Hood Extractor for Downforce - Page 3 - 986 Forum - The Community for Porsche Boxster & Cayman Owners

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.

Very cool and very interested.

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

I'm planning on doing the exact same thing on mine. Will be watching this with great interest!

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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Well yes, ok, technically you’re 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 isn’t 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 dyno’s 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.

double post

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Me too! Similar but different. truegearhead, I wishfully concur. Good post.

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

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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Ditto, and especially the radius of the bend, and such. I love it.... and it's even cooler if it works. Teach me! ;-) Sent from my SM-G970U using Tapatalk

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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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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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?

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/

^^^^^^ above That's exactly why you need to wind tunnel test or use CFD modeling Aero is a fairly complicated thing.

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.

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.

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. 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 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!

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!

I love it. This is a super cool mod, and I'll be following. May end up doing something similar myself.

I have it all pop riveted together (partially). Now I need to file down and clean up the edges, take it apart, wrap the hood and then drill and rivet the rest of it together. All in all I probably have a full day wrapped up into getting to this point. A few lessons learned 1) make template after template, use lots of cardboard. 2) make panels one at a time, rivet them to the hood then recheck your templates for the remaining pieces. dimensions change as the rivets go it. 3) change your jigsaw blades often to keep cuts clean. 4) use tape to protect aluminum surface 5) don't cut aluminum big with the plans to trim. Get the template 100% right then cut 6) measure the straightness constantly, it will want to pull in different directions.

I'm liking your improvements to it. How do you get under the hood now? It looks like it's riveted to the body

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Thanks, I played around with designs that would allow the good to swing up but it would have been a mess. Now to remove the hood it needs to be lifted off. All of the aluminum you see is attached to the hood.

Ok now I’ve really lost. I posted this up on Grassroots and a very knowledgeable guy that’s been helping me told me that due to the size of this thing what I really need is a “cowl flap” and to make it efficient it Really should be adjustable. What if....I used the Boxsters 70mph spoiler deployment system to automatically raise and lower the flap...and change the pitch of the rear wing. It would work in the opposite way the stock wing works though it would maximize downforce below 70mph. I wonder if just reversing the poles on the stock actuators would reverse the direction they operate, anyone know? Alright my crazy pills are wearing off.

Ok I’m back and with some more crazy piles. So using the stock Boxster actuator system won’t work because it deploys at 70 and retracts at 45 or something. I would need a signal that’s consistent at 70. I wonder how the stock system is being signaled, maybe they’re is a consistent signal I could use from it. Anyone know?

If it was me, I would slap an Arduino in there as a control system.

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That's what I was thinking.... but I'm not an arduino pro... and I was afraid he'd ask me how, hahaha. Are you a guy who can build stuff like that? Sent from my SM-G970U using Tapatalk

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Yeah I do it all the time. I haven't done it on a car yet(can't be that different), but that's part of what I have planned for my '97.

There's a few factory vehicles that have a similar hood extractor. Do they have adjustable flaps on them? Seems to me you're starting to over engineer it. The more complex items, the more chances of something breaking and things break a lot on race cars. Just my opinion

I love your guts, not anyone would attempt this.. :-) However, I´m not totally convinced regarding the benefits. If you had a middle radiator I´m sure venting it via the hood is the hardcore way, better than via "the smile" and far better than the standard venting under the car. (given reasonable angles and size of outlet) As a stand alone aero-mod not needed for venting a radiator I'm afraid it will give quite a lot of drag, If I where to attemt it I believe I would go for a smaller outlet, not let the opening go so far forward. Not that you asked for my subjective opinions.. I´m certainly no expert! :-) Love this kind of topics and creative mods, looking forward to upcoming updates!

Love this project. I suggest mixing up some flowviz to see what the airflow looks like across the hood. Based on that you can adjust the angle of attack/radius to keep the airflow attached. EDIT: if you haven’t already, you should read this: https://read.amazon.com/kp/kshare?asin=B01N25RRVT&id=7NWd7w_BSJ-sgyVpdT7aNA&reshareId=JJCPBHBDVCBEFBYGWMRF&reshareChannel=system

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I could do it - could use a 5 or 10 HZ GPS to monitor speed and send a signal to raise or lower the flap. Right now I am running an arduino powered homemade GPS latching speedometer in my car that latches and displays my vMax on the straights and my vMins on the corners. [Edit - tapping into the car signal instead of a GPS would probably be a lot better but I have no idea how to do that ] Check out the digital screen in the lower left of this video. On the left is a real-time GPS speedo, and to the right is the pinned vMin or vMax speed. (The vMin is really useful, when I am trying to build speed through a corner I can peek down on the next straight and see how I did). There is a little overlap due to it being such a tiny screen - the intent was just to display the vMin/vMax but I had the current speed on there for debugging, and just left it there. For this we could define two speeds, one at which the cowl would open and one at which it would close. Separate them by some amount (5 mph?) so that it doesn't oscillate when you are near the threshold speed. If you want to do it, I'm in. I have never controlled a motor before but we could figure it out

You could easily use a voltage divider circuit from the VSS / wheel speed sensor to feed a speed reference into the arduino. From there, map an output to a relay on/off at whatever speed you want to deploy and retract. No gps lag then.

I love this project! Is there any thought to create a slight ridge on the leading edge of the vent? Most motorsport-oriented vents I have seen are slightly "proud" of the plane of the hood. I gather that this creates a small vortex which aids in extraction. Have a look at modern cars like a McLaren P1, or more basic executions like the original racing GT40s.

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Oh yeah that would be a lot easier/better than adding a GPS I am following this so I can learn new things. Since I only have experience with arduino GPS projects that's what everything looks like to me

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That would be very effective, and would lessen the need for such a large duct like truegearhead has made. The same thing on the leading edge of the front wheel wells will give you similar results. Just those two modifications would help lessen lift, which many people call adding downforce. It isn't adding downforce, of course, but if you eliminate 200 lbs of lift, it's the same as adding 200 lbs of downforce.

This was at COTA this weekend. I didn't see any movable flaps on it

Last week I finished the hood and splitter and then headed to the VIR to run the full and grand course. Being the first time out of the season I wasn't able to get any good data, but it is generating some front end downforce but also seems to be impacting my top speed (again hard to gauge since it was so cold out). I did notice that force of the air hitting the inside duct caused the hood to bow up, I think decreasing the angle of the duct would make a huge difference, I'm going to add some plates to help direct the air up wards before it hits the back of the duct and see if that works. In other news man it felt good to get back on the track now that winter is over!!

Any more follow ups? My knowledge base and experience would say the best way to add front downforce, regardless of actually adding downforce verse eliminating lift, would be to ditch the side RADS and use a center only rad in the duct that you built. In stock form the side rads pack air under the nose of the car in front of the tires and even opened up into the wheel while you're packing the wheel well with air which we already know is a high pressure area generally.

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'Truegearhead' has made *SIGNIFICANT* changes to the car since 2020. I'm not sure how often he is on the 986 Forum now, but he updates on another forum regularly here: https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/forget-tt5-lets-go-off-the-deep-end-twin-turbo-v8-widebody-boxster/193174/page1/ Here's some more recent pics:

Thanks for the link, cool to see more of his project!

This may be redundant, but have you tested with yarn yet? Tape yarn to the hood all over, especially the trailing edge of your extractor hole, and see what it does. It'll give you a lot of information about where your high pressure, low pressure and attached/detached flow areas are.