topley

Hi all,
So I know I need to vent the front fenders behind the wheel well but I'm looking for info on where, shape and how big is necessary. I certainly love what 333pg333 has done to his car but extending the vent through the door is beyond what I would be capable of for this car. I have 1" wider fiberglass fenders so it should be a simple matter to do, but should it be above the break line, below the break line, 2 slots one above one below etc? Can I just cut the holes or does it need some kind of interior vent panel (for airflow or for stiffness?) Also what is the purpose of the vents along the top of the fender above the wheels... same thing?
Thanks
Jay

rlm328

Why do you think you need to vent your front wheel wells? My front fenders are 1.5 inches per side (3 inches total) and they are not vented.

951and944S

Just a warning on the fender top vents.

In time attack and other types of solo fast lap venues, the conditions are usually great.

In multi car racing, you practice, qualify and race rain or shine.
The fender top vents put a lot of water in your field of vision, and worse, on a wet but not rainy track, the surface water is muddy, oily and gritty so even if you continue running a slow speed wiper, with no fresh rain to keep your windshield clear, you're gonna have a mess to see through.

T

Paulyy

Pretty sure that's apart of his aero package, not venting.

topley

OK, maybe the word "need" was a bit too much, replace with "I'd like". As to why, I have this from an engineering forum:

"One reason is that the airflows around the fender and underbody ahead of the wheel well tends to be of higher velocity and lower pressure. So when it dumps into the larger (diffusing) volume created by the wheel well, the airflow naturally loses velocity and locally increases in pressure.

To mitigate this situation, closed wheel race car bodywork use spoiler lips at the front edge of the wheel opening, fender louvers above the wheel well, and extractor ducts behind the wheel well. All of these devices serve to create a zone of reduced airflow pressure within the wheel well volume."

And there is a pretty good diagram 3/4 down the page here:

http://superhachi.com/theory/downforce/

So I have seen enough to convince me that it could be a good idea but I know a bad implementation is worse than no implementation.

333pg333

While all cars are different, there are possibly some basic rules of thumb that you can use without too much fear. Venting the fenders is mostly to reduce lift and drag. There's a ton of drag in the wheelhouse area and if you can manage to release that pressure then it's normally going to be a good thing. As Mr T says, the caveat being wet running. I've not run my car in the wet for years so it may be a good point. If you were concerned about this perhaps make the vents so you can run a blocking sheet underneath if you're going to run on a wet track? Venting behind the wheel is also a good thing. Note the 991 GT3RS and what the factory have done. We have gone the whole 9 yards in clearancing some of the door A pillar to allow venting through the door. You wouldn't have to do this to improve your Aero efficiency though. Just got to look around and see what's our there, or make something up yourselves in regards to wider / vented fenders.

333pg333

Oh, and I'd add to this that if you're going to run a large rear wing then it's definitely worth venting various aspects of the front of the car to try and redress the overall aero balance. Generally you're chasing more front downforce when you install large rear wing.

topley

I do do plan on eventually doing a wing (most likely a good aero one...)

Humboldtgrin

How about venting below the break line behind the tire?

Duke

Venting the front fenders is an easy way to reduce a bit of front-end lift.
I happened to have a set of vents, the same as Patrick, for sale. Never installed.
They are just laying around so I'd be happy to sell them

topley

"How about venting below the break line behind the tire? "

Probably what I'm going to do... rectangular opening taller than it is wide under the break line right before the rear of the panel.

"I happened to have a set of vents, the same as Patrick, for sale. Never installed."

Do you mean the top ones above the tire? If so I think for reasons stated earlier in this thread (junk/rain on windshield) I'll stay away from those though they look great for down force. If its something else???

951and944S

Yep, that's what I'd do.

Luckily for my current project race only car, I already have fiberglass front fenders but you could make your own vents if you have a little fitting and welding skill.

Just cut the forward vertical with an .045" thick cutoff wheel, then the upper and lower horizontal cuts. Leaving the rear metal intact, push the front inwards an inch and fit new triangular shaped horizontal upper and lower walls to close it back in.

I would take the vents Duke has but the shipping would be higher than buying in the USA but I would use them on the lower rear 1/4 panel below the rear marker light.

I am cutting out my rear back wheel well to intersect with the lower right and left storage hole in the deck area, then closing it off flush with the deck level and using that as an air extrication conduit to to vent rear wheel well to outside the rear 1/4.

On frontal pressure, part of my aero plan is to notch down the firewall, place a custom tray that reaches the rearward part of the hood and add a wicker lip to help pull air from under the hood.

This way, that air will flow over the windshield and roof as the rest of factory airflow is designed anyway.

Two effects for one modification, reducing front lift and increasing flow over the rear wing.

The factory only sealed the firewall with a weatherstripping to keep engine fumes from entering the HVAC blower motor suction.

T

951and944S

You can find all types of wing profiles with verified data all over the internet.

Pick one with the downforce/drag/chord and width that suits your needs.

Print it out to scale.

I have done this two ways, made my own hot wire cutter, or b) search around competitive R/C airplane sites and find someone who cuts styrofoam wing cores.

Take your printed to scale wing profile, transfer to 1/16th" aluminum sheet and make two exact duplicate end plates.
Fix them together stacked and dress them with a file to eliminate any variable in your cuts and make them exact mirrors. These will be the pattern for the foam cutter.

For about $30-$50 including shipping, you will receive back a perfectly cut foam core. Request that he put a hole bore across the entire core width near the leading edge where you will later install a glued strengthening spar.

From there, you figure your wing mount upright locations, carve out a rectangular section and install an aircraft quality plywood block. Before gluing into the wing core, drill two spaced holes and install two t-nuts from the top down. These will be your threads to attach wing to your uprights or inclination cradle.

Do the same steps to create threads for your wing end plates.

Now you start laying the cloth weight of your choice and epoxy resin over the foam in multiple layers keeping in mind that the core remains flat, true and not twisted until a strength is achieved that satisfies you. Low oz/ weight cloth gives a better finish on your last one or two layers.

That's it.

Prime, paint and install.

You can make your own entire wing with certified performance for $100.

Plus, you get to say you did it when someone asks where you bought it from.

Save the $1500 for tires....

T

rlm328

Here is what I did with my rears. I will send better pictures when I get home. Please note my rear fenders were widened 1-1/2 inches on each side for a total of 3 inches.

topley

Well crap!
Thanks a lot T. My morning is shot now having spent it on youtube looking up how to make my own race car wing. The afternoon isn't looking too good either as I'll probably be bouncing back and forth with how to make my own hot wire setup. Keep it up and I'll NEVER finish this car (I mean I need it drivable, but hopefully never finished, this is all so much fun...)

BTW front fenders are fiberglass (have 3" wider rear quarters but I'll save those for another engine) and I'd really like to do something like this:


Only push the leading edge all the way to the chassis behind the front wheel. Unfortunately I plan on adding the boxster ABS to my car eventually and it'll have to go inside the passenger side fender.