Frontal area and CD of a Porsche 928 S4
07-29-2009, 05:01 PM
#76
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Well it was a good thought! I think the tallest (Y) tire 186 mph+ is what I run which is 26.7" diameter and that would not help you at all. I wonder how you would lower your car back to a reasonable ride height, lets see 3+ inches. 
Stay with your existing setup then and I'll concede the 1/4 & 1/2 mile to you. (No Mark don't go there)
Stay with your existing setup then and I'll concede the 1/4 & 1/2 mile to you. (No Mark don't go there)
07-29-2009, 05:23 PM
#77
Race Car
29.2" diameter:
http://www.tirerack.com/tires/tires....omCompare1=yes
http://www.wheelenhancement.com/inde...lDetail&id=227
I can just imagine 22" wheels on a 928
Dan
'91 928GT S/C
475hp/460lb.ft
http://www.tirerack.com/tires/tires....omCompare1=yes
http://www.wheelenhancement.com/inde...lDetail&id=227
I can just imagine 22" wheels on a 928
Dan
'91 928GT S/C
475hp/460lb.ft
07-29-2009, 05:41 PM
#78
Rennlist Member
so, you are saying you need 80 more hp to make 200hp because they ran at 8am with no headwinds and cooler temps? I would like to hear Marc's take on that as well. do you always have headwinds in the afternoon, and if so, how fast. headwinds at 6000rpm are alot less of a problem than they are at sea level. (about 20% less) but still , it still is more air to push aside. Heat and humidity make the air even more thin, taking proportinoally more HP away, but a squared effect on aero loss, so in theory, it should be better in the hot, just as it is better as you go higher in altitude.
If I remember correctly, didnt Marc run 209mph with the 450hp rwhp engine? The only time it got all pumped up to over 500rwhp was when it was fitted with the ITB's, right? going on memory here, not trying to start something.
is your trap area perfectly level as well. that could be an issue obviously. (for or against )
Im not arguing that what has been seen is real. But, when the right values are placed in the formulas, the end result usually will match. simulators are good enough now, that they can predict results pretty darn close to reality.
my car, bone stock, was able to run 180mph at Nardo (unofficially) I think Nardo is near sea level and the car only had 300rwhp I would imagine, since I tested it 15years later at 293rwhp.
mk
If I remember correctly, didnt Marc run 209mph with the 450hp rwhp engine? The only time it got all pumped up to over 500rwhp was when it was fitted with the ITB's, right? going on memory here, not trying to start something.
is your trap area perfectly level as well. that could be an issue obviously. (for or against )
Im not arguing that what has been seen is real. But, when the right values are placed in the formulas, the end result usually will match. simulators are good enough now, that they can predict results pretty darn close to reality.
my car, bone stock, was able to run 180mph at Nardo (unofficially) I think Nardo is near sea level and the car only had 300rwhp I would imagine, since I tested it 15years later at 293rwhp.
mk
Mark / Heinrich, we are kind of talking about two different sets of data here. When Marc and Susan were running (1997-2003 or so) they had the advantage of starting there runs at 8am. Absolutely no disrespect to them, that was the order the cars ran back then but the old white car you are using as the bench mark could not break 200 under the conditions we run in now. My car has more HP, TQ and better aerodynamics that your bench mark car. I believe Marc would agree with this. Check out the 200 mph club speeds before 2002 compared to today. They were the first cars out. In the desert that means no head wind, no heat and no humidity. That is a substantial advantage. For the last six years the SS class has had to start at the end of all of the "stock or lightly modified" cars. It is routine for the temps to be in excess of 100 degrees. The winds kick up as the desert floor heats and creates a very strong headwind. Many times we do not even leave the start until 2-3 pm. Under these conditions it is very difficult to reach a certified 200+ mph. Just taking HP and altitude figures does not tell the entire story. I stand by my comments that you need 500 rwhp @ sea level in a 928 street car, to break the 200 mph barrier "in ORR competition". We all run the same day, same class and same event. I challenge any street legal, event compliant 928 with less than 450 hp @ sea level to come out and try. If you break 200 through the traps, I will reimburse your hotel and gas fees. I will also learn a great deal from you about how you did it and I will go faster the next time.
Spread sheets and internet calculations are a great starting point for achieving your goals and helping with initial setups. I used them extensively when we built our car. To rely on them exclusively for results of performance in this case is just bench racing.
Respectfully submitted,
Spread sheets and internet calculations are a great starting point for achieving your goals and helping with initial setups. I used them extensively when we built our car. To rely on them exclusively for results of performance in this case is just bench racing.
