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Old 01-26-2020, 03:11 PM   #1
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Hood Extractor for Downforce

I watched a little too much 24 Hours of Daytona this weekend and ended up cutting up my old stock hood (and inside my trunk..yikes). I don't have my splitter installed in these pictures but the idea is to increase front end down force. I have a wing installed on the rear (not at the moment) so I'm trying to balance it out a bit. ITs just roughed in at the moment, any advice? I still need to create the ducting for the trunk.







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Old 01-26-2020, 03:40 PM   #2
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Very cool and very interested.
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Old 01-26-2020, 06:36 PM   #3
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I love it and have been trying to talk a friend who races into doing that. I personally think from a visual perspective the hole needs to be smaller and from an airflow perspective it doesn't need to be that big. Not unless you plan on opening some more in the front. I love your headlight air intake idea too
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Old 01-26-2020, 09:42 PM   #4
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I'm planning on doing the exact same thing on mine. Will be watching this with great interest!
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Old 01-27-2020, 05:50 AM   #5
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Well unless you have access to a wind tunnel, you only have a butt dyno to calculate actual pros or cons.

Non tested changes to aero can have detrimental effects

It does look cool however
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Old 01-27-2020, 06:50 AM   #6
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Quote:
Originally Posted by JayG View Post
Well unless you have access to a wind tunnel, you only have a butt dyno to calculate actual pros or cons.

Non tested changes to aero can have detrimental effects

It does look cool however
Well yes, ok, technically youíre right but this is definitely one of those Internet sayings that is mostly bull****. Sure it requires some work to get right but everyone running in non-restricted class has made aerodynamic improvements without wind tunnels or CFD models with fantastic results. You just have to follow proven aerodynamic approaches. My wing knocked almost 3 seconds off my VIR lap time day one (after spending a year looking for tenths of seconds else where). Sure tunning, testing and suspension adjustments were required after the fact to dial the wing in but this stuff isnít as hard as the internet would lead you to believe. In fact I think aero is easier and lower risk than suspension modifications.

As far as butt dynoís go track guys have a lot of tools at their disposal now. Not only do we have lap times, segment times, etc but we also have GPS data, lateral acceleration,, braking performance etc that can be over-layed with pre-modification laps. I can tell you how this modification is performing on every inch of the track. Really cool stuff.
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Old 01-27-2020, 04:26 PM   #7
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Smile

double post

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Old 01-27-2020, 04:30 PM   #8
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Originally Posted by WizardSmokey View Post
I'm planning on doing the exact same thing on mine. Will be watching this with great interest!
Me too! Similar but different.

truegearhead, I wishfully concur. Good post.
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Old 01-28-2020, 02:50 AM   #9
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I nkocked out most of the aluminum work last night, I still have a couple of pieces to make though. I'm going to drop the hoof off to be wrapped and then I'll pop rivet it all together. I definitively need make the bumper opening larger too. All in all this has been a surprisingly easy project so far





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Old 01-28-2020, 08:31 PM   #10
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Question: How did you determine the dimensions?

I ask because I don't know how size is determined for something like this and I'd like to hear your thinking...
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Old 01-28-2020, 08:39 PM   #11
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Question: How did you determine the dimensions?

I ask because I don't know how size is determined for something like this and I'd like to hear your thinking...
Ditto, and especially the radius of the bend, and such.

I love it.... and it's even cooler if it works.

Teach me! ;-)

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Old 01-29-2020, 09:05 AM   #12
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Not very scientific but I just made it as big as I could while still keeping the transition from the front of the car smooth (still not as big as Porsche 911 RSR's hood extractor (see below)). Also nothing scientific about the radius, I just cut and bent the hood to where I wanted it (again smooth transition from the opening in the grill) and then cut a punch of cardboard up to make templates. Once I had templates I liked I traced them onto aluminum and used a jigsaw to cut the shapes out. To make the 90s I used a dremel to edge in a line and then used a hammer the 90 degree bends in. If you moved the brake fluid reservoir you could get closet to the Porsche design


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Old 01-29-2020, 09:36 AM   #13
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Quote:
Originally Posted by truegearhead View Post
... If you moved the brake fluid reservoir you could get closet to the Porsche design
Great innovation. It's always nice to have a spare hood around. How about cutting deeper and having a bump/cowling for the brake reservoir?
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Old 01-29-2020, 10:50 AM   #14
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Originally Posted by 78F350 View Post
Great innovation. It's always nice to have a spare hood around. How about cutting deeper and having a bump/cowling for the brake reservoir?
Not a bad idea. I think if I were to do it again I’d relocate just the reservoir which I don’t think would be too hard. The hood will more than likely not hing anymore it’ll have to be removed so it would be nice to move the reservoir inside the car.

