I've been looking at adding a flat underbody kit along with my splitter and LRB underbody panels, but I'm concerned about how to mitigate heat issues from the exhaust, which seems would be completely boxed in from front to rear between the underbody panels and the floor of the passenger compartment. Car is street driven but also have plans of track use, thus the smooth underbody and diffuser. I see R-Theory flat underbody has a NACA duct to diff cooling, is exhaust heat not also an issue? Or is there enough air moving through this space from the engine bay around the trans and driveline to do an ample job of evacuating heat? Really worried about heat from turbo and downpipe being an issue. What have you guys with flat underbody panels done to address this?

 

Summit Sticky Shield?
https://www.summitracing.com/parts/hsp-180025 (I picked the largest size for reference)
They do make others, but since you are asking about underbody panels, I was just looking at exterior additions. I am sure you could put some insulation on the interior floor/tunnel but at that point the sheet metal may have heat soaked a bit before it gets to the insulation.

 

Yeah, definitely going to do heatshielding the full length of the tunnel, as well as thermal wrap on the midpipe. Thanks for that link. I'm just wondering on ducting ideas to bring airflow to that region, or if it's really needed. Searching through all the threads here is kinda hit or miss, but I have seen a few things.

 

I'd say it depends entirely on what kind of driving you are doing, but yes, cooling of the drivetrain is a major concern with a flat underbody.

Street or autox? Probably doesn't matter. Definitely add some heat shielding to the trans tunnel so you don't cook yourself though.

Time attack or time trials format? You can probably get away with it, but I'd start to consider a trans cooler depending on length of sessions, how many, how hot your environment is etc.

Endurance or extended HPDE sessions, I.E. 20+ minutes of full tilt turbo miata ripping, you are going to need a trans cooler at the very least. And ideally a diff cooler as well. 6 speeds are the stronger miata trans but they will fail due to heat.

 

In all honesty the car will be street driven about 99% of the time, but I do want to ability to run the occasional autocross and maybe a track day, but nothing competition like time trials or SOLO I (I'm presuming SCCA still calls it that). I've been out of the road race scene for almost 20 years, now I'm mainly just interested in a super fun streetcar that I could put on the track for limited use without melting everything. I learned the hard way the very first time I put a Corvette on a road course back in the late 1990's that heat will eat you alive after just a short stint on track, and stock parts are NOT made for racing LOL.

Basically, I'd rather overbuild to make things more reliable. And yeah, I know that for a street-driven car that sees the track infrequently, a full flat bottom is the epitome of "diminishing returns" with respect to investment $$, so it will likely be the last phase of the car's build and development. But I figured if I'm doing a splitter and diffuser, why not give the center section the same treatment and improve the effectiveness.... because if I didn't it would always bug me that maybe I could have made it just a little better.

 

Ah, in that case flat bottom away! I will say it's kinda a hard parker move to put that on a street car, but I get it. I wouldn't be worried about the heat too much on a street car, other than it cooking you.

I'd argue the sweet spot for a flat bottom is a time attack car, once you get into edurance or hpde cars I think the cooling is more important than the aero benefits. And on a street car there's almost no benefit to be had, really, since the car isn't being pushed to any kind of limit on the street anyways.

And for the record I think a flat bottom is awesome, and I've been drooling over the R theory setup since I saw it on the FM shop car. it looked really nice, but I don't wanna deal with the extra heat on my already very hot high HP turbo track car.

 

I'm a world away from actually doing a flat floor but have given it some thought. I wouldn't bother at all for a non-dedicated/competitive track car, but you've addressed that. No personal experience but I'd be worried about adding a ton of heat and thus additional problems for no real gain.

There is a thread here somewhere with pictures of a slick setup Emilio had with trans/diff coolers in the trunk, exhausting out of the license plate area. There's also a few cars around that took the exhaust around the wheel well and down the side of the car, which by itself I'd assume gets a ton of heat away from the underbody area. Requires a large amount of work cutting and welding the rockers though.

No matter what mitigations you put in place I'd get temp sensors in place so you can monitor trans/diff temps. Seems better to find out there's a problem via data than by needing new parts.

