NACA duct in splitter

[Jnikic]

I'm making a few upgrades to my track rat so that I can start racing wheel-to-wheel next season in NASA GTS series.

Brake cooling and aero are two areas that I think I need to make improvements to be somewhat competitive.

I plan to run a GT racing splitter (https://www.gt-racing.com/products-page/911-bumpers/911-splitter/) below an RSR bumper with the chin spoiler.

What do you guys think about integrating two NACA ducts into the splitter to feed air to the baking plate? Would this be better, or worse, or even as good as the more traditional scoops that most 911 guys are running on the A arms?

Paragon has a backing plate with two inlets - I thought that I might try to run a second hose to the horn grill opening to supplement the flow.

[KTL]

Make your splitter adjustable with some struts/stays like these.

Longacre Spoiler Support - Black

They help ensure the splitter is at a downward angle. You don't want to rely on the shape of the bumper to dicate the angle of attack. You could have a splitter angling slighty upward that would create lift!

I think that's duct in the splitter is a good idea. The NACA duct is designed to do exactly what you are looking at doing- collecting air flowing parallel to the surface. Just have to take a look and see what kind of clearance you have. Brake cooling hoses need to be fairly large to be effective, since the flow is not very strong- especially when there's bends in the hose.

Pros who do this stuff for a living say the area at the outboard area of the front air dam is less effective at grabbing air than most people believe. Especially when you've got a contoured nose like the IROC style. Flat noses like the Peter Gregg style RSR or the 934 style are a bit better at the outboard area. But still, the air is hitting the center and then going thru (if you have a cooler there), up, down or sideways from there. Sideways air doesn't like to turn 90 degrees and go thru a duct. It likes to keep going it's merry way in a sideways direction.

If you have a front cooler, you could put the NACA ducts in the duct behind the cooler (assuming you have something behind the cooler to improve flow- either under the car or out the hood) and route those to the brake backing plates via hoses in the front trunk.

BTW the NACA ducts are nicely priced from Butler Built ButlerBuilt Professional Seat Systems


Also, there's a lot you can do to improve aero on the 911 IMO.

>Ride height/air going under car- less is more
>Front nose is a brick- 993 or 964 style noses are big improvement to cut thru the air. IROC or 934 improve front downforce but not all that aero friendly.
>Frog eye fenders/headlights protrude into airflow alot- convert to slantnose fenders
>Stock flag or aero side mirrors are quite large- install 935 style or small aftermarket mirrors
>Windshield wipers- take one off or both off when dry conditions
>Windshield seal has a lot of surface area- flush lexan or 993 style glass is a big improvement
>Shave rain gutters on windshield pillars or use spendy filler strips or even racer tape
>930 fenders grab a lot of air- canards in the front help
>Venting air from inside fenders helps reduce lift & drag
>Wide tires have considerable frontal area/wind resistance. Especially prevalent on long/fast tracks. Slightly narrower tire can improve top speed at expense of cornering grip. You decide what's most important. Usually grip!!!
>Need good rear spoiler for some rear downforce- wing with uprights to get it @ roof level is a proven improvement. Also add some vortex generators to improve flow coming of trailing edge of roof.
>Vent the rear bumper to let air out. The reverse curve on the lower valance of most rear bumpers is a good air brake/grabber that slows you down
>Air coming in the door windows grabs alot of air near the cage hoop. Can do some improving of flow there with some sheetmetal work.
>Vent the rear window by drilling some large dia. holes in the bottom. Has to be lexan of course.

Long list there and i'm sure there's many things I forgot.

What class are you running? '78 with a 3.4 sounds like a GTS-4 car. Do everything you can within the rules. Maximize weight-to-power ratio, run the fastest tires, do anything within the rules to develop the car.

[Jnikic]

Haven't dyno'd the car just yet, but I expect to be around ~240 rwhp which gives me the latitude to run around 2640 pounds with driver in GTS3. As it sits, with a full interior, a bolt-in cage and two seats, my car is 2450 pounds - add my 170 pounds, and I'm right where I should be once I exchange the interior weight savings for a more robust cage.

For now, I'm sticking with my big torsion bars, so the car won't be a front runner (even if it had the driver) from day 1. There's more than enough room for driver growth before I really need to start worrying, though!

I plan on adding a GT Racing GT3 style trunk lid and 60" wing to add some downforce - coupled with the splitter (thanks on the suggestions) and I think it will certainly help with high speed stability and limit time lost to e36 m3's.

Braking power isn't an issue with my stock brakes, slotted rotors, and pagid blacks - but fade during long, hot sessions is. So I'm hoping that the NACA ducts and the additional horn grill intake will suffice.

