A Porsche 928 competes at Pike's Peak in 2007
07-12-2007 | 12:28 PM
#152
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From: Horicon, WI
2" of clearence?? You do know there are some big rocks that get thrown on the course, right??
07-12-2007 | 02:18 PM
#154
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From: saratoga, ca
Carl, you may want to plug them up. ive measured the pressure in the cabin and it is not higher than ambient. its not like the windshield is open (bucket anaology) you know i have done a lot of testing here. I had the holes originally, and when i finished with my measurements, i replaced the rear window with a new one with no holes and gave my old one to scot 
.
Now, the other thought of venting air under the wing is kind of off too, as if you were able to increase air flow underneath the wing, it would be adding to the air traveling under the wing, right? this means more molecules of air, slightly higher pressure, and it would actually reduced total downforce. your gurney lip is pretty big. you can EASILY do a test as i did to check downforce with that set up. you will get efficiencies about 10x better if you do the risers on the stock wing and tilt it 10 degrees as i did on my GTS wing set up before the GT3 wing i now use. a gurney flap, basically changes the effective wings angle of attack, so if you take a level on the highest point and run down to the leading edge, see how many inches you need to make the level , level. if its any where near 4", that would be 4"/11"(invTan) for degrees. its going to be a big number. you dont want that. big drag, but more importantly, loosing the effectiveness of the wing. I can send you my risers if you want to try them.
Ideally, the gurney lip is a 1/8" to 1/4" lip added to create a little more downforce without increasing drag too much. make it too big and it defeats the purpose.
another note, with the holes closed the rear window is arching down . I actually thought that the window needed straps on the outside, but now after learning more about the air flow dynamics of the 928, i should have put a strap on the inside, as the window actually wants to come down into the car. the only time the window would want to blow out, and your bucket analogy will reign true, will be if you get in a spin where the side window actually becomes the front of the car . then like, Anderson saw at Las vegas, the rear window will blow out. (and the little holes wont stop that either). so, save the weight, seal it up and raise the wing, if you want my advice on that.
another side note, you also have to understand that the top of the wing. (bottom if it was on an airplane) is not really needed. however, the flow over the bottom of the wing (top on an airplane) is where the downforce (lift) comes from. It is very sensitive to flow disturbances. the way you have the ducts set up, looks like it certainly will effect flow negatively over the arch of the wing surface, increasing drag and decreasing downforce.
PM me and we can discuss more.
MK
.Now, the other thought of venting air under the wing is kind of off too, as if you were able to increase air flow underneath the wing, it would be adding to the air traveling under the wing, right? this means more molecules of air, slightly higher pressure, and it would actually reduced total downforce. your gurney lip is pretty big. you can EASILY do a test as i did to check downforce with that set up. you will get efficiencies about 10x better if you do the risers on the stock wing and tilt it 10 degrees as i did on my GTS wing set up before the GT3 wing i now use. a gurney flap, basically changes the effective wings angle of attack, so if you take a level on the highest point and run down to the leading edge, see how many inches you need to make the level , level. if its any where near 4", that would be 4"/11"(invTan) for degrees. its going to be a big number. you dont want that. big drag, but more importantly, loosing the effectiveness of the wing. I can send you my risers if you want to try them.
Ideally, the gurney lip is a 1/8" to 1/4" lip added to create a little more downforce without increasing drag too much. make it too big and it defeats the purpose.
another note, with the holes closed the rear window is arching down . I actually thought that the window needed straps on the outside, but now after learning more about the air flow dynamics of the 928, i should have put a strap on the inside, as the window actually wants to come down into the car. the only time the window would want to blow out, and your bucket analogy will reign true, will be if you get in a spin where the side window actually becomes the front of the car
. then like, Anderson saw at Las vegas, the rear window will blow out. (and the little holes wont stop that either). so, save the weight, seal it up and raise the wing, if you want my advice on that.another side note, you also have to understand that the top of the wing. (bottom if it was on an airplane) is not really needed. however, the flow over the bottom of the wing (top on an airplane) is where the downforce (lift) comes from. It is very sensitive to flow disturbances. the way you have the ducts set up, looks like it certainly will effect flow negatively over the arch of the wing surface, increasing drag and decreasing downforce.
PM me and we can discuss more.
MK
Originally Posted by Carl Fausett
They perform 2 aerodynamic functions: a) they depressurize the cabin and reduce drag, b) they increase downforce of the rear spoiler making it more effective.
Analogy: Imagine you are holding two buckets in the wind. One bucket is normal, on bucket has big holes in the bottom. You know which one will have more drag.
