Corrected Data
Correction factors are another source of controversy in vehicle testing. Because weather conditions vary from day to day, this affects an engine's horsepower output. As a consequence, acceleration times can be effectively compared only if the results are adjusted to a set of standard atmospheric conditions. The most widely recognized correction factors are those the SAE specifies within its horsepower measurement procedure.
SAE correction factors have undergone a revision or two in recent years, and it is our policy to use the one contained in the most recent horsepower measurement procedure, SAE J1349. Turbocharged engine performance is not corrected by this standard, because modern turbocharged engines with electronic controls essentially produce and optimize their own atmosphere.
The old standard, SAE J607, is now considered obsolete by the SAE, but the use of its correction factor produces quarter-mile times that are about 0.3 second quicker than those returned by J1349. Some publications still use J607, ostensibly because they don't want to lose the ability to make comparisons to their library of past data. (Sure, the 0.3-second advantage they get in quarter-mile times has
nothing to do with it.)
If the outdated correction factor is combined with rollout, the results can be dramatic. The following example is based on data from a single run of our
2008 Mitsubishi Lancer GTS long-term test car. Here you can see the effects that the worst-case combination of correction factor and rollout can have on a 0-60 time.
Correction
Rollout
0-60 (sec)
1/4 mile (sec @ mph)
SAE J1349
(current)
without
8.61
with
8.30
16.44 @ 83.85
SAE J607
(obsolete)
without
8.23
with
7.92
16.17 @ 85.03
If you inappropriately apply rollout to 0-60 times and use the outmoded SAE J607 for weather correction, the 0-60 time appears to be 7.9 seconds. We use the more current SAE J1349 and do not use rollout for 0-60 runs, so we would report 8.6 seconds, a difference of some 0.7 second. On quarter-mile runs, where we do include rollout for reasons explained earlier, the difference comes down to correction factor alone, and in this example the difference would round out to 0.2 second and 1.1 mph.
Same car, same run, same raw data file, same ambient conditions, but different data processing — clearly, a lot of tricks can be played by massaging the raw data. And there's a strong temptation to corrupt the data in this way because acceleration times arouse such strong emotions among readers. Enthusiasts want their dream car to be super fast, so those publications that produce the lowest numbers are hailed as professionals, while anyone who gets a lesser number "doesn't know how to drive." We think it's more important to be as correct about performance as possible, so we're scrupulous about our data.
Meanwhile, the weather data we use for the correction calculations comes from a Novalynx WS-18 portable weather station we set up at the track. It records ambient temperature, wind speed and direction, barometric pressure and relative humidity at five-minute intervals throughout the day.
As we can see from above "these corrections" vary widely and sometimes help or hurt data. Since most magazines do not post what the corrected vs non corrected times are or if they use corrections, how are we to know to what degree the numbers have or have not been effected. *Then there are cars like the GT-R, whose performance seems unaffected by weather?* So if correcting test results is a standard practice then Motor Trends corrected sub 12 sec run is very much capable in good conditions. Why then should we take anything away from Motor Trends sub 12 sec 1/4 mile run with the ZL1?