How to calculate hp from quarter mile run
07-22-2012, 12:48 AM
#1
Three Wheelin'
Thread Starter
How to calculate hp from quarter mile run
This may have been posted elsewhere (but I couldn't find it).
There is an interesting formula you can use to calculate hp from the 1/4 mile terminal velocity. It is knows as the Osbourne-Reynolds formula.
It goes like this:
Power = weight x (speed/n)^3 or (in fwhp)
Speed = n x (power/weight)^0.33 (in mph)
n is a constant that is somewhere between 225 and 235. (varies depending on losses in transmission, rolling resistance and cdA)
So in my case my supercharged S2 has the following parameters
Weight = 3003 lb (with driver)
1/4 mile speed = 107.3 mph
therefore using n = 235 (conservative):
Power = 286 hp. My dyno hp is around 245 x 1.17 = 285hp.
By the way, the 1/4 mile ET was 13.45 s with some wheel spin at the start.
There is an interesting formula you can use to calculate hp from the 1/4 mile terminal velocity. It is knows as the Osbourne-Reynolds formula.
It goes like this:
Power = weight x (speed/n)^3 or (in fwhp)
Speed = n x (power/weight)^0.33 (in mph)
n is a constant that is somewhere between 225 and 235. (varies depending on losses in transmission, rolling resistance and cdA)
So in my case my supercharged S2 has the following parameters
Weight = 3003 lb (with driver)
1/4 mile speed = 107.3 mph
therefore using n = 235 (conservative):
Power = 286 hp. My dyno hp is around 245 x 1.17 = 285hp.
By the way, the 1/4 mile ET was 13.45 s with some wheel spin at the start.
07-22-2012, 01:12 AM
#3
Three Wheelin'
Thread Starter
I thought it was an interesting way of equating horsepower to terminal velocity. The actual results weren't the reason for the post here, more the comparison of calculated vs actual. The 60' time makes little difference to the outcome, because by the time you reach the 1/4 mile, the difference is normally only 1 or 2 mph (max) between runs regardless of how poor a launch you get.
07-22-2012, 05:56 PM
#4
Three Wheelin'
Thread Starter
In practice your terminal velocity doesn't vary much. In 6 runs, there was only 1 mph variance even though the ET varied by 0.5s.
Yes the n value is a guess. It is certainly not meant to be as accurate as a dyno, but gives you a reasonable guide.
Yes the n value is a guess. It is certainly not meant to be as accurate as a dyno, but gives you a reasonable guide.
07-23-2012, 07:16 AM
#5
Three Wheelin'
+1 on that. Terminal velocity is more repeatable than 1/4 mile time, and isn't sensitive to off-the-line technique so no need to sidestep the clutch...
07-23-2012, 11:52 AM
#6
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