 

I reckon this falls into the 'nice idea, but ...' category. Some of the issues have been ventilated above, nothing a total deal breaker, but is it worth it? Op seems to think so, his car, his money so ...

One issue I have not seen addressed, is coolant rejected heat. There may be enough opening between the end of the undertray and the start of the flat floor, but that would be the first thing I looked at if I was doing what the OP is proposing. For street, possibly not an issue as the engine is not working hard, but track use where full power is used may be a very different story.

 

Well, concerns duly noted, and yeah it probably creates way more problems than it's worth. One part of my brain says "Well, Lotus engineers obviously thought it was worth it..." but yeah, probably something that if I was to do would only be installed for a track day.

 

As someone that does a lot more planning/daydreaming than actually following through, IMO the proper approach for pretty much every NA/NB street car owner wanting to play with under car airflow is to manage what goes under the car before it gets there. If reliability is a concern, don't add extra thermal stress by keeping heat in the tunnel, and the extra steps for maintenance. Lotus doesn't have to deal with that on their mid engine cars.

Hood vents will pull the radiator airflow up and over instead of under the car, and are proven to reduce both lift and drag, and give a cooling benefit too. A splitter should reduce airflow under the car, and make better use of what does go under the nose. Same story, less lift and drag. Side skirts will keep any air from curling under the sides of the car. Spats/wickers can keep air from packing into the wheelwells. All of those elements will contribute to a better working flat underbody, so you might as well do them first. RGR Engineering makes a lot of aero parts that are very cleanly done, including a soon to be released full underbody setup. The owner has (well, had) a Mazdaspeed Miata and did some on track testing with thermocouples to ensure temperatures were kept in check.

I would have done the hood vents and splitter already to get the double whammy of less drag and more downforce, but my experience (and conventional Miata wisdom) is that Miatas need to get rear downforce first unless you like a car wants to go off track *** end first at 80+ mph. A wing or functional spoiler should be the second aero mod you do on a track car. The first is sealing up the radiator to the front bumper.

 

Have you done any track days before?

Honestly as someone who has been around now for about 20 years in the miata world. You are chasing a problem that you don't need to chase. Put the time, money, and effort of trying to flat bottom your miata into actual track time, or consumables, or a turbo.

I do think it is worth adding heat shields and heat shielding in the trans tunnel to keep heat out of the cabin.

 

Already have hood vents (Singular Motorsports), splitter w/ spats, and side skirts, and am working on/planning radiator sealing and ducting. I'm still in the "doing my homework" stage of all this, thus the thought exercise on ducting air to the tunnel for cooling. As far as rear downforce, the whole point of looking at the flat underbody was the idea that it would increase the effectiveness of a rear diffuser in conjunction with a rear spoiler. I've been off track backwards too many times LOL, definitely want that rear end planed.

 

Track days in the miata? No, I haven't tracked the car as I haven't deemed it sufficiently track ready, that's the reason for all this - Car is riding on almost 30-year-old everything, so she's getting a full suspension rebuild and upgrade with FM coil overs, sway bars, bushings, etc.

As far as me personally, it's been a few years so I'm probably not as quick anymore, but I had about a decade of autocross, time trials and track time before I hung the helmet up, including road racing championships with SCCA and NASA. Building the miata to be a dual purpose car is my attempt to scratch that old itch, I guess you'd say. And she'll definitely be turbo'd.

But you're absolutely right, seat time is the ultimate go-faster investment.

edit: I want to add, almost 30 years ago I was a Corvette autocrosser and time trial guy, 20+ years ago turned Acura/Honda road racer. I'm a complete Miata newb when it comes to all this. I know this community has a massive amount of knowledge, and although I rarely have posted over the last 43-4 years, I've spent so many hours reading, researching, and admiring y'all's work. Definitely be brutally honest, I'm a big boy, I'm gonna' ask things that seem like pretty dumb questions to you guys who are Miata wizards, just understand I'm trying to not make the dumb mistakes I made 30 years ago that caused me to have to do and redo things 2 and 3 times before figuring it out, because I was too proud to ask the guys who actually knew. :cheers: