Documents here are from other authors, and may be used as additional information sources. These are generally those that I think cover technnical topics in more fundamental detail than one can generally find on web forums, magazines, etc.
I have chosen documents that are accessible to those without math skills much beyond second-year algebra--since I myself would have to struggle with them in that case.
As with about everything on this site, the information here will probably be of most use to those who are very interested in the "how and why" of internal combustion engines, not those who are simply looking for a recipe for selecting parts.
"Engine Fundamentals" - Racer Brown - Deals with conceptual topics related to performance modifications. Much of the info has to do with Datsun (Nissan) four-cylinder road racing engines, but it applies equally to Harleys. Read this several times, as it has lots of good information.
"Tech Tips 2000" -- Iskenderian - Topics related to cam design and associated engine components. Much of the info deals with circle track small-block Chevys, but still applies to our engines.
"Daytona Twin-Tec FAQs" - Daytona Twin-Tec LLC - Information related to compression, AFR, and Timing when tuning bikes. Very clear, very concise, and in my opinion, very correct. Should be read by every would-be tuner, as it addresses some of the more common misunderstandings.
"VFI Course Prerequisite Reference" - Powersports Institute - A brief introduction to terminology and concepts related to EFI, specifically as they pertain to S&S VFI modules. Used to introduce potential attendees to some of the concepts they will need to understand while attending the S&S VFI Tuner classes at PSI.
"FM WB Shootout" - Ford Muscle Magazine - A very well-done comparison of several well-known WideBand Controllers. Some surprising info regarding a well-known motorcycle module.
"Back to Basics" - Prof. Gordon Blair - A paper titled "Fundamentals and Empiricism in Engine Design". Very short and basic treatment of the physics to be considered when designing an engine. Mid-level treatment, with a few math formulae. Blair is one of the most quoted Internal Combustion Engine researchers, and is generally considered to be an authority on the subject.
"Exhaust Tech" - Race Engine Technology magazine - An interesting introduction to various considerations in exhaust system design. Explanations and illustrations are worthwhile, and the text is accessible to nearly all levels of reader.
"Riblets" - Brian Douglas Dean - Graduate thesis on the effect of various forms of protrusions and shapes on airflow in ductwork. Inspired by the research and reporting on the effect of sharkskin texture on the ability of sharks to swim through water efficiently.
"Using O2 Sensors to Tune" - NGK - Information that I excerpted from NGK's AFX Wideband Controller User Guide. Generally useful information for the beginning tuner who has been exposed only to incorrect conventional wisdom regarding tuning EFI motorcycles. Includes particular attention to tuning using AFR values.
"VE and Airflow" - Unknown - Presents some fundamental information on the topics.
"Valvetrain Tech" - Unknown - Presents some fundamental information of the pushrod valvetrain, including camshaft dynamics, terminology, valvetrain interactions, etc.
"Engine Dyno Guide" - Motion Software - The user guide for the 1996 MS-DOS version of the Desktop Dynos product. (These are also the people who produce the Dynomation engine simulation program). Beyond the setup and user-guide portions of the manual, it presents some of the concepts used in engine simulation software, along with some very interesting basic information about how engines actually work. I consider this to be one of the better introductory-level documents on the physics of the internal combustion engine, written without any higher-level math. Note that this document was copyrighted, but references a very out-of-date version of their product, and so I hope that they will not mind if I include it here.
"Injector Timing" - From Motec - A presentation of the effect of injector pulse timing (the point at which it begins and/or ends) on engine performance by Motec, who are a manufacturer of aftermarket automotive high-end EFI systems. Especially helpful for those who are not familiar with the topic.
"Realistic Info Is Rare" - Taken from http://www.calsci.com/motorcycleinfo/Horsepower.html website that presents power and performance information in a truthful manner, with a minimum of exaggeration. Since I am a very firm believer in those things, I am impressed—even though I don’t 100% agree with everything written.
I believe that his intent in creating that site was to present material that readers could use to further their own education regarding not only Harleys, but also motorcycles in general. Since I share that objective, I have stolen the majority of what he has written and included it here. I have made no substantial changes, other than correcting some spelling errors and adding some "in my opinion" explanatory footnotes. I also must say that the text seems to have been written in the early-to-mid-90’s, when air-quality concerns first became a part of HD engines. As a result, some of the information, for instance that regarding ignition, is outdated in my opinion.
"Porting Commentary" - A very short document on the subject of porting Harley heads. I found it on the web, but it was not attributed to anyone. It appears to have been written by someone who, while not a professional porter, does understand the concepts well enough. I have added comments as footnotes where I do not fully agree, or do not completely understand what the writer was trying to say. Overall, a worthwhile read at the conceptual/theoretical level.