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For HP
losses per 1000 feet, it is quite common here (in the high deserts of the
Western USA) to see HP correction factors change as much as one percent over
the course of a day. For a bracket racer, this can make as much as a 4/10th
second difference in 1/4 mile ET's, (or something like forty HP in a 400 HP
engine),
Each tenth of a second equates to a full car length change at the end of the
1/4 mile, which can be huge in a race which is often times won or lost by
less than an inch. Even though our local 1/4 mile track is located at some
2700 feet ASL there are times during the spring and early fall when cool,
dry, high pressure days equate to "sea level air" quality.
Staying on top of the HP correction factors is a must when the competition
is laying down "ten thousandths" packages on both ends of the track.
One of the best ways to check HP production is to make a few runs at the
local 1/4 mile track and then work backwards either from ET or MPH to
calculate rear wheel HP. The http://www.speedworldmotorplex.com website
generates solid HP numbers when the ET, MPH and car/driver weight are known.
While dyno generated HP numbers can be fun to play with, there has been too
many times when the figures are just not believable (kind of like EPA
mileage figures for new cars!). Our local dyno guy tested a 240-Z GN Buick
powered car and claimed some 665 HP, yet when the car was tested at the
track it only ran 4/10ths faster than an identical chassis V-8 car which
tested on a different dyno at 492 HP. ET/weight calculations show that both
cars were putting no more than 410 and 450 hp down to the rear wheels (at
the track).
My point is that real time/real world generated race data can add a valuable
witness as to how well the entire car performs.
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