Actually does anyone know if this is possible? The stock reservoir has two nipples on the bottom so theoretically I could just move it into the cabin and run lines (as long as it was higher than the master cylinder). What do you all think?
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Old 01-29-2020, 11:49 AM   #15
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so - downforce *and* reduction in lift. two things happening here.

original centre rad is tilted down; air passes through rad and is deflected down and under the car. this creates upforce (ditto the side rads which deflect air downwards; good diy on pelican about how to vent your wheel well liners to introduce air into the wheel well instead).

this also introduces air under the car which creates lift. car is shaped a bit like an airfoil; air traveling under the car follows a straight line, while air traveling over the car follows a curved, longer path. since the air all has to finish at the same place at the back of the car then the air traveling over the car has to move faster. pv = nrt so this air is less dense and creates a low pressure area above the car - lift. the more air moving under the car and the move lift is created. this is the reason for splitters and side skirts - to limit this air. this is also the reason for rear diffusers - to extend the travel distance of the air under the car and reduce the lift effect; a big enough gain that the latest porsche 911 rsr is mid-engined - to make more room for the diffuser.

so, design your system accordingly. porsche did. the gt3 'smile' vent is the same thing. if done properly w oem parts the centre rad is angled the opposite direction and there is an oem duct that sits behind it to direct air upwards - 99657532530.

have a look at the attache photo - note the low pressure area on the roof - lift. but also notice the low pressure area on the front of the hood - this is where the gt3 vent is located - the low pressure assists w extraction and airflow over the rad. move it further back and the pressure increases. note also the low pressure over the front wheel wells; if you do decide to vent your side radiators into your wheel wells, a great way to vent the wheel wells is a fender vent located in this low pressure area - turn upforce into downforce.

https://www.flickr.com/photos/jalopnik/422044172/
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Old 01-29-2020, 02:18 PM   #16
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^^^^^^ above

That's exactly why you need to wind tunnel test or use CFD modeling
Aero is a fairly complicated thing.
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Old 01-29-2020, 02:49 PM   #17
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well, make the opening too small relative to the front opening and air will have to compress to flow through and most likely get cooling issues as you reduce airflow over the rad. not sure what the issue is with a larger opening; presume air would expand and have less force to oppose any positive pressure over the hood so perhaps again reduced airflow. w the porsche gt3 vent the output is about the same size as the input.

w the 911 rsr posted the vent is quite big - no side rads on this car so there is one big front rad and opening to match. not sure what is up here; i think the way it is set up (airflow back at an angle vs directed straight up for downforce) i think they are looking to actually shorten the travel distance of the upper airflow - for all intents the nose of the car is shorter. lots of trade-offs here that i presume they have addressed - the bigger front rad i presume creates more air resistance than a smoother shape, and the increased airflow over the top of the car (vs slamming the air straight upwards) will increase lift (although not as much as the travel distance has been shortened) but reduce air resistance.

this additional airflow over the top of the car interacts with what is hanging off the back; again downforce vs lift. a big wing hanging off the back will create downforce, while a 'spoiler' (gurney flap?) is designed to kill airflow; creates dead air in front of it which forces the airflow over the top of the car to delaminate from the car and kill the lift effect. this delamination takes work, however, too much and you increase the energy required to push the car through the air. of course a wing and a spoiler work in conjunction; spoiler has to be in clean air to work.

final thought would be to tune the car to the track you are on; are you going so fast that the aero aides are slowing you down (ie,, a rear wing takes more force to push through the air as speed increases)? or is it a more technical track with lots of turning where you want mega downforce at the expense of top speed?

