F
(the correction factor)= 1.18 (29.38/(BP-Vp)x the square root of
(T+460)/537)-0.18. Also, to find Vp, find the temperature on the steam tables
and multiply the corresponding factor by the relative humidity when expressed
as a decimal. (i.e. 70% relative humidity should be expressed as .7) Given the
environmental conditions shown on the dyno sheet, that should yield a
correction factor of 1.0280698.
Just
to further confuse things, the dyno sheet shown in the example for corrected
data was an older one that was calculated from the SAE tables, not from this
formula. The Society of Automotive Engineers (SAE) has since discontinued use
of those tables for data correction and now has adopted the newer,
formula-based means of correcting data. The correction factor that you arrive
at from the tables is somewhat different from the one derived from the
formula. Some of that is due to the fact that all the most current stuff is
calculated using metric measurements; temperature in Centigrade, barometric
pressure in Pascals, and so on. The differences are tiny, but all together
they yield a slightly different correction factor.
I
should point out, however, that it doesn't really matter how you correct your
data only that you do it. Even if the data correction formula or table you use
isn't the most current one, as long as you're consistent with how you adjust
for temperature, humidity, and barometric pressure, you'll be able to get
maximum benefit from your dyno data. Remember, if you don't normalize your
data for different environmental conditions, it will be a lot less useful,
comparison-wise.
In
retrospect, I should have re-made the corrected data printout using the
formula (including the missing factors) that was outlined in the article.
Sorry for any confusion this created. It is gratifying, however, and a measure
of how many readers are seriously interested in this subject, to note how many
of you took the trouble to drag out your calculators and grind through the
formula to discover that I was wrong. Thanks for keeping me on my toes. I'll
try not to lead you astray again.
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