Miata Fender Vents

A splitter makes the majority of its downforce through suction. If you’re fuzzy about what that means, read Top 10 Reasons Your Splitter Sucks. In that article, I beat to death the fact that it’s the underside of the splitter that does most of the work. To make a splitter work better, you need to expand the air under and behind the splitter. The most common way to do this is with splitter diffusers (aka splitter tunnels or splitter ramps).

Splitter diffusers dump air into the wheel wells, which is a convenient area with a lot of volume. But it’s also an area of chaos, with spinning tires that squirt a jet of air perpendicularly across the splitter diffusers, and suspension parts moving rapidly up and down. Getting air to move efficiently through this mess isn’t easy, and so pressure can build in the wheel wells, which reduces the effectiveness of the splitter.

So anything you can do to get air out of the wheel wells will help the splitter make more downforce. As a byproduct, you’ll often get better brake cooling as well. The most common way to extract air from the wheel wells is by installing louvers on the top or sides of the fenders.

You can also cut a slit and push the fender inwards. Or some people pull the rear of the fender away from the body, and extract air from behind the fender.

However you do it, fender vents have been proven to be highly efficient and add downforce with very little drag. Race Louvers test data shows that a top vent alone is worth about 14-17 lbs of downforce at 100 mph, and only a pound or so of drag. Adding another vent behind the fender results in more gains.

But what’s interesting is that adding a third vent doesn’t seem to add any more downforce. Despite the fact that Race Louvers is a business that wants your money, they advise you not to buy a third vent. So there’s definitely a point of diminishing gains with fender vents, and I’ll address one solution to that in the future.

In this article, I’m going to review some of the popular Miata fender vents available, and some DIY solutions for the frugal. Most of these are universal in nature, and can be used on other cars as well.

Race Louvers

I’m going to start with Race Louvers fender vents, because Al from Race Louvers has done a lot of wind tunnel testing and publishes PDFs on his site. Much of this article is based on the testing and data in his articles and videos with AJ Hartman.

Al’s extensive wind tunnel testing shows that louver blades should extend both above and below the bodywork. This applies not only to fender vents, but hood vents as well. Louvers that were only on top of the bodywork, or were only recessed below the bodywork, didn’t work as well as louvers that extended both above and below the skin. Moreover, it appears that the depth below is more significant than the height above. This discovery seems lost on virtually every other manufacturer.

At $289 for the curved version you see above, or $279 for the straight version (less visually interesting), these do not come cheap. But then again, other louvered vents aren’t going to perform as well as these, so you get what you pay for.

On a NB Miata, these tested at 17 lbs of front downforce and virtually no drag. If that doesn’t seem like a lot of downforce, note that any gains you make in front downforce are typically matched with 100-150% more rear downforce. So you could think of this as a means of adding 42 lbs of total downforce.

Now this is just from a single pair of vents on top of the fenders, and more venting can be done behind the fenders. Race Louvers doesn’t have a rear vent for Miatas, but Left Lane Designs does.

Left Lane Designs

Left Lane Designs makes rear fender vents for NA and NB Miatas. NAs use two vents, one on either side of the style line, while NB Miatas have just a single big vent. These are priced at $279 and $289 respectively.

Adding the Left Lane Designs vents to the Race Louvers fender vents could get you as much as 50 lbs of front downforce. At least that’s what Race Louvers got on another test using similar vents, and I think that data can be safely applied to Miatas and other cars. Again, add 100-150% because now you can add more rear downforce.

Singular

For $249 you can get Singular fender louvers, which are the same vents that are on Ryan Passey’s HyperMiata. These laser-cut aluminum vents are visually interesting, in that they have some curvature to the vent shape, rather than the more rectangular venting on the Race Louver vents. This curvature follows the countour of the Miata fenders, and while I haven’t found any data on them, it’s a great look. (I’ll be testing these in a wind tunnel soon.)

One interesting feature of these vents is that for an additional $40 you can get a partial cover so that these vents are legal for Grid Life Touring Cup. GLTC limits the size of fender vents to 45 square inches for some reason, and it seems as arbitrary and quizzical as SCCA Time Trials limiting splitter diffusers to 84 square inches, or NASA ST4-6 limiting hood vents to 3/8″ in height. Whatever people, whatever.

