback.
Frontal area stays the same. Same with the example of the Saturn coupe verus station wagon.
on early 90s Saturns, the front end was the same on both cars, as well as the frontal area.

[JasonG]

The way I'm seing it is this,
The coupe/sedan rear shape creates more eddies creating a negative pressure area on the rear window and trunk.
With the wagon, the airflow seperates more cleanly. This leaves a neutral pressure zone without the reverse holding currrent.


As a Saturn nut, the sc1/sc2 are the same body, the engine is the difference. Single vs dual cams. Same goes for the sw (wagon) and the sl (sedan, sl was the manual everything stripped model, 40 mpg hwy in a 1995).
Sorry to drift OT.