Race Car Rear wing and Aero???? 240 IMSA GTU Car

[cobramatt]

I have been posting pics of my build over on the "Members Projects" page (My Dream LS240Z Car) and I need a little help. The car has an IMSA wide body kit and looks great. However with the 600 rwhp we are going to generate from the built LS engine I am concerned about the present spoiler setup. I am thinking that we need to fabricate a rear wing setup and front aero (to compliment the G nose) to create some more downforce. At 2000 lbs. and that kind of power, at anything past 150 mph its gonna get even lighter. I know there is someone out there that has either modified or fabricated a setup that works. Our plan was to test the car with the present package at the track then take it to the wind-tunnel for Aero testing. I just don't want to re-invent the wheel here.

 

Any suggestion will be helpful. This setup needs to be fully functional, aesthetics are important but I see to many cars at the track that the setup actually causes drag versus downforce.

Thanks,

 

Matt

[JMortensen]

You might start by reading the wind tunnel test results and some of the other threads in the aero section. You can run the spoiler and a wing (link shows a 911 with ducktail and wing that he built from scratch): http://forums.pelicanparts.com/porsche-911-technical-forum/294153-poor-mans-aero-wingin-3.html

 

You'll still need to balance the front end with the back. The back is pretty easy, just keep adding wing until you get what you want. The front is where it gets a little complicated, as it is harder to decide what to do to get downforce there. Your question is potentially huge and open ended, so I'd suggest you do a little research and then come back with some more specific questions. Otherwise you're probably just going to get generic recommendations like building a duct from the front to the radiator, splitter, wing, vortex generators, etc. You'll likely be blazing trails for the rest of us as well.

 

If you get really into this subject you might try a couple books like Competition Car Downforce and Competition Car Aero by McBeath or Race Car Aerodynamics by Katz. Every car is different and without another wind tunnel test what you'll have is a lot of speculation and experimentation, but there are some general overriding principles that should work and be applicable to your car.

 

Looks like a hell of a car. I'll be keeping an eye out to see what you end up with and how it works out for you.

[JMortensen]

Oops, wrong link. Here you go: http://forums.pelica...g-many-pix.html

 

EDIT--I think the first thing I'd do is get rid of that hood. NACA ducts are going to put more air under the hood, and it looks like the louvers face the front too, so they would also push air into the engine compartment. You want the exact opposite.

Edited May 12, 2011 by JMortensen

[cobramatt]

Oops, wrong link. Here you go: http://forums.pelica...g-many-pix.html

 

EDIT--I think the first thing I'd do is get rid of that hood. NACA ducts are going to put more air under the hood, and it looks like the louvers face the front too, so they would also push air into the engine compartment. You want the exact opposite.

 

Thank you for the input. I have read the wind tunnel test results and there are some conflicting opinions about the subject. My thought was to find a clear thread on... "This is how I did it, this is how we tested it, this is how it works, this is how much it costs". That's not asking for too much is it???

 

My thought was for some canards up front. Ducting. A splitter. A diffuser with as much underbody paneling as possible, when I go to the track I don't have a pit crew helping me, so I want it to be functional but accessible. A rear wing that was more than likely made for another car (Porsche GT3???) and fab it to the deck lid. I also have a question about how to reinforce the struts for the wing, as the deck lid thickness is not exactly beefy.

 

Your thoughts on the hood are noteworthy. The louvers are not facing forward though. (and I loved that hood)

 

Matt

[Robftw]

Just taking a glance the immediate problem i see are the naca ducts, also you could probably get away with some canards up front, IMO because of the nature of this project you should start small and move up, Canards / whale tail and go from there. Hate to say it but it could be a very expensive trial and error type of thing with the wind tunnel

[thehelix112]

I'd be:

 

• moving the bonnet vents a bit more forwards. I think you want them where the air is lowest pressure so they suck as much out from the engine bay as possible, and the closer they are to the windscreen the closer they are to that high pressure zone.
  
• experimenting with blocking up as much of the front intake until you notice engine temperatures start to rise. The additional advantage here is if you run canards this might force more air around the sides and over them.
  
• butchering those front and rear guards to vent above and behind the wheel as much as possible. These could act as a scoop and just add drag I think.
  
• flat underbody combined with a nice rear diffuser
  
• front splitter where you can adjust how much it sticks out
  
• rear wing with adjustable height and angle of attack
  
• vortex generators on the roof to try and keep the flow down the hatch attached
  
• extending the fronts of the flares to ensure they completely cover the wheel from on-coming air.
  
• side skirts as low to the ground as applicable for your tracks/suspension setup.
  
• open up the cowl into the interior and run pipes from this point and exit it either out the rear hatch or out where the license plates go. This could help reduce the windscreens high pressure zone which is both drag (bad) and negative lift (good) inducing for perhaps more efficiency on the rear spoilers and lower drag.
  
• recording everything so I don't actually have to do it to know what works.
  

 

In approximately that order. However, this is not based on any actual experience, just complete guess work.

 

Dave

[JIM73240Z]

I would have the rear diffuser near the bottom of the list of items to try. The z car is not exactly clean on the bottom. A belly pan / splitter to the x member and call it good. vg's are a must for a wing. Although there there was a comment from Ernesto Roco who had an insane mustang said that he picked up 200-300 rpm at the end of a straight without the vg's. you can find him on youtube. The further down the splitter is and further out in front, the more down force. Vent the wheel wells, duct the radiator, and some canards. i do not think we can get much more from these old bricks.