R Theory

For slightly less dosh than the Race Louver or Singular fender vents, you can get a similar looking vent from R Theory. However, the similarities are only skin deep. Literally.

If you watch their wool tuft testing video, you can see that the louvers do not extend into the fender. Recall from the Race Louvers data that it’s probably more important to extend below the surface than above. The Singular vents are similar in this fashion, but at least those install below the surface, while the R Theory appear to install on top of the fenders (which not only looks less professional, but probably increases drag as well).

While the $192 price is significantly lower than Race Louvers or Singular, I couldn’t justify the loss in performance of using these. If you want fender vents because they look cool, go for it. If you’re primarily looking for performance, I’d look elsewhere.

Spiked Performance

Spiked Performance sells an inexpensive aluminum fender louver for $169. Like the R Theory vents, they install on top of the fender skin. Spiked vents measure about 33 square inches, and appear to be the smallest here. Based on that, I would expect them to be the least effective (but still worthwhile).

Napp Motorsports made a great installation video, and it looks quite easy with the supplied cutting template and a few rivets.

DCN Performance

I’ve never heard of DCN Performance, but they have NB Miata fender vents that are inexpensive and appear to be properly engineered. You can buy all three vents directly from DCN and Top Miata.

I didn’t see a US supplier, but I did find the lower pair of vents available on Etsy and eBay. At $150 for the lower pair, these are worth looking into.

Amazon plastic shit

Finally there are tons of universal plastic shit vents you can buy on Amazon or eBay. I’m not going to review any of them because I would rather put a stick in my eye than put these on my car. If you’re too cheap to buy decent vents, don’t buy plastic crap, make them yourself.

DIY options

If you think it’s ridiculous to spend nearly $600 on fender vents, or support crappy products made in China, I’m with you on both counts. This whole site is about DIY creativity, and I neither shill for nor take kickbacks from anyone. Let’s DIY us some fender vents!

Fenders holes

If you’re a cheap bastard or just an avid DIYer, then you can make your own fender louvers by just cutting into the sheet metal and bending louvers into it. Follow a pattern like Race Louvers and make them go both above and below the fender skin.

Another option is to say fuck the louvers, and cut a big gaping hole in the fender. In the course I took with JKF Aero, the CFD showed that a big louver-less hole made more front downforce than a fender vent with louvers. So if you’re after the maximum front downforce, and can accept a slight drag increase and loss in rear downforce, get that cutting wheel out grind you a big ass hole in your fenders.

Note that you should put a 3/8” Gurney flap on the leading edge. For hood louvers, you’d want up to 1” high flap, but apparently fender vents should use a smaller leading edge flap.

Fender cuts

Another easy modification is cutting a slit into the fender and pushing the bottom inward. On a NA Miata, a lot of people make a subtle cut on the belt line. But some people make a more aggressive cut higher up, and vent the entire rear of the tire.

I don’t have any data on how effective these fender cuts are compared to louvered vents, but testing this is on my list of things to try on my next trip to the wind tunnel.

3D printed

If you have a 3D printer, Beavis Motorsports has a free STL file you can use to print a “time attack style” fender vent. These sit up quite tall, and you’d think they would be really draggy, but Race Louvers tested a 1 7/8″ tall time attack vent (not this exact vent, but something similar) and it made 14 lbs of downforce and 1 lb of drag, which was the second best of the vents they tested.

I don’t love the look of these, but that’s purely a subjective thing. If you do, then something like this is a good option.

Installation advice

After reading all of the PDFs on the Race Louvers site, and watching too many of his videos, I’ve distilled the following advice, which applies generally to all fender vents.

• Don’t place top-mounted fender vents too far rearward. The vents should end 6″ before the windshield to avoid high pressure air from the windshield interfering with the vent.
• Airflow over a hood or fender is generally straight back, and louvers should be installed perpendicular to this flow. Don’t angle the louver blades more than about 15 degrees or it will reduce their functionality. (The louvers should be more vertical than swept back.)
• Many people have removed their fender liners completely, which is often a mistake. But if you still have fender liners, you’ll get the most downforce by cutting a hole in the fender liner near the louver. If improved cooling is your primary goal, remove the front of the fender liner.