by Robert Loszewski
© February 2013
No use without permission, All Rights Reserved
Dan Adovasio: One of the things I like about this Stingray is the functionality of all the air intakes. They actually cool things, have exit points, and they're not just black pieces of plastic stuck somewhere for a look.
Kirk Bennion: Yes. We worked real hard on all of those inlets. Those are viable, functional pieces. The airflow target was eight cubic meters of airflow per minute at 45 miles per hour, and that was necessary to cool the automatic transmission and rear differential.
I love having all of those functional details, but I'll tell ya, we worked hard on optimizing the function. It's easy to put something there and say, "yeah it pulls in a little air", but when you start putting numbers against it, and you've got to get this much airflow because you've got to be able to cool a heat exchanger that's this big, then it gets tough.
You want to grab air, but you also don't want to increase the drag of the vehicle either. There's still that black magic part of aero where we still don't know everything. We've tried some of those big scoops and it didn't help.
With that [air] inlet, we did a lot to minimize the section around the opening. We maxed its width. Each one of those vanes is strategically placed and tilted per the airflow, so we're optimizing that flow path. The other thing, if you notice, there's like a diagonal weave to that part that fits in. The air will actually come in straight at the bottom. To get to that back corner, it's actually coming through the top and then down. So we had to be able to flow the air from the front and the top as well, and then protect it. We didn't want to have someone park the car somewhere and all of a sudden have chestnuts filled up in their heat exchanger!
[Regarding the mirrors] The goal with mirrors is that you want to keep the air attached for low Cd and then you're also trying to manipulate the air for wind noise. These mirrors are better (than the C6 mirrors). The passenger side mirror is 0.004 counts of a drag hit. So you're looking at something that's a little over 1% of the drag of the vehicle.
Mirrors are hard. We spent a lot of time. Defining where that air attaches to the mirror is so critical - we would use ink drops. We would put rows of ink drops, and then run it to the technician and say "hit the button!" before the ink dries. Then the ink would flow and we would tune the surfaces to do that. We didn't just do that one time. We did it about 30 or 40 times where the whole side of the car was starting to get dark. It was starting to get this black ink spray down the side of it.
The base of this new mirror is actually thicker. Being critical of today's car, we felt that in certain conditions, we had too much mirror vibration and mirror shake. So we actually made the base thicker, which meant that we had to shape it, optimize it and offset it. We were able to get the Cd out of it. [However, with] the frontal area calculation - we added a little bit to that. We were all about trying to minimize the frontal area. So dropping the roof gave us some of that back, [as well as] manipulating the head of the mirror. At the end of the day, this car actually has a smaller frontal area, and that's with a one inch wider track than the previous car.
Corvette Action Center: What's the Cd of the C7 vs. the C6?
Kirk Bennion: We're still finalizing our numbers on the car. In the aero-developmental process, we go from scale to full-size clay to IVER [Integrated Vehicle Evaluation Review - the first prototype cars]. Things that we refine and establish through the build of the vehicle: the big picture - you have to make the aero numbers stick. So what worked in clay, sometimes doesn't always work the same when you get a production part. You [may] find out there's an interaction there, so we sometimes end up going in and adjusting that production part to give us the numbers we saw in clay. When you get that finite; at that threshold, things become sensitive and critical. You can't put the aero in, and just walk away and think, you don't have to worry about that...you don't have to check it again...but you end up checking it again and re-establishing it.
Corvette Action Center: So every little change you make has the potential to make a drastic change down the line?
Kirk Bennion: Corvettes are are super sensitive [in] some areas. I could go up and we could have our sculptor pull this out 10 mils and push that down 10 mils and it won't see it. And then some areas like a mirror, you move it up a mil in the wrong direction and the noise goes up, and the drag [increases].
Jake Drennon: Back on the C6 era, when [the car was introduced, there was a lot of discussion about the front license tag] and someone slipped up with that and said "uh, we didn't even think about that." Where are we with the C7 and a front tag?