regardless, i wouldn't be afraid of playing w aero; faster is faster or it isn't. and if you increase downforce on the front such that your rear now feels too loose, don't devolve, just pin the rear down better.
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Old 01-29-2020, 03:22 PM   #18
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Here's a link to where Fred posted his basic Boxster aero at 180 kph.
https://www.flickr.com/photos/gtcollection/with/32589573982/
Although it's the stock aero, its a good starting point to see where you want to adjust the flow. Center-line cross section of Speed, Density, Pressure, and Turbulence. Just be mindful that as you bend or compress the air, you add more drag.

Edit: *Deleted the pics, they're at the link and posting them all felt like it cluttered up the thread.
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Old 01-29-2020, 04:26 PM   #19
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Quote:
so - downforce *and* reduction in lift. two things happening here.

original centre rad is tilted down; air passes through rad and is deflected down and under the car. this creates upforce (ditto the side rads which deflect air downwards; good diy on pelican about how to vent your wheel well liners to introduce air into the wheel well instead).

this also introduces air under the car which creates lift. car is shaped a bit like an airfoil; air traveling under the car follows a straight line, while air traveling over the car follows a curved, longer path. since the air all has to finish at the same place at the back of the car then the air traveling over the car has to move faster. pv = nrt so this air is less dense and creates a low pressure area above the car - lift. the more air moving under the car and the move lift is created. this is the reason for splitters and side skirts - to limit this air. this is also the reason for rear diffusers - to extend the travel distance of the air under the car and reduce the lift effect; a big enough gain that the latest porsche 911 rsr is mid-engined - to make more room for the diffuser.

so, design your system accordingly. porsche did. the gt3 'smile' vent is the same thing. if done properly w oem parts the centre rad is angled the opposite direction and there is an oem duct that sits behind it to direct air upwards - 99657532530.

have a look at the attache photo - note the low pressure area on the roof - lift. but also notice the low pressure area on the front of the hood - this is where the gt3 vent is located - the low pressure assists w extraction and airflow over the rad. move it further back and the pressure increases. note also the low pressure over the front wheel wells; if you do decide to vent your side radiators into your wheel wells, a great way to vent the wheel wells is a fender vent located in this low pressure area - turn upforce into downforce.
The rads dumping air down is definitely a problem I cut holes in my wheel wells to vent to the wells which also isn't great. I was thinking about venting the rear of the wells so the air didn't eventually go under the car but I'm probably getting carried away. I do have a splitter (or did before I hit a tire wall last year) to help reduce the air under the car, I'll remake it after the hood project.




Quote:
this additional airflow over the top of the car interacts with what is hanging off the back; again downforce vs lift. a big wing hanging off the back will create downforce, while a 'spoiler' (gurney flap?) is designed to kill airflow; creates dead air in front of it which forces the airflow over the top of the car to delaminate from the car and kill the lift effect. this delamination takes work, however, too much and you increase the energy required to push the car through the air. of course a wing and a spoiler work in conjunction; spoiler has to be in clean air to work.
I went went for both (spoiler and wing), the wing resulted in the most lap time and grip improvement but with just the spoiler I increased my top end and saw a small improvement in lateral acceleration. So no downside at all





Quote:
Here's a link to where Fred posted his basic Boxster aero at 180 kph.
https://www.flickr.com/photos/gtcollection/with/32589573982/
Although it's the stock aero, its a good starting point to see where you want to adjust the flow. Center-line cross section of Speed, Density, Pressure, and Turbulence. Just be mindful that as you bend or compress the air, you add more drag.
Great info, last year I rerouted my intake to my rear windshield due to a complication with installing a 2.7L engine with a 2.5L manifold (probably could have made the stock setup fit with some fab but this was way cooler) and ever since I've had people come up to me and tell me its a bad location due to the lower pressure on the rear window. Looking at the graphs it isn't actually that bad!
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Old 01-30-2020, 10:01 AM   #20
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I've got nothing to add but I do like what you're doing! I'm not sure it was the first, but the Ford GT40 was the first car I ever noticed that ever had this kind of extractor!

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