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Horse Power needed to run a certain speed:
On the last page we left off with a drag force of 715. I have found two formulas that help you determine the rear wheel HP needed to run a certain speed with a car. Please don't ask me to show you how these formulas were derived, since I don't know, but I do have some faith in them.
One of them is: HP ={ [(W X R) + D] X V} / 375
where:
W = Vehicle weight in pounds -- R = Rolling resistance (they say 0.008 is a good value for highway speeds; over 150 mph you may need to double that to 0.016) -- V = Vehicle speed in MPH -- D = Drag Force (which we found above).
Again plugging in data from the Stude where W = 3000 lb. -- R = 0.016 -- D = 715 and V = 220 MPH we get 448 HP (rear wheel) needed to go 220 MPH for this car.
The other formula I found is: Rear Wheel HP = ( A X Cd X V (cubed) ) / 146,625
Going back a page A = 20.2 sq. ft. -- Cd = 0.30 and V = 220 MPH we get with this formula that it takes 440 HP to go 220 MPH for the car. ( Again you can find spread sheets for these formulas ( HERE ))
We don't have dyno numbers on the car for 2004 when it ran 219 MPH, but using the Dyno 2000 program and by a gut feeling we think the 406 small block chevy running on alcohol was making over 500 HP at the crank, so probably in the mid 400's at the rear wheels at 6500 rpm, which was about the RPM we were at on the 219 mph run. So the numbers both calculated and real world seem to be very close.
Horse Power needed to run a "new" speed based on a previous speed:
Now that we have a HP for a known speed let's see what happens if we want to predict how much HP we need to run faster. In 2004 we had an exit speed of about 219 with the alcohol motor and then noticed that the "B" motor blown gas record was 231+ MPH. So we thought we just need to run about 13 MPH faster and we might have a record. So if we wanted to run the 232+ we needed for a record how much HP would we need with the blown gas motor vs. what we were making with the alcohol motor? To give us a little cushion let's figure the HP needed to go 240 mph.
Using the formula that it takes 8 times the HP to go twice as fast (you can also find a spread sheet for that ( HERE )) we find that it will take 576 HP (RW -rear wheel) to run the 240 MPH with our car. Like wise if we plug the numbers above into the two previous HP formulas we went through above we come up with 575 Rear Wheel HP (using a car weight of 3000 lb.) and 571 Rear Wheel HP using the 2nd formula. Since the above is Rear Wheel HP a realistic at the crank HP would be around 680 to 700 HP.
Now we start getting a real sense for what Rex was talking about, that it takes a lot more HP to overcoming the "aero drag" and not the weight of the car. We want to run about 20 mph faster, but it is going to take an additional 157 At the Crank HP ( 132 Rear Wheel HP) to do it at the speeds we are at in this example.
How much HP do you think it would take to go from running 40 mph to say 60 mph, still only an increase in 20 mph?? If my calculations are right only about 10 more HP to go from 40 to 60 mph with the same car. Think "aero drag".
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