Kirk Bennion: I would tell you on C6, we wanted a clean appearance. Up until that time, Corvette had integrated plates. When you look at the rest of GM's lineup...actually the rest of the cars in the industry, a lot of cars just have tag offsets. So the thinking was, it's no different than other cars...we should be ok.
When we intro'd that, that's [was] our biggest customer complaint about the exterior of the car - how well you integrate that plate. I would tell you that the Corvette and Camaro customers seem to be lock-stepped. Other brands that we see - not so fussy.
So last night at the reveal, we actually took some shots of what the integrated plate looks like in the grille.
Now the new integrated plate does a couple things. To live on "Corvette Island", you've got to do a couple of things well. So the new plate sits down in the opening, and it actually extends out to the grille bars, and actually improves the Cd of the car, thus improving your fuel economy and giving a nice nesting place for the plate to sit in.
Here's the difference between the C6 and C7: with C6, we had to put the plate in there and we had only one cooling flow target. The idea for that car was, if I was going down the highway and [suddenly] decided to participate in a track event, you could drive that car, hammer it in second and third gear - high rpms...and you'd be fine, but then you had to live with this license plate up in your forehead.
So Tadge established two flow targets. We had a flow target for average daily driving - highway driving, which allowed us to put the plate down in the opening. Now for track conditions, we're telling the customers, "hey, if you're going to run this car at the track, you've got to pull the plate", because at the track, that car is all about pulling the air from that area.
Now one of the things we learned from our race team is, that center air flow - that nine square inches of center air flow is your purest, most forceful air stream. So when you put a plate in there, you help the Cd, but when you're looking for cooling flow at those enormous track speeds, you're blocking off the center of the geyser - where the air is most forceful.
So we've got it designed where it has a quick release. You can pull your plate off. There's two quick release tabs in there, turn it with a screw driver, pop it off and you're ready to go. It's a two minute operation.
Jake Drennon: Speaking of just air in and air out, the top of the hood is for air extraction, and the side vents are extracting air - correct?
Kirk Bennion: [Yes.] Even through our downtime, we were having closed-door meetings on the front end architecture of the car. We said, "we don't have a program yet, but we know that we want to take this radiator configuration, and we want to make it just like our counterpart on the race team".
Corvettes have evolved to where the radiator is laid back and we've had our bottom-breathers. Bottom-breathers, for their day were really good for low drag and speed to a certain extent, but now you've got a [C6] base car that can go 196 mph. When you've got a base car that can go that fast, you can't have bottom-breathers. You'll just fold those air dams under. So we've evolved to this radiator and air dam combination that now needs to be leaned forward. That was the big lesson from the race team. You need to tip it forward and exit the air out on to the hood. In doing so, that reduces the lift, which is huge. It's great for stability at both highway speeds and track speeds, and it also reduces drag of the vehicle and helps optimize the cooling.
Now, you've seen a lot of vents on hoods. If you actually pop the hood [on the C7] over there, this one actually has a duct that picks up 1/3rd of the air going through the radiator and ducts it directly to the hood. It isn't just one of those "we're going to throw a lot of air under the hood and it might come out of here." When we make something functional, we're optimizing its function. We're not just putting it on there to say that it's a real detail. We're doing it as a real detail to where it's optimizing the car.
Changing that front-end architecture was a huge deal.
Jake Drennon: I noticed on the bottom of the front cap, it sort of sticks out there - I'm thinking to hold the front-end down somewhat? Also I understand that there were some regulations that came into play?
Kirk Bennion: Yeah, it's got to be a global car. Part of being that, is you have to design the car to what we call "pedestrian protection" rules/laws. One of them is that you have this lower leg contact area that will catch you - and it's all about catching you first and then, trying to aim the collapse on to the hood.
In doing so, we established that ped-pro contact design in such a way that they will give us some down-force on the front end and a little drag reduction. So they do function on three levels. The industry has kind of evolved where you have to have that. We didn't want to extend the whole chin out and make the car longer so it was still about keeping it tight and keeping the corners back.
Our big win is that we're lower than the C6 hood. We raised the hood to give the Powertrain guys as much room as possible to make their intake merger, but we also lowered the valleys to maintain downward vision [from a driver perspective].