The links presented here are not to your usual sites where you can buy stuff. Those are readily available everwhere else. These are links to gearhead information---the kind of information that most newer riders have not yet been exposed to.
Harley Riders Guide http://www.harley-riders-guide.com/
A collection of articles that present several of the basic concepts surrounding Harley performance. Information is generally better than average. While the site is ad-supported, and some of the articles appear to be directly lifted from other sources, (or maybe it's just my imagination), this does not affect the accuracy of the information.
Depth of information is significantly above the usual magazine level, and is highly recommended as a starting point for the person interested in understanding more about EFI, pipes, engines, and even some information regarding dyno tests that they may not have known before.
Night Rider http://www.nightrider.com
A well-known and informative site, created and maintained by Steve Mullen. Full of all sorts of detailed articles, charts, tables, etc., etc. Every rider should spend some time with this site, even if you don't agree with each part of every section. Steve has put more effort into assembling information and making it available to people everywhere than anyone active today that I am aware of.
It is ad-supported, and in addition, Steve sells the XIED Fuel Injection "tuning" devices, but I have never seen any of the information in the site that I thought was slanted in any way because of that.
This is the go-to online repository for Harley information. Think of it as you would the reference section of your local library, and you won't be far off: it ain't got everything, and what it does have may not be exactly what you wanted, but you have to appreciate the effort if nothing else.
Tilden Technologies http://www.tildentechnologies.com
More than you ever wanted to know about cam design. There are forums available which may have members who know more about the nitty-gritty of cam design, but this guy collects most of it in one spot. Probably over the head of most people, and very difficult--for me at least--to read, but it's all there.
If you are the guy who thinks all you have to do to design a custom cam is spec timing and lift, read this and be humbled. Suddenly Harvey Crane, John Andrews, Tom Sifton, and other bike names from back in the day become giants...
SpeedTalk Forum http://www.speedtalk.com
This site layout is conceptually similar to a magazine, in that it has articles, interviews, and regular sections. The Forums are one of the regular sections. There are several of these forums, but the two I want to mention here are Engine Tech, and Advanced Engine Tech.
The Engine Tech section is for the usual kind of questions and discussion, generally above the level of other motorhead forums, but still a lot of "what cam should I use" type of topics.
Where you want to be is in the Advanced Engine Tech section, on which you must register to access. Here you will find people who are at the highest professional level of design, engineering, building, competition, whatever. There are people in here who "do magic" every day as part of their usual business. Sign up, shut up, and if you pay a lot of attention and study hard, you will learn. Everything in this forum is almost entirely about automotive engine technology, but an IC engine is an IC engine. The engineering and theory are similar. You won't see much about v-twin engines, but when you do, it's even more interesting. For instance, "the effect of the 315/405 firing sequence on intake/exhaust design", or similar topics. if you want to know about cam design, porting, exhaust, and on and on, this is one place to find that.