Best approach for aero improvement on sedan roof?

[19bonestock88]

Hey guys, I've got my first question... I've got a 2007 Chevy Malibu, and I've been getting into eco-modding it, and the easiest gains at the moment are aerodynamic... Generally speaking, is it better to keep flow attached to the back glass of a sedan through use of a vortex generator(like EVO 8/9), and use a lip spoiler like a lot of new cars are using, or is it better to build a partial kamm-back"sunshade" to extend the roofline?

Given the steep angle of the back glass, I highly doubt that any sort of attached airflow is present...

[California98Civic]

Unless you have the knowledge and equipment to tune the position of the vortex generator very precisely, the vortex generators will only help if you get extremely lucky with position. The EVO VGs were very precisely tuned. We can't really hope to duplicate that in our garages. That's something like a consensus on VGs among the most engaged and experienced folks on this forum, from what I have seen.

Also a consensus, a kamm is better than a wing, and a boat tail is better than a kamm. I partial kamm off the top of the window and a flat trunk lid spoiler might be a good combination. An airdam, grill blocking, and wheel skirts are also typical recommendations. They seem to work very well in most scenarios.

[19bonestock88]

My plan for tuning the VG was to make a full width version and experiment with its fore/aft position in relation to the upper back glass seal until tuft testing showed attached flow down the back glass... However, given that profile picture, it appears that I should be building a partial kamm-back... I already have done an air dam extension and rear tire spats, but I don't have hubcaps, just the plain steel wheels... Future plans are lowering at same time as strut replacement(by means of cutting small amount from spring), adding at least a partial undertray, rear wheel skirts, grille blocks, and the kamm-back on the roof...

The end goal is to have a car that is still aesthetically appealing, and can achieve 40mpg highway with cruise set and A/C on if applicable... Given that I set my cruise at 80mph, that's a tall order, but nonetheless it's my goal

[oldtamiyaphile]

I think you can buy replica EVO VG arrays ready to stick on. The air flow over that part of the roof of any modern car is going to be very similar so I wouldn't spend too much time re-inventing the wheel.

[Ecky]

40mpg at 80mph is a tall order indeed. My Civic Hybrid, which is fairly aero for a stock sedan at ~0.28cd and has an incredibly efficient engine, gets between 60 and 70mpg at ~55, but is under 40mpg at 80mph.

[California98Civic]

Originally Posted by oldtamiyaphile

I think you can buy replica EVO VG arrays ready to stick on. The air flow over that part of the roof of any modern car is going to be very similar so I wouldn't spend too much time re-inventing the wheel.

But the VGs on the EVO were designed to direct air to that high spoiler for the sake of downforce, and he will not have one. So he'll simply be adding drag without tue downforce benfit to performance at the high speeds.

[19bonestock88]

I'll keep the VGs in kind for use on my Saturn, maybe(it has a OEM wing)...

But after hearing some opinions, I think that a partial Kamm would be best for the roof, and maybe a lip spoiler on the trunk lid, if I can make a decent one...

I wish I hadn't thrown away the three OEM hubcaps that come with the car, given the easy ways to make smoothies out of them...

I also understand that the 07-up L61 Ecotec has HPTuners support, which opens me up to tune the powertrain for better mileage and power...

[elhigh]

I believe a small spoiler at the lip of your trunklid will help you. Like an open truck bed, it will help form a stable rolling bubble of air in the vacuum behind your backlight, and that acts as a sort of Kammback for the rest of the air flowing over your roof. It's far from perfect but better than nothing - which is what you have now.

Obviously some testing would be needed.

I'll second Ecky: I think 40 at 80 is going to be a really, really hard goal to meet. I'm softpedaling pretty hard just to break 50 at 60 (also a HCH1 like Ecky's), and our cars are both smaller and lighter than yours. But if you can do it, you are The Man.

Aero is your friend. Find every advantage you can take, and take them all. I drove a Malibu from Roanoke VA to Boston MA and had a blast. The trunk is like a Las Vegas magic act - all my stuff AND the wife's AND the kids AND the strollers fit comfortably inside, with plenty of room to spare. As a long-distance cruiser goes, it was one of the nicest trips I've ever taken. And as American cars went at the time, it was super thrifty. But still, I never got better than mid 30s. If you can crack that lofty goal I will be very impressed indeed.

[19bonestock88]

Well, as the car sits, I can get right at 30mpg at 80mph, cruise set and A/C on, here in WV... I have an air dam extension, good for around 1.8" lower than stock, and rear tire spats that are about two inches below my rocker panels, but I ended up getting a 215/60-15 tire instead of the OEM 205/65 due to availability... I think I lost just a little highway mpg, but gained a little on my way to work due to my being able to pitch it into corners a little faster...

[oldtamiyaphile]

Originally Posted by California98Civic

But the VGs on the EVO were designed to direct air to that high spoiler for the sake of downforce, and he will not have one. So he'll simply be adding drag without tue downforce benfit to performance at the high speeds.

Yes, but Autospeed's tuft testing showed attached airflow on the rear window of a sedan after installing VG's.