You may have noticed in your garden-variety 4-door sedan that the noise caused by driving with the driver's window only down is deafening, but if you also crack open a rear window, the noise (and turbulance and drag) is much lower and better. Same thing.
The car, with the windows down for racing - is the bucket. The air curls into the cabin on the 928 just in front of the B pillar, but cannot exit. Its like going down the main straight with the drag parachute extended ( a slight exageration, but an effective visual aid). If we let the air out of the back of the 928, then we reduce drag, and we cool the driver better too as we have improved air exchange in the cabin.
We simply aim the outlet below the spoiler to lower the air pressure beneath the spoiler, therby increasing downforce.
My design - I have not seen it yet anywhere else. I can't be the first to have thought of it - so I 'm sure I'll see it somewhere someday.
Analogy: Imagine you are holding two buckets in the wind. One bucket is normal, on bucket has big holes in the bottom. You know which one will have more drag.
You may have noticed in your garden-variety 4-door sedan that the noise caused by driving with the driver's window only down is deafening, but if you also crack open a rear window, the noise (and turbulance and drag) is much lower and better. Same thing.
The car, with the windows down for racing - is the bucket. The air curls into the cabin on the 928 just in front of the B pillar, but cannot exit. Its like going down the main straight with the drag parachute extended ( a slight exageration, but an effective visual aid). If we let the air out of the back of the 928, then we reduce drag, and we cool the driver better too as we have improved air exchange in the cabin.
We simply aim the outlet below the spoiler to lower the air pressure beneath the spoiler, therby increasing downforce.
My design - I have not seen it yet anywhere else. I can't be the first to have thought of it - so I 'm sure I'll see it somewhere someday.
Last edited by mark kibort; 07-12-2007 at 03:05 PM.
07-12-2007 | 04:10 PM
#155
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Joined: Dec 2001
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From: Horicon, WI
another side note, you also have to understand that the top of the wing. (bottom if it was on an airplane) is not really needed. however, the flow over the bottom of the wing (top on an airplane) is where the downforce (lift) comes from. It is very sensitive to flow disturbances. the way you have the ducts set up, looks like it certainly will effect flow negatively over the arch of the wing surface, increasing drag and decreasing downforce.
07-12-2007 | 04:14 PM
#156
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Joined: Dec 2001
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From: Horicon, WI
Here is a piece of yesterday's dirt track practice session:
The second one on this page:
http://www.928motorsports.com/privatevideos.php
The G-Force data is overlayed on the video, the L-R graph at the bottom is lateral G's, and you can see all I can get in the corners is about .5 G's. The Up-Down axis on the left is acceleration and decel.
Watch the mirror get shooken right off the windshield! The track was very rough and not groomed. That was actually good for us - we found one or two loose things to tie down better. Thats the point of testing...
The second one on this page:
http://www.928motorsports.com/privatevideos.php
The G-Force data is overlayed on the video, the L-R graph at the bottom is lateral G's, and you can see all I can get in the corners is about .5 G's. The Up-Down axis on the left is acceleration and decel.
Watch the mirror get shooken right off the windshield! The track was very rough and not groomed. That was actually good for us - we found one or two loose things to tie down better. Thats the point of testing...
07-12-2007 | 06:36 PM
#158
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Joined: Dec 2001
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From: Horicon, WI
Last photos of the car ready to go on the trailer.
You'll note we raised her up a tad from her asphalt racing height.
Let me take this moment and thank my sponsors for their support.
These things are not cheap, and their support is helping make it happen.
See you there! Connie McNabb is running things back here in WI with help from my daughter Amber and wife Connie while I am gone. Go easy on them!
You'll note we raised her up a tad from her asphalt racing height.
Let me take this moment and thank my sponsors for their support.
These things are not cheap, and their support is helping make it happen.
See you there! Connie McNabb is running things back here in WI with help from my daughter Amber and wife Connie while I am gone. Go easy on them!
07-12-2007 | 09:45 PM
#161
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Joined: Feb 2003
Posts: 29,817
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From: saratoga, ca
If you increase the mass flow, you increase pressure, not lower the pressure. if you were able to speed up the air coming from the roof line to the wing, that could do it too, but thats what the wing is actually doing. Taking the flow of air, and IMPARTING a negative presure on one side. if you add molecules of air to the bottom, it would be counter productive. This is just plain aerodynamic fact, and i can easily prove it for you and help you get more downforce if you need it. You may like how it feels at 100mph, but if you remove the "ports" im positive there will not be much difference. You can do a test with the hatch latch and see how much its compressing at 100mph. Seal up the ports and see if there is a difference.
