Project 718 Cayman T: Part 9 – GT4 Undertray Vanes and Water Pump Mod

It was time to dig into the low-cost OEM+ upgrade goody bag again. Aerodynamics has become increasingly prevalent on cars straight off the showroom floor. Everyone can see the front splitters, rear wings, and diffusers, but hidden underneath are underbody aerodynamic devices that are now common on track focused cars. I decided it was time to address a reliability concern related to the water pump too.

Within the 718 Cayman lineup, the GT4 and GT4RS get underbody aero devices. I decided to go with the modest GT4 undertray turning vane parts which can be bought for under $50; Suncoast Parts calls them Front Fender Liner Lower Spoilers. The GT4RS has a much more extensive array to direct a larger volume of airflow, but I don’t think those components would survive the speed bumps in my complex. If you want to go the GT4RS route, that’ll run you about $260 before taxes at the time of this writing.

The purpose of these front underbody aero devices is to clean up the airflow going underneath the car to the rear diffuser on the GTs to improve diffuser effectiveness. That’s my understanding at least. Of course, my car doesn’t have a rear diffuser. The way I figure, these underbody aero surfaces push air out the sides of the car resulting in less volume of airflow going underneath the rear of the car. I think less airflow going under the rear of the car will improve the intercooler performance; the airflow for the two radiators for the air to water intercooler exit into the engine bay and flow out under the rear of the car. If less airflow from the front of the car is going under the rear of the car, I think the mass flow of air through the radiators will increase. It will be an incremental benefit at best in intake air temperatures and rear lift, but I figure it’s worth a try. As a side note, the 2024 Ford Mustang GT with Performance Pack and the Dark Horse come with very similar air turning vanes with the goal of reducing lift. Ford implemented very similar design concepts on the front of the Mustang as is found on the 718. Imitation is the sincerest form of flattery right?

I got these new gloves from a buddy to try out. They are 8 mil thick with textured surface and way more robust than the blue 5 mil thick gloves I had been using. They are starting to pop up in Advance Auto Parts stores in some states in the US.

The front undertray already has outlines for where to cut and drill for the turning vanes mounting hardware.

The front mounting location for the turning vanes is marked out already too. The hole is for the fastener. The square has to be cut out in order to install the mounting clip.

My new cordless Dremel made quick work of getting the square cut out. I used a drill bit for the screw hole before installing the metal clip.

The turning vane has a horizontal locating tab. The undertray has the cutout location already marked out for that mounting tab too. Sorry for the blurry picture.


  1. Khiem-
    I have admired your many articles, and enjoy reading and rereading them. The title of this one sounded interesting, and the explanations, pictures, etc. were keeping me engaged, and then you casually drop that “a certain Mr. Coleman” drove your car. With you along for the thrilling ride, I assume. I’ve been onboard with Mr. Coleman’s writing, storytelling, and engineering prowess since I picked up my first issue of Sport Compact Car in late 1998. Yes, a 200-plus-page tangible magazine; remember those? I’m currently shopping for a car (a tree fell on the current ride), and that same certain Mr. Coleman is a major factor in considering the Mazda3 that I’ve been eyeing. Sorry to hijack the comments, Khiem, but I really wanted to give a shoutout to him. I hope that was alright. Thanks again for the always enjoyable and educating articles. Enjoy the drive!

    1. I loved reading all things Dave Coleman as well! I wrote a question into SCC when I was in college and he answered it! I love Mazdas; I have a base model 2016 CX-5 that just turned 90k miles. I recently took it on a road trip with one of my dogs hitting up some of the best roads in California and it was still fun. I had to put the auto transmission in Sport mode on some sections and the transmission shift strategy was great for the twisty roads. Of course in manual mode, it has the proper sequential shifter orientation of downshift forward, upshift rearward. Lifetime average of 27mpg driving around LA. I literally just had a brand new Mazda3 sedan for a rental car and got almost 39mpg over 350 miles of mostly two-lane cruising. A lot of new cars have lane keep assist which is very annoying. Dave helped make the system be as minimally annoying as possible. The only thing that’s not optimal is the brake pad compound which doesn’t have a ton of initial bite. Though it is an improvement over a CX-30 I drove a couple years ago, but not as good as my 2016. Seems to be a bean counter change. Dave can’t fix everything. I’ll recommend anything in the Mazda lineup if you’re looking for a driver’s car.

    1. I’d found that before too. I do think it is vehicle/overall package dependent. The only thing I know for certain is that they direct air from the middle of the car outwards. I vaguely remember reading somewhere they also interact with the air swirl coming off the front tires which helps clean up the underbody airflow. I think acting as an air curtain to prevent the dirty air off the front tires from going under the rear.

      This link is to an artist representation of the airflow under the GT4RS with the more extensive turning vanes array underneath the car. It does show the vanes redirecting the air outward right into where my rocker panels got hammered. Huh.

  2. My 718’s rocker panels aren’t as peppered with rocks like your is. But the under panel definitely is.

  3. Never understood why my 718S water pump started leaking at 20,000 miles and 3 years. Makes sense. Thanks for your 718 articles, they’ve been great reading! PS I have the APR tune and wish I tried the AP…

    1. If the car is bone stock, I think APR is a best option. It’s when you start modifying things where the pro-tune has benefits.

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