Respectfully submitted,
07-29-2009, 05:52 PM
#79
Rennlist Member
sorry, i have to.
ah, no I dont.
ah, yes I do. your 440hp and a 2.2 would not have any problem at any distance even with 26.7 tires vs any stockish 350rwhp.
as factual, actual data point. I found it really interesting when I did my 60 -100 tests with the new stroker motor that I could accelerate at the same rate with a one gear taller gear shift. (i.e. 4th gear with the stroker will accelerate the same as the old 3rd gear with the engine with 50hp less). thats with a full 70% less gearing advantage. (similar to what you would find in ONLY 1st gear with the old 3.09 comparison. ) What this means, the net net of extreme gearing advantages are worth about 50hp best case in 3rd gear, and after that, the trade offs begin.
(In first, the gains would be near double for that 1 gear and short period of time, ignoring slip, traction, etc)
sorry, I just couldnt help myself.
mk
ah, no I dont.
ah, yes I do. your 440hp and a 2.2 would not have any problem at any distance even with 26.7 tires vs any stockish 350rwhp.
as factual, actual data point. I found it really interesting when I did my 60 -100 tests with the new stroker motor that I could accelerate at the same rate with a one gear taller gear shift. (i.e. 4th gear with the stroker will accelerate the same as the old 3rd gear with the engine with 50hp less). thats with a full 70% less gearing advantage. (similar to what you would find in ONLY 1st gear with the old 3.09 comparison. ) What this means, the net net of extreme gearing advantages are worth about 50hp best case in 3rd gear, and after that, the trade offs begin.
(In first, the gains would be near double for that 1 gear and short period of time, ignoring slip, traction, etc)
sorry, I just couldnt help myself.
mk
Well it was a good thought! I think the tallest (Y) tire 186 mph+ is what I run which is 26.7" diameter and that would not help you at all. I wonder how you would lower your car back to a reasonable ride height, lets see 3+ inches.
Stay with your existing setup then and I'll concede the 1/4 & 1/2 mile to you. (No Mark don't go there)
Stay with your existing setup then and I'll concede the 1/4 & 1/2 mile to you. (No Mark don't go there)
07-29-2009, 05:54 PM
#80
Rennlist Member
didnt sterling post a photoshop picture of the 928 slammed and with larger wheels?
29.2" diameter:
http://www.tirerack.com/tires/tires....omCompare1=yes
http://www.wheelenhancement.com/inde...lDetail&id=227
I can just imagine 22" wheels on a 928
Dan
'91 928GT S/C 475hp/460lb.ft
http://www.tirerack.com/tires/tires....omCompare1=yes
http://www.wheelenhancement.com/inde...lDetail&id=227
I can just imagine 22" wheels on a 928
Dan
'91 928GT S/C
475hp/460lb.ft
07-29-2009, 06:09 PM
#81
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sorry, i have to.
ah, no I dont.
ah, yes I do. your 440hp and a 2.2 would not have any problem at any distance even with 26.7 tires vs any stockish 350rwhp.
as factual, actual data point. I found it really interesting when I did my 60 -100 tests with the new stroker motor that I could accelerate at the same rate with a one gear taller gear shift. (i.e. 4th gear with the stroker will accelerate the same as the old 3rd gear with the engine with 50hp less). thats with a full 70% less gearing advantage. (similar to what you would find in ONLY 1st gear with the old 3.09 comparison. ) What this means, the net net of extreme gearing advantages are worth about 50hp best case in 3rd gear, and after that, the trade offs begin.
(In first, the gains would be near double for that 1 gear and short period of time, ignoring slip, traction, etc)
sorry, I just couldnt help myself.
mk
ah, no I dont.
ah, yes I do. your 440hp and a 2.2 would not have any problem at any distance even with 26.7 tires vs any stockish 350rwhp.
as factual, actual data point. I found it really interesting when I did my 60 -100 tests with the new stroker motor that I could accelerate at the same rate with a one gear taller gear shift. (i.e. 4th gear with the stroker will accelerate the same as the old 3rd gear with the engine with 50hp less). thats with a full 70% less gearing advantage. (similar to what you would find in ONLY 1st gear with the old 3.09 comparison. ) What this means, the net net of extreme gearing advantages are worth about 50hp best case in 3rd gear, and after that, the trade offs begin.
(In first, the gains would be near double for that 1 gear and short period of time, ignoring slip, traction, etc)
sorry, I just couldnt help myself.
mk
He does not have 350 hp. He has 475 hp. I assume it is supercharged.