Plus, with pressure tests that i have already reported on, that region at the back of the hatch is actually a higher pressure zone. the air is probably coming back in to the cabin at high speed. (one of the reasons i pluged the holes up, is that i was getting "smoked " out with exhaust fumes! this means the air was coming back in. you can certainly use the vents to change the buffeting characteristics, for lower interior noise, and thats what you may have done. you also may be cooler by the fact that because you have an open vent at the rear hatch, the air could be ventilating back in. all you need to do is put some wool streamers to proof this. the rear hatch area is being pushed down due to higher pressure vs the cabin pressure. ive already done the tests. I may seem like what you have done could increase downflow, but think about it. you are adding to the air flow (if lets say it is moving out of the window holes) and that would not speed up the air and lower the pressure, it would be adding to it, actually detracting to a certain extent, the downforce.
Think about this, the reasons we vent the hood in conjuction with the splitter, is to vent the air to the lower pressure zone on top of the hood. (besides the base of the windshield, the hood is a lower pressure zone. under the car is a higher zone of pressure (lift, bad!) so, we vent the air coming in the engine comparment to the lower pressure zone. its all about differential pressure.
you dont HAVE to agree, but its a pretty known science.
by the way, the car looks great. I cant wait to see how you do! be safe!!
MK
Plus, with pressure tests that i have already reported on, that region at the back of the hatch is actually a higher pressure zone. the air is probably coming back in to the cabin at high speed. (one of the reasons i pluged the holes up, is that i was getting "smoked " out with exhaust fumes! this means the air was coming back in. you can certainly use the vents to change the buffeting characteristics, for lower interior noise, and thats what you may have done. you also may be cooler by the fact that because you have an open vent at the rear hatch, the air could be ventilating back in. all you need to do is put some wool streamers to proof this. the rear hatch area is being pushed down due to higher pressure vs the cabin pressure. ive already done the tests. I may seem like what you have done could increase downflow, but think about it. you are adding to the air flow (if lets say it is moving out of the window holes) and that would not speed up the air and lower the pressure, it would be adding to it, actually detracting to a certain extent, the downforce.
Think about this, the reasons we vent the hood in conjuction with the splitter, is to vent the air to the lower pressure zone on top of the hood. (besides the base of the windshield, the hood is a lower pressure zone. under the car is a higher zone of pressure (lift, bad!) so, we vent the air coming in the engine comparment to the lower pressure zone. its all about differential pressure.
you dont HAVE to agree, but its a pretty known science.
by the way, the car looks great. I cant wait to see how you do! be safe!!
MK
Originally Posted by Carl Fausett
I'm sorry, but I disagree. We increased flow under the wing, which will lower the pressure beneath the wing, creating more downforce from the top. The results so far have been very good at speeds in excess of 100 MHP. We like them a lot. Two small side benefits that have also been nice is the inside of the cabin is much quiter and a lot less hot.
07-12-2007 | 11:14 PM
#162
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Joined: Mar 2002
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From: California
Great Thread!
There is lots of work there Carl.....WOW. Beautiful merge with those DEVEK L2 Headers!
Is the run half asphalt/half road base?
Mark - you are wise beyond your years!! Now can you guess what happens to the sunroof at 200 mph?
Cheers and good luck and be safe!!
Marc
There is lots of work there Carl.....WOW. Beautiful merge with those DEVEK L2 Headers!
Is the run half asphalt/half road base?
Mark - you are wise beyond your years!! Now can you guess what happens to the sunroof at 200 mph?
Cheers and good luck and be safe!!
Marc
07-13-2007 | 03:05 AM
#163
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Joined: Feb 2003
Posts: 29,817
Likes: 185
From: saratoga, ca
mine are what Marc, 6 years old now, and still look like jewelry! a little tough to take on and off, but worth it!
At 200mph, i have to imagine the sunroof just gets sucked out in to orbit??
Right?
Carl, that video is nuts, is there more asphalt now? looks like one wrong move can send you down a 5000ft cliff!!!
MK
At 200mph, i have to imagine the sunroof just gets sucked out in to orbit??
Right?
Carl, that video is nuts, is there more asphalt now? looks like one wrong move can send you down a 5000ft cliff!!!
MK
Originally Posted by marc@DEVEK
Great Thread!
There is lots of work there Carl.....WOW. Beautiful merge with those DEVEK L2 Headers!
Is the run half asphalt/half road base?
Mark - you are wise beyond your years!! Now can you guess what happens to the sunroof at 200 mph?
