I was wondering just how much performance is lost with the taga top off on a C4. Was any testing ever done? Is it noticeable from 0 -60 ? Top end ?. I would think the drag would be increased greatly with the air being traped behind the seats.

 

There is going to be slim to none in increased drag at speeds under 120mph. First off, that little pocket the targa opens up is going to create a space of low pressure that the high pressure air is going to glide right over. It is arguable that it will even increase efficiency upon the same principle as a golf ball. The intents create a surface of low pressure areas that decrease drag. Thus, a regular golf ball will fly farther than a smooth golf ball.

Performance wise with a stock c4, IMO we are talking about a difference measured in 0.00009 of a second if you were to do a quarter mile run. And again would it be that much faster or slower? Even if you have your launch down to a consistent science, the variances in that action affect the outcome ten fold vs. targo/no targa. In short, it is something you shouldn't concern yourself with.

 

0-60 times may actually be improved if the targa was not on/in the car at all, due to reduced weight. Any increased aero drag due to a removed targa would probably not be very much below 60mph.

But above 60mph, top speed would most certainly be affected with the top off, as well as high-speed stability, due to disrupted airstream flow over the back-end body surfaces. With the targa removed, I get a lot of turbulence at higher speeds and that's got to have some kind of effect -- it's not like the outside air is completely flying over the cabin space, it's blowing inside noticeably.

 

I believe what you are experiencing when you say "it's blowing inside noticeably" is the air in your cabin being rotated though the friction experienced as the high pressure differential passes over your cabin. Think of it like this, the high pressure is red and your cabin air is blue. As the red flows over the top its comes into contact with the blue, and through friction it pushes the blue air in a turbulent motion that you believe is outside air coming in.

Of course there will be transfer of blue and red air, (I'd say the blue air is completely diluted/cycled out several times a minute at 55mph) but its not as significant as you think. The question is: is the friction of the high pressure red air creating more drag over the targa top than it is over the low pressure blue air with that pocket exposed.

Here is a good example, If I am out in 40 degree weather with the targa off, but the windows rolled up and the heater on, I stay nice and toasty because the 40 degree high pressure air is flowing right over me while the low pressure that contains the heat just blows around in my cabin.

Friction over a hard surface, or friction over a low density pocket of air. Which will create more drag? This is a question I don't think anyone here will know.

All this said, I am only holding to what I say when we are at speeds of under 120+ mph. Any higher and I believe the high pressure air has enough force to run into the pocket of low pressure acting as a sort of catch or air brake thus creating drag.

As for the weight reduction +1, If they will let you race with the targa off, then I'd do it.

 

Don't really notice much difference in terms of straight line performance on a smooth road. Rough twisties are a whole different story.

As for the wind buffeting, get one of the various hatch vents available from most of the parts suppliers. For about $30, it works wonders with noise and turbulence during top off performance, locking the hatch in an open position about 2" wide. Only downside is some annoying reflection off the top of the rear bumper if you have a lighter color paint scheme.

 

I don't to hijack the thread but, I have the same question with regard to convertibles. I wonder whether the wind drag is greater with the top up or with it down.

 

I'd say much more drag. The 2012 Mercedes-Benz SLK350 loses 14% fuel efficiency with the top down.