07-29-2009, 06:55 PM
#82
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so, you are saying you need 80 more hp to make 200hp because they ran at 8am with no headwinds and cooler temps? I would like to hear Marc's take on that as well. do you always have headwinds in the afternoon, and if so, how fast. headwinds at 6000rpm are alot less of a problem than they are at sea level. (about 20% less) but still , it still is more air to push aside. Heat and humidity make the air even more thin, taking proportinoally more HP away, but a squared effect on aero loss, so in theory, it should be better in the hot, just as it is better as you go higher in altitude.
If I remember correctly, didnt Marc run 209mph with the 450hp rwhp engine? The only time it got all pumped up to over 500rwhp was when it was fitted with the ITB's, right? going on memory here, not trying to start something.
is your trap area perfectly level as well. that could be an issue obviously. (for or against )
Im not arguing that what has been seen is real. But, when the right values are placed in the formulas, the end result usually will match. simulators are good enough now, that they can predict results pretty darn close to reality.
my car, bone stock, was able to run 180mph at Nardo (unofficially) I think Nardo is near sea level and the car only had 300rwhp I would imagine, since I tested it 15years later at 293rwhp.
mk
If I remember correctly, didnt Marc run 209mph with the 450hp rwhp engine? The only time it got all pumped up to over 500rwhp was when it was fitted with the ITB's, right? going on memory here, not trying to start something.
is your trap area perfectly level as well. that could be an issue obviously. (for or against )
Im not arguing that what has been seen is real. But, when the right values are placed in the formulas, the end result usually will match. simulators are good enough now, that they can predict results pretty darn close to reality.
my car, bone stock, was able to run 180mph at Nardo (unofficially) I think Nardo is near sea level and the car only had 300rwhp I would imagine, since I tested it 15years later at 293rwhp.
mk
It is almost always calm in the morning but there are always headwinds and cross winds at these events in the afternoon. I have heard reports from the finish line (before we run) of 10 to 25 mph on course. On two way races we may get the advantage on way but not the other. Our traps are not flat but they are the same for everyone. I can tell you for a fact that when you are hit with the wind gusts your speed falls very quickly. I can remember at the
Gamblers going downhill into the trap and my speed fell from 208 to 195 on one gust. It took me another mile or two to get back up to 203 before I crossed the line.
I am not trying to argue with anyone but these races and speeds are not accomplished in a test tube. They are on the closed public highway out in the middle of the desert. It is not as easy as checking a spreadsheet and making 440 hp and going out and running 200 mph because theoretically it should work. We don't have a wind tunnel, we experiment by ourselves and gleen any info from other cars at the event. If you were to build your ORR car to the specs people are throwing around you would be very dissappointed with the results on your first race. There are lots of racers that have been trying to join our exclusive (and disfunctional) little club for years and never make it. I do believe that my gearing is costing me maybe a few mph but short of having the gears machined for me there is no combination of Porsche gearsets that would work.
Got to go and make some money to feed my habit.
Last edited by Tahoe Shark; 07-29-2009 at 07:14 PM.
07-29-2009, 07:13 PM
#83
Rennlist Member
07-29-2009, 08:20 PM
#84
Instructor
The 928 rear-end, as attractive as that big, juicy butt is, is one of the biggest problems in terms or aerodynamics.
Somebody should come up with a Peter Brock-designed Cobra Daytona Coupe-style rear bumper cover. Brock based this on the aerodynamic ideas of Wunibald Kamm, and is called the Kamm-tail or Kammback. It was reported to have improved top speed and fuel economy by 25% over the more rounded earlier rear.
http://www.secondstrike.com/Coupe/Brock.htm
This back-end style is also seen on the Ferrari Dino.
If you've seen a 928 with the rear cover off, you'd see there is an opportunity to do this. The underlying structure is recessed quite a bit under the cover.
Somebody should come up with a Peter Brock-designed Cobra Daytona Coupe-style rear bumper cover. Brock based this on the aerodynamic ideas of Wunibald Kamm, and is called the Kamm-tail or Kammback. It was reported to have improved top speed and fuel economy by 25% over the more rounded earlier rear.
http://www.secondstrike.com/Coupe/Brock.htm
This back-end style is also seen on the Ferrari Dino.
If you've seen a 928 with the rear cover off, you'd see there is an opportunity to do this. The underlying structure is recessed quite a bit under the cover.
Thats an interesting read on the Daytona. I've been familiar with this Kamm Back aero trick. Lost of early vintage race cars adopted this I'm sure.