[Jnikic]

Originally Posted by KTL

Stock flag or aero side mirrors are quite large- install 935 style or small aftermarket mirrors

930 fenders grab a lot of air- canards in the front help

Venting air from inside fenders helps reduce lift & drag

Vent the rear bumper to let air out. The reverse curve on the lower valance of most rear bumpers is a good air brake/grabber that slows you down

935 mirrors seem like an easy enough change, so that's definitely getting added to the list!

With the dive planes, are these still helpful even without deep side skirts? From what I understand, they don't add downforce explicitly, but smooth air flow along the side of the car ...

Do you have any examples of vented 930 fenders? I've seen the 74 3.0 RSR versions, but I'm running (73 2.8) RSR 9" fenders, so they're basically the same as turbo fenders - but they're FRP so I can easily chop them up ...

Same with the rear bumper, that seems easy enough once I have a sense of what the right way to do this is. I'm going to start searching!

Thanks for all the great suggestions ...

[Mahler9th]

These cars have been around forever as you know, and folks have tried lots of things to increase performance. Folks adhering to tight racing rules, and folks prepping an "anything goes" car. And everything in between. Same with budgets.

Brake ducting mods are generally well-explored in my opinion. Taking air from scoops on the a-arms is a time-tested approach, and it is very likely that commercial products are still available. Or one could fabricate their own scoops out of materials like FRP, CF or some metal.

Taking air from ducts in the front bumper is also a well-explored approach. This is the approach I use.

I have one very clever friend (who just retired as a chief engineer for a very famous Porsche racing team) who installed NACA ducts in the hood of his personal 911 race car years ago. Hoses then routed air in the front trunk area through the chassis and back to the spindle-mounted backing plates. Very clever.

I have never seen anyone use NACAs on a splitter and I am not sure why one would want to do that.

Of course, routing hoses can be a challenge with some approaches. I decided on SRP manifolds years ago for my application. This type of thing is easy to make yourself if you have an application for it.

Of course it makes sense to ensure your backing plate covers the entire backside of your rotors so that whatever air is ducted goes where you want it. It is pretty easy to add material to commercially available backing plates or to fab up your own. I did the later and used carbon-kevlar.

There are plenty of threads on brake ducting approaches here in these forums. Perhaps you can use some of the ideas shared herein.

As for mirrors, I know of several cars, including mine, that are equipped with a single driver-door mounted product: The Vitaloni Californian. Works fine. Cheap and easy to deal with.

[aston@ultrasw.c]

I'm not quite seeing it.

From what I understand NACA ducts work by re directing the laminar flow from a flat surface with the air flow at 90 deg to the surface.

At the front the air is impinging at 0 deg and bunching up like a bow wave.

I think simple ducts facing directly into the air flow will work well enough.

[KTL]

Originally Posted by Jnikic

935 mirrors seem like an easy enough change, so that's definitely getting added to the list!

With the dive planes, are these still helpful even without deep side skirts? From what I understand, they don't add downforce explicitly, but smooth air flow along the side of the car ...

Do you have any examples of vented 930 fenders? I've seen the 74 3.0 RSR versions, but I'm running (73 2.8) RSR 9" fenders, so they're basically the same as turbo fenders - but they're FRP so I can easily chop them up ...

Same with the rear bumper, that seems easy enough once I have a sense of what the right way to do this is. I'm going to start searching!

Thanks for all the great suggestions ...

The canards/dive planes do help. It's been said alot that they don't add downforce per se'. They direct the air around the side of the car and reduce drag. Air that avoids entering the wheel opening and avoids going under the car results in an overall drag reduction. Meaning, the directive effect of the dive planes outweighs the drag increase of the dive planes themselves

I don't have any pictures handy of the vented fenders. Basically it's just a cutout on the trailing side of the flare that lets the air out. Typically I see them screened to avoid the openings from allowing the rocks, sand, rubber bits, etc. from marking up the side of the car and the front of the rear flares.

[winders]

The canards, or dive planes, generate little down force directly. The vortex they create that goes down the side of the car is the main benefit. The vortex draws air out from the wheel wells and out from underneath the car improving down force.

They actually increase drag but improve the overall downforce of the car.

Scott

[Walt Fricke]

I share the skepticism about NACA ducts being of any value in a splitter. Where is the flat surface with air flow parallel to it from which to draw, with little increase in drag, air to head to the brakes? I could see it in the hood, but not the splitter. The only flat surface I know of on a splitter is where you have a kind of undertray extending rearward under the car. If you put a NACA duct there, you would be drawing from a kind of laminar flow.

