Underbody Panel Effectiveness...
 

Hi everyone. Happy Easter! I'm new to the forum here, and new to these cars. I was wondering if anyone has done a full underbody panel kit with diffuser with a stock (height) suspension? Do they have a noticeable effect with that height? I know that an underbody kit works better the lower the car is, but for a DD I don't think I want to go that low. To back up, my last car was a '99 Prelude SH on Tein coilovers and was set to have about 3" clearance to the lowest point. The car looked cool, but it has its draw-backs. #1 I scrapped the ground a lot (of course), #2 I couldn't go through a car wash because I would bottom out on the guide rail. My plans for my FR-S is to do a full underbody kit to increase the downforce at interstate speeds, but I still want to go through the wash and I dont want to destroy the $600 diffuser every time I back out of my driveway. Has anyone done these aero mods with a stock height? Thanks everyone

Not stock height but was on STi springs (only 10mm drop) with my diffuser/suspension covers/unbody panels, I never scraped the diffuser. You should be totally fine.

Also you won't be adding downforce but rather reducing lift and drag.

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Smoothing the air below and lowering pressure underneath the car, maybe? :iono:

Does our "sports" car generate enough momentum to kick-start underbody aero-shenanigans?

All the info:

https://www.verus-engineering.com/informative-packets

Oh dang. According to those CP charts, yeah, lowering pressure underneath definitely does help. :w00t:

It is also quite efficient.

That's good to know. I would hate to damage such a pretty piece like a diffuser. I might get a set of TRD springs at some point, they have a 1" drop. That can't be any worse than the Prelude lol. Thanks

That's awesome. Thanks

I know I'm not going to get F1 effects here. Those panels start working right about where I'm having troubles, so every bit will help. I have an Electro-Mechanical degree, not Aeronautical.

1" drop is pretty mild, you should be okay everywhere but the steepest driveway entrances/exits. I'm at around -2.5" right now with a splitter and diffuser and as long as I am paying attention, I don't scrap.

Aero engineer here. For sure way way less effective at stock ride height. If you look at the Ventus stuff you see their model is slammed. Also, the application is for cruising at freeway speeds? A smidge more downforce/less lift is going to make no real difference for you. If you want it for looks that's fine. But it's not going to be doing anything useful for you, I don't think...

I am starting to get all the pieces for a flat bottom why because racecar. So far I have the following

Splitter up front
OEM Under Panels
Verus Trans Tunnel and rear Suspension covers

Just need the diffuser and diff cover :)

Thanks for the replies everyone. Maybe I should have started with a different question rather than a question about a solution. Does anyone else feel the steering starting to get loose at 80mph? It feels like the front end is lifting a tiny bit and I start loosing that tight steering feel

how hard or sharp are you turning..? I regularly take highway loops at 80+ and I don't feel like I'm losing control of the front end even when it goes over uneven expansion joints...plus naturally steering feel should get lighter at higher speeds or am I missing something here.

No turns, just straight line 80+. If that's the way cars are suppose to be then maybe I was missing something. It just feels a little less grippy up there. Maybe I'll just drive slower lol:laughabove:

Toss a front splitter and some canards on, that should add the downforce you're looking for at high speeds.

If it's feeling noticeably less grippy at only 80, I'd make sure the basics are right before adding downforce.

How's the alignment?

Lets start with alignment specs, tire pressures and tires.

If you are tracking some of those panels could lead to heat building up in your rear diff because it does not get any air, do a search on it. On the street it’s not likely an issue,

I just got the car a few weeks ago, so I haven't really done anything to it yet. It came with 235/40-18 up front and 265/35-18 in the back. I forget the name of the tires, but they're brand new and Z rated. Stock suspension, TRD swaybar.

Well there's your problem. Put the same size tires on all four corners and report back.

I bet that car had other mods and they returned it to mostly stock before selling. I’d start with alignment.


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It did come with the wheels, TRD swaybars, TRD catback, Crawford intake spacers and HKS air filter. I'll get an alignment here soon.

They are same diameter. Just different width.

I had the following, did wonders for high speed stability.

Greddy Gracer Front Lip
Beatrush Undertray
JDM Aero Underpanels
OEM 86 Rear Spoiler
Verus Front Wheel Deflectors
Verus Trans Tunnel Covers
Verus Rear Aero Spats
FT-86 Speedfactory Rear diff covers
FT-86 Speedfactory Rear Diffuser
TRD Front Fender Fins
TRD Trunk Stabilizer cover
TRD Aero Turbulator
STI Side Skirts

The rear diff is still relatively exposed, but i did have rear diff cover with extra capacity and “heatsink fins”

High speed stability can be achieved with exactly zero of those modifications. That said, I would love to drive a car with all of those modifications to compare it directly to one without. I'm skeptical I would feel any difference.

For the street it's useless... Got any links? Curious more than anything else..

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Different width is the problem. Makes the car an understeery mess. Not sure if it contributes to feeling light at speed, but I havent driven a factory alignment in so long (let alone on the highway)...

I had almost the same setup when I bought my car.
While I wouldn't have called it floaty, it was definitely understeery AF compared to now with the stock tire sizes on OEM wheels.

Find some used OEMs for cheap and lose the boat anchors.

Know
 

Do a search, it was on this forum. There was a discussion that a shop had higher temps in the diff with all the aero. I don’t think it’s deal closer, just something to think about with changing fluids more often and such.

You should pick up a bit of mpg and should be able to carry some more speed on the track due to less drag and the car being more stable

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In theory I totally agree. In practice I'm skeptical I could feel much.

You can't always feel what the stop watch saw.

Width doesn’t matter for his issue.

I was also staggered 225/255 zero issue.

At 100mph+ it’s beyond noticeable the difference.

Take a car with zero aero,
Add only a real functional front splitter. And prepare to hate driving at 100mph+.

Then add a diffuser to the above to rebalance it and prepare to love driving at 100mph+ the feeing is night and day.

From a drag reduction stand point you won’t really feel that. IIRC the drag reduction at 100mph for everything combined was the equivalent to about 10whp or 2.5whp at 50mph, I calculated it out once. Which i guess adds up but 10whp isn’t that noticeable.

From a downforce / reducing lift stand point though, It’s huge. Below 80 you dont really feel it, at 80 you start to feel it by 100 it’s very noticeable and by 120 you couldn’t imagine driving without it.

Huh. Interesting. I'd love to experience that first hand. [emoji106]

Thanks everyone for the comments. I got an alignment, it didn't do much. I tracked down the wheels that came on the car, they're Avid.1 SL01's 8.5" & 9.5". Before the end of Fall I'll get two of the 8.5" wheels and mount either 225's or 235's all the way around. I really haven't put too much thought in the width yet, I'm open to suggestions for a spirited DD. I think a good chunk of my stability problem is the OEM suspension that has 114k on it. I did see wet spots on the shock boots indicating a leak. I'm going to get KYB GR2's and Tein H Tech springs very soon because I notice a descent amount of bounce when I drive. Does anyone know if a ~.6" rear drop will mandate adjustable toe bars? I'm planning on getting a camber/toe kit, but I don't have all the cash to do it all at once.

Your stock adjustment for toe will get you back in spec, you'll need adjustable rear lower control arms if you want to adjust the camber. Not really necessary for such a mild drop if you're fine with whatever camber in the rear comes naturally. I would advise against KYB's, you could get Koni's or better yet Bilsteins. They are miles more advanced than KYB GR2's. Look into Eibach or Racecomp springs.

If you're on a budget I suggest looking for lightly used 17+ dampers and springs.