[Spridgets] Power required for a given CDa to reach a specific speed.

[WeslakeMonza1330 at aol.com]

Hi Ron & List,
 
I worked backwards using exact power values to get the speed values.   I 
suppose it doesn't matter that the speeds are round up or down rather than the 
 power.  The chart still doesn't look exactly the way I wanted to but will  
do for now.  For example I can't get the legend reversed so it matches the  
data.
 
The spread sheeting/data challenge is to get this data overlaid to rpm for  
various gears.  The objective is to show that more power doesn't always  
generate more speed.  This is because if the car is over-geared  the 
additional power required to raise the speed is demanded at a  higher rate than the 
power increase of the engine for the same rpm.  Put  another way the car is 
said to be drag limited rather than power or rev  limited.
 
Looking at the data the difference in speed, for cars with identical power, 
 is 9 mph (actually 8 but never mind!).  However, the car with the best  CD 
achieve max speed at 6,218 rpm. While the car with the worst CD achieves  
max speed (which is lower by 8/9 mph) at 5,810 rpm.  Except the car with  the 
worst CD doesn't actually reach 114 mph because the engine is not making 
100  bhp at 5810 (it's making at say 6000 - 6300rpm)
 
Here are the charts if it doesn't make sense now it will when I've had more 
 time to work on the charts.  The examples are for a car with a 5 speed on 
a  3.9 diff etc and for the higher drag car using 4th instead of 5th won't 
help it  because that would need 6785 rpm at which engine still doesn't make 
100 bhp -  being the other side of peak power.
 
Regards
 
 
Danel
 
 
In a message dated 19/11/2017 16:18:12 GMT Standard Time, soavero at yahoo.com 
 writes:

Daniel,
I honestly cannot remember, but the equation with the 146,000 in the  
denominator is the equation I am "familiar" with, meaning I remember it ;).  It's 
too clean to be a conversion factor. I vaguely remember it being in  
Stokes' equation. I will look when I'm not tapping into a phone. 


By the way, the full radius stacks I mentioned were the ones you gave me  
years ago when I'd sent those aviation books for your brother. They were for 
a  45 DCOE. The stock ones were better at midrange but yours sung at high 
RPM.  I'm pulling 8500 RPM in this video. 


https://youtu.be/Iwsf5jr1kpE

Ron Soave


On Nov 19, 2017, at 7:42 AM, _WeslakeMonza1330 at aol.com_ 
(mailto:WeslakeMonza1330 at aol.com)   wrote:



 
Hi Ron and list,
 
I want to create a graph in Excel that shows how much power required  for a 
given CDa to reach a specific speed.
 
To this I need a formulae that I can use in Excel.  I have a  library of 
technical motoring books that puts to shame virtually any public  lending 
library in the UK and maybe some colleges.  However, I can't  find the exact 
details of what I need.
 
The Forbes Aird Aerodynamics book includes a formulae along with a  value 
that I don't see referenced in the text.
 
The Colin Campbell Sports car book is about the same but with a  different 
value I don't see referenced in the text, which is:
 
Drag Horsepower (I'm ignoring non-drag horsepower for now) =  Coefficient 
of drag x frontal area x mph cubed  DIVIDED by  146,000.  So what is the 
value of 146,000?
 
Thanks
 
 
Daniel
 
 
 
 
 
 




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