TLDR: They worked a tiny amount on the e36 and made lift (the opposite of downforce) on the Corvette.
Vortex generators (VGs) are one of those mods that show up on everything from Evos to full-blown race cars, usually lined up across the back window or roofline. The promise is simple: they're supposed to help air stick to the car and, in theory, feed cleaner air to your wing for more downforce. But does that actually happen, or are they just there to look aggressive?
At Nine Lives Racing, we put it to a vote in our Facebook group, Trackable Aero, and vortex generators won. So we ran a full CFD (computational fluid dynamics) study — real simulation data, not guesswork — to find out whether roof VGs actually help, hurt, or do nothing at all.
The Setup
We tested two very different cars on purpose:
- A C5 Corvette — chosen because it has the shallowest, laziest sloping roofline we could find.
- A BMW E36 — chosen because it has the sharpest, most abrupt roofline we could find.
The idea was to bracket the problem: if VGs behave differently on a gentle slope rear window of a corvette versus a sharp one like a e36 , we'd actually learn something about when they work instead of just whether they work.
We also broke from convention on placement. Most cars with VGs (the Evo being the classic example) mount them at the back of the roof, right before the glass drops off. But on airplanes, vortex generators almost always go on the front of the wing, not the back — because their real job is delaying airflow separation before it happens, not fixing it after the fact. So alongside the standard rear-roof placement, we also tested VGs mounted at the front of the roof, to see if the aircraft logic actually translates to cars.
Quick Refresher: What a Vortex Generator Actually Does
A VG's job is to take air that would otherwise separate from the surface — going turbulent and "falling away" — and instead spin it into a vortex. That swirling motion re-energizes the airflow and helps it stay attached to the body longer than it naturally would.
This is exactly why airplanes use them: at a high angle of attack, wings can stall when air detaches from the top surface. A well-placed VG delays that separation and buys you more usable angle of attack before things go wrong.
On a car, the theory is similar. Rear glass — especially on something with a steep rear window like a sedan or an Evo — is notoriously hard to keep air attached to. VGs are meant to help that air stay glued to the glass, which should mean cleaner, more energized air reaching your rear wing.
Formula 1 teams use vortices even more aggressively — not just to keep flow attached, but sometimes deliberately to keep flow separated, using vortices off the floor and bargeboards to manage the interaction between the tires and the underbody. Vortices aren't inherently "good" or "bad" — they're a tool, and how you use them determines the outcome.
Confirming the VGs Were Actually Working
Before looking at any downforce numbers, we checked the CFD flow visualization to confirm the VGs were doing their basic job — generating vortices at all.
On the E36, the iso-surface plots clearly showed multiple distinct vortices peeling off the rear-mounted VGs — four of them, in fact. So mechanically, the parts were functioning exactly as intended. Worth noting: we intentionally sized these VGs larger than anything you'd actually run in production, purely so the CFD software (which struggles to resolve very small features) could clearly capture the effect. This was about proving the concept as dramatically as possible, not about testing a shippable part.
E36 Results: A Small, Real Gain
Comparing the baseline run (no VGs) against the rear-mounted VG run, the pressure maps on the back glass showed a visible change — the low-energy, "detached" purple zone shrank noticeably once the VGs were added. That's the flow reattachment working as designed.
The hard numbers, converted from the raw output:
- Front-mounted VGs: negligible effect — roughly a 3 lb downforce increase. Barely worth mentioning.
- Rear-mounted VGs: picked up about 17 N·m, which works out to roughly 12 lbs of added downforce, against a drag increase of about 8 N·m (roughly 5.9 lbs).
That's roughly a 2:1 lift-to-drag ratio for the added components. For context, an average road car sits somewhere around a 2:1 to 3:1 overall lift-to-drag ratio, so what the VGs added is in the same ballpark — not spectacular, but not nothing either. Bottom line for the E36: rear VGs gave a small, genuine downforce gain. Front-mounted VGs, despite matching the aircraft convention, did essentially nothing here.
Corvette Results: The Opposite Outcome
This is where it got interesting. The Corvette's flow visualization showed an even more dramatic vortex effect than the E36 — bigger, more obvious structures forming off the rear VGs. If you judged purely by "are vortices being created," the Corvette test looked like the more successful one.
But the actual force numbers told a different story:
- Drag increased about 7% (roughly 7 lbs).
- Downforce decreased.
So more visible vortex activity did not translate into more downforce — it went backwards.
The X-ray pressure plots explain why. Pictured above on the Corvette, the airflow over the back glass was already well attached before adding any VGs — the baseline run showed clean, green (low-loss) flow across that whole area, with no separation problem to fix. Because there was nothing to reattach, the VGs weren't solving a problem — they were just adding energy and turbulence into air that was already behaving. That extra energy from the vortices dumped straight into the glass and generated a large hot spot of lift, not downforce. Critically, the rear wing's pressure signature was completely unchanged between the baseline and VG runs — the wing wasn't seeing any benefit at all. Every bit of the change was localized to the glass itself, and it was working against the car.
The Takeaway: It's Entirely Vehicle-Dependent
The core conclusion from this test is that vortex generators are not a universal upgrade — their effectiveness depends entirely on whether your car's roofline has a flow separation problem in the first place:
- Sharp, abrupt rooflines (E36, E30, and similar) with attachment issues: VGs can help, even if the gain is modest.
- Gentle, already-attached rooflines (Corvette and similar): VGs actively hurt — more drag, less downforce, because they're stirring up air that didn't need help.
There's also a broader warning here: if you add vortex generators to a car without a wing behind them, you could easily be making things worse, not better. The VGs can generate net lift on the glass itself with nothing downstream to convert that energized airflow into usable downforce. If your goal is more downforce, a wing built with a proper lift-to-drag ratio will get you there far more reliably than VGs alone.
Verdict: If you're running a Corvette-style shallow roofline, skip the VGs entirely. If you've got a sharp-backed car like an E36, they may be worth it — but temper expectations, since the gains here were single-digit pounds, not a game-changer.
if you would like to download the runs and all the data, it's available here.
This test was run and shared publicly by Nine Lives Racing as part of their ongoing free aero testing series for the Trackable Aero community. Full CFD data and future tests are available through their forum.