4th gen aero

Do you still run multiple cameras on your car? If so, can you do a yarn test on the front of the car? Either on the centerline? or on the side fender? If you mount a camera on the rear window you can yarn the back to see if the flow changes there. If less air is going under, you should see the flow change post rear bumper.

good call I haven't done any tests in that area yet, would be good to see what kind of data could be gathered in that zone. I do still have the capability to run a multicam set up. Some with/without splitter set ups with yarn is a good idea. So not only from an obvious front of car perspective, but you're saying that I may see changes at the back of the car in how the yarn is reacting with/without the front splitter yeah? Never thought about that

Always wondered what could be gained from fabbing up some slick bottom panels for the underbody. I've always been pleasantly surprised that this car came with partial slick body underpanels from the factory.

the underside is surprisingly smooth if you have the front underbody plastic in place. Even the rear tire tray smooths out that area pretty well.

I'm curious what would happen if the rear bumper skin was removed. Looking at its shape, it appears to catch the air and change shape. If you were to remove it and add a diffuser instead sticking out a couple feet, it might solve the negative pressure issues of the tailgate

yarn stick - https://legacygt.com/forums/attachment.php?attachmentid=250566&d=1496675739

Of all the yarn attempts, I found the 8" long stick taped to a location to be the most useful. It most simulates the smoke used in actual wind tunnels. The hard part is getting the camera mounted to catch the data without becoming data. Blue Painters tape was good for the yarn. Although I used 100mph tape to attach the camera and sticks. 2 - 3 inches of yarn is more than enough to "see" the air flow. Any more than that, and you lose resolution.

I don't think so but I'd like to see it done. I'll get there one day. There's a neon at rallycross that the guy cut up his rear bumper to allow for a larger diffuser and it's had my stuck on the idea of making a custom rear bumper and diffuser and setting it up into an under body tray.

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i have a rear cut of a specb in my shop it's surprisingly smooth. Adding the rear bumper skin doesn't help much, but it doesn't hurt much either. Looking at the plots above, it didn't affect the rear output much.

Tape some strings to the rear hatch, mount a gopro, make a test run to 100mph.

tape the area between the mufflers from the spare tire tray to the bumper skin, repeat

remove rear bumper skin, repeat.

One of those runs will be better than the rest.

Sgt.Gator made a full tray for his sti. Claims it helped

I think that would be great. The rear bumper skin looks like it acts as a big parachute. I believe just closing/smoothing that area would help quite a bit.

One of the things that is all the rage in the Miata community is the "pool noodle upgrade". People mount a pool noodle tucked up under the rear bumper and one owner claims it reduces drag by 7 pounds at 75 mph - that's 1.4 horsepower and in a car that can cruise at that speed at about 30 horsepower, that is noticeable. Other people have cut the rear bumper up to the cross beam that it mounts to so there is no "parachute".

A full underbody panel would have to tolerate exhaust heat in certain places and would be dangerous in the event of an exhaust leak. No more hogzaust for you!

I've thought about the heat tolerance. Also clearance for the rear suspension.

If you channeled it instead of running a completely flat bottom, it might work better. I've seen that some cars do that to allow clearance of the trans and exhaust tunnels. I was also thinking about just cutting two big holes for my turned down exhaust tips....

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I've thought about the heat tolerance. Also clearance for the rear suspension.

 

If you channeled it instead of running a completely flat bottom, it might work better. I've seen that some cars do that to allow clearance of the trans and exhaust tunnels. I was also thinking about just cutting two big holes for my turned down exhaust tips....

 

 

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you can add heat reflecting mats to the panels to keep heat where it needs to be.

Of all the aero mods possible, the flat bottom is the least beneficial. Unless you have done others first. Also if you have a tmic, you need the opening at the back of tray to draw air through the i/c. If you have a fmic, you need ducting to draw air out of the engine bay.

Also you want a tapered flat bottom, closer to ground at front and higher (slightly) in rear. Otherwise you generate lift. Ideally, you want air to go around rather than under, so there's a low pressure zone under the car.

Of all the aero mods possible, the flat bottom is the least beneficial. Unless you have done others first. Also if you have a tmic, you need the opening at the back of tray to draw air through the i/c. If you have a fmic, you need ducting to draw air out of the engine bay.

Good point about the TMIC. That's interesting, from what I've heard flat-bottom is usually the biggest impact.

Good point about the TMIC. That's interesting, from what I've heard flat-bottom is usually the biggest impact.

Might be platform dependant. We already have a pretty low drag because aside from the rear diff area the underside is pretty smooth honestly. At least compared to others. Between that and the note about the top mount intercooler.

When compared to a splitter, lip, side skirts, and tmic there might be so much air getting under the car that the smoothness and exit doesn't matter as much as getting air to not go under the car.

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^ Basically that. It's simple in some respects. Ideally zero air goes under the car. You want the splitter to be basically scraping the ground. Minimizing the air going through the nose into the engine bay. That air has to go somewhere. Ideally any air going into the engine bay gets exhausted over the top of the car. Look at high end sports cars with the venting behind the rad. Though some these days do a good job of hiding the channeling of the air. Obviously some of this is easier to achieve then others.

It might not look great but gaffers tape covering as many holes as you can will help and should be easily removed. It's what nascar uses it so should be fine for your track car.

Tape/cover as much as you can before over heating. And get the spitter as close to the ground as possible.

Gaffers tape is crazy money, but comes off without taking paint off. There's a cheaper option nicknamed 100mph tape ( ive tested gopro to 140 with it). For sealing on inside, just about any duct type tape will work.

When sealing off, you'll need a helper but you can use a weed blower to test your progress. Use safety glasses.

For the radiator, you can use a directional screen to manage air flow (side effect of greatly reducing damage) - https://www.walmart.com/ip/Nomex-Honeycomb-Radiator-Guard-Protector-Screen-Mesh-15-x-22/513524281?wmlspartner=wmtlabs&adid=22222222223255879529&wmlspartner=wmtlabs&wl0=e&wl1=s&wl2=m&wl3=74491998164086&wl4=pla-4578091562179935&wl5=&wl6=&wl7=&%20wl10=Walmart&wl12=513524281_10000013338&wl14=radiator%20protector%20screen&veh=sem

The smoother the outside is the better the airflow works. A vertical wall is better than a concave ( c shape), however the wall still needs to move the air somewhere else otherwise it's drag

 

Tape/cover as much as you can before over heating. And get the spitter as close to the ground as possible.

this is a track only mod. Using it on the street is a good way to force garage time

Good point about the TMIC. That's interesting, from what I've heard flat-bottom is usually the biggest impact.

Assuming a typical pre 2000's car. Pretty much every car,truck sold today has massive aero work already implemented. The 3rd gen wagon vs 4th was a big difference.

I just had a look at my stock usdm rear bumper. The bottom edge of the bumper skin overlaps the spare tire tray. It's really smooth under there.

Just spent the balance of my Amazon points on this guy:

https://www.amazon.com/gp/product/B018VO5GI2/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Should help answer some of the airflow questions, and provide some better estimates of boundary conditions for any future CFD work.

What about airflow of the lower vs the upper grill? During monte Carlo the lower of the msport fiesta was getting blocked and causing overheating.

I've thought about putting led fogs in the lower and you could use the info for ducting and blocking possibly?

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