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ENGINE PERFORMANCE – TECH TIPS DYNAMOMETER TESTING AND FORD RACING CALIBRATIONS A question we at Ford Racing Performance Parts are often asked is “Why do certain companies claim to make more power with their power upgrade kits than you do with your kits?” To address that question properly, it is necessary to understand some of the intricacies of chassis dynamometer testing, as well as some of the compromises that must be made in order to simultaneously meet our standards for performance, emissions, durability and safety. CHASSIS DYNAMOMETER TESTING There are two main types of chassis dynamometers (dynos) in widespread use today. • An inertia dyno uses a large spinning drum that is accelerated by the drive wheels of the test vehicle. Power is then computed by knowing the inertia of the drum and how quickly it was accelerated. Torque can then be calculated by knowing the speed of the drum. • An eddy-current dyno absorbs and measures power by rotating a metallic disc through a magnetic field. Without getting into which dyno is more “correct” under what conditions, and why, we will simply say that these two types of dynos typically do not always give the same result even with all else being equal. It is generally not possible to accurately compare numbers from one type of dyno with those from the other type of dyno. Each type of dyno has its own advantages and disadvantages, but as long as all the tuning work is done on the same type of dyno, it doesn’t really matter which one is used. With any dyno testing there is a need for correction factors that are applied to the raw numbers the dyno actually measures. These correction factors are an attempt to correct for varying atmospheric conditions such as humidity, barometric pressure and air temperature. The two most common standards are SAE J1349 and SAE J607 (sometimes known as “STD” on some dynos). How correction factors are calculated is given in the “Crate Engine” section of this catalog. For this article, understand that these correction factors will give results that are different from each other, with SAE J1349 typically about 4% lower than SAE J607. OEMs will almost always quote J1349 corrected numbers when advertising horsepower and torque. Some “tuning” shops will report STD numbers because they are always higher than SAE. Be sure to ask which correction factor is being used when comparing dyno numbers! Whenever comparing dyno results, always be sure that the numbers are corrected to the same standard. Despite these correction factors, atmospheric conditions can play an additional role in terms of ignition timing. The correction factors account only for the change in the density of the air due to atmospheric conditions and cannot account for things like engine borderline spark sensitivity. As inlet air temperature increases, the PCM will generally retard spark to prevent detonation using the particular octane of fuel for which it was calibrated. Correction factors cannot account for this because different engine designs can have different spark sensitivity and different For important information about the proper usage of performance parts, please see page 14. See pages 286-292 for important safety, emissions and warranty information. www.fordracingparts.com 149


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