But the air under the car is hotter than the air even a bit higher up. More importantly, the air under the car is at a lower pressure (or you sure hope it is) than the air flowing up over it. That's what downforce (or less lift) is all about. But drawing air from a low pressure area doesn't seem like the way to get more air to flow by itself through a tube to the brakes.

I think the air piling up in front of the front spoiler is apt to be at a higher pressure, and it is what most use with 911s.

There is a trend to go to Cup/GT3 style brake cooling, doing away with hoses. A vertical diverter an the back of the spindle assembly or thereabouts catches air flowing backward through the wheel well. Canards or the like create a lower pressure area across the outside of the wheel well opening. The differential in pressures pushes/pulls air through the brake rotors. I've heard of reports of amazingly cool rotor temperatures on older cars retrofit with this.

[Mahler9th]

I think it is fairly important to consider that the terms "dive planes" and "canards" are used pretty loosely in many discussions. Some of the "doo dads" folks apply to the front bumpers of their race cars may generate downforce directly-- it depends on the design and embodiment.

I have never seen a technical article that describes/discusses dive planes and/or canards on am air cooled Porsche 911 application. Perhaps there is one somewhere though.

I am pretty sure that if the design and embodiment are technically valid, then sure one could consider indirect aero benefits greater than any direct downforce. But I have seen some pretty wild "doo dads" on the front of amateur race cars that may add direct downforce.

I have not seen any standardized approaches for front bumper doo dads for any of the commonly used air cooled 911 front bumper shapes, nor any aero analysis for any one-of approach.

I have experimented with some shapes for my bumpers as have some of my friends. There are threads on this.

As for added aero benefit versus drag cost, I am unaware of any analysis or testing.

The only 911 aero work of which I am aware that likely had an engineer involved is the stuff the Crawfords did for the Kelly Moss 993.

[Cajundaddy]

These cars have been around for a long time so resist the temptation to re-invent the wheel. See what the front runners with a similar car and setup are doing and do that. Clean sheets of paper and revolutionary ideas often get expensive when things go wrong at 130mph.

[js154]

Originally Posted by Jnikic

I'm making a few upgrades to my track rat so that I can start racing wheel-to-wheel next season in NASA GTS series.

Brake cooling and aero are two areas that I think I need to make improvements to be somewhat competitive.

I plan to run a GT racing splitter (https://www.gt-racing.com/products-page/911-bumpers/911-splitter/) below an RSR bumper with the chin spoiler.

What do you guys think about integrating two NACA ducts into the splitter to feed air to the baking plate? Would this be better, or worse, or even as good as the more traditional scoops that most 911 guys are running on the A arms?

Paragon has a backing plate with two inlets - I thought that I might try to run a second hose to the horn grill opening to supplement the flow.

I've done this, it would really only be of benefit if you were trying to reduce drag and brake cooling wasn't an issue, such as at tracks like Daytona or road America. That way you can seal up the front of the car better.

It works, it definitely makes getting the bumper cover on and off trickier, but if you're looking for every last bit, and you've already maxed out your brake options and.put good thought into your actual aero needs ( as opposed to Big Wing and dive planes, becuz everyone else in yrs, yo!) Then it can make a difference.

I know it helps at Lime Rock which is easy on brakes, and by closing up the front ducts and running splitter ducts, you can gain a little more splitter reinforce while at the same time evacuating a little more air from underneath. Long story short, more front grip, which for my car (Gulf E30M3) definitely helped.

[3rd_gear_Ted]

Originally Posted by winders

The canards, or dive planes, generate little down force directly. The vortex they create that goes down the side of the car is the main benefit. The vortex draws air out from the wheel wells and out from underneath the car improving down force.

They actually increase drag but improve the overall downforce of the car.

Scott

Also for my NON ABS 911 track car the canards on the 964 nose really helps for high speed braking, the areo loading on the front nose outer edges helps equally "lever" the front brakes to bite better. My braking point off the Roval at Fontana into the infield got moved up big time.
The increased use of front areo devices IMHO is about the only change I've noticed in 911 racing set ups over the years that I've seen.

I still dream about a perfectly smooth car bottom though, therapy has helped a lot.

[RichardNew]

Back up for a minute. Do you have any idea how hot your brakes actually are?

Before you start reinventing things check the temps on both your rotors and your calipers.

Brake Temps 101



Richard Newton

[KTL]

Originally Posted by 3rd_gear_Ted

I still dream about a perfectly smooth car bottom though, therapy has helped a lot.

For some reason this Excellence article always comes to mind when someone mentions the smooth underbody. Gretener indicated that it made a noticeable improvement!

A Wolf in Wolf’s Clothing : page 5 | Issue 223 | Excellence