 

Jimbo

[JMortensen]

Thank you for the input. I have read the wind tunnel test results and there are some conflicting opinions about the subject. My thought was to find a clear thread on... "This is how I did it, this is how we tested it, this is how it works, this is how much it costs". That's not asking for too much is it???

You have to keep in mind what the test was done for. It wasn't done to see what would make maximum downforce. It was done on relatively stock Z's to see what the typical bolt on aero mods did. In that respect the test was very effective and answered your questions directly. I think you can pretty well just look at the numbers and draw your own conclusions.

 

It could have been a lot more useful to you and me if it were done specifically to see what worked to make maximum downforce, but that really wasn't the purpose and it's not really fair to the guys who did the testing to complain about it now. Most (all?) of the arguments surrounding the wind tunnel testing have to do with things that WEREN'T tested, where people are speculating on whether something could have worked, or whether X which wasn't in the test works better than Y which was. Only real way to settle those arguments is another test.

 

If your louvers face back, then you might just cover the NACA ducts in the hood. I know Richard (260DET) did that on one of his hoods..

[260DET]

I'll stick to just one aspect, front aero, seeing that's what I'm working on right now and assuming you that have read all the other comments. You need a splitter and undertray to start with. Splitter is pretty simple, there is plenty of info around about them. The thing I found difficult to get was info on was an effective undertray. My conclusion was flat is fine provided it's angled slightly higher at the back than the front by a few degrees. Diffusers into the front wheel wells are good (it's a low pressure area) as are big front guard vents to exhaust wheel well air, either out the top of the guards or out along the side, or both.

 

You are on the money with querying aero, with big power aero is crucial. But, bugger me, front aero is hard to work out

Edited May 13, 2011 by 260DET

[jt1]

You have to keep in mind what the test was done for. It wasn't done to see what would make maximum downforce. It was done on relatively stock Z's to see what the typical bolt on aero mods did. In that respect the test was very effective and answered your questions directly. I think you can pretty well just look at the numbers and draw your own conclusions.

 

It could have been a lot more useful to you and me if it were done specifically to see what worked to make maximum downforce, but that really wasn't the purpose and it's not really fair to the guys who did the testing to complain about it now. Most (all?) of the arguments surrounding the wind tunnel testing have to do with things that WEREN'T tested, where people are speculating on whether something could have worked, or whether X which wasn't in the test works better than Y which was. Only real way to settle those arguments is another test.

 

Well spoken.

 

jt

[wondersparrow]

I always wondered about tilting the rad and scooping all of the air coming through it out the top through a hole in the hood. Much like the Blue Oval Z. My instinct is that would reduce the front end lift by reducing the ammt of air hitting the firewall. And the air hitting the scoop should create some downforce. OR it might just move that air to hitting the front wind shield and have little to no effect. It would obvuously not work with a mechanical fan, but it is one of the things I was toying with doing on my track car.

 

My 2 cents...

[JMortensen]

I always wondered about tilting the rad and scooping all of the air coming through it out the top through a hole in the hood. Much like the Blue Oval Z. My instinct is that would reduce the front end lift by reducing the ammt of air hitting the firewall. And the air hitting the scoop should create some downforce. OR it might just move that air to hitting the front wind shield and have little to no effect. It would obvuously not work with a mechanical fan, but it is one of the things I was toying with doing on my track car.

The air going through the radiator isn't moving very fast, so the downforce created by the slope of the ramp isn't going to be much if anything. What exhausting the radiator air out the top (or the sides) does is prevent if from going under the car and particularly under the hood, and that's where the downforce comes from.

 

I love the Janspeed hillclimb video for showing the effect of air under the hood. Right at the end there you can see the thin FG hood flexing up on the sides and in the middle due to the relative pressure underneath and on top of the hood. Just think of how hard you would have to push on that hood to get that kind of movement. That's upforce created by running without a radiator exhaust.

[wondersparrow]

The air going through the radiator isn't moving very fast, so the downforce created by the slope of the ramp isn't going to be much if anything. What exhausting the radiator air out the top (or the sides) does is prevent if from going under the car and particularly under the hood, and that's where the downforce comes from.

 

I love the Janspeed hillclimb video for showing the effect of air under the hood. Right at the end there you can see the thin FG hood flexing up on the sides and in the middle due to the relative pressure underneath and on top of the hood. Just think of how hard you would have to push on that hood to get that kind of movement. That's upforce created by running without a radiator exhaust.

 

I can speak from experience on that too. One of the tracks near me as a 1/2 mile front straight. My fg hood actually started to crack in the middle on the sides and I needed to reinforce it. Next time out, I noticed the cheapy cotter pin style hood pins at the back of the hood were starting to bend, so I upgraded them. I really need to do some work to stop that much air from getting under the hood and/or getting it out. I was either going to exhaust out the hood or build some ducts and take it out the back of the fenders.

 

I have a relatively low powered car (L24 GT3) so I am more concerned about keeping the front end down than rear end traction. I am pretty set on dealing with that before putting any form of rear wing on. A rear wing will just make the front end even lighter.

Edited May 13, 2011 by wondersparrow

[JMortensen]

I would err on the side of too much aero derived rear traction rather than too little. Oversteer at high speeds is not desireable, regardless of your hp.

[JIM73240Z]

good lord, was that hood made from paper. crap there were huge holes opening up at what seemed like slow speeds.

 

Jimbo

[JMortensen]

I'm sure the hood is very thin, but put it in perspective too. Let's say you can generate .1 psi pressure differential on the hood, and the hood is 36 x 48 inches (not actual dimensions, BTW). That would give you 172.8 lbs of force on the hood, from only .1 psi of pressure differential.