Whenever I'm looking at my 928's ***, at least once a day
I've always thought that it has this Kamm design built into it somewhat. Take a look at where your rear number plate or ID plate is located. It's somewhat resembles that only a little smaller scale to the true Kamm Back car. Another this is that the early 928's rear is shorter than the S4 shape onwards. You guys with the rear bumber over-riders placed between the number plates should remove them to increase this area to the rear.
In the Daytona article, it was decide to shorten the rear rather than making it longer and sleeker like a A/C fusealage, to create the 'virtual tail'. In other words to cut the air off the rear of the car sooner, not to extend and smoothen out.
I think the (early) 928 could benifit from a rear undertray that took advantage of the rear P/U shape to create that Kamm area. With the little rear hatch spoiler combined with the rear mod would definately make the car 'aero' faster.
Any frontal area of most high performace cars can cut through the air well. Passenger A/C are bullnosed because they are sub sonic. It's they way the air travells over the shape after this that makes the difference. Proven by that Daytona.
Cheers,
Glenn
'81 928
AU
07-29-2009, 08:52 PM
#85
928 Collector
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Agreed. It should be inexpensive, there are good plasti-mold shops available these days. In fact, a Rennlister was recently asking on here, which 928 parts he could plasti-mold for us.
07-29-2009, 08:53 PM
#86
Rennlist Member
Tim, as far as head winds, certainly that would be a HUGE difference. If that was the difference between the devek AM runs and how they now run you guys in the afternoon, I certainly understand.10mph difference in winds is HUGE HP, as I have to correct myself for making it sound trivial. your true air speed just went up to 5% while your ground speed didnt change . thats another 60hp required to drive the extra aero. there you go, 500hp vs 440hp.
. thats another 60hp required to drive the extra aero. there you go, 500hp vs 440hp.
07-29-2009, 08:55 PM
#87
07-31-2009, 05:47 PM
#89
Race Director
ORR racing is its own unique animal...& its safe to say Tim knows his stuff, George too...since they have the only 928's that are tech certified for "unlimited"....sure a street car might have the power-gearing to do it...but it doesn't have the safety equipment or driver experience to pull it off either
Having a 928 that on paper will do 200+ is one thing...... I'm sure Todd's 675whp supercharged monster could do it with tall enough gears (think he is running 2.54 so he might not make 200) & Louie too......
Just for fun, assuming the widow is .36cd (maybe more for the rear flares) in order to redline 5th gear (6200rpm) at 181mph (335/30-18 tires) would take 329hp.....but is that wheel HP or crank HP...if its crank then I should be able to do it????
Having a 928 that on paper will do 200+ is one thing...... I'm sure Todd's 675whp supercharged monster could do it with tall enough gears (think he is running 2.54 so he might not make 200) & Louie too......
Just for fun, assuming the widow is .36cd (maybe more for the rear flares) in order to redline 5th gear (6200rpm) at 181mph (335/30-18 tires) would take 329hp.....but is that wheel HP or crank HP...if its crank then I should be able to do it????
07-31-2009, 06:16 PM
#90
Rennlist Member
If you got that off the calculator, its rear wheel hp. flares wont change the Cd that much, its more about the frontal area increase. (but certainly both are factors). again, the holbert machine ran 180mph at nardo, so 300rwhp can certainly do that under the right conditions and aero enhancments.
ORR racing is its own unique animal...& its safe to say Tim knows his stuff, George too...since they have the only 928's that are tech certified for "unlimited"....sure a street car might have the power-gearing to do it...but it doesn't have the safety equipment or driver experience to pull it off either
Having a 928 that on paper will do 200+ is one thing...... I'm sure Todd's 675whp supercharged monster could do it with tall enough gears (think he is running 2.54 so he might not make 200) & Louie too......
Just for fun, assuming the widow is .36cd (maybe more for the rear flares) in order to redline 5th gear (6200rpm) at 181mph (335/30-18 tires) would take 329hp.....but is that wheel HP or crank HP...if its crank then I should be able to do it????
Having a 928 that on paper will do 200+ is one thing...... I'm sure Todd's 675whp supercharged monster could do it with tall enough gears (think he is running 2.54 so he might not make 200) & Louie too......
Just for fun, assuming the widow is .36cd (maybe more for the rear flares) in order to redline 5th gear (6200rpm) at 181mph (335/30-18 tires) would take 329hp.....but is that wheel HP or crank HP...if its crank then I should be able to do it????