Cheers and good luck and be safe!!
Marc
There is lots of work there Carl.....WOW. Beautiful merge with those DEVEK L2 Headers!
Is the run half asphalt/half road base?
Mark - you are wise beyond your years!! Now can you guess what happens to the sunroof at 200 mph?
Cheers and good luck and be safe!!
Marc
07-13-2007 | 03:17 AM
#164
Captain Obvious
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Joined: Aug 2003
Posts: 22,846
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From: Cambridge, Ontario, Canada
Originally Posted by mark kibort
If you increase the mass flow, you increase pressure, not lower the pressure. if you were able to speed up the air coming from the roof line to the wing, that could do it too, but thats what the wing is actually doing. Taking the flow of air, and IMPARTING a negative presure on one side. if you add molecules of air to the bottom, it would be counter productive. This is just plain aerodynamic fact, and i can easily prove it for you and help you get more downforce if you need it. You may like how it feels at 100mph, but if you remove the "ports" im positive there will not be much difference. You can do a test with the hatch latch and see how much its compressing at 100mph. Seal up the ports and see if there is a difference.
Plus, with pressure tests that i have already reported on, that region at the back of the hatch is actually a higher pressure zone. the air is probably coming back in to the cabin at high speed. (one of the reasons i pluged the holes up, is that i was getting "smoked " out with exhaust fumes! this means the air was coming back in. you can certainly use the vents to change the buffeting characteristics, for lower interior noise, and thats what you may have done. you also may be cooler by the fact that because you have an open vent at the rear hatch, the air could be ventilating back in. all you need to do is put some wool streamers to proof this. the rear hatch area is being pushed down due to higher pressure vs the cabin pressure. ive already done the tests. I may seem like what you have done could increase downflow, but think about it. you are adding to the air flow (if lets say it is moving out of the window holes) and that would not speed up the air and lower the pressure, it would be adding to it, actually detracting to a certain extent, the downforce.
Think about this, the reasons we vent the hood in conjuction with the splitter, is to vent the air to the lower pressure zone on top of the hood. (besides the base of the windshield, the hood is a lower pressure zone. under the car is a higher zone of pressure (lift, bad!) so, we vent the air coming in the engine comparment to the lower pressure zone. its all about differential pressure.
you dont HAVE to agree, but its a pretty known science.
by the way, the car looks great. I cant wait to see how you do! be safe!!
MK
Plus, with pressure tests that i have already reported on, that region at the back of the hatch is actually a higher pressure zone. the air is probably coming back in to the cabin at high speed. (one of the reasons i pluged the holes up, is that i was getting "smoked " out with exhaust fumes! this means the air was coming back in. you can certainly use the vents to change the buffeting characteristics, for lower interior noise, and thats what you may have done. you also may be cooler by the fact that because you have an open vent at the rear hatch, the air could be ventilating back in. all you need to do is put some wool streamers to proof this. the rear hatch area is being pushed down due to higher pressure vs the cabin pressure. ive already done the tests. I may seem like what you have done could increase downflow, but think about it. you are adding to the air flow (if lets say it is moving out of the window holes) and that would not speed up the air and lower the pressure, it would be adding to it, actually detracting to a certain extent, the downforce.
Think about this, the reasons we vent the hood in conjuction with the splitter, is to vent the air to the lower pressure zone on top of the hood. (besides the base of the windshield, the hood is a lower pressure zone. under the car is a higher zone of pressure (lift, bad!) so, we vent the air coming in the engine comparment to the lower pressure zone. its all about differential pressure.
you dont HAVE to agree, but its a pretty known science.
by the way, the car looks great. I cant wait to see how you do! be safe!!
MK
07-13-2007 | 06:25 AM
#165
Drifting
Joined: May 2002
Posts: 2,777
Likes: 2
From: New Zealand
the car looks fantastic carl! am looking forward to watching the video link when i get home tonight
i have to say im with the aero opinions expressed here, the back end of the car has poorer airflow than around the front windows ie air will be venting from the front window area - where has it come from?? either through front ventilation channels or through the back hatch ie you will have air coming in from the back hatch.
i agree with mark though and think the wool/string test would definitely ahow whats going on - pretty easy to do as well
i have to say im with the aero opinions expressed here, the back end of the car has poorer airflow than around the front windows ie air will be venting from the front window area - where has it come from?? either through front ventilation channels or through the back hatch ie you will have air coming in from the back hatch.
i agree with mark though and think the wool/string test would definitely ahow whats going on - pretty easy to do as well