P3Driver, I have researched and copied roughly 1,300 patents pertinent to the Wankel and closely related engine designs. These involved extensive searches over several months.
Broadly speaking, US utility patents are valid for 20 years. These are the regular numeric series with no letters before the numeric part and which include Wankel engines and components: for example, patent 6,543,210. This term was extended from 17 years some years back, in case you have heard about the 17-year figure. Other types of US patents use a letter in front of the serial number, such as design patent D123,456, but these generally would not apply here.
The few patents that are still good date from 1985 and newer and are relatively few in number. Generally they cover such things as stratified charge and other refinements to the basic rotary design through computer simulation and research. Most aspects of the basic Wankel design were patented in the late 1950s-early 1970s, and those patents have long expired.
Wankel patents appeared in three waves: (1) one cresting in the mid-1960s, covering the basic engine design and concepts; (2) one cresting in the mid-1970s, even larger than the first, covering refinements and new configurations (e.g., apex seal configurations and materials). During this heyday, well over 100 US patents appeared annually. These patents, too, have expired. A distinct trough followed this second wave in the late 1970s-mid-1980s. Finally, (3) a third, much smaller wave based on the computer research mentioned peaked in the late 1980s-early 1990s. These patents are generally still valid because of the 20-year rule.
Mazda's oldest designs are no longer covered. The expiration of the basic patents led to the abandonment of the Wankel licensing scheme in the early 1980s; NSU's ceasing to exist as a manufacturer, the wholesale desertion by most licensees of Wankel development in the 1970s, and Audi's abandonment of Wankel development by the mid-1980s didn't help. Consequently, as far as I know, you can knock yourself out producing and selling a variation of one of the older rotary designs without worrying about patent infringement or NSU-Wankel licensing hassles. (Note: I'm not engaged in giving legal advice; talk to a lawyer just to be sure, if you're serious.)
Mazda has so far applied for no distinct US patents for the internals of the RENESIS design. (By contrast, it has applied for numerous patents on the novel RX-8 body structure and styling.) That is because the basic side-port design was patented back in the early 1960s and its use as the exhaust port would be considered obvious by the US Patent and Trademark Office.
Some of Mazda's 1980s patents for the 13B that are still in effect would still cover other features of the RENESIS design. The only patent pending on anything to do with the RENESIS in the US is one for the engine positioning/mounting system, if I recall correctly.
I hope this helps. You can do basic patent searches on the USPTO website or on gb.espacenet.com. Take some time and explore the latter, as you can bring up US and European patents back to 1920 once you learn how to search and then narrow your searches. Good luck.
Doing a quick estimate, the RX-7 is producing about 80% of its peak torque at 2000rpm with the 6-port forced closed.
